Kaifeng Chen

Multiple reference Fourier transform holography with soft x rays

The authors demonstrate multiple reference source Fourier transform holography with soft x rays. This technique extends the detection limit of high resolution lensless imaging by introducing spatial multiplexing to coherent x-ray scattering. In this way, image quality is improved without increasing the radiation exposure to the sample. This technique is especially relevant for recording static images of radiation sensitive samples and for studying spatial dynamics with pulsed light sources. Applying their technique in the weak illumination limit they image a nanoscale test object by detecting ∼2500 photons. The observed enhancement in the signal-to-noise ratio of the image follows the square root of the number of reference sources.

Suppressing sub-bandgap phonon-polariton heat transfer in near-field thermophotovoltaic devices for waste heat recovery

We consider a near-field thermophotovoltaic device with metal as the emitter and semiconductor as the photovoltaic cell. We show that when the cell is a III-V semiconductor, such as GaSb, parasitic phonon-polariton heat transfer reduces efficiency in the near-field regime, especially when the temperature of the emitter is not high enough. We further propose ways to avoid the phonon-polariton heat transfer by replacing the III-V semiconductor with a non-polar semiconductor such as Ge. Our work provides practical guidance on the design of near-field thermophotovoltaic systems for efficient harvesting of low-quality waste heat.

High-performance near-field electroluminescent refrigeration device consisting of a GaAs light emitting diode and a Si photovoltaic cell

We consider a near-field electroluminescent refrigeration device. The device uses a GaAs light emitting diode as the cold side, and a Si photovoltaic cell as the hot side. The two sides are brought in close proximity to each other across a vacuum gap. The cooling is achieved by applying a positive bias on the GaAs light emitting diode. We show that the choice of GaAs and Si here can suppress the non-idealities for electroluminescent cooling purposes: GaAs has a wide bandgap with low Auger recombination, and Si is a non-polar semiconductor which leads to significantly reduced sub-bandgap heat transfer. We show that by using this configuration in the near-field regime, the cooling power density can reach 105 W/m2 even in the presence of realistic Auger recombination and Shockley-Read-Hall recombination. In addition, with photovoltaic power recovery from the Si cell, the efficiency of the device can be further improved. Our work points to the significant potential of combining near-field heat transfer with a...

Electroluminescent refrigeration by ultra-efficient GaAs light-emitting diodes

Electroluminescence—the conversion of electrons to photons in a light-emitting diode (LED)—can be used as a mechanism for refrigeration, provided that the LED has an exceptionally high quantum efficiency. We investigate the practical limits of present optoelectronic technology for cooling applications by optimizing a GaAs/GaInP double heterostructure LED. We develop a model of the design based on the physics of detailed balance and the methods of statistical ray optics, and predict an external luminescence efficiency of ηext = 97.7% at 263 K. To enhance the cooling coefficient of performance, we pair the refrigerated LED with a photovoltaic cell, which partially recovers the emitted optical energy as electricity. For applications near room temperature and moderate power densities (1.0–10 mW/cm2), we project that an electroluminescent refrigerator can operate with up to 1.7× the coefficient of performance of thermoelectric coolers with ZT = 1, using the material quality in existing GaAs devices. We also predict superior cooling efficiency for cryogenic applications relative to both thermoelectric and laser cooling. Large improvements to these results are possible with optoelectronic devices that asymptotically approach unity luminescence efficiency.

Near-field Thermophotonic Systems for Low-Grade Waste Heat Recovery

Low-grade waste heat contains an enormous amount of exergy that can be recovered for renewable-energy generation. Current solid-state techniques for recovering low-grade waste heat, such as thermoelectric generators and thermophotovoltaics, however, are limited by low conversion efficiencies or power densities. In this work, we propose a solid-state near-field thermophotonic system. The system consists of a light-emitting diode (LED) on the hot side and a photovoltaic (PV) cell on the cold side. Part of the generated power by the PV cell is used to positively bias the LED. When operating in the near-field regime, the system can have power density and conversion efficiency significantly exceeding the performance of current solid-state approaches for low-grade waste-heat recovery. For example, when the gap spacing is 10 nm and the hot side and cold side are, respectively, 600 and 300 K, we show that the generated electric power density and thermal-to-electrical conversion efficiency can reach 9.6 W/cm2 and 9.8%, respectively, significantly outperforming the current record-setting thermoelectric generators. We identify the alignment of the band gaps of the LED and the PV cell, the appropriate choice of thickness of the LED and PV cell to mitigate the effect of non-radiative recombination, and the use of highly reflective back mirrors as key factors that affect the performance of the system. Our work points to the significant potential of photonic systems for the recovery of low-grade waste heat.

MESH: A free electromagnetic solver for far-field and near-field radiative heat transfer for layered periodic structures

Abstract We describe MESH ( M ultilayer E lectromagnetic S olver for H eat transfer), a free software that combines rigorous coupled wave analysis (RCWA) and scattering matrix formalism to simulate the radiative heat transfer both in the near-field and far-field regimes for layered three-dimensional structures made of planar layers. Each layer can have in-plane one-dimensional or two-dimensional periodicity. In this paper, we provide detailed discussions of the algorithms of MESH, which enables it to be a flexible tool for different types of radiative heat transfer simulations. We also discuss aspects of the codes related to parallelization and user scripting. Program summary Program Title: MESH Program Files doi: http://dx.doi.org/10.17632/zx9v3bf3hf.1 Licensing provisions: GNU General Public License 3 (GPL) Programming language: C, C++. External routines: Lua[1], Python[2] and LAPACK and BLAS linear-algebra software[3], and optionally MPI message-passing interface[4]. Armadillo[5] is already contained in MESH. Nature of problem: Far-field and near-field radiative heat transfer in layered periodic structures. Solution method: Fourier modal method (rigorous coupled wave analysis) and the scattering matrix method. [1] R. Ierusalimschy, L.H. de Figueiredo, W.C. Filho, Lua an extensible extension language, Software: Practice and Experience 26 (1996) 635652. http://www.lua.org . [2] Python Software Foundation. Available at http://www.python.org [3] MKL: https://software.intel.com/en-us/intel-mkl [4] T.M. Forum, MPI: A Message Passing Interface, in: Supercomputing 93, Portland, OR, 878883, 1993 [5] Conrad Sanderson and Ryan Curtin. Armadillo: a template-based C++ library for linear algebra. Journal of Open Source Software, Vol. 1, pp. 26, 2016. http://dx.doi.org/10.21105/joss.00026

Passive cooling of solar cells with a comprehensive photonic approach

We present a comprehensive photonic approach for passive cooling of solar cells by simultaneously performing radiative cooling while also selectively utilizing the sunlight. We design a photonic cooler made of multilayer dielectric stack that can strongly radiate heat through its thermal radiation while also significantly reflecting the solar spectrum in sub-band gap and ultraviolet regime. We show that applying this photonic cooler on solar panel can cool the solar cell by over 5.6K in a typical terrestrial operating condition. Our technique points to an optimal photonic approach for passive cooling of solar cells and can be readily implemented as a retrofit for current photovoltaic modules to improve both efficiency and reliability.

Light-Emitting Diodes for Solid-State Refrigeration

Electroluminescence, the conversion of electrons into externally emitted photons, is an intrinsic cooling process in a light-emitting diode as long as the applied voltage is less than the photon energy. When the diode is sufficiently efficient so that cooling due to electroluminescence surpasses heating due to internal losses, it becomes a solid-state refrigerator. We present the theoretical performance limits of a solid-state refrigerator that combines an optimized GaAs light-emitting diode and a GaAs photovoltaic cell. We show that at moderate power densities, this optoelectronic refrigerator can outperform thermoelectric coolers in cooling efficiency and is also a viable technology for cryogenic cooling applications.