program has been a key part of the ARL research portfolio for decades, and it continues to be the leader in this country in solid-state laser development, and particularly in the critical 3-5 µm region that is critical to the Army. The optical sciences and photonics program at ARL has demonstrated the ability to balance the protection and support of strong existing programs with a willingness to pursue promising new fields in which ARL has not worked previously. For this achievement, the leadership is commended.

Integrated Photonics

The motivation undergirding all of the research presented in the integrated photonics area appears to be twofold: the development of new optical materials and devices having capability not available previously and yet closely tied to Army applications; and interfacing micro- and nano-electronics to optical devices and arrays so as to realize a new generation of optoelectronic systems providing control of optical systems at electronic speeds. With regard to the former, presentation included the development of precision microwave source for use as a clock/frequency standard that was based on micro-resonator optical frequency combs in which the phase noise has been suppressed by use of a self-referencing circuitry more than 10 dB better than a state-of-the-art Wenzel “Golden” Quartz resonator within performance of phase noise of −130 dBc/Hz at 1 kHz for a 10 GHz signal. Frequency combs and atomic clocks will undoubtedly be the workhorses as frequency standards in future Army communications systems, and, for the frequency comb in particular, the magnitude of the phase noise determines its frequency stability. In coordination with the precision microwave source, a new environmentally stable resonator is being developed. Research was facilitated by a collaboration of ARL with California Institute of Technology (Cal Tech) and University of Maryland, Baltimore, Campus—both leaders in the field of optical microresonators. This partnership is likely to yield new resonators that are designed specifically for Army communications applications.

An example of the development of new optical/infrared materials for Army needs is the long-wave infrared (LWIR) window research effort. By combining the properties of nanomaterials with a “host” material, promising results have been obtained in the pursuit of LWIR materials that do not suffer from the drawbacks of long-standing (conventional) materials such as zinc-selenide.

A significant, well-knit R&D effort devoted to the integration of electronic and photonic devices and systems was also presented. An optical beam-former designed for electronic warfare (EW) applications was described. Operating in the telecommunications band (1,300-1,600 nm), an RF phased array beam-former technology will be based on a true time delay splitter that uses a unique slow-light phenomena for the elongated delays necessary. The resonator is based on an indium tin oxide (ITO) epsilon-near-zero (ENZ) metamaterial that has been perfected internally to ARL. The custom air-core ITO-ENZ resonators are being fabricated in ARL’s cleanroom facility. Combination of these elements will lead to a precision, long-hold-over optical clock for positioning, navigation, and timing applications. Slow-light simulations of the resonator have been performed, and the ARL team is working closely with AIM Photonics of Albany, New York, to fabricate the splitter and delay structures for use with an RF demonstration platform. ARL has participated in eight fabrication “runs” that were conducted to date at the AIM foundry, and ARL staff are working remotely with the AIM fabrication facility on a continuing basis. This level of interaction is crucial to the success of the effort.

In research efforts to develop optical phased arrays, ARL is exploring several different scanning and control architectures, with edge and vertical scanning having been achieved to date with a maximum deflection of 11 degrees for single beams. In the electronic/photonic integrated circuit (E-PIC) program, ARL is exploring the AIM foundry’s ability to collocate electronic and photonics components specifically designed for AIM’s process design kit. These results are encouraging and suggest that the capability for electronic steering of infrared laser beam arrays is achievable in the near term. The development of E-PIC chips is in partnership with AIM Photonics and has demonstrated numerous passive and some active electronic elements. The goal of this effort is to integrate transistors, capacitors, and other electronic

components directly onto photonic integrated circuits (PICs), thus enabling both electronic processing and photonic functionality on the same chip.

Another R&D effort draws on the PIC sensing element and a photonic sensor array platform technology under development at ARL to realize wearable biosensors for warfighter health and sensor monitoring. The technology described combines a PIC sensor array with a microfluidic chamber in order to obtain a lightweight, yet sensitive, biomolecular sensor. Achieving the necessary sensitivity in a low-cost and lightweight sensor is a challenge. It was solved with an on-chip Mach-Zehnder interferometer. The shift of the interferometer’s spectral modes and alterations in the free-spectral range (FSR), in particular, results in extraordinary sensitivity that makes this technological approach a practical, fieldable solution.

This is a strong program. The emphasis on interlocking optical/electronic devices or systems, which can be regarded as separable “platforms,” is a wise approach because it allows small teams to focus on a particular optoelectronic functionality. At the same time, however, the individual teams are in close communication with all other teams, and, in some cases, individual researchers are members of two or more teams. In addition, the collaborations with multiple experts and organizations with unique capabilities—such as the ARL Biotechnology branch, CCDC Chemical Biological Center, Cal Tech, AIM Photonics, and Naval Research Laboratory (NRL)—are definitely assets to this R&D effort. Insular research efforts invariably “reinvent the wheel,” but the integrated photonics effort at ARL appears to be able to move more quickly because of the balance of internal and external expertise. The quality of the researchers is impressive.

Quantum Information Science

Quantum information science (QIS) is currently an extremely active area of research, with many facets that extend beyond physics into other disciplines such as computer science and mathematics. Research on quantum materials and devices is central to the development of fundamental scientific understanding as well as for technological applications. The focus at ARL is appropriately on aspects of quantum materials issues that are particularly relevant or even unique to Army needs or that draw on unique Army resources and facilities. This allows the research to have an impact even in a very large and competitive field. This effort is leveraged by outside collaborations, most notably through the Quantum Technology Institute at the University of Maryland, College Park, as discussed further below. In addition, ARL QIS experts act as a source of information and advice for non-expert decision makers on what is practical and what is hype in the development of technologies such as sensors, precision navigation technologies, and distributed-entanglement secure communications systems. This role is especially important, because in most cases there is a very long time frame and considerable investment to get these projects to applications.

The promise of neutral Rydberg atom arrays for quantum information was recently highlighted in the literature—for example, in a May 2020 Nature Physics News and Views article. While most schemes being pursued in Europe and China require fast routing of single photons, ARL has accomplished spin wave multiplexing with a one million cold atom ensemble that currently stores 10 spin waves but, in principle, could store up to 1,000 by multiplexing via dynamic classical dressing beams while informational photons emit into a TEM00 optical cavity mode.¹ The significance of this contribution is highlighted by publication in Physical Review Letters featured as a “Physics Synopsis” in December 2019.

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¹ The TEM00 mode is the lowest-order transverse mode. It has the lowest threshold, smallest beam waist and divergence, and it contains no nodes in the output beam transverse intensity distribution.

is employed as a paradigm to extend sensor protection capabilities beyond the confines of more conventional material solutions. There is an ongoing search for nonlinear optical materials and components that are activated by high energy or laser intensity, and only activate when a system is exposed to a threat laser. The sensor protection presentations encompassed these guiding principles.

Six posters were presented that formed a coherent set of projects that support the overall goal of sensor protection with numerous members taking part is several projects. The first three posters discussed the impact of continuous wave (CW) kW level 1-micron lasers and high-intensity fs lasers and the laser-matter interaction of materials of interest to the Army. The fourth poster discussed the development of tunable micro-engineered metamaterials designed to serve as optical notch filters in the LWIR region from 8 to 12 microns. The final two posters discussed the development and characterization of transition metal chromophores and their unique ground and excited state properties that result in reverse saturable absorption (RSA) and two-photon absorption (2PA), with the goal of providing sensor protection. The entire poster session was impressive and well received. For example, the sensor protection scientific team is commended for the clever iridium chemistry that is being pursued to develop broadband reverse saturable absorption materials and the GMR filters.

High-Power CW Damage Measurements and Protection of Optical Components

Most of the work around the country regarding laser-induced damage of IR optics has focused on pulsed laser damage. ARL is potentially the only research group investigating the CW laser-induced damage threshold of IR optics at 1 micron using 1.5 kW and 10 kW lasers. These researchers are also investigating high-reflecting metastructures to increase the damage threshold and increase the laser acceptance angle. This research is designed to determine the resiliency of this IR optics against 1-micron radiation and to make recommendations as to the suitability of specific optics in any next generation combat vehicle system. Given the unfortunate prevalence of the use of high-power, CW 0.5-micron—that is, green—laser pointers to blind airline pilots and police officers, frequency doubling these high-power 1-micron lasers to 0.5 microns will be straightforward to investigate materials for eye protection.

Characterizing Ultrashort Pulse Laser Filamentation in Optical Materials

Using NIR and SWIR ultrashort pulse lasers, measurements of the threshold pulse energy for filamentation and supercontinuum generation in relevant optical media is being performed. The Army is interested in characterizing these effects to develop unique applications that exploit such extreme nonlinear optical effects as well as to identify areas for protection to aid the service member. A goal is to determine how these nonlinear effects scale with laser power and wavelengths as long as 10 microns. Simulations are used to design experiments, and experimental results check the simulations, resulting in enhanced models of ultrashort pulse laser filamentation. The propagation of ultrashort pulses through nonlinear media produces a wide array of interesting optical effects.

Mid-Infrared n₂ Characterization in Infrared Transmitting Materials

Over the past few years, MWIR laser propagation, primarily in the 3-5 µm regime, has garnered a great deal of interest owing to high transmission in the atmosphere. MWIR ps and fs Z-scan measurements determined nonlinear refraction, n₂, and two-photon absorption (2PA, α₂) coefficients of optical materials of interest to the Army. This group was able to model the high-harmonic and supercontinuum generation they observed using second-, third-, and fourth-order nonlinearities. One of the parameters that determines the onset of filamentation is nonlinear refraction. This parameter has been investigated extensively over several decades in a multitude of materials, primarily in the visible and

SWIR spectral regimes, but very few efforts have been conducted in the MWIR region owing to the lack of capable ultrashort pulse sources. ARL is developing a unique capability for ultrashort pulse MWIR nonlinear characterization in order to determine the n₂ of Army-relevant materials as well as experimentally investigate laser-matter interactions in the visible, SWIR, MWIR, and LWIR regimes by acquiring high-energy ultrashort pulse sources with tunable frequency conversion into the MWIR and LWIR spectral regions. There is the possibility of initiating a CRADA with Ohio State University to study other effects from ultrashort pulses such as damage in Army-relevant materials.

LWIR Tunable Notch Filter Using Metamaterials

The absence of tunable notch filters in the LWIR region from 8 to12 microns is being addressed by the development of novel micro-engineered metamaterials based on a subwavelength grating on top of a Ge/ZnSe planar waveguide. These notch filters are based on the GMR effect in dielectric materials, pioneered at the University of Texas, Arlington. At present, Ge/ZnSe appears to be the only materials combination that produces a tunable filter in this spectral range. The transmission experiments were performed with a tunable quantum cascade laser system, resulting in very good agreement between experiment and simulation. Looking toward the future, ARL has started a new cooperative agreement with the University of Texas, Austin, and initiated a collaboration with the National Institute of Standards and Technology (NIST) to use their excellent cleanroom facility and expertise. These appear to be the only tunable notch filters in the LWIR from 8 to12 microns.

ARL Development of Reverse Saturable Absorption Materials Through Multi-University Collaborations

Reverse saturable absorption (RSA) is a nonlinear optical process that occurs when a material absorbs light more strongly in its excited state as compared to that of its ground state. This is in direct contrast to saturable absorbers (SAs) used in laser cavities to mode lock or Q-switch, which become more transparent—that is, bleach—under high irradiation. ARL seeks RSA materials that function across various wavelength regimes and time scales, and has robust in-house synthetic and photophysical characterization efforts for the development of RSA materials. Transition metal chromophores containing ruthenium and iridium metal centers and their unique ground and excited state properties have resulted in RSA and 2PA providing sensor protection. ARL utilizes state-of-the-art organic synthesis techniques to develop new RSA materials as well as excellent spectroscopy instrumentation for nonlinear optical characterization. In addition to ARL’s robust in-house synthetic efforts and photophysical characterization, this team initiated several cross-functional multi-university cooperative agreements to augment its capabilities with University of Central Florida, North Carolina State University (NCSU), University of Buffalo (UB), University of Houston (UH), and University of New Mexico (UNM). Looking forward, this group has been funded for a second year of cooperative agreements with NCSU, UB, UH, and UNM. There is also a collaboration with the U.S. Military Academy, West Point, chemistry faculty and cadets, as well as ARL summer student researchers for increased synthetic throughput.

Synthesis and Characterization of Novel, Broadband Nonlinear-Absorbing Chromophores

Iridium-based transition metal chromophores have resulted in RSA signatures as demonstrated by open-aperture, picosecond Z-scan measurements. These transition metal chromophores have a highly modular architecture, which is adaptable to microwave-assisted synthesis, and utilizes relatively inexpensive starting materials. One major advantage of these chromophores is that they can be

synthesized in a microwave reactor that greatly reduces the production time to about 1 hour instead of hours to days.

Advanced Solid-State Lasers

Mid-IR laser development constituted four short poster presentations. The focus of this research team is on new materials as well as new strategies for implementing compact, fieldable lasers operating in the atmospheric transmission window at about 3 to 5 microns. Overall, this program is a well-thought-out and integrated as well as innovative effort. This work is unique, is of high quality, is well integrated with Army needs, and has the potential for impact well beyond the needs of the U.S. Army.

The group has dedicated significant effort into developing a platform for experimentally evaluating new laser materials, including the ability to measure the thermal conductivity, spectroscopy, excited state lifetimes, and dynamics of these materials. In combination with materials growers—with a strong collaboration with Fisk University—this group has identified new BaF₂-host candidate laser materials and demonstrated their basic potential as a direct diode-pumped laser material. Other host crystals, specifically consisting of heavy-element materials combinations that exhibit low phonon frequencies to suppress quenching of the laser transition, are in-house ready for this evaluation. Mid-IR lasers have seen an upswelling of interest in recent years. In the field of laser science as a whole, entire eras of innovation can be identified by the discovery and development of a single new laser material. The ARL effort is unique and is thus a national resource. A review of the literature in the area of new laser materials makes this apparent, because no other domestic efforts in developing these BaF2-based materials could be identified. Moreover, there is no other fully domestic effort in both growing and characterizing laser crystals. It is noted that the Fisk University group has in the past been primarily focused on scintillator materials, and the ARL group was instrumental in encouraging this group to broaden the scope of its crystal growing.

The researchers within this group uniformly exhibited a good grasp of their research, as well as how their work integrates into the overall effort, and how their work compares with outside research as well as alternative approaches such as nonlinear parametric generation of mid-IR. They are also exploring novel approaches for implementation of these mid-IR laser materials, such as the concept that laser performance in the mid-IR can be enhanced through simultaneous or sequential lasing at several wavelengths as a means of mitigating the quantum defect and transporting heat from the laser material. Continued development of these concepts is needed to the point where the utility of this approach (or lack of utility) becomes clear.

Challenges and Opportunities

Integrated Photonics

The optical frequency comb research could aggressively explore and possibly developed in-house resonators beyond those available from Cal Tech. Although the latter are leaders worldwide in performance, their design is not necessarily optimized for Army applications. The design and fabricate PICs and E-PICs with AIM Photonics is a productive relationship, but the number of device fabrication “runs” is limited because ARL is the sole customer of this facility. It is, therefore, advisable to design chips that simultaneously test as many optical/electronic devices and subsystem designs as possible at one time to optimize the rate of progress toward deployable chips and systems. The group needs to explore working with additional companies with experience in IR materials.

developed decades ago are now key enablers for many of the quantum science, energy, sensor protection, and materials technologies discussed elsewhere in this report. Technologies that make new regions of the spectrum accessible to coherent light will open new possibilities and new capabilities. National security, prosperity, and preeminence are all inextricably linked to mastery and control of every region of the electromagnetic spectrum.

ELECTRONICS AND OPTOELECTRONICS

The research themes of the electronics and optoelectronics areas are diamond electronics, microdevices, materials-driven antenna design, optical power devices, emerging materials, and RF and digital electronics.

Overall, the research projects in the electronics and optoelectronics area were of high quality and comparable to other first-class research organizations. The projects evaluated were well thought out and motivated by Army needs—two examples are emerging materials that are being explored as a means to increase information and data assessment in the field; and materials-driven antenna design that will address the Army’s expanding communications and size, weight, and power as well as cost (SWaP-C) needs. There was a good mix of exciting high-risk projects and those that are in support of Army near-term applications.

Accomplishments and Advancements

Diamond Electronics

The Army has significant need for efficient, high-power electronics. Diamond offers potential advantages over the current standard of GaN-based devices owing to its higher thermal conductivity. The effort is currently focused on improving device performance by introducing transition metal oxides or boron nitride to stabilize the diamond surface. The target power metric of 50 W/mm has been identified as necessary for this technology to supplant the current state-of-the-art GaN.

The scientific quality of the work is excellent and makes a significant contribution to the field. The device fabrication and characterization facilities are well suited to development of a robust technology, and the expertise of the researchers is well matched to the needs of the program. Device and material modeling expertise seems to be well integrated with the processing program. There are strong collaborations with vendors and university researchers. The group has achieved state-of-the-art direct current (DC) characteristics utilizing solid oxide cap layers.

Microdevices

This poster session concentrated on making new devices useful to the Army using ARL materials capabilities. Piezoelectrics materials are an ARL strength. ARL has a Cooperative Research and Development Agreement (CRADA) with the Kurt J. Lesker Company, an organization well known for depositing thin films using atomic layer epitaxy. ARL and Lesker collaboratively deposited piezoelectrics for three-dimensional (3D) piezo-microelectromechanical system (MEMS); this is a unique and potentially important accomplishment. These studies compare very favorably with those at Penn State University and NCSU.

that operate over ultra-wideband; (2) creation of hardware architecture to run a reconfigurable neural network to enable real-time data processing; and (3) protection of integrated circuits on-demand printing of dielectric materials, with the ability to tune properties and performance.

The ultimate goals of developing specific electronics supply and protecting them are timely and will have high impact, and the research topics addressed have identified areas in which significant advances are needed to realize overmatch for the safety and security of the Army. In general, these projects are in relatively nascent stages, with preliminary effort in the broadband transmitter architectures showing the furthest progress.

The novel broadband transmitter architecture has shown important successes using two different approaches to access ultra-broadband with high precision. This has resulted in multiple patents and conference proceedings.

Challenges and Opportunities

Diamond Electronics

Issues with thermal and device stability as well as leakage at small gate lengths are important challenges to address; thus, the plan going forward to focus more attention on the area of stability is certainly warranted. In order to enhance ultimate success, ARL needs to bring additional capability in-house in the area of diamond surface cleaning and/or diamond epitaxial growth. In-house epitaxial growth would substantially increase the probability of success. Additionally, the material modeling efforts needs to focus on improving reliability for a particular heterostructure in order to accelerate progress on improving ultimate circuit reliability.

Microdevices

The applications chosen, RF filters, microelectromechnical systems (MEMS), and integrated optics, are of interest to the Army, but the advantage of these material approaches on devices needs to be demonstrated.

Hybrid manufacturing using ARL’s varied material capabilities could produce some new and useful packaging for the Army.

Computational origami is a new and unusual technique for forming complex 3D structures in metal. The technique has been proven and well demonstrated, it now needs to be applied to a useful structure and tested to see how it compares to approaches that are more conventional.

Materials-Driven Antenna Design

Given the maturity and relatively high cost of the antennas from metaferrite materials, materials development for lower cost is a good future direction. The printing of composite filaments for dielectric materials with variable properties is rapidly maturing, and further work on demonstrating utility is needed. The project centering on composite magnetic materials with low loss is in a nascent stage relative to the others. Here, while simulation results are promising, further testing and experience are needed. In addition to design and fabrication, ARL needs to do computer simulation performance modeling.

polyaniline) with ionically conductive polymers (e.g., sulfonated poly-ether-ether-ketone, or SPEEK), and electrospun fibers made from poly-4-vinyl pyridine and from poly-acrylonitrile. In many cases, the composite materials include integrated catalysts—for example, Pt-group-metal particles, which catalyze fuel oxidation or oxidant reduction. Actuation is achieved by treatment with various chemicals including solvents, aqueous solutions of varying pH, and chemical fuels or oxidants. Fuel is in most cases hydrogen gas and oxidant is oxygen gas. Actuation mechanisms likely derive from a combination of solvent swelling/deswelling, ion exchange, and oxidation/reduction of components in the fibers. A 1 kg load was lifted more than 1 cm by a solvent-actuated fiber that was the program target and is more than 10× larger than current state of the art. Reversible redox actuation of a finger-shaped object was demonstrated with approximately 2 Hz frequency, and a light-activated fuel-powered actuator was shown to exhibit a work capacity of 50 J/kg that is approximately 6 times larger than a typical mammalian muscle work capacity.

Energy Harvesting for Fuel Flexibility

The overall goal of this effort is to operationalize foundational research to unburden the soldier/squad with a focus on logistics reduction, mobility, situational awareness, and survivability. Specifically, energy supply in the battlefield is limited by logistics resupply and the amount of fuel and number of batteries carried, thus energy harvesting is an important focus of ARL research in energy sciences. ARL is the leading organization researching needed energy for future Army systems and capabilities.

The seven projects presented in this area were (1) enabling multifunctional materials design and analysis for energy and power; (2) photomechanics of plasmonic photoelectrocatalysts; (3) polarization fields in iii-nitride polarization field enhanced solar water splitting; (4) electro-oxidation of urea for hydrogen generation and cleaner water; (5) efficient hydrogen generation from hydrolysis of nanostructured aluminum alloys; (6) synthesis and investigation of new catalytic materials for the effective in situ hydrodeoxygenation of biomass derived oil; and (7) single-atom catalysis for conversion of methane (natural gas) to liquid fuels.

Overall, the projects are deemed successful and well aligned with the needs of the soldier. Researchers in this area have published 53 peer-reviewed publications and received 7 patents since 2018. A wide range of energy harvesting/conversion technologies were observed with a common theme for most projects being catalysis for energy conversion. The development of a heuristic methodology for the design of multifunctional materials, which is 4 to 6 orders of magnitude less expensive than finite-element analysis, is noteworthy. The technique was verified and validated for mechanics and multiphysics satisfying Onsager reciprocity relations. Progress is reported on some relatively high-risk schemes for coupling solar energy into electrochemical reactions to create and use chemical fuels, with a particular focus on catalysis. The power of harvesting solar energy in this way is impressive. ARL researchers have created molecular beam epitaxy-grown pseudomorphic layers of InGaN whose strain coherency supports polarization fields in the c-plane direction, which is useful for solar energy harvesting. Catalyst materials and reactor designs are demonstrated for value-added chemical and fuel production, particularly involving biomass hydrodeoxygenation and methane conversion to liquid fuels.

Thermal Science

The need for efficient electronic cooling is important as device heat flux continues to increase. To mitigate the heat generation, engineering solutions are put in place for thermal management; however, these solutions are optimized for peak-load conditions and many systems only operate at peak load a very small percentage of the time, and only for a fixed duration. Phase change materials (PCMs) absorb heat in transience. Because the effective heat capacitance increases significantly during phase change, it enables absorbing a large quantity of heat with theoretically no change in temperature difference and hence minimal thermal budget and reduced maximum junction temperature, T_jmax, for applications ranging from

batteries to electronic/photonic devices. The performance of PCMs is related to its Figure of Merit (FOM), and FOM = kpL, where k, p, and L are the thermal conductivity, density, and heat of fusion. Three posters were presented: (1) 3D fabrication of NiTi PCMs and characterization in partnership with Texas A&M University (TAMU); (2) a novel technique to measure thin-film thermal conductivity, and (3) development of a MATLAB-based ParaPowertool as an open source resource for co-design and exploration of PCMs.

In the first poster, collaborating with TAMU, the team adopted a multitiered approach to fabricate, characterize, and model traditionally manufactured and additively manufactured NiTi alloys. NiTi represents a 3D printable, high-conductivity, form-stable, high-capacity, solid-state PCM; and the parameters can be adjusted and optimized during fabrication. The facilities at both TAMU and ARL are excellent. Excellent progress is being made including publications at conference proceedings and journals. The path forward is to exceed both the literature reported thermal conductivity and latent heat, which is an ambitious but doable goal.

In the second poster, the team addresses the problem of measuring the thermal conductivity of thin films, which is important in evaluating FOM. Using state-of-the-art characterization methodologies at ARL for thermal properties of thin films, the team was first to report the thermal conductivity of 4-element NiTi-based shape memory alloy, and then deduce the effect of parameters such as Cu concentration.

In the third poster, the team developed tools that can survey design space to understand trade-offs inherent in phase-change thermal load leveling. The team released an open source code called ARL ParaPower, where co-design and parametric exploration are accessible to researchers. This is quite significant, as the alternative of multiphysics modeling, using computational techniques, is very CPU (central processing unit) intensive and for all practical purposes cannot be used as a design tool on its own. The hierarchical design approach that has been implemented positions ParaPower as the back-of-the-envelope calculator and then couples it to more detailed computational multiphysics models. ParaPower will be quite handy for teaching, such as in an undergraduate heat transfer or thermodynamics class, as well as being quite useful for research purposes. The work can be showcased at events such as the Semi-Therm Fall Thermal Workshop or present tutorials at ITherm conferences and Electronic Components and Technology Conference (ECTC).

This is a very strong team with potential for important breakthroughs. The results achieved to date are very good. It is worth pointing out that the projects are not stand-alone and that they complement each other and will reduce development lead time. The team is working closely with universities such as TAMU and University of Maryland on material development, and they appreciate the importance of interns as a great way to educate future students in the field. This is also a potential path to recruiting, which ARL seems to have already implemented.

Energy Storage and Batteries

The energy storage and batteries efforts within the energy sciences group seeks to establish the fundamental knowledge base of electrical energy storage technologies. This emphasizes batteries, to enable the development of devices that are, above all, safe, while delivering high energy, high power, fast recharge, and long cycle life. A number of the efforts were well defined and integrated, while others were somewhat less so. The effectiveness of today’s warfighter increasingly hinges on the capabilities of multiple devices that depend almost exclusively on portable electric power. Thus, there is a great impetus to develop electrical energy storage technologies that can deliver high energy for length of use, high power for intense use, with fast recharge to minimize “off” time, with long cycle life to minimize replacement, and that are above all, safe. This last point is critical because in numerous cases the battery would be in contact with or very close to the warfighter.

Without a doubt, the most important and impressive accomplishment has been the development of the ultra-concentrated 63 m electrolyte (42 m LiTFSI + 21 m (Me)₃Et N·TFSI). This accomplishment has

enabled the development of water-based 4.0 V lithium-ion batteries. This paradigm-changing development has enabled the development of nonflammable lithium-ion batteries. In essence, the ultra-concentrated electrolyte expands the voltage stability window by delaying the evolution of hydrogen and oxygen. This body of work was extremely well integrated with theory and modeling going hand-in-hand with experimental developments. The theory component was excellent and set the ground for the technological aspects of the work, including batteries based on halide intercalation and a 4.0 V nonflammable lithium-ion battery.

There was a poster related to the development of zinc metal batteries. While this is an area that has been studied extensively for many years, there is a great deal of divergence in the published literature. The ARL group is developing and establishing a testing protocol intended to “organize” and provide a common footing for those results. While the program is still in early stages, the outlook appears promising.

There were also presentations dealing with more traditional battery materials and technologies, including the use of NMC-811 (NiMnCo) as fast-charging cathodes, the use of nano-scale Si/silicide anodes, as well as the development of a low-temperature protocol for the synthesis of TNO (TiNb₂O₇) for fast-charging anodes. These studies were not considered to be at the same level of excellence as the work described above.

The computational and experimental work on the ultra-concentrated electrolyte is excellent, and the group is commended for the very high quality of the work, its excellent integration, and the large number of high-quality publications. This area is uniquely associated with ARL, and is considered to be of the very highest quality in the battery community. The work on Zn metal batteries is getting started and has a good trajectory. The work on more traditional materials was not at the level as that described above.

Alternative Power

The alternative power efforts within the energy science group includes work on wireless power transmission by several means, power conversion by thermal (pyroelectric) methods, energy storage in 3D dielectric capacitors, and energy conversion by electrolysis (focus on electrocatalysis of alkaline oxygen evolution reaction—OER) and by fuel-flexible combustion via an emerging type of combustion flame. A common theme in the work is that all the projects involve some form of energy transmission, reception, or conversion. Each project improves or seeks to improve on the state of the art for energy transmission or conversion efficiency in ways that would be beneficial to the Army, including warfighter power, autonomous vehicle charging, and wearable sensors. Army operations are increasingly energy-intensive, and technologies that provide Army teams in the field with energy via efficient transmission or conversion from battlefield resources are highly valued.

A group of projects focuses on wireless energy transmission via various transmitted and received waves, with each project focusing on a different type of wave and distance for transmission. Electrical-induction-based wireless energy transmission is accomplished using mechanically stretchable inductor coils made from liquid metals entrapped in closed channels within silicone polymer sheets. Wireless energy transmission efficiencies above 90 percent are achieved for stretchable single inductor coils in near contact and near 50 percent for coils at a distance approximately twice the coil diameter. These transmission efficiencies are among the highest reported for stretchable and wearable materials, and the ARL team is among the best in the world in this area. High-frequency acoustical wireless energy transmission is well suited for longer distances—for example, tens of cm—and is accomplished through air and metals and through air-filled metal tubes. Effective strategies for focusing energy at transmission and reception points are developed and provide improved transmission efficiencies. Acoustical energy transmission lacks an electromagnetic signal and is thus well suited for clandestine operations. Piezoelectric resonators are coupled with custom electronic circuitry to create very-high-gain voltage-to-voltage amplifiers for use with low-power wake-up receivers. Voltage gains above 100 are obtained for a single-stage resonator made from quartz, for which a very high Q-factor for the mechanical resonance is

found to be the most critical factor for achieving high amplification. These high gains are a significant improvement on prior work and are highly beneficial for low-power wake-up receiver devices—for example, for remote sensing.

Advanced pyroelectric materials are showing potential for heat-to-electrical energy conversion that could be accomplished to utilize waste heat and to transmit energy at a distance via high-power lasers. Three-dimensional dielectric capacitors are created from a 3D-printed carbon mesh onto which is deposited a layer of Al-doped HfO₂—a well-known antiferromagnetic dielectric—followed by back-filling the structure with liquid metal to provide conductors on each side of a thin dielectric. High-energy storage is achieved per unit volume of dielectric with fast response and long cycle life for relatively low applied biases—less than one-quarter of the dielectric breakdown field strength—as is expected for the chosen dielectric material. Approaches to using pyroelectric materials for heat-to-electricity conversion were also presented, in one instance using a laser-irradiated pyroelectric film to charge a battery.

Work on electrocatalyst development for oxygen evolution in aqueous alkaline electrolytes was reported for a combination of a FeNiCo oxide spinel with a MoS₂ support. The combination of the FeNiCo oxide with MoS₂ yields an electrocatalyst with particularly low overpotentials for the OER. Last, ARL has learned important new aspects of the blue whirl, an emerging type of liquid-fuel-fed flame that is particularly flexible with regard to fuel source. Details on flow dynamics within the flame have been elucidated, and devices for containing the flame have been created for coupling with heat-to-electrical or heat-to-mechanical energy conversion devices such as a Stirling engine.

The overall technical work quality is high, with some work being of very high quality. The wireless energy transmission work and the low-power wake-up receiver work is equivalent to or better than that from the best groups in the world. The ARL team has a holistic view on wireless energy transmission that likely gives it a valuable perspective on how best to approach energy transmission for various application details. Pyroelectric energy conversion work is of high quality and is generating significant publications in high-impact journals. The capacitive energy storage and electrochemical conversion work advances the state of the art but is generally similar to other related works.

Nuclide Power

The nuclide power efforts focus on providing power to greatly extend and sustain Army operations at multiple scales by looking beyond fossil fuels and conventional technologies.

The three projects presented in this area were (1) nuclide power to move beyond fossil fuels, (2) first demonstration of nuclear excitation by electron capture for radioisotope energy release, and (3) maximizing beta interactions in textured energy converters. Researchers in this area have published 17 peer-reviewed publications since 2018.

Key objectives of this program include the determination of feasibility of switching radioisotopes from long-lived excited states (isomers) to short-lived ground state using NEEC for novel, disruptive power sources, and maximization of power-source power density for sensors and communications electronics. The latter is aimed to be achieved through use of energy-dense radioisotopes (RIs) and textured wide-bandgap semiconductors for decades of persistent sensing (IoBT) and communications (SatCom) with a goal of 10 mW/cc. The ARL team has for the first time achieved an experimental demonstration of NEEC. This is an important advancement that could enable creation of future portable nuclide-based electrical power sources. Incremental progress is being made using conventional radioisotopes to create beta-batteries. A betavoltaic power source with power density of 0.8 W/kg has been achieved with a goal of 10 W/kg.

This relatively small program has achieved excellent results. The program has significant collaborations with many external organizations that enhance the impact. A small accelerator will be installed at ARL soon that will move this program forward. There is currently only one full-time person dedicated to the betavoltaics program. Additional researchers would be necessary to continue growing this effort.

dramatic advance, and ARL needs to pursue such efforts. Zn metal batteries represent a promising venue for Army applications. The initial efforts for developing and establishing well defined testing protocols will be of great value to the Army and to the community at large. This is an early effort with great potential. Work on fast-charging anode and cathode materials such as NTO and surface-treated NMC811 provides important incremental advances toward the Army goal of 10 C charging. Work on silicon anodes could contribute to the development of a promising anode material.

Alternative Power

Wave-based wireless energy transmission offers excellent opportunities for technology advancement for the Army. The already-high efficiencies for energy transmission could be further increased by improvements in transmitter and receiver design. Some of the technologies in this project group could be developed to the point where integration into a wearable or field-deployable device may be appropriate. This would challenge the team to test its technology in nonoptimal situations—to test performance and develop applications that align with the unique advantages of the various approaches to wireless energy transmission. Experience gained from such testing would inform the work to develop improved materials and devices.

The 3D capacitor work has made significant advancements beyond the usual planar cells used in dielectric material testing. Three-dimensional structures will almost certainly be needed for high-power applications, and this is a good approach. Even so, the devices as configured are likely to have a low mass fraction of dielectric material, and therefore a low system-level energy density. It would be useful to model the expected system-level energy density and compare it with existing state of the art devices. A focus on increasing mass fraction of active material in the devices while retaining high internal surface area and high-applied fields would likely be beneficial. Concerns about liquid metal penetration into small pores could also be met and be resolved using various surface treatments as noted by the ARL team.

The pyroelectric material work appears to be making steady but significant progress toward useful devices for using waste heat for electrical energy generation. Electrocatalyst work for alkaline OER is advancing a field that is also rapidly advancing in the scientific community at large. Field-deployable configurations will almost certainly involve electrocatalyst integration into alkaline exchange membranes, and the ARL team is planning work in this direction. Work on advanced flames is intriguing insofar as it appears to greatly improve fuel flexibility. Efficiency determination relative to conventional fuel-fed generators is desired, as are quantitative statements about what kinds of fuels are acceptable. Performance metric comparison with other fuel-to-electrical energy conversion devices such as solid-oxide fuel cells, which can also have high fuel flexibility, would be prudent.

An emphasis on the ways in which ARL work is best-in-the-world would be beneficial. Work on blue whirl flames is at a relatively early stage and has made progress, but it is still difficult to tell how transformational the work would be. A more specific focus comparing that work with work on other combustion technologies, and other fuel-to-electricity technologies, would be beneficial.

OVERALL QUALITY OF THE WORK

The overall quality of the management is of high caliber. Researchers are well qualified for their work. ARL’s work in preparing for the review was superb, particularly given the uncertainties surrounding this first virtual review.

Most of the projects presented are excellent and, in some cases, world class and may have a pervasive impact on the Army. The scientific soundness and the use of fundamental sciences are outstanding. The project portfolio fits well with both global thrusts and the national agenda.

Collaborations throughout the optical sciences and photonics program were diverse and strong, to the extent that the research advances emanating from these partnerships demonstrates that “the whole is

greater than the sum of the parts.” That is, the time and effort invested by ARL researchers in developing these collaborations have resulted in a substantial return to the ARL research effort. The fundamental research conducted in the quantum information sciences group is especially impressive and the demonstration of the first communications receiver based on Rydberg atoms is a major milestone of which all of ARL can be proud. The successful effort to store information in spin waves is also promising and a credit to this research team. The advanced solid-state lasers group continues to be one of the “crown jewels” for ARL by driving infrared laser technology with the recent achievement of lasing at 3 microns in new, low phonon energy hosts such as barium fluoride and yttrium lithium fluoride. The sensor protection scientific team is commended for the clever iridium chemistry that is being pursued to develop broadband reverse saturable absorption materials and the GMR filters. Among the impressive accomplishments of the integrated photonics research team is the dramatic improvement in the performance of optical frequency combs and the successful demonstration of electrically steerable phased arrays.

Overall, the research projects in the electronics and optoelectronics area were of high quality and comparable to other first-class research organizations. The projects evaluated were well thought out and motivated by Army needs—two examples are emerging materials being explored as a means to increase information and data assessment in the field, and materials-driven antenna design that will address the Army’s expanding communications and SWaP-C needs. There was a good mix of exciting high-risk projects and those that support Army near-term applications.

In energy science, clear linkages between ARL research programs and Army needs were, in most cases, well presented. Some strong and highly visible programs are internationally recognized and are likely to create opportunities to improve effectiveness of Army personnel in the field. In addition, there are less visible although essential efforts providing incremental advances specific to Army needs. The team working on aqueous lithium-ion battery materials and systems is making exceptional advancements in the S&T of electrical energy storage with lithium-ion batteries. Aqueous electrolytes are nonflammable and thus dramatically safer than conventional electrolytes for military use. This ARL team has advanced the science of ultra-concentrated aqueous electrolytes that has enabled the use of high-voltage electrodes that previously were incompatible with aqueous systems. Work by the ARL team spans a broad range from computational modeling of interfacial chemistry to fabrication of cells of a size (approximately 5 Ah) suitable for field use. This is a wide range of activity for a relatively small group that has deservedly garnered positive international recognition. Work on wireless energy transmission is also making excellent progress with capabilities, for local wireless energy transmission (centimeters to meters) using electronic and acoustical waves, and is among the best in the world.

RECOMMENDATIONS

The setup for measuring the thermal conductivity of various laser advanced solid-state laser materials as a function of temperature is clearly a workhorse characterization setup with extensive and useful data presented. The group could explore expanding this experiment to include other measurements critical to evaluating laser operation. For thermal lensing, the variation in the index of refraction with temperature dn/dT and the coefficient of thermal expansion a(T) both also contribute to modeling of thermal lensing in laser materials. For some applications, the nonlinear index n₂ is also important. Furthermore, characterization of temperature-dependent excited-state lifetimes yields important insight into relaxation pathways. Not everything can be done in one setup, and some measurements are easier than others are. However, the ARL group is in an excellent position to raise its visibility as the “go to” group for materials developers seeking a laser materials characterization capability in the United States, which would prove to be of considerable benefit to both the ARL and U.S. laser efforts.