Light-Emitting Metasurfaces: Simultaneous Control of Spontaneous Emission and Far-Field Radiation.

Light-emitting sources and devices permeate every aspect of our lives and are used in lighting, communications, transportation, computing, and medicine. Advances in multifunctional and "smart lighting" would require revolutionary concepts in the control of emission spectra and directionality. Such control might be possible with new schemes and regimes of light-matter interaction paired with developments in light-emitting materials. Here we show that all-dielectric metasurfaces made from III-V semiconductors with embedded emitters have the potential to provide revolutionary lighting concepts and devices, with new functionality that goes far beyond what is available in existing technologies. Specifically, we use Mie-resonant metasurfaces made from semiconductor heterostructures containing epitaxial quantum dots. By controlling the symmetry of the resonant modes, their overlap with the emission spectra, and other structural parameters, we can enhance the brightness by 2 orders of magnitude, as well as reduce its far-field divergence significantly.

Viscoelastic optical nonlocality of low-loss epsilon-near-zero nanofilms.

Optical nonlocalities are elusive and hardly observable in traditional plasmonic materials like noble and alkali metals. Here we report experimental observation of viscoelastic nonlocalities in the infrared optical response of epsilon-near-zero nanofilms made of low-loss doped cadmium-oxide. The nonlocality is detectable thanks to the low damping rate of conduction electrons and the virtual absence of interband transitions at infrared wavelengths. We describe the motion of conduction electrons using a hydrodynamic model for a viscoelastic fluid, and find excellent agreement with experimental results. The electrons' elasticity blue-shifts the infrared plasmonic resonance associated with the main epsilon-near-zero mode, and triggers the onset of higher-order resonances due to the excitation of electron-pressure modes above the bulk plasma frequency. We also provide evidence of the existence of nonlocal damping, i.e., viscosity, in the motion of optically-excited conduction electrons using a combination of spectroscopic ellipsometry data and predictions based on the viscoelastic hydrodynamic model.

An all-dielectric metasurface as a broadband optical frequency mixer.

A frequency mixer is a nonlinear device that combines electromagnetic waves to create waves at new frequencies. Mixers are ubiquitous components in modern radio-frequency technology and microwave signal processing. The development of versatile frequency mixers for optical frequencies remains challenging: such devices generally rely on weak nonlinear optical processes and, thus, must satisfy phase-matching conditions. Here we utilize a GaAs-based dielectric metasurface to demonstrate an optical frequency mixer that concurrently generates eleven new frequencies spanning the ultraviolet to near-infrared. The even and odd order nonlinearities of GaAs enable our observation of second-harmonic, third-harmonic, and fourth-harmonic generation, sum-frequency generation, two-photon absorption-induced photoluminescence, four-wave mixing and six-wave mixing. The simultaneous occurrence of these seven nonlinear processes is assisted by the combined effects of strong intrinsic material nonlinearities, enhanced electromagnetic fields, and relaxed phase-matching requirements. Such ultracompact optical mixers may enable a plethora of applications in biology, chemistry, sensing, communications, and quantum optics.

Huygens' Metasurfaces Enabled by Magnetic Dipole Resonance Tuning in Split Dielectric Nanoresonators.

Dielectric metasurfaces that exploit the different Mie resonances of nanoscale dielectric resonators are a powerful platform for manipulating electromagnetic fields and can provide novel optical behavior. In this work, we experimentally demonstrate independent tuning of the magnetic dipole resonances relative to the electric dipole resonances of split dielectric resonators (SDRs). By increasing the split dimension, we observe a blue shift of the magnetic dipole resonance toward the electric dipole resonance. Therefore, SDRs provide the ability to directly control the interaction between the two dipole resonances within the same resonator. For example, we achieve the first Kerker condition by spectrally overlapping the electric and magnetic dipole resonances and observe significantly suppressed backward scattering. Moreover, we show that a single SDR can be used as an optical nanoantenna that provides strong unidirectional emission from an electric dipole source.

Ultrafast all-optical tuning of direct-gap semiconductor metasurfaces.

Optical metasurfaces are regular quasi-planar nanopatterns that can apply diverse spatial and spectral transformations to light waves. However, metasurfaces are no longer adjustable after fabrication, and a critical challenge is to realise a technique of tuning their optical properties that is both fast and efficient. We experimentally realise an ultrafast tunable metasurface consisting of subwavelength gallium arsenide nanoparticles supporting Mie-type resonances in the near infrared. Using transient reflectance spectroscopy, we demonstrate a picosecond-scale absolute reflectance modulation of up to 0.35 at the magnetic dipole resonance of the metasurfaces and a spectral shift of the resonance by 30 nm, both achieved at unprecedentedly low pump fluences of less than 400 µJ cm-2. Our findings thereby enable a versatile tool for ultrafast and efficient control of light using light.Metasurfaces are not adjustable after fabrication, and a critical challenge is to realise a technique of tuning their optical properties that is both fast and efficient. Here, Shcherbakov et al. realise an ultrafast tunable metasurface with picosecond-scale large absolute reflectance modulation at low pump fluences.

Experimental verification of epsilon-near-zero plasmon polariton modes in degenerately doped semiconductor nanolayers.

We investigate optical polariton modes supported by subwavelength-thick degenerately doped semiconductor nanolayers (e.g. indium tin oxide) on glass in the epsilon-near-zero (ENZ) regime. The dispersions of the radiative (R, on the left of the light line) and non-radiative (NR, on the right of the light line) ENZ polariton modes are experimentally measured and theoretically analyzed through the transfer matrix method and the complex-frequency/real-wavenumber analysis, which are in remarkable agreement. We observe directional near-perfect absorption using the Kretschmann geometry for incidence conditions close to the NR-ENZ polariton mode dispersion. Along with field enhancement, this provides us with an unexplored pathway to enhance nonlinear optical processes and to open up directions for ultrafast, tunable thermal emission.

Resonantly Enhanced Second-Harmonic Generation Using III-V Semiconductor All-Dielectric Metasurfaces.

Nonlinear optical phenomena in nanostructured materials have been challenging our perceptions of nonlinear optical processes that have been explored since the invention of lasers. For example, the ability to control optical field confinement, enhancement, and scattering almost independently allows nonlinear frequency conversion efficiencies to be enhanced by many orders of magnitude compared to bulk materials. Also, the subwavelength length scale renders phase matching issues irrelevant. Compared with plasmonic nanostructures, dielectric resonator metamaterials show great promise for enhanced nonlinear optical processes due to their larger mode volumes. Here, we present, for the first time, resonantly enhanced second-harmonic generation (SHG) using gallium arsenide (GaAs) based dielectric metasurfaces. Using arrays of cylindrical resonators we observe SHG enhancement factors as large as 10(4) relative to unpatterned GaAs. At the magnetic dipole resonance, we measure an absolute nonlinear conversion efficiency of ∼2 × 10(-5) with ∼3.4 GW/cm(2) pump intensity. The polarization properties of the SHG reveal that both bulk and surface nonlinearities play important roles in the observed nonlinear process.

Parents' Incomes and Children's Outcomes: A Quasi-Experiment.

We examine the role that an exogenous increase in household income due to a government transfer unrelated to household characteristics plays in children's long run outcomes. Children in affected households have higher levels of education in their young adulthood and a lower incidence of criminality for minor offenses. Effects differ by initial household poverty status. An additional $4000 per year for the poorest households increases educational attainment by one year at age 21 and reduces having ever committed a minor crime by 22% at ages 16-17. Our evidence suggests that improved parental quality is a likely mechanism for the change.

Posttraumatic stress without trauma in children.

OBJECTIVE: It remains unclear to what degree children show signs of posttraumatic stress disorder (PTSD) after experiencing low-magnitude stressors, or stressors milder than those required for the DSM-IV extreme stressor criterion. METHOD: A representative community sample of 1,420 children, ages 9, 11, and 13 at intake, was followed annually through age 16. Low-magnitude and extreme stressors as well as subsequent posttraumatic stress symptoms were assessed with the Child and Adolescent Psychiatric Assessment. Two measures of posttraumatic stress symptoms were used: having painful recall, hyperarousal, and avoidance symptoms (subclinical PTSD) and having painful recall only. RESULTS: During any 3-month period, low-magnitude stressors occurred four times as often as extreme stressors (24.0% compared with 5.9%). Extreme stressors elicited painful recall in 8.7% of participants and subclinical PTSD in 3.1%, compared with 4.2% and 0.7%, respectively, for low-magnitude stressors. Because of their higher prevalence, however, low-magnitude stressors accounted for two-thirds of cases of painful recall and half of cases of subclinical PTSD. Moreover, exposure to low-magnitude stressors predicted symptoms even among youths with no prior lifetime exposure to an extreme stressor. CONCLUSIONS: Relative to low-magnitude stressors, extreme stressors place children at greater risk for posttraumatic stress symptoms. Nevertheless, a sizable proportion of children manifesting posttraumatic stress symptoms experienced only a low-magnitude stressor.

Micromachined Accelerometers With Optical Interferometric Read-Out and Integrated Electrostatic Actuation.

A micromachined accelerometer device structure with diffraction-based optical detection and integrated electrostatic actuation is introduced. The sensor consists of a bulk silicon proof mass electrode that moves vertically with respect to a rigid diffraction grating backplate electrode to provide interferometric detection resolution of the proof-mass displacement when illuminated with coherent light. The sensor architecture includes a monolithically integrated electrostatic actuation port that enables the application of precisely controlled broadband forces to the proof mass while the displacement is simultaneously and independently measured optically. This enables several useful features such as dynamic self-characterization and a variety of force-feedback modalities, including alteration of device dynamics in situ. These features are experimentally demonstrated with sensors that have been optoelectronically integrated into sub-cubic-millimeter volumes using an entirely surface-normal, rigid, and robust embodiment incorporating vertical cavity surface emitting lasers and integrated photodetector arrays. In addition to small form factor and high acceleration resolution, the ability to self-characterize and alter device dynamics in situ may be advantageous. This allows periodic calibration and in situ matching of sensor dynamics among an array of accelerometers or seismometers configured in a network.

Childhood psychiatric disorders and young adult crime: a prospective, population-based study.

OBJECTIVE: While psychopathology is common in criminal populations, knowing more about what kinds of psychiatric disorders precede criminal behavior could be helpful in delineating at-risk children. The authors determined rates of juvenile psychiatric disorders in a sample of young adult offenders and then tested which childhood disorders best predicted young adult criminal status. METHOD: A representative sample of 1,420 children ages 9, 11, and 13 at intake were followed annually through age 16 for psychiatric disorders. Criminal offense status in young adulthood (ages 16 to 21) was ascertained through court records. RESULTS: Thirty-one percent of the sample had one or more adult criminal charges. Overall, 51.4% of male young adult offenders and 43.6% of female offenders had a child psychiatric history. The population-attributable risk of criminality from childhood disorders was 20.6% for young adult female participants and 15.3% for male participants. Childhood psychiatric profiles predicted all levels of criminality. Severe/violent offenses were predicted by comorbid diagnostic groups that included both emotional and behavioral disorders. CONCLUSIONS: The authors found that children with specific patterns of psychopathology with and without conduct disorder were at risk of later criminality. Effective identification and treatment of children with such patterns may reduce later crime.

Traumatic events and posttraumatic stress in childhood.

CONTEXT: Traumatic events are common and are related to psychiatric impairment in childhood. Little is known about the risk for posttraumatic stress disorder (PTSD) across different types of trauma exposure in children. OBJECTIVE: To examine the developmental epidemiology of potential trauma and posttraumatic stress (PTS) in a longitudinal community sample of children. METHODS: A representative population sample of 1420 children aged 9, 11, and 13 years at intake were followed up annually through 16 years of age. Main Outcome Measure Traumatic events and PTS were assessed from child and parent reports annually to 16 years of age. Risk factors and DSM-IV disorders were also assessed. RESULTS: More than two thirds of children reported at least 1 traumatic event by 16 years of age, with 13.4% of those children developing some PTS symptoms. Few PTS symptoms or psychiatric disorders were observed for individuals experiencing their first event, and any effects were short-lived. Less than 0.5% of children met the criteria for full-blown DSM-IV PTSD. Violent or sexual trauma were associated with the highest rates of symptoms. The PTS symptoms were predicted by previous exposure to multiple traumas, anxiety disorders, and family adversity. Lifetime co-occurrence of other psychiatric disorders with traumatic events and PTS symptoms was high, with the highest rates for anxiety and depressive disorders. CONCLUSIONS: In the general population of children, potentially traumatic events are fairly common and do not often result in PTS symptoms, except after multiple traumas or a history of anxiety. The prognosis after the first lifetime trauma exposure was generally favorable. Apart from PTSD, traumatic events are related to many forms of psychopathology, with the strongest links being with anxiety and depressive disorders.

Service costs of caring for adolescents with mental illness in a rural community, 1993-2000.

OBJECTIVE: Costs of treating child psychiatric disorders fall on educational, primary care, juvenile justice, and social service agencies as well as on psychiatric services. The authors estimated multiagency mental health costs by integrating service unit costs with utilization rates in an 11-county area. Using psychiatric diagnoses made independently of service use records, the authors calculated costs across agencies as well as the extent of unmet need for psychiatric care. METHOD: Annual parent and child reports were used to measure mental health care needs and units of service across 21 types of settings for the population-based Great Smoky Mountain Study sample of 1,420 adolescents from ages 13 to 16. Unit costs for services were generated from information from service providers and records. The authors calculated costs overall, costs by type of service, and costs by diagnosis. RESULTS: Average annual costs per adolescent treated were $3,146. Juvenile justice and inpatient/residential facilities accounted for well over half of the total costs. Costs for youths with two or more diagnoses were twice as much as costs of those with a single disorder. Among adolescents with service needs, 66.9% received no services. Public health insurance was associated with higher rates of specialty mental health care than either private insurance or no insurance. CONCLUSIONS: Annual costs across all services were three to four times greater than recent health insurance estimates alone. Many costs for adolescents with mental health problems were borne by agencies not designed primarily to provide psychiatric or psychological services. Only one in three adolescents needing psychiatric care received any mental health services.

Test-Retest Reliability of the Preschool Age Psychiatric Assessment (PAPA).

OBJECTIVE: To examine the test-retest reliability of a new interviewer-based psychiatric diagnostic measure (the Preschool Age Psychiatric Assessment) for use with parents of preschoolers aged 2 to 5 years. METHOD: A total of 1,073 parents of children attending a large pediatric clinic completed the Child Behavior Checklist 1 1/2-5. For 18 months, 193 parents of high scorers and 114 parents of low scorers were interviewed on two occasions an average of 11 days apart. RESULTS: Estimates of diagnostic reliability were very similar to those obtained from interviews with parents of older children and adults, with kappas ranging from 0.36 to 0.79. Test-retest intraclass correlations for DSM-IV syndrome scale scores ranged from 0.56 to 0.89. There were no significant differences in reliability by age, sex, or race (African American versus non-African American). CONCLUSIONS: The Preschool Age Psychiatric Assessment provides a reasonably reliable standardized measure of DSM-IV psychiatric symptoms and disorders in preschoolers for use in both research and clinical service evaluations of preschoolers as young as 2 years old.

Relationships between poverty and psychopathology: a natural experiment.

CONTEXT: Social causation (adversity and stress) vs social selection (downward mobility from familial liability to mental illness) are competing theories about the origins of mental illness. OBJECTIVE: To test the role of social selection vs social causation of childhood psychopathology using a natural experiment. DESIGN: Quasi-experimental, longitudinal study. POPULATION AND SETTING: A representative population sample of 1420 rural children aged 9 to 13 years at intake were given annual psychiatric assessments for 8 years (1993-2000). One quarter of the sample were American Indian, and the remaining were predominantly white. Halfway through the study, a casino opening on the Indian reservation gave every American Indian an income supplement that increased annually. This increase moved 14% of study families out of poverty, while 53% remained poor, and 32% were never poor. Incomes of non-Indian families were unaffected. MAIN OUTCOME MEASURES: Levels of Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition, psychiatric symptoms in the never-poor, persistently poor, and ex-poor children were compared for the 4 years before and after the casino opened. RESULTS: Before the casino opened, the persistently poor and ex-poor children had more psychiatric symptoms (4.38 and 4.28, respectively) than the never-poor children (2.75), but after the opening levels among the ex-poor fell to those of the never-poor children, while levels among those who were persistently poor remained high (odds ratio, 1.50; 95% confidence interval, 1.08-2.09; and odds ratio, 0.91; 95% confidence interval, 0.77-1.07, respectively). The effect was specific to symptoms of conduct and oppositional defiant disorders. Anxiety and depression symptoms were unaffected. Similar results were found in non-Indian children whose families moved out of poverty during the same period. CONCLUSIONS: An income intervention that moved families out of poverty for reasons that cannot be ascribed to family characteristics had a major effect on some types of children's psychiatric disorders, but not on others. Results support a social causation explanation for conduct and oppositional disorder, but not for anxiety or depression.

Prevalence and development of psychiatric disorders in childhood and adolescence.

BACKGROUND: This longitudinal community study assessed the prevalence and development of psychiatric disorders from age 9 through 16 years and examined homotypic and heterotypic continuity. METHODS: A representative population sample of 1420 children aged 9 to 13 years at intake were assessed annually for DSM-IV disorders until age 16 years. RESULTS: Although 3-month prevalence of any disorder averaged 13.3% (95% confidence interval [CI], 11.7%-15.0%), during the study period 36.7% of participants (31% of girls and 42% of boys) had at least 1 psychiatric disorder. Some disorders (social anxiety, panic, depression, and substance abuse) increased in prevalence, whereas others, including separation anxiety disorder and attention-deficit/hyperactivity disorder (ADHD), decreased. Lagged analyses showed that children with a history of psychiatric disorder were 3 times more likely than those with no previous disorder to have a diagnosis at any subsequent wave (odds ratio, 3.7; 95% CI, 2.9-4.9; P<.001). Risk from a previous diagnosis was high among both girls and boys, but it was significantly higher among girls. Continuity of the same disorder (homotypic) was significant for all disorders except specific phobias. Continuity from one diagnosis to another (heterotypic) was significant from depression to anxiety and anxiety to depression, from ADHD to oppositional defiant disorder, and from anxiety and conduct disorder to substance abuse. Almost all the heterotypic continuity was seen in girls. CONCLUSIONS: The risk of having at least 1 psychiatric disorder by age 16 years is much higher than point estimates would suggest. Concurrent comorbidity and homotypic and heterotypic continuity are more marked in girls than in boys.

Obesity and psychiatric disorder: developmental trajectories.

OBJECTIVES: To identify age-related trajectories of obesity from childhood into adolescence, and to test the association of these trajectories with the development of psychiatric disorders (conduct disorder, oppositional defiant disorder, attention-deficit/hyperactivity disorder, substance abuse, depression, and anxiety). METHODS: White children (N = 991) 9 to 16 years old from the Great Smoky Mountains Study, a representative sample of rural youth, were evaluated annually over an 8-year period for height, weight, psychiatric disorder, and vulnerabilities for psychiatric disorder. Longitudinal analyses on the repeated measures data were conducted using developmental trajectory models and generalized estimating equation models. RESULTS: Obesity was 3 to 4 times more common than expected from national rates using Centers for Disease Control and Prevention 2000 criteria. Four developmental trajectories of obesity were found: no obesity (73%), chronic obesity (15%), childhood obesity (5%), and adolescent obesity (7%). Only chronic obesity was associated with psychiatric disorder: oppositional defiant disorder in boys and girls and depressive disorders in boys. CONCLUSIONS: In a general population sample studied longitudinally, chronic obesity was associated with psychopathology.

Psychiatric disorder, impairment, and service use in rural African American and white youth.

BACKGROUND: The Caring for Children in the Community Study examined the prevalence of DSM-IV psychiatric disorders and correlates of mental health service use in rural African American and white youth. METHODS: Four thousand five hundred youth aged 9 to 17 years from 4 North Carolina counties were randomly selected from school databases. Parents completed telephone questionnaires about their children's behavior problems. A second-stage sample of 1302 was identified for recruitment into the interview phase of the study, and 920 (70.7%) of these were successfully interviewed at home using the Child and Adolescent Psychiatric Assessment and related measures of service use. RESULTS: Weighted back to general population estimates, 21.1% of youth had 1 or more DSM-IV psychiatric disorders in the past 3 months. Prevalence was similar in African American (20.5%) and white (21.9%) youth. The only ethnic difference was an excess of depressive disorders in white youth (4.6% vs 1.4%). Thirteen percent of participants (36.0% of those with a diagnosis) received mental health care in the past 3 months. White youth were more likely than African American youth to use specialty mental health services (6.1% vs 3.2%), but services provided by schools showed very little ethnic disparity (8.6% vs 9.2%). The effect of children's symptoms on their parents was the strongest correlate of specialty mental health care. CONCLUSIONS: In this rural sample, African American and white youth were equally likely to have psychiatric disorders, but African Americans were less likely to use specialty mental health services. School services provided care to the largest number of youths of both ethnic groups.