3
Electromagnetic Spectrum Sciences

INTRODUCTION

The electromagnetic spectrum sciences (EMSS) competency develops novel approaches to: sensing and operating across the entire electromagnetic environment; counter-sensing across the electromagnetic (EM) spectrum; protection from EM effects; and developing emerging radio frequency (RF) concepts for radars and electronic warfare (EW).¹ The vision of the EMSS competency is to deliver EM solutions to the U.S. Army that create enduring overmatch in multi-domain operations entailing EM spectrum warfare, advanced signal processing, sensing, and communications. The competency key philosophy is a focus on the investment in, and execution of, foundational and scientific research across the spectrum of materials-to-devices-to-structures, and phenomena, modeling, and techniques to ensure that the Army can access the EM spectrum in congested, contested, and constrained environments endemic to multi-domain operations where autonomy will drive operationally relevant timescales for operation.² Within the EMSS competency are two “core competencies,” which include the front end technologies core competency, that focuses on research related to the development, modeling, and characterization of EM sensing components, sensor concepts, antenna design, and EM propagation phenomenology (a topic that was not presented in this review), and the electronic warfare core competency which focuses on research related to new methods and concepts for electronic attack, electronic protection, and EW support through integration of concepts, underpinning hardware, and algorithm development.³ On July 18–20, 2023, the Panel on Assessment of the Electromagnetic Spectrum Science received presentations and laboratory tours largely focused on the front-end technologies core competency. For this review, four focal areas were defined within the front end technologies core competency for which the panelists were briefed: (1) materials for EMSS, (2) devices and heterostructures, (3) EM phenomena and structure, and (4) systems and signal processing. Below is the summary of the findings of each of these thrust areas.

___________________

¹ Core competency descriptions in this passage come from U.S. Army Combat Capabilities Development Command (DEVCOM) Army Research Laboratory (ARL), 2022, “Foundational Research Competencies and Core Competencies,” March.

² R. Del Rosario and J. Qui, 2023, “Story of the EMSS Competency,” U.S. Army Combat Capabilities Development Command (DEVCOM) Army Research Laboratory (ARL), document for the Army Research Laboratory Technical Assessment Board, July 18.

³ Ibid.

TECHNICAL QUALITY OF THE CORE COMPETENCY

Front-End Technologies Core Competency

Materials for EMSS

Achievements and Advancements

Overall, the work being carried out in the Materials for EMSS portfolio is on a par with other leading research institutions and the intramural teams and extramural managers have a good understanding of the underlying science and research that is being conducted by other groups working on materials and devices. Several presentations and posters exemplified the high quality work performed in-house and through extramural collaborations.

On the individual project level, a presentation that stood out as being particularly successful was “Ultraviolet (UV) Single-Photon Sensing,” which focused on the development of efficient UV detection technology using silicon carbide (SiC) adopted innovative approaches to solve complex challenges. The project was an effort between extramural collaborators and intramural ARL scientists. SiC has emerged as a promising material for UV detection due to its unique properties, including high absorption at shorter wavelengths. However, the performance of SiC-based devices, such as charge-coupled devices (CCDs), complementary metal-oxide semiconductor (CMOS) sensors, and avalanche photo diodes (APDs), are hindered by low quantum efficiency in the shorter wavelength range. This limitation is primarily attributed to poor photo-excited carrier collection resulting from band bending at the surface.

To overcome this challenge, the researchers on this project have adopted a newer scientific technique known as “delta doping.”⁴ Delta doping has emerged as a promising approach to enhance the quantum efficiency of silicon-based UV detectors. Delta doping involves the introduction of dopant atoms in a thin layer just below the Si surface. The dopant atoms create a localized region with modified electronic properties, effectively counteracting the adverse effects of band bending. By carefully selecting the type and concentration of dopant atoms, delta doping can be tailored to improve the collection efficiency of photo-excited carriers. The introduction of dopants near the surface modifies the electric field distribution and reduces the band bending, facilitating the efficient collection of photo-excited carriers. This could lead to a significant enhancement in the quantum efficiency of SiC-based CCDs, APDs, and CMOS sensors in the UV range. Moreover, delta doping offers additional advantages beyond mitigating band-bending effects. It can also improve the carrier mobility and reduce the surface recombination rate, further enhancing the device performance. Additionally, the technique is compatible with standard semiconductor fabrication processes, making it feasible for large-scale production of SiC-based UV detectors.

The extramural work described in the presentation “Designer Materials and Emergent Phenomena for Next Generation Devices” on multiferroics goes from basic materials science to devices to applications. The collaborations between the extramural investigator at the University of California, Berkeley, and ARL are strong. The extramural PI’s work on complex oxides and devices is best in class. The work of the principal investigator (PI) from Drexel University that gave the presentation “Simultaneous Robust Thin-Film Ferroelectricity and Ferromagnetism” was also highlighted. Again, this is very high quality work. The results from the bismuth ferrite–barium titanate (BiFeO₃–BaTiO₃) system are very interesting. He works closely with the PI from the University of California, Berkeley, and their teams appear to be well integrated.

___________________

⁴ Nikzad et al., 2012, “Delta-Doped Electron-Multiplied CCD with Absolute Quantum Efficiency Over 50% in the Near to Far Ultraviolet Range for Single Photon Counting Applications,” Applied Optics 51(3):365–369, https://doi.org/10.1364/AO.51.000365.

Research Portfolio Opportunities

The vision of the EMSS materials and devices activity for the extramural research at ARL appears to be the discovery of novel functional materials with extraordinary electronic, photonic, magnetic, and thermal properties. Because the quality of new materials is highly dependent on growth conditions, it may be important to consider whether it may be more advantageous to have these growth capabilities in-house at ARL. Since the approach that the intramural teams are taking is dependent on controlling the diamond-dielectric interface, the surface science of films that are grown in-house may be important, because those interfaces are going to be very sensitive to the diamond surface quality. ARL plans to add chemical vapor deposition (CVD) diamond research in 2024, and it will be important for ARL to quickly demonstrate they can grow films on par with best in class. ARL can leverage its external partners to get to best in class as quickly as possible.

Materials growth and surface science⁵ supported by ARL’s extramural efforts could benefit its intramural efforts, especially if there are strong collaborations with the external groups. Materials growth could be brought in house, and extramural collaborations could inform and bolster the surface science expertise and research, especially if the work could be done on films grown at ARL—such intramural and extramural activities could be very complementary.

To maintain a leadership position, it is important to have a comprehensive program that includes materials growth and processing, as well as device design and fabrication. It is also important to maintain a strong understanding of competing technologies, such as other wide-bandgap materials like gallium oxide and hexagonal boron nitride. At this point, it appears that ARL’s intramural activities are committed to diamond, but they should keep an open mind to other materials of this kind. Additionally, as mentioned, areas of research, specifically in the diamond materials and devices area, that would benefit the overall program include in-house thin-film diamond growth, as well as interface and surface analysis.

In the area of thermal management materials (this area is also relevant to the energy conversion core competency), there were few details provided, but with all of the interest in high-temperature operation of semiconductor devices, materials for thermal management is an important area of research. While much of the activity in the field is around making devices that can operate at higher temperature, it is just as important to manage the temperature. Thermal management is an important piece of the high-temperature electronics total system. Companies that work in high-power and high-temperature electronics all have development activities on thermal management. If ARL currently does not have research activities in this area, it may be worthwhile to consider adding more focus. In many ways, thermal management is more straight forward than trying to obtain devices that will operate at higher temperatures

Opportunities Identified for Individual Projects

This section provides opportunities that were identified for individual projects that were shown during the review. Commentary on these project is provided to raise the overall impact of the portfolio.

• “Ultrafast Studies of Thermal and Electric Transport in UWBG Semiconductors”—this intramural presentation provided a clear explanation on a pump pulse technique that was used to measure the interface thermal boundary resistance between diamond and gold. Efforts to mitigate the impedance of an identified bottleneck, however, did not appear to be promising.

___________________

⁵ Since hydrogen passivation of the diamond surface is important in order to control the carrier concentration in the channel, it is important to understand the surface chemistry of diamond, especially with respect to hydrogen passivation. It will be important to determine the conditions needed to fully terminate the diamond surface with hydrogen, using techniques such as hydrogen dosing experiments. It will also be important to determine the thermal stability of the hydrogen passivation to understand the stability of the interface during subsequent processing, such as dielectric and contact deposition. The thermal stability of the passivation can be determined using techniques such as thermal desorption spectroscopy, total reflection Fourier Transform Infrared spectroscopy, and X-ray photoelectron spectroscopy to understand the C-H bonding on the diamond surface.

ARL may wish to address this bottleneck by looking at other state-of-the-art research.⁶

• “Ultraviolet Single-photon Sensing”—while the research methodologies and approaches employed by the teams across the materials for EMSS portfolio appear to be sound, one area of improvement was found in this project. The goal of this project is to optimize single-photon sensing devices (operating in the Geiger Mode in the UV range) based on 4H-SiC. The specific technical goals are to attain high single-photon detection efficiency within the wavelength interval 230–350 nm with large, uniform detection area with minimal defect density. The PI’s group has used numerical modelling to understand how wafer growth conditions affect detection uniformity. The PI showed results on current status of 4H-SiC detectors, emphasizing that the quantum efficiency peaks at 265 nm, but falls to very low values for wavelengths longer than 300 nm (near UV range). In addition, the single-photon detection efficiency is significantly non-uniform. The PI exhibited color maps and three-dimensional (3D) perspective maps of the spatial variation of the detector efficiency in Geiger mode over 100 × 100 micron (Mu) areas. The anisotropy was attributed to mobility/impact ionization as well as substrate miscut. One of the achievements is showing that 4H-SiC non-reach through separated absorption charge multiplication devices can achieve a quantum efficiency of 32 percent at wavelength 340 nm, which is three times higher than that of PIN devices. A focus of the research is to understand non-uniformity of detector efficiency in linear mode and Geiger mode responses. To date this has been investigated mostly by numerical modelling and Monte Carlo simulation. Experimental tests appear to be still on the drawing board. There seems to be over-reliance on numerical modelling and Monte Carlo simulations to improve detector efficiency and less effort directed toward measurements on actual devices to test various hypotheses. For instance, it is not clear if the number of factors listed for the spatially non-uniform detector response (i.e., miscut, mobility variation, and impact ionization) are being tested by systematic measurements on tailored samples. This is obviously harder but seems be of central importance. This, and similar techniques could be further explored.

Devices and Heterostructures

Accomplishments and Advancements

The work within the devices and heterostructures focal area of the front end technologies core competency was very high caliber, and at par with leading institutions. The researchers demonstrated a broad understanding of the research conducted elsewhere, although there are a few suggestions on how their knowledge can be bolstered below. They are also utilizing sound research methodologies. Several examples below reflect the strong research methodologies and solution-based approaches this area of the competency is employing.

The scientific quality of the core competency’s work in the area of novel broadband transmitter architectures based on frequency multipliers is excellent and is state of the art as exemplified by the presentation “RF Circuits for Army Applications: Novel Broadband Transmitter Architecture based on Frequency Multipliers.” As demonstrated by the research team’s publications, they are making major contributions to the challenging area of broadband transmitters in the microwave/millimeter community and this could potentially lead to some changes in conventional transmitter architectures in the wireless communications area as a whole.

The team shows a thorough understanding of the problem at hand, as well as the technologies conventionally utilized to develop these systems. The methodologies utilized by this team are sound and

___________________

⁶ M. Malakoutian, D. Field, N. Hines, S. Pasayat, S. Graham, M. Kuball, and S. Chowdhury, 2021, “Record-Low Thermal Boundary Resistance Between Diamond and GaN-on-SiC for Enabling Radiofrequency Device Cooling,” ACS Applied Materials and Interfaces 13(50):60553–60560, https://doi.org/10.1021/acsami.1c13833.

logical. The research focus of RF circuits for Army applications is well integrated with the devices and heterostructures focal area and the relevance of the research undertaken in ultra-wide bandgap (UWBG) semiconductor materials is obvious for this RF circuit development.

Research Portfolio Opportunities

While there are no major risks identified with this research and development (R&D) effort, it could be noted that there has actually been a lot of research undertaken in RF over fiber applications about pre-distorting signals and related areas to overcome nonlinearities in these links. As such, the ARL researchers may want to consider or evaluate that work. There is a lot of literature on this and ARL needs to make sure research in this area is on par and can advance state of the art.^7,8 Additionally, if this research is to move into the wireless communications domain then researchers need to also look into latency as next generation networks have very demanding requirements.

It should also be noted that the extramural presentation “Electromagnetic Entanglement and Spin-Momentum Locking for Advanced Sensing” was an example of excellent research in electromagnetic entanglement and spin-momentum locking, which has the potential for uncovering new sensing concepts and for advancing sensing technologies. By combining the principles of electromagnetic entanglement and spin-momentum locking, scientists and engineers can create ultra-sensitive and ultra-fast sensing devices. Such advanced sensors can have great applications in various fields, such as quantum communication. The integration of these quantum phenomena into sensing technologies has the potential to revolutionize the way one perceives and interacts with the world around them, unlocking new possibilities for scientific discovery and technological innovation. Spin-momentum locking, on the other hand, is a fascinating phenomenon observed in certain materials where the spin direction of particles is intrinsically linked to their momentum. This unique behavior offers precise control over the spin properties of particles by manipulating their momenta. In sensing, this could lead to the development of highly sensitive and accurate detectors capable of measuring even the tiniest changes in momentum and, consequently, spin.

EM Phenomena and Structure

Achievements and Advancements

The quality of science within the EM phenomena and structure research thrust is high and mostly on par with other research institutions nationally and internationally. Some work likely exceeds the quality of similar work performed elsewhere. The research methodologies used by the teams were sound, and there were no risks identified to the overall portfolio of not meeting its objectives. The balance of expertise in the teams is good and is appropriate to reach the stated goals. Below is a summary of some of the high quality work that was presented at the review and some suggestions on how individual projects may be improved.

The scientific quality found within the presentation “Additive Manufacturing (AM) for Antennas and RF Devices” is very good and is on par with some of the top research institutions in the United States. The research being undertaken is very pertinent and relevant to many present day and future wireless systems. The quality of the science found in the presentation “Multi-Function Metasurfaces and Electromagnetic Skins” is also very good and is consistent with much of the research being conducted in this area around the world. The research teams on both projects show a good understanding of the problems at hand, what is the state of the art, and how to potentially advance these technical areas. The methodologies and techniques used here are sound and appropriate. Overall, the direction of these

___________________

⁷ M. Hadi, 2021, “Mitigation of Nonlinearities in Analog Radio Over Fiber Links Using Machine Learning Approach,” ICT Express 7:253–258, https://doi.org/10.1016/j.icte.2020.11.002.

⁸ A. Matthews and P. Yadav, 2013, “Suppression of Nonlinearity Induced Distortions in Radio Over Fiber Links,” IJECET 4(4):51–60.

research efforts represent sound scientific decisions based on current science. Additionally, the team in the AM area is making some good advancements in the relevant technologies.

Research Portfolio Opportunities

The panel did not identify overarching trends across the EM phenomena and structure that could be helpful to upper ARL management. For this reason, they chose instead to focus more on a review of some of the individual projects within the portfolio.

Opportunities Identified for Individual Projects

This section provides feedback on individual projects to help to raise the overall impact of the portfolio.

• “Surface/Interface Driven Phenomena in Hybrid Quantum Materials.” This extramural presentation focused on the four topics: (1) search for the Majorana bound state, (2) a superconducting diode displaying ideal non-reciprocity, (3) anomalous magnetism induced by altering the Berry curvature in a magnetic layer, and (4) proximity magnetization in magnetic heterostructures. There is currently an intense search for the exotic excitation called Majorana (roughly “half an electron” that is its own antiparticle). The Majorana bound state is predicted to form at the interface of a topological insulator in proximity with a superconductor and a ferromagnetic contact pad. The PI has explored a novel geometry comprised of a film of the magnetic insulator europium sulfide (EuS) evaporated on top of an Au film in proximity with a superconductor, vanadium. He has published intriguing scanning tunneling microscope results consistent with Majorana excitations trapped at the edge of the EuS film. In a second project, the PI has uncovered non-reciprocal supercurrent flow in vanadium sandwiched between EuS and Pt. The supercurrent flows in only one direction selectable by the applied magnetic field, which implies an ideal superconducting diode. In general, the Berry curvature is now known to be the cause of the intrinsic anomalous hall effect observed in all ferromagnets, and in heterostructures of Cr₂Te₃ grown on SrTiO₃, and the researcher has found that the Berry curvature can be tuned by the strain field at the interfaces. The strain produces large effects in the hysteretic loops of the anomalous hall effect (AHE) response. Finally, in heterostructures comprised of alternating layers of MnBi2Te4-Bi2Te3 and MnTe, proximity magnetization also produces a rich array of quantum Hall effect (QHE) behavior.

  On the whole, the PI is investigating a set of interesting phenomena arising from how topological effects (Berry curvature) can couple with magnetization or supercurrent to produce highly unusual phenomena, some of which may be realized as future devices. As the field matures, it may be advisable to narrow the spread of research directions. The time seems appropriate to focus on a few promising projects, reduce the uncertainties, and to bring novel effects into sharper focus. It may be necessary to abandon the less promising directions.

• “Axion Electrodynamics in Topological Films and Devices,” the axion is a particle/excitation in quantum field theory (proposed by Frank Wilczek) that plays a fundamental role in symmetry breaking in a universe that has multiple degenerate vacuum states. The extramural presenter summarized the project’s accomplishments in the past 3 years, which include 60 publications (including submissions) of the six PIs. The topics researched include the EM response of systems displaying the Quantum Anomalous Hall Effect (QAHE), artificial layered topological materials, intrinsic spin Hall torque in a Chern insulator, and the coupling of topological effects to magnetism in devices.

  Recent research has revealed how Axion insulators are realized in a topological insulator in a magnetic field. Using MBE, a group out of the University of California, Los Angeles, has grown layered films in which the top and bottom surfaces can be subjected to fields of

opposite directions (by tuning the remnant fields of the magnetization). The emergence of the axion state is observed by measuring the QAHE. The two researchers associated with this project have used Kerr rotation techniques to probe the optical properties of the Axion state by measuring the Kerr rotation angle. Two PIs on this project have proposed an alternate way to probe and measure the Axion angle by detecting the quadrupole moment in axionic waveguides. Using devices made from MoTe2/WSe2 layers, one PI has demonstrated that in the QAHE state, topological properties allow switching of the magnetization using ultralow currents. These investigations are of uniformly high quality, involving researchers who are very active in the field of topological quantum matter. The demonstration that the exotic electronic properties of Axion insulators can be meaningfully probed by transport and optical measurements is impressive. Although most of the results are currently of interest at the fundamental level, one may anticipate future devices with unique capabilities exploiting the topological properties.

• “Metasurfaces and Electromagnetic Skins.” This presentation falls under the Novel Materials and Concepts for RF Electronics project (2021–2023). This interesting and worthwhile endeavor is a follow on to their earlier 6.1 metamaterial research. Its goal is to provide a low-profile, ultra-wide-band (UWB) radiating structure that supports multiple functions in a military system and can be used as an EM skin for special applications like radar cross section reduction of the antenna aperture, deception against radar operation, and EW activities. However, ensuring the desired UWB behavior and flexibility are monumental challenges, and apparently, the success of the program has been limited. Although this line of research may not be entirely fruitful, investigating low-profile UWB radiating structures that support multiple functions is worth pursuing, given the need for distributed broadband multifunctional RF systems. If they have not already done so, the investigators may want do an extensive literature search on UWB arrays, because numerous UWB arrays have been developed over the two last decades, including Balanced Antipodal Vivaldi Antenna, Frequency-Scaled Ultra-Wide Spectrum Element Array, Planar Folded Vivaldi, Planar UltraWideband Modular Array, Reconfigurable Electromagnetic Interface, Sliced Notch Array, Superstrate-Enhanced Substrate-Loaded Array, and Tightly Coupled Dipole Array. One of the major risks with UWB arrays is an inability of achieving low reflections from the feed points of the individual elements back into the feed network (voltage standing wave ratio [VSWR] less than 2) of the array across its entire desired operating bandwidth. An associated problem is the array has to be thicker to have a low VSWR at the lowest frequencies of the band, and this problem is magnified for operating bands that go below 1 GHz. ARL may want to consider creating in-house experimental capabilities or connecting with extramural partners who can help with advancing experimental demonstrations, to accelerate the advancement of this area.

Systems and Signal Processing

Achievements and Advancements

The presentations and overall portfolio within EMSS systems and signal processing focus area highlighted a wide range of advanced research that is generally well planned and offers significant likelihood of technical success. Specific niche capabilities of research were found to be world class and on par with leading institutions. For example, ARL is working at the cutting edge in ground-penetrating radar algorithms and cognitive EW. There was also quite a bit of excellent work that, even if evolutionary (as most science is), and building on legacy systems, was consistent with the state of the art in the community. Two key elements were identified as distinguishing the world-class areas of research achievement; these were (1) experimental validation of theoretical results and (2) clarity of requirements

for near-term and long-term objectives. The research methodologies used within the competency were also sound.

Research Portfolio Opportunities

In viewing this portfolio of projects, it is clear that challenges related to EM environments have been successfully identified by ARL, such as transient battlefield vision in near future, congested spectrum, dynamic access, and distributed processing. It is also clear that ARL has been collaborating with many national laboratories and universities. These collaborations are valuable, and it is suggested that these efforts could be increased to develop a collaboration network. In addition to university outreach, increasing collaborations with Established Program to Stimulate Competitive Research states, may have potential to benefit ARL. The systems and signal processing team may also consider increasing its collaboration with industry and with two other DoD research laboratories, the Air Force Research Laboratory (AFRL) and the Naval Research Laboratory (NRL). Both organizations have world-class personnel in systems and signal processing, and so it may be beneficial to establish more communication with these organizations if ARL is not already doing so.

The systems and signal processing team may also consider working more closely with the network, cyber, and computational sciences (NC&CS) competency to achieve data analytics and networking technologies among different sensor systems. The importance of developing networking technologies in lock-step with the corresponding sensors and systems to be networked cannot be overstated. Different sensors and systems have particular features and requirements that cannot be assumed to be independent of how they can be networked. As such, a close collaboration with the NC&CS competency toward co-design is suggested. Additionally, the systems and signal processing team can increase its emphasis on over-the-air experimental verification for multi-year theoretical signal processing studies, and provide greater focus to real-world training data and standardization within ARL radio frequency machine-learning (RFML) efforts.

Furthermore, increasing the collaboration and coordination between intramural and extramural research efforts may enhance the support for the general objectives of the systems and signal processing focal area. Although some level of coordination is mentioned between these two groups of activities, an enhancement of predefined processes to coordinate and utilize the results of extramural activities for intramural ARL development may be beneficial.

Overall, ARL’s systems and signal processing team displays a solid and methodical approach for executing the research defined. Intramural research in particular makes use of world-class laboratories, advancements in measurement processes, and talented staff. In addition, its approach of having sufficiently long duration 6.1 and 6.2 efforts (3–8 years) is excellent; anything shorter may produce limited results.

While the systems and signal processing projects are utilizing artificial intelligence (AI) and machine learning (ML) in the current projects, the AI/ML area is an opportunistic research area for the general EMSS competency, and increased focus on this area would help the general competency objectives. Increasing efforts in AI/ML based EMSS efforts could help projects such as “Cognitive EW Research Roadmap” and “E/H Sensing.” Sensing and security capabilities at higher frequencies such as mm-wave and THz frequencies are also seen as important areas.

Efforts toward confidence metrics and experimental data collection for training AI/ML efforts are highly critical; one could even say “mandatory.” Obtaining and employing real measured data is a key discriminator for ARL’s RFML research (e.g., Army Rapid Capabilities Office challenge, Defense Advanced Research Projects Agency Radio Frequency Machine Learning Systems [RFMLS], and the Multi-Channel Airborne Radar Measurement [MCARM] database), even if the use of such data must be managed at Controlled Unclassified Information (CUI) or classified levels. The strengthening of experimental measurement facilities, both internally and externally in collaboration with extramural partners, toward generating large training data sets for AI/ML algorithm training is recommended. This

effort will also be important toward validation of theoretical and computational research being undertaken within ARL’s intramural research activities and through extramural efforts.

One of the assessment criteria questions asks whether there are any specific areas where the research may be at major risk of not meeting its objectives and to provide reasoning. For this question, the following observations and suggestions are offered.

The total number in the workforce within the competency and in specific subareas is important to carry out the research to achieve competency goals. In this regard, some risks are perceived regarding the comparably small number of internal ARL systems and signal processing researchers. A related risk is attrition caused by retirement. It will be important to ensure that ARL keeps a focus on how to address the transition of knowledge from retiring researchers. At least one expert researcher has recently retired, and one very good fairly recent hire left for the National Telecommunications Information Administration. From the poster sessions, it was apparent that ARL is vigorously attempting to address this issue by involving and eventually hiring the students of its excellent extramural team of researchers. This is an important recruitment pipeline. Moving forward, it will continue to be important to put special consideration on the three Rs of the labor-force (retirement, recruitment, and retention), because it takes longer to develop expertise (recruitment) than to lose it overnight (retirement, retention).

One potential way for ARL to reach its scientific objectives faster would be a deeper engagement with government led consortia that engage academia and industry. Additionally, relevant to the ongoing general topic of active RF systems, including synthetic aperture radar (SAR), SDR/RFSoC (radio frequency system-on-chip) and ground penetrating radar,⁹ the following questions and ideas may present new research lines, if ARL is not already addressing them, that could potentially help support the scientific objectives of the EMSS competency:

1. How to meet transmit power requirements for each application while being more spectrally clean?
2. How to pivot from noise-limited to increasingly interference-limited operation?
3. How can a radar achieve greater multi-mode use of fixed spectrum resources?
4. How to leverage what is known as cognitive RF for increasingly complex and dynamic environments? The lead of the intramural EMSS, EW, and spectrum control team is partially addressing this question.
5. Can innovative antenna elements and arrays be developed that are conducive to multifunctional RF co-design?¹⁰

The presentations shown during the review did not have a strong data analysis focus and there were no presentations received on manipulation of multi-modal and heterogeneous data sets. Possible areas of research, if not already under way, to address data analysis could be a focus on multi-modality data-fusion approaches enabled by AI/ML, first for RF-spectrum modalities, subsequently for EM-spectrum modalities, and then for non-EM modalities like acoustics, etc. Relevant questions include, but are not limited to:

___________________

⁹ A new technology based on multiple robotic ground vehicles has been developed. This system can operate as a very-high-resolution multi-static synthetic aperture radar system capable of producing high-resolution 3D images of buried objects. See A.V. Muppala, A. Alburadi, A.Y. Nashashibi, H.N. Shaman, and K. Sarabandi, 2023, “A 223-GHz FMCW Imaging Radar with 360° FoV and 0.3° Azimuthal Resolution Enabled by a Rotationally Stable Fan-Beam Reflector,” IEEE Transactions on Geoscience and Remote Sensing 61:1–9, https://doi.org/10.1109/TGRS.2023.3284715.

¹⁰ E. Mokole and S. Blunt, 2023, “Some Current and Recent RF-Spectrum Research and Development, Applications, Management, and Interference Mitigation,” 2023 IEEE Conference on Antenna Measurements and Applications, https://ieeexplore.ieee.org/document/10352837/authors#authors.

1. Can the severe limitations of traditional stovepiped data and associated processing in modern big data EM environments be mitigated or overcome by using raw data across the entire EM spectrum to define ontological concepts and categories for different modalities as inputs to autonomy engines?
2. A supporting but equally important question is: How does one characterize and handle vast amounts of disparate data? A possible approach is:
   1. Organize disparate data using fundamental physics within each RF modality.
   2. Define an ontology¹¹ of concepts and categories for each modality (domain) that characterizes the properties of that individual modality and then characterize the relations among the modalities. Construct an ontology by identifying and designing appropriate architecture(s) to feed autonomy engines.
   3. Fuse inputs from disparate modalities within autonomy engines. A conceptual exemplar for fusing the electromagnetic modalities of EW, radar, and electro-optical/infrared (EO/IR) is depicted in Figure E-1, Appendix E. The basic notion is to replace the legacy data formats in the modality stovepipes within the dashed box with some to-be-determined physics-based data-fusion process.

Opportunities Identified for Individual Projects

During the July 2023 assessment, a total of seven presentations were shown that focused on the work within the EMSS Systems and Signal Processing research thrust area. The review also included a poster session where the work of several researchers, including those in the EW core competency were highlighted. Takeaways, as they relate to technical quality of the research and opportunities to improve the impact of individual projects are captured in the following bullet points:

• “Physical Layer Secure Design of Dual-Function Sensing-Communication Systems.” This project focused on dual-function radar-communication systems, which have been studied for the last three decades under a variety of different names. This work, however, offers some interesting theoretical results related to time-modulated arrays and the use of intelligent reflective surfaces for enhanced physical security. The theoretical results, by the PI on this project, have spanned nearly two decades, and she is a recognized expert in the area. Still, the path to future maturation and implementation in fielded systems is unclear. If her theoretical research can be validated with prototypes in laboratory and field measurements, it could be expanded to multifunction RF, which subsumes sensing and communications.
• “Algorithm Development for SAR.” GPR is a historical area of intramural research for ARL dating back to the early 1990s work on UWB radar. The project on rotary-wing, drone-based, down-looking ground-penetrating radar (DL-GPR) is the latest instantiation of this storied history. The advent and proliferation of drones, the miniaturization of requisite onboard hardware, increased computational speed and memory, and the improvement of position registration of host platforms have reinvigorated this field. The DL-GPR effort has significant implications for humanitarian demining throughout the world, as well as various other applications related to observations of groundwater, subsurface soil profile characteristics, and bathymetry. Other agencies within the United States, including the National Aeronautics and Space Administration and the U.S. Geological Survey, are also supporting cutting-edge research along these lines. This work addresses a valuable technical challenge with an effective system approach (high-pass filtering, principal component analysis, change detection, and compressed sensing). It was not clear, however, whether the research team was aware of newer systems and whether both CARABAS and the Mirage are still operational.

___________________

¹¹ An ontology is a set of concepts and categories in a subject area or domain that shows their properties and the relations among them.

The team is advised to seek more recent systems to continue its low-frequency synthetic aperture radar (SAR) work. The team reported on physics-based GPR analyses, including for surface-bounce return identification algorithms, which was impressive and on-par with the state-of-the-art. As the work builds on such a long legacy of prior demonstrations, it is suggested that ARL periodically perform broader industry and literature surveys, especially on work supported by other U.S. government agencies and non-U.S. efforts that have concentrated in this area since the 1990s, to ensure that the discriminating image-classification algorithms are in fact the leading option. For example, the research team may find value in the work by Alex Yarovoy’s group at the Technical University Delft in the Netherlands and Felix Vega’s group at the Technology Innovation Institute in the United Arab Emirates. The drone-based system has the highly desirable advantage of avoiding injury to its operator; however, a long-standing problem of achieving sufficiently accurate own-position registration for such a light vehicle is required to accomplish the desired high-resolution images of targets. Since the algorithms are completely data driven with no assumptions about the ground surface, they incur a very high computational cost. However, that cost has the possibility of being retired given the continued size reduction and increased speed of modern computational hardware, which bodes well for future success. Investment in development of physics-based computational renditions of the drone-based system and observational environment is also warranted. Another area of research that is important yet seldom addressed is sensor calibration, which is a prerequisite for performing physics-based analyses and retrievals with DL-GPR data.

• “Cognitive EW Research Roadmap (EW).” This excellent presentation offered very good insight into the broader application space within the EMSS by investigating how to map EW into cognitive space. This area is extremely important to scientific work. Given how rapidly the academic and industrial capabilities are growing, the team may want to update its first-year analysis of available systems and techniques periodically. In particular, focusing on training data (normalization of collection processes, labeling, and training measures) and standardization across researchers may offer greater value than replicating experiments within ARL. With respect to training data, the importance of researchers building an ecosystem that moves beyond strictly synthetic and captured data and creates efficient data generation, channel models, data augmentation, and cyborg data sets¹² (e.g., the use of real-world RF training data; similar industry and academic efforts use a balance of synthetic, augmented, and collected data), cannot be underestimated. Consequently, a major issue is identifying real-world data sets, if they exist, and gaining access to them. If they do not exist, seeking collaborations to ensure their collection is recommended. For example, the radar community has been feeding from AFRL’s MCARM database¹³ for 25 years to improve research and evaluate theoretical results on space-time adaptive processing. Nothing else like it exists in that community, and it has been invaluable in pushing realistic research. The example of the EW use case provided in the presentation is interesting and the project might want to look into very successful data-fusion predecessors like the U.S. Navy’s technology demonstration project, the Vessel Tracking Program, and its progeny (MASTER, Sealink Advanced Analysis, Comprehensive Maritime Awareness Joint Capability Technology Demonstration, etc.) to obtain some lessons learned. These programs digitized and automated the manual process of gathering, correlating, and interpreting multiple sources of data about ships at sea around the world. They used a layered-defense approach that incorporated sensors (electronic intelligence, signal characteristics of maritime navigation radars, and

___________________

¹² For this review, several research search engines were used to generate the research profiles: Google Scholar, World of Science (Clarivate), SPIE, IEEE Xplore, and Elsevier.

¹³ Ibid.

acoustics), automatic identification systems, more than 300 databases, and information feeds from national technical means (e.g., satellites) to open-source information. Among the databases are the U.S. Coast Guard’s Maritime Global Awareness Network and the Office of Naval Intelligence’s Seawatch.

COMPETENCY PORTFOLIO OF SCIENTIFIC EXPERTISE

Both the intramural and extramural teams supporting the materials for EMSS; devices and heterostructures; and EM phenomena were of very high quality. The team involved with the transmitter architectures, in particular, were excellent. Overall, the balance of the E/H Sensing team also seems appropriate for the goals of this R&D program. The qualifications of both teams involved with the AM for antennas and RF devices and the metasurfaces are also very good.

As previously mentioned, ARL plans to add CVD diamond research in 2024, and it will be important for ARL to quickly demonstrate they can grow films on par with best-in-class. If this work is done in-house, this might also be an area where bringing in a post-doc with directly relevant experience could speed up the process.

For the systems and signal processing research thrust, one identified challenge is the division of intramural staff effort across projects, since scientific staff in this focus area is relatively small. Reducing the number of projects that staff are working on may be helpful, as staff can then gain greater expertise in the higher priority scientific areas, which have been identified with concrete contributions to the portfolio’s goals.

Furthermore, the systems and signal processing research thrust area could be bolstered with additional expertise in EM phenomenology, computational methods, inverse scattering, and generally speaking, quantitative analysis of SAR and GPR data. This seemed true especially for both the intramural researchers and also for extramural collaborators. Additionally, AI expertise is critical, as only cursory uses of AI/ML methods were included in the presentation portfolio. Given how much is happening in this area, the absence of work and expertise in that domain was noticeable. ARL may need to consider ways to increase this focus and expertise. Furthermore, for the outreach efforts of the systems and signal focus area, student internships are considered highly beneficial to the general goals of the competency, and continuing efforts in this area are suggested.

Finally, publication records are not the best metric for determining the quality of scientific credentials for an institution when research within it is largely produced as CUI, or in a classified environment. It is therefore understandable that after a scoping review of online information,¹⁴ the intramural ARL staff that were identified through the read-ahead literature in this review appear to have lower citations records (62 percent had less than 1,000 citations) than those ARL is collaborating with in academic environments. Still, in line with the ARL’s request for commentary on the qualifications of its teams, citation records serve as one indicator of how publicly available information has been received by the scientific community. The review found that three intramural managers and three intramural researchers had good-to-excellent citations counts, with these managers reflecting an exceptional number of citations. The extramural researchers (whose research is unclassified, and so more publicly available) had a respectable number of exceptionally high numbers of citations. From this information, it is clear that ARL has some strong scientific intramural managers who have connected to the research community where they are able. Additionally, the extramural efforts at ARL are connecting with top notch, well-respected extramural researchers for their chosen EMSS areas from academia, industry, government, University Affiliated Research Centers (UARCs) and federally funded research and development centers (FFRDCs). The intramural scientists are encouraged to continue publishing where they can (e.g., when information is not restricted to the public), as publications may build more connections to the broader research community.

___________________

¹⁴ Ibid.

The extramural researchers were also found to have earned a significant number of high quality awards and were also found to be in a great many professional societies (IEEE, American Association for the Advancement of Science, American Physical Society, International Society for Optical Engineering [SPIE], Sloan, National Academy of Engineering, and Optical Society of America). Also the intramural researchers appear to engage in professional societies (e.g., IEEE), however the number of these connections was difficult to determine, as online search results were understandingly more limited, since the protocol of cleared individuals to keep identifying information off the web, less information is available. The competency is encouraged to seek out awards for its more publicly releasable scientific studies as a means of connecting further to the broader research community.

COMPETENCIES FACILITIES AND RESOURCES

The facilities and research for the materials for EMSS, devices and heterostructures, and EM phenomena and structure were all found to be appropriate for the work ARL is doing. Specifically, the facilities and resources for the RF circuits and for additive manufacturing for antennas, RF devices, metasurfaces, and EM skins were well suited to the research the teams were conducting. The experimentation in ARL’s in-house laboratories is well conceived and well done, and the addition of extramural laboratories and measurements really boosts their capability. The laboratory tours were excellent and well-articulated by the personnel, and the facilities were very good

If diamond growth and hydrogen surface passivation will move in-house, ARL would need to consider the techniques that would typically be needed for this work, and the equipment that would be required (e.g., grazing angle FTIR [Fourier Transform Infrared] spectroscopy, UV photoelectron spectroscopy, X-ray photoelectron spectroscopy, and thermal desorption).

OVERALL SCIENTIFIC QUALITY OF THE COMPETENCY

The work performed in the materials for EMSS research thrust is on a par with other leading research institutions nationally and internationally. Taken as a whole, the research for this aspect of materials compares favorably with NRL and the Fraunhofer Society for the Advancement of Applied Research, which seems to follow a similar organizational paradigm relative to academic interactions, and exceeds that of AFRL. The quality is very much bolstered by the extramural researchers. The intramural teams and extramural managers have a good understanding of the underlying science and research conducted elsewhere. The researchers also used sound research methodologies and there were no risks identified of the core competency not meeting their objectives. The qualifications of teams were very high quality and there were no identified gaps in the portfolio of scientific expertise. ARL plans to add CVD diamond research in 2024, and it will be important for ARL to quickly demonstrate they can grow films on par with best-in-class. If this work is done in-house, bringing in a post-doc with directly relevant experience could accelerate the process.

Both intramural and extramural work presented within the devices and heterostructures focal area was of very high caliber and at par with leading institutions. The expertise of the intramural and extramural researchers and the facilities supporting the competency were exemplary. The researchers largely demonstrate a broad understanding of the research conducted elsewhere and the research methodologies and solution-based approaches the teams are utilizing are excellent. There were no risks identified to this research thrust in ARL not achieving its goals.

The quality of science within the EM phenomena and structure research thrust is also very high and at par with other research institutions nationally and internationally. Some work likely exceeds the quality of similar work performed worldwide. The scientific expertise is very high caliber and the research methodologies used by the teams were sound. There were no risks to the overall portfolio of not

meeting its objectives. The facilities and resources supporting EM phenomena and structure are appropriate and no additional resources were identified as being needed.

The systems and signal processing portfolio highlighted a wide range of advanced research that was well planned and offers significant likelihood of technical success. While not all projects were at par with top universities and research institutions, specific niche capabilities were found to be world class, for example, ARL is working at the cutting edge in ground-penetrating radar and cognitive EW. Other research appears to be evolutionary work that extends upon legacy systems, and such work is also high quality. Projects demonstrating particular scientific benefit include emerging work to harness sub-THz transmission frequencies, protective EMSS technologies using porous silicon, and fabrication of substantially ruggedized circuits on non-planar substrates. Research plans demonstrate a good understanding of the relevant literature and methods.

The experimentation in ARL’s in-house laboratories is well conceived and well done, and the addition of extramural laboratories and measurements boosts their capability. The laboratory facilities are very good. More broadly, some risks are considered across the competency regarding the comparably small number of intramural researchers¹⁵ compared to the extramural research activities, as well as potential attrition caused by retirement. The small size of the current intramural workforce inherently limits the scope and depth of topics that can be investigated internally. Perhaps the number of intramural researchers could be increased to ensure critical mass across research activities. ARL may also benefit from bringing more of the extramural research efforts into internal ARL development. Finally, reducing the number of projects that current staff are working on may be helpful, as staff can then gain greater expertise in the higher priority scientific areas, which have been identified with concrete contributions to the portfolio’s scientific goals.

Furthermore, AI expertise is critical to the competency as a whole, as only cursory uses of AI/ML methods were included in the presentation portfolio. Given how much is happening in this area, the absence of work and expertise in that domain was noticeable and ARL may need to consider ways to increase this focus and expertise. Increasing the focus on AI/ML would help the general competency objectives and could aid such projects as “Cognitive EW” and “E/H Sensing.”

In addition, the presentations shown during the review did not have a strong data analysis focus and there were no presentations received on manipulation of multi-modal and heterogeneous data sets. A possible area of research, if not already underway, to address data analysis could be a focus on multi-modality data-fusion approaches enabled by AI/ML, first for RF-spectrum modalities, subsequently for EM-spectrum modalities, and then for non-EM modalities like acoustics and similar.

Efforts toward confidence metrics and experimental data collection for training AI/ML efforts are highly critical. Obtaining and employing real measured data is a key discriminator for ARL’s RFML research. The strengthening of experimental measurement facilities, both internally and externally in collaboration with extramural partners, for the purpose of generating large training data sets for AI/ML algorithm training is suggested. This effort will also be important toward validation of theoretical and computational research being undertaken within ARL and through its extramural efforts.

In terms of connecting to the broader community, the expansion of academic and industrial surveys as it connects back to the portfolio is encouraged. Particularly on projects where efforts build on long legacies of intramural research, a continually updated assessment of external capabilities (academia, industry, U.S. government laboratories, FFRDCs, UARCs, entities in nation states, etc.) may help determine future directions for the scientific research to keep it connected to the cutting edge.

ARL may also consider increasing the cooperation between intramural and extramural efforts including an enhancement of predefined processes to coordinate and utilize the results of extramural activities for internal ARL development. Finally, the intramural researchers may increase their focus on

___________________

¹⁵ From the materials presented at the July 18–20, 2023, panel meeting, an estimate of the number of extramural and intramural managers was determined to be approximately 105 individuals. This included 42 extramural researchers/principal investigators, 28 intramural researchers, 30 intramural managers (some of whom perform research and development), and 5 extramural managers.

publications and seeking awards as a means of making better connections to the broader research community.

Finally, with the proliferation of many wireless systems and radars, the electromagnetic spectrum has become very congested. Many communication systems and sensor receivers are prone to self-jamming (co-site interference) and jamming by the adversaries. Distributed, ultra-wide-band and multi-band systems with interferer cancelation capabilities and full-duplex operation could also be studied for both communication and radar systems. The application and improvement of low-frequency systems for communication in non-line-of-sight and urban scenarios as well as geolocation could also be reexamined. Another area that could be researched is under-utilized sub-millimeter-wave band systems. Such systems can provide very high bandwidth and secure communication as they can produce very narrow beams with relatively small apertures.