Michael A. Marciniak

Optical characterization of molecular beam epitaxially grown InAsSb nearly lattice matched to GaSb

Molecular beam epitaxially (MBE)-grown InAsSb nearly lattice matched to (001) GaSb substrates has been studied by infrared absorption, photoluminescence (PL), and double crystal x-ray diffraction (DCXRD). The absorption measurements, made at temperatures of 6–295 K, resulted in determinations of the temperature and compositional dependencies of the energy gap and the absorption coefficients for InAs1−xSbx (0⩽x⩽0.192). Temperature- and laser excitation power-dependent PL measurements showed only a single band edge peak for the ternary samples (Δa/a⩽+0.623%). Both low temperature PL linewidths (as narrow as 4.3 meV) and observations of LO-phonon replicas indicate the good quality of this material. However, careful analysis of the PL data indicates that even this good material may have a tendency for phase separation resulting in compositional inhomogeneity as reported previously for MBE-grown InAsSb. (004) DCXRD measurements resulted in lattice mismatches between −0.629%⩽Δa/a⩽+0.708% for these samples, whil...

Temperature dependence of the direct band gap energy and donor–acceptor transition energies in Be‐doped GaAsSb lattice matched to InP

The direct band energy (Eg) and donor–acceptor (D,A) transition energies are mapped as a function of temperature for Be‐doped GaAsSb lattice matched to InP. Photoluminescence (PL) measurements over the temperature range 2 K≤T≤300 K yield two emission peaks, one of lower intensity and one of higher intensity. The lower intensity peak is believed to be Be related, while the higher intensity peak is from residual impurities. The emission energies of both PL peaks increase linearly with respect to the logarithm of excitation intensity, indicating the peaks are (D,A) transitions. Measurement of Eg was achieved using optical absorption spectroscopy over the range 14 K≤T≤300 K. A least squares fit of the absorption data using the Varshni equation produces a closed form expression for Eg(T) with coefficients α=13.5×10−4 eV/K, and β=135 K.

High electrical activation efficiency obtained from Si-implanted Al0.18Ga0.82N

Si-implanted Al0.18Ga0.82N has been studied by Hall-effect measurements to produce good n-type layers for use in both electronic and optoelectronic devices. Silicon ions were implanted at 200keV with a dose ranging from 5×1014to5×1015cm−2 at room temperature, and the samples were annealed from 1100to1250°C for 5–25min with a 500‐A-thick AlN cap. Nearly 100% electrical activation efficiency for the sample having a dose of 5×1014cm−2 and 94% for a dose of 1×1015cm−2 were achieved after annealing at 1250 and 1200°C for 25min, respectively. Furthermore, this excellent electrical activation was obtained with much lower anneal temperature than the generally perceived 1350°C or higher anneal temperatures. This proves that a longer anneal time at lower anneal temperature (1200°C) is a better alternative than a shorter anneal time at higher anneal temperature (⩾1350°C). We believe that this accomplishment is very important in that the ion implantation technology can now be utilized for device fabrication of group ...

Optical characterization of silver nanorod thin films grown using oblique angle deposition

Nanorods are metamaterial structures that have been shown to have wide application, ranging from biomedical uses to photovoltaic materials. These materials have unique optical characteristics. In this paper, two silver (Ag) nanorod thin-film samples are created using Glancing Angle Deposition (GLAD) at both near-room temperature (∼300 K) and cryogenic temperature (∼100 K). Generalized ellipsometry is used to measure the optical constants of the samples. The strong difference between the optical constants of the constituent materials and those of these thin films shows that the characteristics of the samples are due to how their metamaterial structures are defined.The principle optical axes of the films align well with the morphological characteristics of the nanostructures. The axis with the greatest index of refraction remains aligned to the principle axes but shifts orientation with respect to morphological characteristics between samples. Experimental results show differences in both magnitude and characteristics of the nanorod indexes. Reflectance and transmittance measurements are performed to extract absorptance data. The room temperature deposited sample shows a higher overall absorptance, while the cryogenic sample shows a clear orientation-dependent absorptance. Polarization data is analyzed to show that the 100 K thin film exhibits polarization-dependent absorptance, while the 300 K samples absorptance has a strong orientation dependence.

Atmospheric-turbulence-effects correction factors for the laser range equation

The capability to detect an optical target using laser illumination is typically assessed using an equation commonly referred to as the laser range equation. In practice, however, the laser range equation often produces unreliable predictions when compared to actual results under field conditions. The lack of accuracy is due in large part to the failure of the range equation to account for the effects of atmospheric turbulence on the illuminating laser beam. Retrodirective reflections from a corner cube and a simple lens-mirror system, used as a surrogate for a lens-detector optical system, were studied using near-infrared laser illumination. Each optic was tested under a variety of atmospheric conditions in order to assess the effect of atmospheric turbulence on the returned power. Using established theory, a corrective term for use in the laser range equation that accounts for turbulence-induced beam spreading is developed and compared to experimental results. Additionally, a method for correcting the lab-measured optical cross section of a focused optical system in order to account for the defocusing effects of turbulence is developed. With these corrective terms, the laser range equation was modified to provide accurate return-power predictions under varied atmospheric conditions.

A worldwide physics-based high spectral resolution atmospheric characterization and propagation package for UV to RF wavelengths

The Air Force Institute of Technologys Center for Directed Energy (AFIT/CDE) developed the High Energy Laser End-to-End Operational Simulation (HELEEOS) model in part to quantify the performance variance in laser propagation created by the natural environment during dynamic engagements. As such, HELEEOS includes a fast-calculating, first principles, worldwide surface-to-100 km, atmospheric propagation and characterization package. This package enables the creation of profiles of temperature, pressure, water vapor content, optical turbulence, atmospheric particulates and hydrometeors as they relate to line-by-line layer transmission, path and background radiance at wavelengths from the ultraviolet to radio frequencies. Physics-based cloud and precipitation characterizations are coupled with a probability of cloud free line-of-sight algorithm for all possible look angles. HELEEOS was developed under the sponsorship of the High Energy Laser Joint Technology Office. In the current paper an example of a unique high fidelity simulation of a bi-static, time-varying five band multispectral remote observation of laser energy delivered on a test object is presented. The multispectral example emphasizes atmospheric effects using HELEEOS, the interaction of the laser on target and the observed reflectance and subsequent hot spot generated. A model of a sensor suite located on the surface is included to collect the diffuse reflected in-band laser radiation and the emitted radiance of the hot spot in four separate and spatially offset MWIR and LWIR bands. Particular care is taken in modeling the bidirectional reflectivity distribution function (BRDF) of the laser/target interaction to account for both the coupling of energy into the target body and the changes in reflectance as a function of temperature. The architecture supports any platform-target-observer geometry, geographic location, season, and time of day; and it provides for correct contributions of the sky-earth background. The simulation accurately models the thermal response, kinetics, turbulence, base disturbance, diffraction, and signal-to-noise ratios.

Examining the validity of using a Gaussian Schell-model source to model the scattering of a fully coherent Gaussian beam from a rough impedance surface

Abstract. Military applications that use adaptive optics (AO) often require a point source beacon at the target to measure and correct for wavefront aberrations introduced by atmospheric turbulence. However, turbulence prevents the formation of such a point beacon. The extended beacons that are created instead have finite spatial extents and exhibit varying degrees of spatial coherence. Modeling these extended beacons using a Gaussian Schell-model (GSM) form for the autocorrelation function would be a convenient approach due to the analytical tractability of Gaussian functions. We examine the validity of using such a model by evaluating the field scattered from a rough impedance surface using a full-wave computational technique called the method of moments (MoM). The MoM improves the fidelity of the analysis since it captures all the physics of the laser-target interaction, such as masking, shadowing, multiple reflections, etc. Two rough-surface targets with different roughness statistics are analyzed. The simulation results are verified with experimental bidirectional reflectance distribution function measurements. It is seen that for rough surfaces, in general, the scattered-field autocorrelation function is not of a GSM form. However, under certain conditions, modeling an extended beacon as a GSM source is legitimate. This analysis will aid in understanding the behavior of extended beacons and how they affect the overall performance of an AO system.

High-energy laser weapons: technology overview

High energy laser (HEL) weapons are ready for some of today’s most challenging military applications. For example, the Airborne Laser (ABL) program is designed to defend against Theater Ballistic Missiles in a tactical war scenario. Similarly, the Tactical High Energy Laser (THEL) program is currently testing a laser to defend against rockets and other tactical weapons. The Space Based Laser (SBL), Advanced Tactical Laser (ATL) and Large Aircraft Infrared Countermeasures (LAIRCM) programs promise even greater applications for laser weapons. This technology overview addresses both strategic and tactical roles for HEL weapons on the modern battlefield and examines current technology limited performance of weapon systems components, including various laser device types, beam control systems, atmospheric propagation, and target lethality issues. The characteristics, history, basic hardware, and fundamental performance of chemical lasers, solid state lasers and free electron lasers are summarized and compared. The elements of beam control, including the primary aperture, fast steering mirror, deformable mirrors, wavefront sensors, beacons and illuminators will be discussed with an emphasis on typical and required performance parameters. The effects of diffraction, atmospheric absorption, scattering, turbulence and thermal blooming phenomenon on irradiance at the target are described. Finally, lethality criteria and measures of weapon effectiveness are addressed. The primary purpose of the presentation is to define terminology, establish key performance parameters, and summarize technology capabilities.

Wave-optics modeling of aberration effects in optical cross section measurements

Laser radar is increasingly used to generate high resolution spatial maps of targets. The sensitivity, and hence utility, of laser radar (and laser rangefinders) depend on the laser cross section (LCS) of the target. A significant contribution to overall LCS can be the optical cross sections (OCSs) of its optical components. Optical systems, therefore, should be analyzed to quantify their OCSs. This work develops a better understanding of OCS measurements. Specifically, the measurements of a single-lens optical system with a substrate of known reflectivity at and near focus are discussed. Current practice fails to accurately predict the OCS of this setup; diffraction-limited theory is typically used near focus, where it predicts an OCS much greater than the measured value, and ray tracing is used as the substrate is moved away from focus, where it successfully captures the general trend but fails to predict deviations caused by diffraction and interference effects. We use a wave-optics approach to account for the effects of aberrations. A model is created to accurately predict the OCS of the single-lens system. The results, along with predictions of the current theory, are compared to experimental data. In general, wave optics produces more accurate OCS predictions than ray tracing.

Permittivity and Permeability Tensor Extraction Technique for Arbitrary Anisotropic Materials

Computing the permittivity and permeability of complex materials has previously relied on a series of simplifying assumptions to enable analysis. The most restricting requirement is that the optical axes of the material must align with the laboratory frame of reference. This requirement cannot be met for a large group of materials, including crystalline structures and metamaterials such as tilted nanorods. Currently, designing the optical characteristics of these structures would require ellipsometric analysis, which uses an error-correction-based technique. Here, a new technique built upon the underlying physics of ellipsometry is proposed to extract arbitrary permittivity and permeability tensors using a set of off-axis measurements. This new permittivity and permeability tensor extraction technique allows all 18 elements of the permittivity and permeability tensors to be nonzero and extracts them, given a set of reflectance and transmittance measurements. Several materials are analyzed here, including a) an isotopic plane of known permittivity, b) an anisotropic aligned structure, and c) a tilted-nanorod-based sample that cannot be measured using traditional methodologies. The isotropic plane shows very low error (<; 10-4%) in the x and y tensor measurements and around 1% error in the z tensor measurement at higher (metallic) permittivities. The aligned structures characteristics are compared to measurements made with traditional techniques and show excellent agreement between the techniques. The tilted nanorod characteristics are analyzed and used to predict the reflection and transmission coefficients at other angles. The predictions compare very well with the computational electromagnetic simulations, showing at most 5% error over the range examined.