Suicide among LGBT youth

From Wikipedia, the free encyclopedia
https://en.wikipedia.org/wiki/Suicide_among_LGBT_youth

Research has found that attempted suicide rates and suicidal ideation among lesbian, gay, bisexual, transgender (LGBT) youth are significantly higher than among the general population.

In the United States, one study has shown the passage of laws that discriminate against LGBT people may have significant negative impacts on the physical and mental health and well-being of LGBT youth; for example, depression and drug use among LGBT people have been shown to increase significantly after the passage of discriminatory laws. By contrast, the passage of laws that recognize LGBT people as equal with regard to civil rights, such as laws supporting same-sex marriage, may have significant positive impacts on the physical and mental health and well-being of LGBT youth.

Bullying of LGBT youth is a contributing factor in many suicides, even if not all of the attacks have been specifically regarding sexuality or gender. Since a series of suicides in the early 2000s, more attention has been focused on the issues and underlying causes in an effort to reduce suicides among LGBT youth. Research by the Family Acceptance Project has demonstrated that "parental acceptance, and even neutrality, with regard to a child's sexual orientation" can bring down the attempted suicide rate.

Reports and studies

See also: List of LGBT-related suicides

Clinical social worker Caitlin Ryan's Family Acceptance Project (San Francisco State University) conducted the first study of the effect of family acceptance and rejection on the health, mental health, and well-being of LGBT youth, including suicide, HIV/AIDS, and homelessness. Their research shows that LGBT youths "who experience high levels of rejection from their families during adolescence (when compared with those young people who experienced little or no rejection from parents and caregivers) were more than eight times [more] likely to have attempted suicide, more than six times likely to report high levels of depression, more than three times likely to use illegal drugs, and more than three times likely to be at high risk for HIV or other STDs" by the time they reach their early 20s.

Numerous studies have shown that lesbian, gay, and bisexual youth have a higher rate of suicide attempts than do heterosexual youth. According to a Trevor Project 2023 survey, 18% of LGBT youth have attempted suicide, a rate 2 times higher than teenaged general population. This higher prevalence of suicidal ideation and overall mental health problems among gay teenagers compared to their heterosexual peers has been attributed to minority stress, bullying, and parental disapproval.

Parents with higher levels of education or belonging to different ethnicities do not seem to provide significant impact on LGBT+ suicide statistics.

In terms of school climate, "approximately 25 percent of lesbian, gay and bisexual students and university employees have been harassed due to their sexual orientation, as well as a third of those who identify as transgender, according to the study and reported by the Chronicle of Higher Education." Research has found the presence of gay–straight alliances (GSAs) in schools is associated with decreased suicide attempts; in a study of LGBT youth, ages 13–22, 16.9% of youth who attended schools with GSAs attempted suicide versus 33.1% of students who attended schools without GSAs.

"LGBT students are three times as likely as non-LGBT students to say that they do not feel safe at school (22% vs. 7%) and 90% of LGBT students (vs. 62% of non-LGBT teens) have been harassed or assaulted during the past year."

An international study found that suicidal LGBT people showed important differences with suicidal heterosexuals, in a matched-pairs study. That study found suicidal LGBT people were more likely to communicate suicidal intentions, to search for new friends online, and to find more support online than did suicidal heterosexuals.

The black transgender and gender non-conforming community has been found to face discrimination to a higher degree than the rest of the transgender community, which is due to the intersection of racism and transphobia. Research has found that this community experiences a higher level of poverty, suicide attempts, and harassment, while the effects of HIV and being refused health care due to transphobia and/or racism are greater as well.

A survey by the National LGBTQ task force found that amongst the black respondents 49% reported having attempted suicide. Additional findings were that this group reported that 26% are unemployed and 34% reported an annual income of less than $10,000 per year. 41% of respondents reported homelessness at some point in their lives, which is more than five times the rate of the general US population. Also, the report revealed that the black transgender or gender non-conforming community reported 20.23% were living with HIV and that half of the respondents who attended school expressing a transgender identity or gender non-conformity reported facing harassment. 27% of black transgender youth reported being physically assaulted, 15% were sexually assaulted and 21% left school due to these instances of harassment.

A more recent survey by The Trevor Project revealed that 21% of African American LGBT youth have attempted suicide throughout 2021. Amongst Native American youth, 31% of LGBT youth have attempted suicide, and amongst Latin American youth, 18% of survey respondents admitted they have attempted suicide in the past year.

A 2022 study found that the use of gender-affirming hormone therapy in transgender and nonbinary youth was associated with a significant decrease in depression and suicidality.

Familial acceptance

Familial responses to LGBT youth identities differ from person to person. They range from acceptance to outright rejection of the LGBT individual. "Family connectedness" is important in an LGBT youth's life because it will help establish a positive mental health. One of the negative outcomes of LGBT youth confiding in family members about their sexual identities is the risk of being kicked out of their homes. When these youth do not have the support and acceptance of their family, they are more likely to turn to other riskier sources.

Amongst transgender youth, these effects are even more pronounced. In a sample of 84 transgender youth, those that reported being strongly supported by their parents, had a 93% lower suicide attempt rate (a 14-fold difference). In a separate survey of nearly 34,000 LGBT youth, those with supportive families reported a suicide attempt rate that was less than half of those without supportive families.

Impact of same-sex marriage

Across OECD countries, the legalisation of same-sex marriage is associated with reductions in the suicide rate of youth aged 10–24, with this association persisting into the medium term. The establishment of the legal right of same-sex marriage in the United States is associated with a significant reduction in the rate of attempted suicide among children, with the effect concentrated among children of a minority sexual orientation.

A study of nationwide data from across the United States from January 1999 to December 2015 revealed that the recognition of same-sex marriage is associated with a significant reduction in the rate of attempted suicide among children, with the effect being concentrated among children of a minority sexual orientation (LGBT youth), resulting in approximately 134,000 fewer children attempting suicide each year in the United States. Comparable findings are observed outside the United States. A study using cross-country data from 1991 to 2017 for 36 OECD countries found that same-sex marriage legalization is associated with a decline in youth suicide of 1.191 deaths per 100,000 youth, with this reduction persisting at least into the medium term.

OECD countries

A study of country-level data across 36 OECD countries from 1991 to 2017 found that same-sex marriage legalization reduced the suicide rate of youth aged 10–24 by 1.191 deaths per 100,000 youth, equal to a 17.90% decrease. This decline was most pronounced in males for whom the suicide rate fell by 1.993 compared to a decrease of 0.348 for female youth, corresponding to decreases of 19.98% and 10.90%, respectively. The study worked by exploiting common factors in the youth suicide rate across time between the sample countries to econometrically estimate what the suicide rate would have been in the absence of same-sex marriage legalization for the countries and years that same-sex marriage was legal. The impact of same-sex marriage legalization could then be inferred by comparing this estimated counterfactual to the observed data across time, thereby enabling inferences to be interpreted causally. By virtue of this design, the researchers were able to establish that the association persisted at least into the medium term and that countries that recently adopted same-sex marriage (the Netherlands was the first country to legalize same-sex marriage in 2001 and, as of 2017, 18 of the 36 sample countries had followed suit) also experienced declines in youth suicide. These findings indicate that future legalization in other developed countries would also engender a decrease in youth suicide over time.

United States

A study of nationwide data from January 1999 to December 2015 revealed an association between states that established same-sex marriage and reduced rates of attempted suicide among all schoolchildren in grades 9–12, with a rate reduction in all schoolchildren (LGB and non-LGB youth) in grades 9–12 declining by 7% and a rate reduction among schoolchildren of a minority sexual orientation (LGB youth) in grades 9–12 of 14%, resulting in approximately 134,000 fewer children attempting suicide each year in the United States. The gradual manner in which same-sex marriage was established in the United States (expanding from 1 state in 2004 to all 50 states in 2015) allowed the researchers to compare the rate of attempted suicide among children in each state over the time period studied. Once same-sex marriage was established in a particular state, the reduction in the rate of attempted suicide among children in that state became permanent. No reduction in the rate of attempted suicide among children occurred in a particular state until that state recognized same-sex marriage. The lead researcher of the study observed that "laws that have the greatest impact on gay adults may make gay kids feel more hopeful for the future".

Other research shows that while this nationwide study has shown an association between states that established same-sex marriage and reduced rates of attempted suicide among all schoolchildren in grades 9–12, it does not show causation. According to Julie Cerel, director of the Suicide Prevention & Exposure Lab at the University of Kentucky, LGBTQ children "experience much more interpersonal stress from schools, from peers and from home". The Centers for Disease Control and Prevention survey found that more than 1 in 5 young adults (22%) attempted suicide in 2021. Stigma and violence against LGBTQ teens has greatly contributed to their mental health.

Developmental psychology perspectives

The diathesis-stress model suggests that biological vulnerabilities predispose individuals to different conditions such as cancer, heart disease, and mental health conditions like major depression, a risk factor for suicide. Varying amounts of environmental stress increase the probability that these individuals will develop that condition. Minority stress theory suggests that minority status leads to increased discrimination from the social environment which leads to greater stress and health problems. In the presence of poor emotion regulation skills this can lead to poor mental health. Also, the differential susceptibility hypothesis suggests that for some individuals their physical and mental development is highly dependent on their environment in a "for-better-and-for-worse" fashion. That is, individuals who are highly susceptible will have better than average health in highly supportive environments and significantly worse than average health in hostile, violent environments. The model can help explain the unique health problems affecting LGBT populations including increased suicide attempts. For adolescents, the most relevant environments are the family, neighborhood, and school. Adolescent bullying – which is highly prevalent among sexual minority youths – is a chronic stressor that can increase risk for suicide via the diathesis-stress model. In a 2011 study of American lesbian, gay, and bisexual adolescents, Mark Hatzenbuehler found that a more conservative social environment elevated risk in suicidal behavior among all youth and that this effect was stronger for LGB youth. Furthermore, he found that the social environment partially mediated the relation between LGB status and suicidal behaviour. Hatzenbuehler found that even after such social as well as individual factors were controlled for, however, that "LGB status remained a significant predictor of suicide attempts."

Institutionalized and internalized homophobia

Institutionalized and internalized homophobia may also lead LGBT youth to not accept themselves and have deep internal conflicts about their sexual orientation. Parents may abandon or force children out of home after the child's coming out.

Homophobia arrived at by any means can be a gateway to bullying which can take many forms. Physical bullying is kicking, punching, while emotional bullying is name calling, spreading rumors and other verbal abuse. Cyber bullying involves abusive text messages or messages of the same nature on Facebook, Twitter, also social media networks. Sexual bullying includes inappropriate touching, lewd gestures or jokes.

Bullying may be considered a "rite of passage", but studies have shown it has negative physical and psychological effects. "Sexual minority youth, or teens that identify themselves as gay, lesbian or bisexual, are bullied two to three times more than heterosexuals", and "almost all transgender students have been verbally harassed (e.g., called names or threatened in the past year at school because of their sexual orientation (89%) and gender expression (89%)") according to Gay, Lesbian and Straight Education Network's Harsh Realities: The Experiences of Transgender Youth In Our Nation's Schools.

Projects

Several NGOs have started initiatives to attempt a reduction of LGBT youth suicides, such as The Trevor Project and the It Gets Better Project. Actions such as Ally Week, Day of Silence, and Suicide intervention have helped to combat both Self-harm and violence against LGBT people.

Policy responses

A number of policy options have been repeatedly proposed to address this issue. Some advocate intervention at the stage in which youth are already suicidal (such as crisis hotlines), while others advocate programs directed at increasing LGBT youth access to factors found to be "protective" against suicide (such as social support networks or mentors).

One proposed option is to provide LGBT-sensitivity and anti-bullying training to current middle and high school counselors and teachers. Citing a study by Jordan et al., school psychologist Anastasia Hansen notes that hearing teachers make homophobic remarks or fail to intervene when students make such remarks are both positively correlated with negative feelings about an LGBT identity. Conversely, a number of researchers have found the presence of LGBT-supportive school staff to be related to "positive outcomes for LGBT youth". Citing a 2006 Psychology in the Schools report, The Trevor Project notes that "lesbian, gay, bisexual, transgender and questioning (LGBTQ) youth who believe they have just one school staff member with whom they can talk about problems are only 1/3 as likely as those without that support to... report making multiple suicide attempts in the past year."

Another frequently proposed policy option involves providing grant incentives for schools to create and/or support Gay–Straight Alliances, student groups dedicated to providing a social support network for LGBT students. Kosciw and Diaz, researchers for the Gay, Lesbian and Straight Education Network, found in a nationwide survey that "students in schools with a GSA were less likely to feel unsafe, less likely to miss school, and more likely to feel that they belonged at their school than students in schools with no such clubs." Studies have shown that social isolation and marginalization at school are psychologically damaging to LGBT students, and that GSAs and other similar peer-support group can be effective providers of this "psychosocial support".

Early interventions for LGBT youth

Be proactive and understanding

Educators can be proactive in helping adolescents with gender identity and the questions/issues that sometimes come with them. Normalizing education about sexualities and genders can help prevent adolescents from resorting to suicide, drug abuse, homelessness, and many more psychological problems. Van Wormer & McKinney (2003) relate that understanding LGBT students is the first step to suicide prevention. They use a harm reduction approach, which meets students where they are to reduce any continued harm linked with their behaviors. They relate that creating a supportive and culturally diverse environment is crucial to social acceptance in an educational setting.

LGBT role models/resources

It could be beneficial to hire LGBT teachers to serve as role models and support LGBT students. Many of the resources in the U.S. are crisis-driven- not prevention-driven. In order to prevent suicide for LGBT adolescents, it needs to be the other way around. Furthermore, studies show that counselors and teachers need to be trained in self-awareness, sexuality and sexual diversity with themselves and with students. Researchers also suggest inviting gay/lesbian and bisexual panels from colleges or universities to conduct classroom discussions. Education and resources is potentially key to helping LGBT students and families. According to researcher Rob Cover, role models and resources benefit LGBT youth only if they avoid replicating stereotypes and provide diverse visual and narrative representations to allow broad identification.

Having a PFLAG (Parents Families, and Friends of Lesbians and Gays) and GSA Club are possible resources to promote discussions and leadership roles to LGBT students. These resources extend outside of school and in the community. (Greytak, E. A., Kosciw, J. G., & Boesen, M. J. 2013) report that when schools have a GSA or Gay Straight Alliance club or a club promoting social awareness and camaraderie of sorts, supportive educators, inclusive curricula, and comprehensive policies that LGBT students were victimized less and had more positive school experiences. This would allow LGBT students to be positive and want to be in school.

Teach tolerance and examine a school's climate

Examine a school's climate and teach tolerance – Teaching Tolerance is a movement, magazine and website which gives many tools and ideas to help people be tolerant of one another. It demonstrates that the classroom is a reflection of the world around us. Educators can use Teach Tolerance's website and book to download resources and look up creative ways to learn more about LGBT students and teaching tolerance to their students in the classroom. It helps schools get started with anti-bullying training and professional development and resource suggestions. It even relates common roadblocks and tips to starting a GSA club.

Research shows that a collaborative effort must be made in order to prevent LGBT students from being bullied and/or committing suicide. Teachers, administrators, students, families, and communities need to come together to help LGBT students be confident. Each school has its own individuality, its own sense of "self", whether it be the teachers, administrators, students, or the surrounding community. In order to tackle the issue of bullying for LGBT students it needs to start with understanding the student population and demographic where the school lies. Educating students, faculty, staff, and school boards on LGBT issues and eliminating homophobia and transphobia in schools, training staff on diversity acceptance and bullying prevention, and implementing Gay–Straight Alliances is key to suicide prevention for LGBT students (Bacon, Laura Ann 2011). Adolescents grow and are shaped by many factors including internal and external features (Swearer, Espelage, Vaillancourt, & Hymel, 2010).

The school climate must foster respect. Thus, setting the tone for administration, teachers, professionals who enter the building, parents and most importantly the students. People, in general, need to understand their own misconceptions and stereotypes of what being LGBT is. Unless students and adults are educated on the LGBT community, than stereotypes and negative attitudes will continue to exist (Knotts, G., & Gregorio, D. 2011). The GMCLA (Gay Men's Chorus of Los Angeles) use music and singing as a vehicle for changing the attitudes and hearts of people in schools nationwide. Their goal is to bring music to standards-driven curriculum to youth with the purpose of teaching content in innovative and meaningful ways. They instill in students and staff techniques to foster positive meaning of the social and personal issues dealt with in school and society.

Gay, L. (2009) has generated a guide to helping school safety/climate and fostering positive interpersonal relationships through "The Safe Space Kit". This tool helps teachers create a safe space for LGBT students. One of the most effective ways for an educator to create a safe space is to be a supportive ally to LGBT students. This kit has numerous tools for teachers and schools to utilize to help transgender youth, including: a hard copy of "The Safe Space Kit" includes the "Guide to Being an Ally", stickers and two Safe Space posters. Even utilizing something just to promote awareness, such as using "The Safe Space Kit" could be a good first step for schools to promote responsiveness to LGBT students. Providing some supports rather than none at all can benefit LGBT youth tremendously now and in the future (Greytak, et al. 2013).

OBPP (Olweus Bullying Prevention Program)

OBPP is an anti-bullying program designed by psychologist Dan Olweus utilized in schools in Europe, Canada and the U.S. Reductions in bullying were due to parent training, playground supervision, home-school communication, classroom rules, and training videos. Furthermore, Swearer, et al. (2010) discuss a "dosage effect" in which the more positive and consistent elements included in a program, the more the likelihood that bullying would decrease. Success in one school does not guarantee success in another because each school has its own social climate. The OBPP is effective but still needs to be analyzed further, since there are many things to consider when implementing this technique within a large school.

Steps To Respect

Steps To Respect is an anti-bullying campaign which can be beneficial in schools as well – it is a comprehensive guide for teachers, administrators, and students utilizing in class lessons and training helping schools foster positive social-emotional skills and conflict resolution. If schools are able to change peer conduct and norms, increase student communication skills, and maintain adult prevention and intervention efforts, the positive effects of their work will strengthen over time (Frey, Edstrom & Hirschstein 2005) and continue to grow as each class progresses through the school system.

Photoluminescence

From Wikipedia, the free encyclopedia
https://en.wikipedia.org/wiki/Photoluminescence

Fluorescent solutions under UV light. Absorbed photons are rapidly re-emitted under longer electromagnetic wavelengths.

Photoluminescence (abbreviated as PL) is light emission from any form of matter after the absorption of photons (electromagnetic radiation). It is one of many forms of luminescence (light emission) and is initiated by photoexcitation (i.e. photons that excite electrons to a higher energy level in an atom), hence the prefix photo-. Following excitation, various relaxation processes typically occur in which other photons are re-radiated. Time periods between absorption and emission may vary: ranging from short femtosecond-regime for emission involving free-carrier plasma in inorganic semiconductors up to milliseconds for phosphoresence processes in molecular systems; and under special circumstances delay of emission may even span to minutes or hours.

Observation of photoluminescence at a certain energy can be viewed as an indication that an electron populated an excited state associated with this transition energy.

While this is generally true in atoms and similar systems, correlations and other more complex phenomena also act as sources for photoluminescence in many-body systems such as semiconductors. A theoretical approach to handle this is given by the semiconductor luminescence equations.

Forms

Schematic for the excitation-relaxation processes of photoluminescence

Photoluminescence processes can be classified by various parameters such as the energy of the exciting photon with respect to the emission. Resonant excitation describes a situation in which photons of a particular wavelength are absorbed and equivalent photons are very rapidly re-emitted. This is often referred to as resonance fluorescence. For materials in solution or in the gas phase, this process involves electrons but no significant internal energy transitions involving molecular features of the chemical substance between absorption and emission. In crystalline inorganic semiconductors where an electronic band structure is formed, secondary emission can be more complicated as events may contain both coherent contributions such as resonant Rayleigh scattering where a fixed phase relation with the driving light field is maintained (i.e. energetically elastic processes where no losses are involved), and incoherent contributions (or inelastic modes where some energy channels into an auxiliary loss mode),

The latter originate, e.g., from the radiative recombination of excitons, Coulomb-bound electron-hole pair states in solids. Resonance fluorescence may also show significant quantum optical correlations.

More processes may occur when a substance undergoes internal energy transitions before re-emitting the energy from the absorption event. Electrons change energy states by either resonantly gaining energy from absorption of a photon or losing energy by emitting photons. In chemistry-related disciplines, one often distinguishes between fluorescence and phosphorescence. The former is typically a fast process, yet some amount of the original energy is dissipated so that re-emitted light photons will have lower energy than did the absorbed excitation photons. The re-emitted photon in this case is said to be red shifted, referring to the reduced energy it carries following this loss (as the Jablonski diagram shows). For phosphorescence, electrons which absorbed photons, undergo intersystem crossing where they enter into a state with altered spin multiplicity (see term symbol), usually a triplet state. Once the excited electron is transferred into this triplet state, electron transition (relaxation) back to the lower singlet state energies is quantum mechanically forbidden, meaning that it happens much more slowly than other transitions. The result is a slow process of radiative transition back to the singlet state, sometimes lasting minutes or hours. This is the basis for "glow in the dark" substances.

Photoluminescence is an important technique for measuring the purity and crystalline quality of semiconductors such as GaN and InP and for quantification of the amount of disorder present in a system.

Time-resolved photoluminescence (TRPL) is a method where the sample is excited with a light pulse and then the decay in photoluminescence with respect to time is measured. This technique is useful for measuring the minority carrier lifetime of III-V semiconductors like gallium arsenide (GaAs).

Photoluminescence properties of direct-gap semiconductors

In a typical PL experiment, a semiconductor is excited with a light-source that provides photons with an energy larger than the bandgap energy. The incoming light excites a polarization that can be described with the semiconductor Bloch equations. Once the photons are absorbed, electrons and holes are formed with finite momenta ${\displaystyle \mathbf{k}}$ in the conduction and valence bands, respectively. The excitations then undergo energy and momentum relaxation towards the band-gap minimum. Typical mechanisms are Coulomb scattering and the interaction with phonons. Finally, the electrons recombine with holes under emission of photons.

Ideal, defect-free semiconductors are many-body systems where the interactions of charge-carriers and lattice vibrations have to be considered in addition to the light-matter coupling. In general, the PL properties are also extremely sensitive to internal electric fields and to the dielectric environment (such as in photonic crystals) which impose further degrees of complexity. A precise microscopic description is provided by the semiconductor luminescence equations.

Ideal quantum-well structures

An ideal, defect-free semiconductor quantum well structure is a useful model system to illustrate the fundamental processes in typical PL experiments. The discussion is based on results published in Klingshirn (2012) and Balkan (1998).

The fictive model structure for this discussion has two confined quantized electronic and two hole subbands, e₁, e₂ and h₁, h₂, respectively. The linear absorption spectrum of such a structure shows the exciton resonances of the first (e1h1) and the second quantum well subbands (e₂, h₂), as well as the absorption from the corresponding continuum states and from the barrier.

Photoexcitation

In general, three different excitation conditions are distinguished: resonant, quasi-resonant, and non-resonant. For the resonant excitation, the central energy of the laser corresponds to the lowest exciton resonance of the quantum well. No, or only a negligible amount of the excess, energy is injected to the carrier system. For these conditions, coherent processes contribute significantly to the spontaneous emission. The decay of polarization creates excitons directly. The detection of PL is challenging for resonant excitation as it is difficult to discriminate contributions from the excitation, i.e., stray-light and diffuse scattering from surface roughness. Thus, speckle and resonant Rayleigh-scattering are always superimposed to the incoherent emission.

In case of the non-resonant excitation, the structure is excited with some excess energy. This is the typical situation used in most PL experiments as the excitation energy can be discriminated using a spectrometer or an optical filter. One has to distinguish between quasi-resonant excitation and barrier excitation.

For quasi-resonant conditions, the energy of the excitation is tuned above the ground state but still below the barrier absorption edge, for example, into the continuum of the first subband. The polarization decay for these conditions is much faster than for resonant excitation and coherent contributions to the quantum well emission are negligible. The initial temperature of the carrier system is significantly higher than the lattice temperature due to the surplus energy of the injected carriers. Finally, only the electron-hole plasma is initially created. It is then followed by the formation of excitons.

In case of barrier excitation, the initial carrier distribution in the quantum well strongly depends on the carrier scattering between barrier and the well.

Relaxation

Initially, the laser light induces coherent polarization in the sample, i.e., the transitions between electron and hole states oscillate with the laser frequency and a fixed phase. The polarization dephases typically on a sub-100 fs time-scale in case of nonresonant excitation due to ultra-fast Coulomb- and phonon-scattering.

The dephasing of the polarization leads to creation of populations of electrons and holes in the conduction and the valence bands, respectively. The lifetime of the carrier populations is rather long, limited by radiative and non-radiative recombination such as Auger recombination. During this lifetime a fraction of electrons and holes may form excitons, this topic is still controversially discussed in the literature. The formation rate depends on the experimental conditions such as lattice temperature, excitation density, as well as on the general material parameters, e.g., the strength of the Coulomb-interaction or the exciton binding energy.

The characteristic time-scales are in the range of hundreds of picoseconds in GaAs; they appear to be much shorter in wide-gap semiconductors.

Directly after the excitation with short (femtosecond) pulses and the quasi-instantaneous decay of the polarization, the carrier distribution is mainly determined by the spectral width of the excitation, e.g., a laser pulse. The distribution is thus highly non-thermal and resembles a Gaussian distribution, centered at a finite momentum. In the first hundreds of femtoseconds, the carriers are scattered by phonons, or at elevated carrier densities via Coulomb-interaction. The carrier system successively relaxes to the Fermi–Dirac distribution typically within the first picosecond. Finally, the carrier system cools down under the emission of phonons. This can take up to several nanoseconds, depending on the material system, the lattice temperature, and the excitation conditions such as the surplus energy.

Initially, the carrier temperature decreases fast via emission of optical phonons. This is quite efficient due to the comparatively large energy associated with optical phonons, (36meV or 420K in GaAs) and their rather flat dispersion, allowing for a wide range of scattering processes under conservation of energy and momentum. Once the carrier temperature decreases below the value corresponding to the optical phonon energy, acoustic phonons dominate the relaxation. Here, cooling is less efficient due their dispersion and small energies and the temperature decreases much slower beyond the first tens of picoseconds. At elevated excitation densities, the carrier cooling is further inhibited by the so-called hot-phonon effect. The relaxation of a large number of hot carriers leads to a high generation rate of optical phonons which exceeds the decay rate into acoustic phonons. This creates a non-equilibrium "over-population" of optical phonons and thus causes their increased reabsorption by the charge-carriers significantly suppressing any cooling. Thus, a system cools slower, the higher the carrier density is.

Radiative recombination

The emission directly after the excitation is spectrally very broad, yet still centered in the vicinity of the strongest exciton resonance. As the carrier distribution relaxes and cools, the width of the PL peak decreases and the emission energy shifts to match the ground state of the exciton (such as an electron) for ideal samples without disorder. The PL spectrum approaches its quasi-steady-state shape defined by the distribution of electrons and holes. Increasing the excitation density will change the emission spectra. They are dominated by the excitonic ground state for low densities. Additional peaks from higher subband transitions appear as the carrier density or lattice temperature are increased as these states get more and more populated. Also, the width of the main PL peak increases significantly with rising excitation due to excitation-induced dephasing and the emission peak experiences a small shift in energy due to the Coulomb-renormalization and phase-filling.

In general, both exciton populations and plasma, uncorrelated electrons and holes, can act as sources for photoluminescence as described in the semiconductor-luminescence equations. Both yield very similar spectral features which are difficult to distinguish; their emission dynamics, however, vary significantly. The decay of excitons yields a single-exponential decay function since the probability of their radiative recombination does not depend on the carrier density. The probability of spontaneous emission for uncorrelated electrons and holes, is approximately proportional to the product of electron and hole populations eventually leading to a non-single-exponential decay described by a hyperbolic function.

Effects of disorder

Real material systems always incorporate disorder. Examples are structural defects in the lattice or disorder due to variations of the chemical composition. Their treatment is extremely challenging for microscopic theories due to the lack of detailed knowledge about perturbations of the ideal structure. Thus, the influence of the extrinsic effects on the PL is usually addressed phenomenologically. In experiments, disorder can lead to localization of carriers and hence drastically increase the photoluminescence life times as localized carriers cannot as easily find nonradiative recombination centers as can free ones.

Researchers from the King Abdullah University of Science and Technology (KAUST) have studied the photoinduced entropy (i.e. thermodynamic disorder) of InGaN/GaN p-i-n double-heterostructure and AlGaN nanowires using temperature-dependent photoluminescence. They defined the photoinduced entropy as a thermodynamic quantity that represents the unavailability of a system's energy for conversion into useful work due to carrier recombination and photon emission. They have also related the change in entropy generation to the change in photocarrier dynamics in the nanowire active regions using results from time-resolved photoluminescence study. They hypothesized that the amount of generated disorder in the InGaN layers eventually increases as the temperature approaches room temperature because of the thermal activation of surface states, while an insignificant increase was observed in AlGaN nanowires, indicating lower degrees of disorder-induced uncertainty in the wider bandgap semiconductor. To study the photoinduced entropy, the scientists have developed a mathematical model that considers the net energy exchange resulting from photoexcitation and photoluminescence.

Photoluminescent materials for temperature detection

In phosphor thermometry, the temperature dependence of the photoluminescence process is exploited to measure temperature.

Experimental methods

Photoluminescence spectroscopy is a widely used technique for characterisation of the optical and electronic properties of semiconductors and molecules. The technique itself is fast, contactless, and nondestructive. Therefore, it can be used to study the optoelectronic properties of materials of various sizes (from microns to centimeters) during the fabrication process without complex sample preparation. For example, photoluminescence measurements of solar cell absorbers can predict the maximum voltage the material could produce. In chemistry, the method is more often referred to as fluorescence spectroscopy, but the instrumentation is the same. The relaxation processes can be studied using time-resolved fluorescence spectroscopy to find the decay lifetime of the photoluminescence. These techniques can be combined with microscopy, to map the intensity (confocal microscopy) or the lifetime (fluorescence-lifetime imaging microscopy) of the photoluminescence across a sample (e.g. a semiconducting wafer, or a biological sample that has been marked with fluorescent molecules). Modulated photoluminescence is a specific method for measuring the complex frequency response of the photoluminescence signal to a sinusoidal excitation, allowing for the direct extraction of minority carrier lifetime without the need for intensity calibrations. It has been used to study the influence of interface defects on the recombination of excess carriers in crystalline silicon wafers with different passivation schemes.

Stealth technology

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Stealth_technology 

F-117 stealth aircraft

PL-01 stealth tank

Surcouf French stealth frigate

Stealth technology, also termed low observable technology (LO technology), is a sub-discipline of military tactics and passive and active electronic countermeasures, which covers a range of methods used to make personnel, aircraft, ships, submarines, missiles, satellites, and ground vehicles less visible (ideally invisible) to radar, infrared, sonar and other detection methods. It corresponds to military camouflage for these parts of the electromagnetic spectrum (i.e., multi-spectral camouflage).

Development of modern stealth technologies in the United States began in 1958, where earlier attempts to prevent radar tracking of its U-2 spy planes during the Cold War by the Soviet Union had been unsuccessful. Designers turned to developing a specific shape for planes that tended to reduce detection by redirecting electromagnetic radiation waves from radars. Radiation-absorbent material was also tested and made to reduce or block radar signals that reflect off the surfaces of aircraft. Such changes to shape and surface composition comprise stealth technology as currently used on the Northrop Grumman B-2 Spirit "Stealth Bomber".

The concept of stealth is to operate or hide while giving enemy forces no indication as to the presence of friendly forces. This concept was first explored through camouflage to make an object's appearance blend into the visual background. As the potency of detection and interception technologies (radar, infrared search and tracking, surface-to-air missiles, etc.) have increased, so too has the extent to which the design and operation of military personnel and vehicles have been affected in response. Some military uniforms are treated with chemicals to reduce their infrared signature. A modern stealth vehicle is designed from the outset to have a chosen spectral signature. The degree of stealth embodied in a given design is chosen according to the projected threats of detection.

History

Camouflage to aid or avoid predation predates humanity, and hunters have been using vegetation to conceal themselves perhaps as long as people have been hunting. The earliest application of camouflage in warfare is impossible to ascertain. Methods for visual concealment in war were documented by Sun Tzu in his book The Art of War in the 5th century BC, and by Frontinus in his work Strategemata in the 1st century AD.

In England, irregular units of gamekeepers in the 17th century were the first to adopt drab colours (common in 16th century Irish units) as a form of camouflage, following examples from the continent.

During World War I, the Germans experimented with the use of Cellon (Cellulose acetate), a transparent covering material, in an attempt to reduce the visibility of military aircraft. Single examples of the Fokker E.III Eindecker fighter monoplane, the Albatros C.I two-seat observation biplane, and the Linke-Hofmann R.I prototype heavy bomber were covered with Cellon. However, sunlight glinting from the material made the aircraft even more visible. Cellon was also found to degrade quickly from both sunlight and in-flight temperature changes, so the effort to make transparent aircraft ceased.

In 1916, the British modified a small SS class airship for the purpose of night-time reconnaissance over German lines on the Western Front. Fitted with a silenced engine and a black gas bag, the craft was both invisible and inaudible from the ground but several night-time flights over German-held territory produced little useful intelligence and the idea was dropped.

Diffused lighting camouflage, a shipborne form of counter-illumination camouflage, was trialled by the Royal Canadian Navy from 1941 to 1943. The concept was followed up for aircraft by the Americans and the British: in 1945 a Grumman Avenger with Yehudi lights reached 3,000 yards (2,700 m) from a ship before being sighted. This ability was rendered obsolete by radar.

Chaff was invented in Britain and Germany early in World War II as a means to hide aircraft from radar. In effect, chaff acted upon radio waves much as a smoke screen acted upon visible light.

The U-boat U-480 may have been the first stealth submarine. It featured an anechoic tile rubber coating, one layer of which contained circular air pockets to defeat ASDIC sonar. Radar-absorbent paints and materials of rubber and semiconductor composites (codenames: Sumpf, Schornsteinfeger) were used by the Kriegsmarine on submarines in World War II. Tests showed they were effective in reducing radar signatures at both short (centimetres) and long (1.5 metre) wavelengths.

In 1956 the CIA began attempts to reduce the radar cross-section (RCS) of the U-2 spyplane. Three systems were developed, Trapeze, a series of wires and ferrite beads around the planform of the aircraft, a covering material with PCB circuitry embedded in it, and radar-absorbent paint. These were deployed in the field on the so-called dirty birds but results were disappointing, the weight and drag increases were not worth any reduction in detection rates. More successful was applying camouflage paint to the originally bare metal aircraft; a deep blue was found to be most effective. The weight of this cost 250 ft in maximum altitude, but made the aircraft harder for interceptors to see.

In 1958, the U.S. Central Intelligence Agency requested funding for a reconnaissance aircraft to replace the existing U-2 spy planes, and Lockheed secured contractual rights to produce it. "Kelly" Johnson and his team at Lockheed's Skunk Works were assigned to produce the A-12 (or OXCART), which operated at high altitude of 70,000 to 80,000 ft and speed of Mach 3.2 to avoid radar detection. Various plane shapes designed to reduce radar detection were developed in earlier prototypes, named A-1 to A-11. The A-12 included a number of stealthy features including special fuel to reduce the signature of the exhaust plume, canted vertical stabilizers, the use of composite materials in key locations, and the overall finish in radar-absorbent paint.

In 1960, the USAF reduced the radar cross-section of a Ryan Q-2C Firebee drone. This was achieved through specially designed screens over the air intake, and radiation-absorbent material on the fuselage, and radar-absorbent paint.

The United States Army issued a specification in 1968 which called for an observation aircraft that would be acoustically undetectable from the ground when flying at an altitude of 1,500 feet (457 m) at night. This resulted in the Lockheed YO-3A Quiet Star, which operated in South Vietnam from late June 1970 to September 1971.

During the 1970s the U.S. Department of Defense launched project Lockheed Have Blue, with the aim of developing a stealth fighter. There was fierce bidding between Lockheed and Northrop to secure the multibillion-dollar contract. Lockheed incorporated into its bid a text written by the Soviet-Russian physicist Pyotr Ufimtsev from 1962, titled Method of Edge Waves in the Physical Theory of Diffraction, Soviet Radio, Moscow, 1962. In 1971 this book was translated into English with the same title by U.S. Air Force, Foreign Technology Division. The theory played a critical role in the design of American stealth-aircraft F-117 and B-2. Equations outlined in the paper quantified how a plane's shape would affect its detectability by radar, termed radar cross-section (RCS). At the time, the Soviet Union did not have supercomputer capacity to solve these equations for actual designs. This was applied by Lockheed in computer simulation to design a novel shape they called the "Hopeless Diamond", a wordplay on the Hope Diamond, securing contractual rights to produce the F-117 Nighthawk starting in 1975. In 1977 Lockheed produced two 60% scale models under the Have Blue contract. The Have Blue program was a stealth technology demonstrator that lasted from 1976 to 1979. The Northrop Grumman Tacit Blue also played a part in the development of composite material and curvilinear surfaces, low observables, fly-by-wire, and other stealth technology innovations. The success of Have Blue led the Air Force to create the Senior Trend program which developed the F-117.

Principles

Stealth technology (or LO for low observability) is not one technology. It is a set of technologies, used in combinations, that can greatly reduce the distances at which a person or vehicle can be detected; more so radar cross-section reductions, but also acoustic, thermal, and other aspects.

Radar cross-section (RCS) reductions

Main article: Radar cross-section

Almost since the invention of radar, various methods have been tried to minimize detection. Rapid development of radar during World War II led to equally rapid development of numerous counter radar measures during the period; a notable example of this was the use of chaff. Modern methods include Radar jamming and deception.

The term stealth in reference to reduced radar signature aircraft became popular during the late eighties when the Lockheed Martin F-117 stealth fighter became widely known. The first large scale (and public) use of the F-117 was during the Gulf War in 1991. However, F-117A stealth fighters were used for the first time in combat during Operation Just Cause, the United States invasion of Panama in 1989.

Vehicle shape

Aircraft

Main article: Aircraft design process

The F-35 Lightning II offers better stealthy features (such as this landing gear door) than prior American multi-role fighters, such as the F-16 Fighting Falcon

The possibility of designing aircraft in such a manner as to reduce their radar cross-section was recognized in the late 1930s, when the first radar tracking systems were employed, and it has been known since at least the 1960s that aircraft shape makes a significant difference in detectability. The Avro Vulcan, a British bomber of the 1960s, had a remarkably small appearance on radar despite its large size, and occasionally disappeared from radar screens entirely. It is now known that it had a fortuitously stealthy shape apart from the vertical element of the tail. Despite being designed before a low radar cross-section (RCS) and other stealth factors were ever a consideration, a Royal Aircraft Establishment technical note of 1957 stated that of all the aircraft so far studied, the Vulcan appeared by far the simplest radar echoing object, due to its shape: only one or two components contributing significantly to the echo at any aspect (one of them being the vertical stabilizer, which is especially relevant for side aspect RCS), compared with three or more on most other types. While writing about radar systems, authors Simon Kingsley and Shaun Quegan singled out the Vulcan's shape as acting to reduce the RCS. In contrast, the Tupolev 95 Russian long-range bomber (NATO reporting name 'Bear') was conspicuous on radar. It is now known that propellers and jet turbine blades produce a bright radar image; the Bear has four pairs of large (5.6-meter diameter) contra-rotating propellers.

Another important factor is internal construction. Some stealth aircraft have skin that is radar transparent or absorbing, behind which are structures termed reentrant triangles. Radar waves penetrating the skin get trapped in these structures, reflecting off the internal faces and losing energy. This method was first used on the Blackbird series: A-12, YF-12A, Lockheed SR-71 Blackbird.

The most efficient way to reflect radar waves back to the emitting radar is with orthogonal metal plates, forming a corner reflector consisting of either a dihedral (two plates) or a trihedral (three orthogonal plates). This configuration occurs in the tail of a conventional aircraft, where the vertical and horizontal components of the tail are set at right angles. Stealth aircraft such as the F-117 use a different arrangement, tilting the tail surfaces to reduce corner reflections formed between them. A more radical method is to omit the tail, as in the B-2 Spirit. The B-2's clean, low-drag flying wing configuration gives it exceptional range and reduces its radar profile. The flying wing design most closely resembles a so-called infinite flat plate (as vertical control surfaces dramatically increase RCS), the perfect stealth shape, as it would have no angles to reflect back radar waves.

YF-23 S-duct engine air intake conceals engine from probing radar waves

In addition to altering the tail, stealth design must bury the engines within the wing or fuselage, or in some cases where stealth is applied to an extant aircraft, install baffles in the air intakes, so that the compressor blades are not visible to radar. A stealthy shape must be devoid of complex bumps or protrusions of any kind, meaning that weapons, fuel tanks, and other stores must not be carried externally. Any stealthy vehicle becomes un-stealthy when a door or hatch opens.

Parallel alignment of edges or even surfaces is also often used in stealth designs. The technique involves using a small number of edge orientations in the shape of the structure. For example, on the F-22A Raptor, the leading edges of the wing and the tail planes are set at the same angle. Other smaller structures, such as the air intake bypass doors and the air refueling aperture, also use the same angles. The effect of this is to return a narrow radar signal in a very specific direction away from the radar emitter rather than returning a diffuse signal detectable at many angles. The effect is sometimes called "glitter" after the very brief signal seen when the reflected beam passes across a detector. It can be difficult for the radar operator to distinguish between a glitter event and a digital glitch in the processing system.

Stealth airframes sometimes display distinctive serrations on some exposed edges, such as the engine ports. The YF-23 has such serrations on the exhaust ports. This is another example in the parallel alignment of features, this time on the external airframe.

The shaping requirements detracted greatly from the F-117's aerodynamic properties. It is inherently unstable, and cannot be flown without a fly-by-wire control system.

Similarly, coating the cockpit canopy with a thin film transparent conductor (vapor-deposited gold or indium tin oxide) helps to reduce the aircraft's radar profile, because radar waves would normally enter the cockpit, reflect off objects (the inside of a cockpit has a complex shape, with a pilot helmet alone forming a sizeable return), and possibly return to the radar, but the conductive coating creates a controlled shape that deflects the incoming radar waves away from the radar. The coating is thin enough that it has no adverse effect on pilot vision.

K32 HMS Helsingborg, a stealth ship

Ships

Main article: Naval architecture

Ships have also adopted similar methods. Though the earlier Arleigh Burke-class destroyer incorporated some signature-reduction features. the Norwegian Skjold-class corvette was the first coastal defence and the French La Fayette-class frigate the first ocean-going stealth ship to enter service. Other examples are the Dutch De Zeven Provinciën class frigates, the Taiwanese Tuo Chiang stealth corvette, German Sachsen-class frigates, the Swedish Visby-class corvette, the USS San Antonio amphibious transport dock, and most modern warship designs.

Materials

Non-metallic airframe

Dielectric composite materials are more transparent to radar, whereas electrically conductive materials such as metals and carbon fibers reflect electromagnetic energy incident on the material's surface. Composites may also contain ferrites to optimize the dielectric and magnetic properties of a material for its application.

Radar-absorbent material

Main article: Radiation-absorbent material

Skin of a B-2 bomber.

Radiation-absorbent material (RAM), often as paints, are used especially on the edges of metal surfaces. While the material and thickness of RAM coatings can vary, the way they work is the same: absorb radiated energy from a ground- or air-based radar station into the coating and convert it to heat rather than reflect it back. Current technologies include dielectric composites and metal fibers containing ferrite isotopes. Ceramic composite coating is a new type of material systems which can sustain at higher temperatures with better sand erosion resistance and thermal resistance. Paint comprises depositing pyramid-like colonies on the reflecting superficies with the gaps filled with ferrite-based RAM. The pyramidal structure deflects the incident radar energy in the maze of RAM. One commonly used material is called iron ball paint. It contains microscopic iron spheres that resonate in tune with incoming radio waves and dissipate most of their energy as heat, leaving little to reflect back to detectors. FSS are planar periodic structures that behave like filters to electromagnetic energy. The considered frequency-selective surfaces are composed of conducting patch elements pasted on the ferrite layer. FSS are used for filtration and microwave absorption.

Radar stealth countermeasures and limits

Low-frequency radar

Shaping offers far fewer stealth advantages against low-frequency radar. If the radar wavelength is roughly twice the size of the target, a half-wave resonance effect can still generate a significant return. However, low-frequency radar is limited by lack of available frequencies (many are heavily used by other systems), by lack of accuracy of the diffraction-limited systems given their long wavelengths, and by the radar's size, making it difficult to transport. A long-wave radar may detect a target and roughly locate it, but not provide enough information to identify it, target it with weapons, or even to guide a fighter to it.

Multiple emitters

Stealth aircraft attempt to minimize all radar reflections, but are specifically designed to avoid reflecting radar waves back in the direction they came from (since in most cases a radar emitter and receiver are in the same location). They are less able to minimize radar reflections in other directions. Thus, detection can be better achieved if emitters are in different locations from receivers. One emitter separate from one receiver is termed bistatic radar; one or more emitters separate from more than one receiver is termed multistatic radar. Proposals exist to use reflections from emitters such as civilian radio transmitters, including cellular telephone radio towers.

Moore's law

By Moore's law the processing power behind radar systems is rising over time. This will eventually erode the ability of physical stealth to hide vehicles.

Ship wakes and spray

Synthetic aperture sidescan radars can be used to detect the location and heading of ships from their wake patterns. These are detectable from orbit. When a ship moves through a seaway it throws up a cloud of spray which can be detected by radar.

Acoustics

See also: Acoustic signature, Aircraft noise, and Helicopter noise reduction

Acoustic stealth plays a primary role for submarines and ground vehicles. Submarines use extensive rubber mountings to isolate, damp, and avoid mechanical noises that can reveal locations to underwater passive sonar arrays.

Early stealth observation aircraft used slow-turning propellers to avoid being heard by enemy troops below. Stealth aircraft that stay subsonic can avoid being tracked by sonic boom. The presence of supersonic and jet-powered stealth aircraft such as the SR-71 Blackbird indicates that acoustic signature is not always a major driver in aircraft design, as the Blackbird relied more on its very high speed and altitude.

One method to reduce helicopter rotor noise is modulated blade spacing. Standard rotor blades are evenly spaced, and produce greater noise at a given frequency and its harmonics. Using varied spacing between the blades spreads the noise or acoustic signature of the rotor over a greater range of frequencies.

Visibility

Further information: Military camouflage, Active camouflage, Aircraft camouflage, and Ship camouflage

The simplest technology is visual camouflage; the use of paint or other materials to color and break up the lines of a vehicle or person.

Most stealth aircraft use matte paint and dark colors, and operate only at night. Lately, interest in daylight Stealth (especially by the USAF) has emphasized the use of gray paint in disruptive schemes, and it is assumed that Yehudi lights could be used in the future to hide the airframe (against the background of the sky, including at night, aircraft of any colour appear dark) or as a sort of active camouflage. The original B-2 design had wing tanks for a contrail-inhibiting chemical, alleged by some to be chlorofluorosulfonic acid, but this was replaced in the final design with a contrail sensor that alerts the pilot when he should change altitude and mission planning also considers altitudes where the probability of their formation is minimized.

In space, mirrored surfaces can be employed to reflect views of empty space toward known or suspected observers; this approach is compatible with several radar stealth schemes. Careful control of the orientation of the satellite relative to the observers is essential, and mistakes can lead to detectability enhancement rather than the desired reduction.

Infrared

Main article: infrared signature

See also: Infrared countermeasure

An exhaust plume contributes a significant infrared signature. One means to reduce IR signature is to have a non-circular tail pipe (a slit shape) to minimize the exhaust cross sectional area and maximize the mixing of hot exhaust with cool ambient air (see Lockheed F-117 Nighthawk). Often, cool air is deliberately injected into the exhaust flow to boost this process (see Ryan AQM-91 Firefly and Northrop Grumman B-2 Spirit). The Stefan–Boltzmann law shows how this results in less energy (Thermal radiation in infrared spectrum) being released and thus reduces the heat signature. In some aircraft, the jet exhaust is vented above the wing surface to shield it from observers below, as in the Lockheed F-117 Nighthawk, and the unstealthy Fairchild Republic A-10 Thunderbolt II. To achieve infrared stealth, the exhaust gas is cooled to the temperatures where the brightest wavelengths it radiates are absorbed by atmospheric carbon dioxide and water vapor, greatly reducing the infrared visibility of the exhaust plume. Another way to reduce the exhaust temperature is to circulate coolant fluids such as fuel inside the exhaust pipe, where the fuel tanks serve as heat sinks cooled by the flow of air along the wings.

Ground combat includes the use of both active and passive infrared sensors. Thus, the United States Marine Corps (USMC) ground combat uniform requirements document specifies infrared reflective quality standards.

Reducing radio frequency (RF) emissions

See also: Radio wave

In addition to reducing infrared and acoustic emissions, a stealth vehicle must avoid radiating any other detectable energy, such as from onboard radars, communications systems, or RF leakage from electronics enclosures. The F-117 uses passive infrared and low light level television sensor systems to aim its weapons and the F-22 Raptor has an advanced LPI radar which can illuminate enemy aircraft without triggering a radar warning receiver response.

Measuring

The size of a target's image on radar is measured by the radar cross section (RCS), often represented by the symbol σ and expressed in square meters. This does not equal geometric area. A perfectly conducting sphere of projected cross sectional area 1 m² (i.e. a diameter of 1.13 m) will have an RCS of 1 m². Note that for radar wavelengths much less than the diameter of the sphere, RCS is independent of frequency. Conversely, a square flat plate of area 1 m² will have an RCS of σ=4π A² / λ² (where A=area, λ=wavelength), or 13,982 m² at 10 GHz if the radar is perpendicular to the flat surface. At off-normal incident angles, energy is reflected away from the receiver, reducing the RCS. Modern stealth aircraft are said to have an RCS comparable with small birds or large insects, though this varies widely depending on aircraft and radar.

If the RCS was directly related to the target's cross-sectional area, the only way to reduce it would be to make the physical profile smaller. Rather, by reflecting much of the radiation away or by absorbing it, the target achieves a smaller radar cross section.

Tactics

Stealthy strike aircraft such as the Lockheed F-117 Nighthawk, are usually used against heavily defended enemy sites such as command and control centers or surface-to-air missile (SAM) batteries. Enemy radar will cover the airspace around these sites with overlapping coverage, making undetected entry by conventional aircraft nearly impossible. Stealthy aircraft can also be detected, but only at short ranges around the radars; for a stealthy aircraft there are substantial gaps in the radar coverage. Thus a stealthy aircraft flying an appropriate route can remain undetected by radar. Even if a stealth aircraft is detected, fire-control radars operating in C, X and Ku bands cannot paint (for missile guidance) low observable (LO) jets except at very close ranges. Many ground-based radars exploit Doppler filter to improve sensitivity to objects having a radial velocity component relative to the radar. Mission planners use their knowledge of enemy radar locations and the RCS pattern of the aircraft to design a flight path that minimizes radial speed while presenting the lowest-RCS aspects of the aircraft to the threat radar. To be able to fly these "safe" routes, it is necessary to understand an enemy's radar coverage (see electronic intelligence). Airborne or mobile radar systems such as AWACS can complicate tactical strategy for stealth operation.

Research

After the invention of electromagnetic metasurfaces, the conventional means to reduce RCS have been improved significantly. As mentioned earlier, the main objective in purpose shaping is to redirect scattered waves away from the backscattered direction, which is usually the source. However, this usually compromises aerodynamic performance. One feasible solution, which has extensively been explored in recent time, is to use metasurfaces which can redirect scattered waves without altering the geometry of a target. Such metasurfaces can primarily be classified in two categories: (i) checkerboard metasurfaces, (ii) gradient index metasurfaces. Similarly, negative index metamaterials are artificial structures for which refractive index has a negative value for some frequency range, such as in microwave, infrared, or possibly optical. These offer another way to reduce detectability, and may provide electromagnetic near-invisibility in designed wavelengths.

Plasma stealth is a phenomenon proposed to use ionized gas, termed a plasma, to reduce RCS of vehicles. Interactions between electromagnetic radiation and ionized gas have been studied extensively for many purposes, including concealing vehicles from radar. Various methods might form a layer or cloud of plasma around a vehicle to deflect or absorb radar, from simpler electrostatic to radio frequency (RF) more complex laser discharges, but these may be difficult in practice.

Several technology research and development efforts exist to integrate the functions of aircraft flight control systems such as ailerons, elevators, elevons, flaps, and flaperons into wings to perform the aerodynamic purpose with the advantages of lower RCS for stealth, via simpler geometries and lower complexity (mechanically simpler, fewer or no moving parts or surfaces, less maintenance), and lower mass, cost (up to 50% less), drag (up to 15% less during use), and inertia (for faster, stronger control response to change vehicle orientation to reduce detection). Two promising approaches are flexible wings, and fluidics.

In flexible wings, much or all of a wing surface can change shape in flight to deflect air flow. Adaptive compliant wings are a military and commercial effort.^[63][64][65] The X-53 Active Aeroelastic Wing was a US Air Force, Boeing, and NASA effort.

In fluidics, fluid injection is being researched for use in aircraft to control direction, in two ways: circulation control and thrust vectoring. In both, larger more complex mechanical parts are replaced by smaller, simpler, lower mass fluidic systems, in which larger forces in fluids are diverted by smaller jets or flows of fluid intermittently, to change the direction of vehicles. Mechanical control surfaces that must move cause an important part of aircraft radar cross-section. Omitting mechanical control surfaces can reduce radar returns. BAE Systems has tested two fluidically controlled unmanned aircraft, one starting in 2010 named Demon, and another starting in 2017 named MAGMA, with the University of Manchester.

In circulation control, near the trailing edges of wings, aircraft flight control systems are replaced by slots which emit fluid flows.