RT&L FOCUS AREA(S): General Warfighting Requirements TECHNOLOGY AREA(S): Electronics OBJECTIVE: Develop midinfrared light emitting diodes (LEDs) with optical cavities electromagnetically engineered at a subwavelength scale to enhance wall-plug efficiency and brightness of devices beyond the current state-of-the-art technology and to demonstrate multipixel mid-infrared LED arrays. DESCRIPTION: Efficient light-emitting diodes (LEDs) operating in the midinfrared spectral range, that includes both mid-wave infrared (MWIR) (3-5 micron) and long-wave infrared (LWIR) (8-12 micron) wavelength regions, are highly desired for the use in systems for infrared scene projection (IRSP), chemical sensing, and spectroscopy. Hardware-in-the-loop (HITL) testing of infrared (IR) guided weapons necessitates infrared imagery to provide target signatures with high fidelity in a simulated environment with sufficient brightness. The capability to engage IR weapon and aircraft sensors and seekers with high-brightness, high-definition imagery of targets and backgrounds in HITL simulation is essential in the test and evaluation of the systems, such as threat detection and missile warning systems. Current IR scene projectors based on resistive emitter array technology have performance shortcomings such as low output radiance, slow frame rates, and small frame size. Compared to thermal sources, midinfrared LEDs can offer substantially higher radiance, modulation speeds, and significantly larger frame size over existing technologies. However, current devices are still highly inefficient. External wall-plug efficiency of state-of-the-art MWIR LEDs is currently below 0.5% at room temperature [Refs 1, 2] and that of LWIR LEDs is at least one order of magnitude lower [Ref 3]. Low wall-plug efficiency leads to low brightness of MWIR and LWIR LED systems. Low efficiency and brightness of midinfrared LEDs primarily result from a combination of low internal quantum efficiency (IQE) of light generation and low light extraction efficiency. IQE is limited by rapid non-radiative carrier recombination, which is dominated by strong Auger recombination at high pump currents [Ref 4]. As a result, IQE is estimated to be approximately 10% in the state-of-the-art MWIR LEDs operating around 3 microns [Ref 2] and drops quickly at longer wavelengths. For LWIR LEDs, IQE is well below 1% [Ref 4]. Furthermore, mid-infrared LEDs suffer from low extraction efficiency at 2% resulting from a narrow total internal reflection cone in LED materials [Ref 1, 2]. Parasitic voltage drops in the semiconductor heterostructure also have a negative effect on the midinfrared LED efficiency, although this factor has relatively minor effect compared to the two factors mentioned above [Refs 2, 3]. Electromagnetic engineering of LED optical cavities at a subwavelength scale can dramatically enhance light emission in the near- and far-infrared bands [Refs 5, 6, 7]. Subwavelength LED cavities can produce strong Purcell enhancement of spontaneous emission rates, which leads to drastic improvements in IQE, and enables optimal radiative emission rates of the photons in the cavity mode to free space, which improves output efficiency [Refs 5, 6, 7]. This SBIR topic seeks to investigate if similar approaches may dramatically enhance midinfrared LED efficiency and to demonstrate high-performance midinfrared LED arrays based on this technology. Proposed approaches should design, fabricate, and characterize midinfrared LEDs with optical cavities electromagnetically engineered at a subwavelength scale to enhance wall-plug efficiency and brightness of devices beyond the current state of the art. The threshold and final objective wall-plug efficiencies of this MWIR LED arrays are 10% and 15%, respectively. Multipixel LED arrays based on this technology for high-fidelity, HITL testing should be demonstrated. PHASE I: Design, develop, and demonstrate the feasibility of brightness and wall-plug efficiency enhancement of midinfrared LEDs using subwavelength optical cavity structuring to enhance spontaneous light emission rates into the LED material and out-coupling rates of light from the LED material to free space. The Phase I effort will include prototype plans to be developed in Phase II. PHASE II: Fabricate and characterize a single element midinfrared LED prototype, with wall-plug efficiency at room temperature. Based on the new LED geometry, demonstrate feasibility of fabricating multipixel LED arrays. Fabricate and completely characterize the prototype with a 64x64 pixel addressable LED array. Prepare a report that summarizes the experimental evaluation and validation of performance characteristics of the developed system. PHASE III DUAL USE APPLICATIONS: Fully develop and transition a 512x512 pixel addressable LED array-based dynamic IR scene projector per specifications based on the research and development of results developed during Phase II for DoD applications. This type of high brightness, high-fidelity infrared scene projectors can be used as HITL testing of thermal imaging cameras used by firefighters. In direct projection, images are projected directly into the camera; in indirect projection, images are projected onto a diffuse screen, which is then viewed by the camera. These high performance LED-based scene projectors can also be used in virtual reality for testing of IR search, track and rescue operations systems, and calibration for any spectrally sensitive IR remote sensing instrument. REFERENCES: Meyer, R.J.; Bewley, W.W.; Merritt, C.D.; Kim, M.; Kim, C.S.; Warren, M.V.; Canedy, C.L. and Vurgaftman, I. Mid-infrared interband cascade light-emitting devices with improved radiance [Paper presentation]. Proceedings of the SPIE OPTO: Quantum Sensing and Nano Electronics and Photonics XV, San Francisco, CA, United States, 10540, 1054009-1, January 27-February 1, 2018. https://doi.org/10.1117/12.2288009 Ermolaev, M.; Lin, Y.; Shterengas, L.; Hosoda, T.; Kipshidze, G.; Suchalkin, S. and Belenky, G. GaSb-Based Type-I Quantum Well 3 3.5- m Cascade Light Emitting Diodes. IEEE Photonics Technology Letters, 30(9), May 1, 2018, pp. 869-872. https://doi.org/10.1109/LPT.2018.2822621 Das, N.C.; Bradshaw, J.; Towner, F. and Leavitt, R. Long-wave (10 m) infrared light emitting diode device performance. Solid-State Electonics, 52(11), November 2008, pp. 1821-1824. https://doi.org/10.1016/j.sse.2008.09.003 Krier, A. Physics and technology of mid-infrared light emitting diodes. Philosophical Transactions of the Royal Society A, 359(1780), March 15, 2001, pp. 599-618. https://doi.org/10.1098/rsta.2000.0745 Hoang, T.B.; Akselrod, G.M.; Argyropoulos, C.; Huang, J.; Smith, D.R. and Mikkelsen, M.H. Ultrafast spontaneous emission source using plasmonic nanoantennas. Nature Communications, 6, 7788, July 27, 2015. https://doi.org/10.1038/ncomms8788 Akselrod, G.M.; Argyropoulos, C.; Hoang, T.B.; Cirac , C.; Fang, C.; Huang, J.; Smith, D.R. and Mikkelsen, M.H. Probing the mechanisms of large Purcell enhancement in plasmonic nanoantennas. Nature Photonics, 8, October 14, 2014, pp. 835-840. https://www.nature.com/articles/nphoton.2014.228 Madeo, J.; Todorov, Y.; Gilman, A.; Frucci, G.; Li, L.H.; Davies, A.G.; Linfield, E.H.; Sirtori, C. and Dani, K.M. Patch antenna microcavity terahertz sources with enhanced emission. Applied Physics Letters, 109(14), 141103, October 4, 2016. https://doi.org/10.1063/1.4963891
