9
Physics
The panel met on November 6-8, 2018, at the National Academies of Sciences, Engineering, and Medicine (National Academies) facility in Washington, D.C., to review the In-House Laboratory Independent Research (ILIR) program projects in physics conducted in 2018 at the following U.S. Army Research, Development, and Engineering Centers (RDECs): Aviation and Missile Research, Development, and Engineering Center (AMRDEC); Communications–Electronics Research, Development, and Engineering Center (CERDEC); Natick Soldier Research, Development, and Engineering Center (NSRDEC); and the Space and Missile Defense Command (SMDC). The panel received overview presentations on the ILIR programs at each RDEC and technical presentations describing the projects. During each presentation the panel engaged in question-and-answer sessions with the presenter, and a general discussion with RDEC staff after the panel had formulated initial impressions and developed additional questions during its closed-session deliberations, conducted after the RDEC staff had concluded their presentations.
AVIATION AND MISSILE RESEARCH, DEVELOPMENT, AND ENGINEERING CENTER
Project: Collision Broadening and Hyperfine State-Changing Collisions at Very Low Densities in Atomic Vapor Cells
This research is directed toward noninvasive diagnostics of atomic vapor cells for improving accuracy of rubidium (Rb)-based optical and microwave frequency reference standards in sensors and clocks. A further aim is to improve knowledge of long-range interatomic potentials for Rb-X collisions.
The presentation reported results of the measurement of hyperfine-resolved collision parameters for Rb-X collisions at low densities (1013 cm−3) of 52S1/2 → 52P3/2 optical transitions. Saturated absorption precision spectroscopy with megahertz resolution was carried out. Stimulated Raman scattering was used to measure ground-state hyperfine level splittings. Direct measurement of excited-state hyperfine
level intervals was reported using frequency-locked lasers. Numerical solutions and experimental results for resonance widths were then compared.
The work could be relevant to noninvasive, all-optical diagnostics of atomic vapor cells and constitutes a good implementation of high-resolution spectroscopy. However, pressure broadening by rare gas collisions is a very old and well-studied topic. Neither the methodology nor the systems studied in the project are novel or are of a groundbreaking nature. Still, this research adds to an existing database of spectroscopic information.
Project: Continuum Electrodynamics
There is a long ongoing debate on some fundamental aspects of the macroscopic theory of continuum electrodynamics, the so-called Abraham–Minkowski controversy, arising from the inconsistency of Maxwellian continuum electrodynamics theorems with conservation laws. This remains true even if the “fix” is applied by viewing continuum electrodynamics as a subsystem and then inserting hypothetical material momentum by hand.
The researcher provided theoretical work in an attempt to resolve the Abraham–Minkowski controversy. The impact area is projected to be solid-state sensors and emerging applications in metamaterials, optical cloaking, negative refraction, optically induced forces, and nanotechnology.
The approach is apparently somewhat different from what has been tried before. However, the researcher reported that it was not possible to get the results published because of objections of the peer-review journal’s reviewers. Furthermore, no justification was provided regarding the claimed relevance of this activity to the development of a comprehensive theory of continuum electrodynamics targeting the development of smaller, faster, cheaper optical devices based on sensitive and/or novel light-matter interactions.
This topic has a very long history and has been the subject of considerable debate over the past 100 years or so. Numerous theoretical arguments have been advanced and experiments carried out over the years, but their outcome appears unable to satisfy the various parties involved and bring this issue to its resolution. The key point is that there is absolutely no issue at the level of microscopic electrodynamics, and the controversy is really a result of the phenomenological steps involved in the description of macroscopic dielectrics.
The present work does not provide a satisfactory theoretical resolution to this issue, nor does it propose any experimental test that might allow for a quantitative test of the arguments being advanced. For this reason, it does not appear to offer any significant and convincing new angle to this century-old discussion, and, as such, its impact is rather limited.
Project: Nested Plasmonic Resonances and Linear and Nonlinear Optical Properties of Epsilon-Near-Zero Metamaterials
The goal of this research is to study optics at the nanoscale. It focuses most specifically on situations when the electric permittivity of a medium interfaced with the vacuum approaches zero, in which case the boundary conditions for the longitudinal displacement vector at the interface imply an intra-medium field becoming extremely large (ideally approaching infinity). However, under realistic conditions the singularity is mitigated by absorption. A careful description of the dynamics of the bound and free electrons includes a detailed treatment of linear and nonlinear dispersions at the interface and inside the volume. Using an in-house developed pulse propagation method at the interface of the medium with the vacuum leads to a number of predictions, involving in particular nonlocal effects, second-harmonic generation, third harmonic generation, the onset of plasmon oscillations, plasma frequency redshifts, and
more. One important result of this work is the observation that this behavior need not rely on the use of composite or metamaterials but can also be observed as a consequence of the presence of a layer of free electrons (or more precisely of the non-vanishing boundary electron wave function). The resulting volume and surface charge distributions in systems are as conceptually simple as the interface of noble metal mirrors and air or at metal-oxide and metal-metal boundaries.
A review of research activities, going back to 2011, was presented during the review. Discussions of several peer-reviewed papers by the research team were also presented starting with “Singularity-driven Second- and Third-Harmonic Generation at e-near-zero Crossing Points,”1 which follows the initial observation by Engheta of peculiar linear and nonlinear optical properties of e-near-zero materials. The ILIR reported was extensive and covered 7 years of activities.
This fundamental research is of excellent quality and combines theory and experiment in a convincing and effective fashion. It involves senior scientists, postdoctoral researchers, and academic collaborators from the United States and Europe, thanks in particular to the additional funding that the principal investigator (PI) was able to secure. The authors are to be applauded for these efforts to bring complementary expertise and resources toward the realization of this project.
As can be expected in basic research, this program has developed over several years, culminating in the present focus and accomplishments. It has resulted in a number of peer-reviewed papers in some of the leading journals—three in the past year—as well as in a number of conference presentations and, importantly, significant international recognition for AMRDEC. The presentation gave an excellent and useful introduction to the broader context of this work. Unfortunately, the presentation covered much of the past work on this project, rather than concentrating on the results of the past couple of years, which would have given a better understanding of the recent progress and future plans.
COMMUNICATIONS–ELECTRONICS RESEARCH, DEVELOPMENT, AND ENGINEERING CENTER
Project: Defect Analysis and Transport Measurement in Infrared Detector Devices
The Electron Beam-Induced Current (EBIC) technique was used to non-destructively measure minority carrier diffusion lengths in novel infrared (IR) detectors. Secondary electrons are used for imaging. Key questions addressed include the following: Can EBIC be used to adequately characterize transport in devices with a complex layer structure? Will the measurement be sufficient to handle regions of interest where the diffusion length is potentially on the order of, or larger than, the thickness of the absorber? Can the results of the study be directly correlated with knowledge of defect states and device fabrication methods?
The EBIC technique has long been used (starting with early studies of silicon devices) to characterize carrier transport, in addition to the imaging of electrically active material defects and integrated circuit failure analysis. Coupled with classical diffusion equations, it is used to obtain the diffusion lengths for a well-understood device.
Improved understanding of material properties affecting performance could lead to widespread performance and cost improvements. The techniques can also serve as diagnostics for finished products in order to understand failure mechanisms.
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1 M.A. Vicenti, D. de Ceglia, A. Ciattoni, and M. Scalora, 2011, Singularity-driven second and third harmonic generation at e-near-zero crossing points, Physical Review A84:063826.
That said, in-depth analysis and theoretical quantitative modeling is lacking in this project. Extensive data were collected, but not enough effort was given to developing a broader picture and guiding principles. Time-resolved photoluminescence provides carrier lifetimes, but all other results are at steady state and highly averaged. Time-resolved transport studies could be most helpful in the development of the project, but such studies were not attempted.
Project: Electrochemical Deposition of ZnTe Passivation Layers for Infrared Detectors
III-V semiconductor-based infrared (IR) detectors are potentially seen as alternatives to mercury cadmium telluride (MCT) detectors; however, their device performance is often limited because it is affected by surface leakage current. IR detector pixels with InAs-like surfaces suffer from this surface leakage. Surface passivation of IR detectors is a significant technological challenge, and surface passivation of the III-V material could help reduce the leakage current problems. Passivation material needs to be conformal and the technology for deposition needs to be low-cost and scalable. Zinc telluride (ZnTe) is an attractive material with a large bandgap (2.24 eV), a surface Fermi level in bandgap (depleted surface), and a 6.1Å lattice constant (which matches relevant IR detector materials). Recently, ZnTe has been found to be a good surface passivating material and success has been reported for molecular beam epitaxy (MBE)-grown epitaxial ZnTe. The ILIR project is directed toward exploring an alternate technique using electrochemical deposition on III-V semiconductor surfaces for passivation.
Electrochemical deposition from an acidic aqueous bath was used to deposit the wide-gap II-VI semiconductor ZnTe as a surface passivation layer on narrow-gap III-V IR detector structures with InAs-like surfaces exhibiting surface leakage current. The work explored important practical questions: Can a high-quality, semi-insulating ZnTe layer be electrodeposited on relevant III-V IR detector materials? Can a high-quality interface be prepared in a wet chemical environment? Does ZnTe grown electrochemically exhibit strong passivating behavior?
Electrochemical deposition is inexpensive, conformal, and scalable. ZnTe electrodeposition was successfully carried out, and the results suggest that ZnTe deposited by electrochemical deposition may be a promising conformal passivant for III-V-based IR detectors. These results are supported by Raman scattering and Hall measurements.
Successful deposition of ZnTe on planar surfaces is reported, showing a strong reduction in surface electron population. Post-deposition heating was shown to be a critical process. Conformal deposition was also demonstrated. Improved uniformity will be needed for device applications.
The work on this project is good; however, it is too narrowly focused. The presentation did not include models or transferable results to other materials and devices. Additional methods for characterizing the morphology of the deposited layers will be needed to make the results more quantitative. Questions about the quality of ZnTe layers, either epitaxial or amorphous, have yet to be answered. Furthermore, no comparisons were made with existing results of passivation of InAs-based devices, which have been passivated using MBE-grown epitaxial ZnTe.
Project: Physics-Based Simulation as a Diagnostic and Design Tool for Optoelectronics
For material systems in which there is considerable parameter variation—run-to-run or vendor-to-vendor—there is a need to propagate the uncertainty through a single input, single output model. Problems arise from the fact that test chips contain as many as 30 devices, and measurements are performed over a wide temperature and voltage range. It is important to process that large data set with device-to-device variability without introducing operator bias during the fitting. Potential value includes virtual prototyping and testing to reduce costs and person-hours for in-the-laboratory and field tests. A deeper
understanding of material behavior and parameters, which impact sensor capabilities, would allow more informed decision making for improved performance and higher manufacturing yield.
A combination of dedicated measurements and test device analysis were used to create a library for IR detector materials such as mercury cadmium telluride (HgCdTe) and III-V. This library was used in drift-diffusion simulations of IR detectors to determine immeasurable quantities on known devices or measurable quantities on unknown devices. Iterative numerical solutions of coupled drift-diffusion, and Poisson equations (via finite element [FE] methods) were used in the simulations. The results were used to design and to help the growth and fabrication of devices.
Large test data sets were reduced to compact material models through fitting to semiconductor device equations. Additionally, automated parameter synthesis from device test data through simulated annealing is being carried out, and graphical tools are also being developed. Applications were made to analyze test device data from an in-house Ga-free Type-II SLS nBn IR detector growth campaign.
Performance-limiting mechanisms of test detectors were addressed, and attempts were made to correlate material properties such as carrier mobilities, lifetimes, and trap densities to device parameters. InAs/InAsSb superlattice materials were also tested.
The work is systematic and technically sound. It could have a large impact on industries by providing device design and optimization protocols. However, this will require substantially more in-depth and broader modeling than done heretofore, and the researcher will need to specify error bars and formulate generalizable rules that go beyond the specific materials and devices. Efforts along these lines are lacking, and the use of machine learning algorithms to process the data more effectively could speed up the process. Such analyses are very commonly used in the integrated circuit fabrication industry for yield improvement. Connecting with others with such analytic expertise may provide guidance for more effective outcomes.
Project: Unbiased Test Target for Nonlinear EO/IR System Evaluation
The Army needs a test target to directly measure imaging system performance without human bias, especially given nonlinear processing. Current performance metrics rely on linear models based on linear measurements such as modulation transfer function (MTF) and noise equivalent temperature difference (NETD). Test targets are often contextual and subjective, and so a standard testing protocol is needed to make comparisons among systems, create reliable specifications across all systems, and monitor systems in real time. The project is designed to measure the information loss from the original encoded image to the collected image and use this information loss as an image quality metric. An important part of the project is the development of the decoding algorithm for determining a robust deep learning architecture that takes into account a wide range of simulated effects, including blur, noise, sampling, distortion and warping, dynamic range compression, saturation, noise suppression, and other nonlinear effects.
The research group has created a fractal optical test pattern consisting of about 40 black dots of different sizes and spacing on a white background. The optical system under evaluation looks at the test pattern to discern the black dots, and the image is evaluated by a machine learning scoring algorithm, which outputs the percentage of these dots that are detected. The research group also looked at the Night Vision Integrated Performance Model, which is extensively used to quantify the performance (i.e., Receiver Operating Characteristics [ROC]) of an electro-optical (EO) sensor or detector. This program extends this model to include or simulate human detection of the image. Such an approach is needed because it eliminates the bias of human vision and interpretation (i.e., soldier testing) and is very important when nonlinear optical transfer or imaging is encountered, due to atmospheric turbulence, image distortion optics, dust clouds, or visual impairment. The researchers are now finalizing the test pattern, which is expected to be finished at the end of 2018, and they expect to optimize their algorithm during 2019.
Their approach appears to be sound from an engineering point of view; however, it is not considered to be basic research. Moreover, it is too soon to judge the correctness or utility of the researchers’ approach at this time. This work needs to be put in context with other imaging techniques and evaluation systems such as those presented in optical imaging conferences (e.g., International Society of Photographic Instrumentation Engineers and Imaging and Visual Science at the Optical Society of America [OSA]). Additionally, this work needs to be compared with past and present research being carried out elsewhere, in particular in relation to the machine algorithms used for image recognition associated with 3D point cloud analysis for Light Detection and Ranging (LIDAR) sensing of objects in autonomous vehicles. It is important to also quantify the degree to which unbiased testing can be compared to current techniques as long-term improvement goals.
NATICK SOLDIER RESEARCH, DEVELOPMENT, AND ENGINEERING CENTER
Project: Atomic-Scale Quantum Rectification
The major goal of this project is to study the potential of nonlinear metamaterials for optical rectification of visible and IR radiation and, more generally, to study the potential of metal-insulator-metal (MIM) and memristor diodes to act as platforms for nonlinear metamaterials. A key element of this work is the use of plasmonic resonances to enhance the fields involved in the rectification process, with a long-term goal of developing single-photon detectors in the IR regime. Toward this goal, this project has developed a model of the tunneling potential (trapezoidal potential) between the two metallic layers that allows for an exact description of the image potential and the inclusion of thermal effects.
The authors report the first evidence of quantum rectification in two platforms, an Au-Al2O3-Al rectifying antenna, known as rectenna, and an oxidized Al (AlOx) memristor. Memristors, or memory resistors, are a type of passive circuit element that maintains a relationship between the time integrals of current and voltage across a two-terminal element. As such, memristors allow access to a history of applied voltage.2
This is a significant project that goes beyond the relatively modest limits of a single ILIR project. NSRDEC contributions were funded with ILIR dollars, which enabled active collaboration with the Army Research Office (ARO) and Brown University. This research involves collaborations with groups at Brown University (performing nano-rectifying antenna design and analysis); the University of Massachusetts, Lowell (UML); Seoul National University (South Korea); Massachusetts Institute of Technology; Lincoln Laboratory (performing fabrication and characterization); and collaborators at CERDEC, ARDEC, and the Army Research Laboratory’s Sensors and Electron Devices Directorate (ARL-SEDD). It also involves undergraduate and graduate students at the partner academic institutions.
Future work will aim at analyzing and quantifying quantum rectification in the platforms that have been demonstrated, to pursue a proof of rectification and its isolation from other effects in the devices, and to explore the potential of these systems in ARDEC and CERDEC applied research programs that could benefit from the use of rectifying antenna receivers.
The various participants in this project are active on the international scene, attend a number of meetings, and have presented invited papers at international conferences. They have several submitted recent papers in high impact peer-reviewed journals. This is a strong research program, at the boundary between basic and applied research, and it is apparently motivated by the specific current goals of NSRDEC.
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2 Science and Advocacy at Memristor, “What Are Memristors?” https://www.memristor.org/reference/13/what-are-memristors, accessed February 26, 2019.
However, a general discussion of the position and impact of this work within in the broader context of the extremely active worldwide research efforts on metamaterial nonlinear optics—for example, in the context of terahertz generation—would have been instructive during the review.
Project: Coupled Dynamic Interactions for Flow-Induced Vibrations of Braided Cord Models
This project involves a fundamental study of vibrational instabilities induced on braided textile parachute cords by the ambient airflow. It is motivated by the fact that currently deployed Army gliding parachutes using these cords can develop high drag that has important operational implications due to reduced standoff. The interaction of the airflow with these cords can also generate significant noise that has additional operational implications. While the braided cords are employed in a variety of applications, only their use in parachutes has an aerodynamic component, and so there is a very limited set of interested stakeholders (i.e., Army, Air Force, and the National Aeronautics and Space Administration [NASA]). The topic is therefore highly appropriate for fundamental study by the Army. The ultimate goal of the research is to develop improved understandings to allow the reduction of the drag and noise.
The researcher provided an excellent introduction to the basic physical phenomena and demonstrated a solid understanding of the problem. A comprehensive suite of approaches is being applied to study the problem including theoretical stability analysis, experiments, and high-fidelity computational techniques. The effort is very effectively leveraging collaborations and related ongoing activities. Water tunnel experiments are being performed at the Air Force Academy, and wind tunnel experiments involving advanced molecular tagging velocimetry are being conducted at Michigan State University (in work that is funded by ARO). Three different computational analyses are being performed: fluid-structural interactions (FSI), direct numerical simulation (DNS) of the turbulent flow, and Reynolds-averaged Navier–Stokes (RANS) phenomenological turbulence modeling. The FSI simulations are being performed at the University of Massachusetts, Lowell (UML). The DNS calculations are being performed by a collaborator at the Institut Supérieur de l’Aéronautique et de l’Espace (ISAE) in France. The RANS code involves interaction with the development of a low Reynolds number capability for Kestrel, a general computational fluid dynamics (CFD) code under development for the Department of Defense (DoD). While these are each state-of-the-art approaches, none of them includes all of the physics required to fully predict the coupled effects of the aerodynamic flow on the vibrational behavior of the parachute cords. Additionally, cyber-physical fluid dynamic analyses are being conducted at UML.
This impressive project is an excellent example of a well-executed, fundamental study involving applied physics and engineering, which is seeking to address a problem of direct operational relevance to the Army. It is also considered a very good example of curiosity-driven research. Some recommendations to further improve the project include the following:
Recommendation: NSRDEC should develop clear metrics for the evaluation of progress in this project.
Recommendation: NSRDEC should consider whether the methodologies under development in this project could find application to additional scientific problems.
Recommendation: NSRDEC should consider expanding the scope of the conditions studied and the presentation of data to develop general results that could benefit a broader set of scientific problems.
SPACE AND MISSILE DEFENSE COMMAND
Project: Advanced Beam Control for High-Energy Lasers
Typical adaptive optics systems use a beacon illuminator laser to create a point source that is imaged onto a Shack–Hartmann wavefront sensor. Some difficulties arise when the atmospheric turbulence causes coherent interference of the beacon illuminator laser and the target surface roughness causes a speckled pattern in the light returned. This creates branch points across the pupil plane seen by a wavefront sensor. Branch points are a Pi-Phase shift and have no irradiance at that location. This project looks at innovative adaptive optics techniques that overcome this difficulty. Two approaches include performing iterative hill-climbing type algorithms on a deformable mirror with an image of the far-field laser spot as a metric and investigating techniques to move the branch points across the pupil plane of the wavefront sensor and reconstruct the phase.
The research involves developing atmospheric turbulence measurement instrumentation that will be useful to interpret and even control a high-energy laser beam system propagating through atmospheric turbulence, dust, and haze near the ground. The researchers studied and compared two techniques, which are the metric-based adaptive optics system that optimizes a bright spot and the Shack–Hartmann wavefront sensor that measures the fundamental optical wavefront so that more precise correction can be made during the freeze or decorrelation time of the atmosphere. The researchers developed an excellent outdoor testing site (optical trailer: range 50 to 750 m) and have made several turbulence measurements using 12 W 532 nm laser and turbulence generators and phase plates. Their research has quantified the number of control channels and rubber mirror actuators needed, and they found that slope discrepancies could be used to find branch points for turbulence and wavefront correction. They have also developed a side car aperture metric adaptive optics approach. They plan to analyze these different approaches during FY19.
Overall, the research is important and relevant to the High-Energy Laser (HEL) program. The researchers plan to set up a Shack–Hartmann wavefront sensor range, measure atmospheric turbulence Cn2, and test the system under different propagation conditions. It was encouraging to see that they are also communicating with similar research groups at the Air Force and Navy and with the High-Energy Laser Joint Technology Office (HEL-JTO).
Project: All Weather Tracker Research
This project seeks to develop improved approaches for the tracking of targets for ground-based laser weapon systems, which is challenging due to the background clutter of clouds and haze and the distortion caused by a dirty atmosphere. In particular, the basic idea of the research is seeking to increase target contrast using light polarization properties.
The research is focused on understanding the effects on the polarization state of light from airborne particulates, such as dust and soot. A computational model is being developed to analyze the effects of particle scattering on the polarization state through analysis of the Stokes vector. The standard approach for analysis of spherical particles employs Mie scattering. However, the particles of interest are known to be nonspherical, and so a more complex (T-matrix) method, developed in 2006, is being implemented. Because of long computational run times, the scattering code algorithm is being parallelized for execution on graphics processor units (GPUs).
There have been prior basic research efforts studying the effects of irregularly shaped particle scattering on light polarization, and it was not made clear how the SMDC work is breaking new ground.
Reviewing prior work may give researchers a greater understanding of directions for this project. Further, there appear to have been no journal publications that have resulted from this project to date.
Recommendation: SMDC should conduct a review of the literature to understand prior work relevant to the project and to identify unique research challenges that are relevant to Army tracking scenarios.
Project: Atmospheric Propagation Characterization
This project seeks to develop improved understanding of the negative effects of the structure of the atmosphere on the propagation of laser beams. Such effects include laser beam distortion by turbulence, scattering by particulates such as dust and rain, and molecular absorption. Detailed characterization of these effects is needed to develop beam control techniques to optimize the performance of laser-based systems. The goals of the project are to understand how lasers propagate through the atmosphere, characterize atmospheric turbulence using different diagnostic sensors, and develop and verify models of turbulence in the atmosphere.
For atmospheric propagation, the performance of two different diagnostics were compared: Shack–Hartmann wavefront sensors (SHWFS) and scintillometers. Data were collected on a range at Redstone Arsenal and involved the measurement of parameters characterizing the effect of the atmosphere on laser beam distortion and spot diameter. The data show important variations between different diagnostics and between two different units of the same instrument. Work has also begun on the development of new wavefront sensors.
CFD is being employed to model turbulence in the atmosphere. In an effort to improve on phenomenological turbulence models that do not work well in all circumstances, high-fidelity CFD is being applied.
There have been significant prior efforts expended in atmospheric beam propagation by other elements of the Department of Defense (DoD) and other federal agencies (e.g., the National Oceanic and Atmospheric Administration [NOAA]). It was not made clear how the SMDC work is breaking new ground.
Recommendation: SMDC should work with the DoD High-Energy Laser Joint Technology Office (HEL-JTO) to identify basic research challenges of laser beam propagation that are unique to Army interests, to determine the aspects of beam propagation that are most important to system performance, and to determine the levels of accuracy required from instrumentation and modeling that are used to characterize these effects.
Project: Direct Diode High-Energy Laser Research
This project plans to identify combinations of present and emerging diode laser technologies and approaches and leverage investments and progress made on a master oscillator power amplifier (MOPA) for an innovative direct diode phased array system with coherent beam combining methods, as well as a spectral combination method using on-chip photonics pioneered in the telecommunications industry for high-density, robust, spectral beam combining of the thousands of emitters.
Many HEL systems use diode pumped fiber lasers to obtain high power. This research into direct diode lasers involves combining output power from multiple diode lasers directly, bypassing some of the thermal problems associated with the diode pumped fiber laser approach. The researchers have made an excellent survey of different beam combining techniques (e.g., spectral grating, arrayed waveguide gratings [AWG], and coherent phase beam combining) and are developing two laboratory systems: a
coherent beam combining tapered amplifier phase array using two 980 nm diode lasers and an AWG combiner system using four 1550 nm diode lasers. Both of these systems have shown good proof of concept in the laboratory and will be further tested during the next year. They have a goal for coherent beam combining to reach the 16 W level next year.
Overall, the research appears to be promising. The researchers have a working relationship with other leaders in this field such as the laser beam–combining program at the Massachusetts Institute of Technology’s Lincoln Laboratory. This work is important for developing HEL laser beam control within the Army and is essential for tying basic laser beam research to engineering development HEL programs. However, the researchers need to pay serious attention to the thermal management issues, which have been funded under several Small Business Innovation Research programs to small industrial organizations.
Project: Hybrid Diode-Pumped Rare Gas Laser Research
Gas lasers have reached hundreds of kilowatts using monolithic aperture beam paths at high beam quality but have historically relied on hazardous gases like iodine, fluorine, and oxygen or highly corrosive alkali metal vapor. This new approach consists of diode-pumping a xenon-helium (Xe-He) plasma in an attempt to develop a laser system with the advantages of both diode-pumped lasers and gas lasers, while attempting to minimize the intrinsic negatives present in both schemes. Xenon will have a naturally broader pump transition compared to alkali metals, reducing the linewidth constraints of the pump diodes. The expectation of the project is to investigate a new type of laser gain medium to improve size, weight, and power requirements.
This research reported the use of diode lasers to directly pump Xe-He gas for obtaining laser action in Xe at 979.8 nm. Previous work in this area was done with alkali lasers (e.g., sodium), which have the disadvantage of solidification of the alkali atoms on the cell windows. The researchers used inert metastable Xe to overcome some of these problems. The hybrid diode-pumped diode Xe-He laser is a three-level lasing scheme composed of optically pumping (at a wavelength of 904.5 nm) the Xe metastable state, 6s[3/2]2 (produced via electrical discharge in the gas), up to 6p[5/2]2, which then collisionally decays to the 6p[1/2]1 state to create a population inversion between 6p[1/2]1 and 6s[3/2]2, resulting in laser action at 979.9 nm. The experiments also provided data on fluorescence lifetime. Tunable diode laser absorption spectroscopy (TDLAS) for in-depth analysis of the plasma condition and gain properties was reported. The research successfully generated laser action in He-Xe plasma after secondary optical pumping using 904.5 nm diode, at a threshold pump power of ∼10 W. At the maximum available optical pump power of ∼90 W, laser power at the wavelength of 979.9 nm was ∼343 mW.
This research is of sound quality and represents excellent basic laser research. Unfortunately, the maximum generated output of the Xe-He laser was low due to the low density (1012 cm3) of the Xe metastable atoms. The probability that such a laser can be scaled to HEL levels appears to be low. Aspects of the new research conducted in pulsed radio frequency (RF) and diode laser pumping might be published to see if it also has applications to other high-energy gas laser systems; the two-stage pumping technique (i.e., pulsed RF discharge plus diode pumping) and diagnostics developed may be utilized in other HEL systems.
SMDC Crosscutting Findings
SMDC’s projects on All Weather Tracker Research, which has been going on since FY16, and Atmospheric Propagation Characterization, which has been going on since FY11, appear to lack clear research plans and metrics to assess progress.
Recommendation: SMDC should identify clear metrics used to gauge technical progress in its research on the All Weather Tracker Research and Atmospheric Propagation Characterization projects.
OVERARCHING FINDINGS AND RECOMMENDATIONS
For organizations and staff members primarily involved in applied research activities, the ability provided by the ILIR program to perform basic research is seen as an empowering mechanism for individuals to make long-term fundamental impacts in support of institutional responsibilities.
Most of the research that was reviewed, however, is not basic research in the context of ARO or the outside world, but it is good solid research that can be used for support of system development at the centers within the boundary conditions or within the designated scope of the center, agency, or laboratory. It also appears that many of the ILIR-funded activities were directly supporting applied research activities.
Many presentations focused narrowly on specific devices and materials related to ongoing applied concerns. The principal investigators need to be encouraged to look at the broader picture and make efforts to develop models and concepts that could be generalized and widely used by others. ILIR needs to empower researchers to look beyond the day-to-day activities at the center and not be limited to activities only having direct impact on the center’s mission.
The current narrow focus is possibly a result of reviews and selection done only at the RDEC level; there could be benefit from using outside reviewers in the project selection process. Sometimes it is valid that the criteria for project selection be just to build research knowledge at the basic research level within a given center. A small number of ILIR programs do seem to build research knowledge at the basic level within its center, and this is excellent.
Conducting basic research activities in an otherwise applied research environment can be very challenging because of a different philosophy and methodology with which such activities are executed. These differences include travel to meetings for interacting with external individuals in the field and outside collaborations. This leads to a question: Do the ILIR activities lead to isolation of the performers from day-to-day colleagues? If a center does not have a laboratory dedicated to basic research, it could consider building or designating a facility for basic research programs.
These centers have established several partnerships with colleagues in academia; however, these were, by and large, chosen because of geographical convenience and familiarity with local universities. These may not necessarily be the colleagues who are most familiar with the Army-relevant basic research environment. It would be helpful to extend such collaborations to universities at the national level to better interact with the best research groups in a particular subject. Every effort needs to be made to expand the collaboration horizons to reach out to experts in the field. The ILIR program needs to act as a bridge to develop strong ties with the relevant basic research world outside the particular Research, Development, and Engineering Command centers. It is important that the ILIR participants have freedom to get involved in conferences, visits, and exchanges.
The 3-year horizon of many ILIR programs can be challenging for basic research, and during the review it would have been useful to receive information about how the interim progress is evaluated and whether the management is able to provide guidance when challenges arise. This is where expanding exchanges with a broader research community would help. Alternatively, perhaps an established procedure by which small external visiting committees of appropriate experts in the field visit the center(s) may help.
The amount of ILIR funding is quite small, and there needs to be a focus on getting the most impact from the limited resources. Too many projects are supported at a low level. It may be important to
consider whether it makes sense to conduct fewer projects with stronger support for each. The centers need to provide broad guidance regarding the type of ILIR to be funded, but tying the ILIR to a specific applied research problem subverts the intent of ILIR.
CERDEC, NSRDEC, and SMDC all had projects that could have included more metrics used to gauge technical progress in their research.
Recommendation: CERDEC, NSRDEC, and SMDC should specify metrics used to gauge technical progress in their research.
Recommendation: AMRDEC, CERDEC, NSRDEC, and SMDC should promote interactions of their researchers with the broader scientific community in order to advance greater scientific understandings that will enhance their projects.
