arXiv subjects

Daniel Cui

Publications and source records attributed to Daniel Cui.

9 recordsLinked to original sources

arXiv2026

Experimental demonstration of dual-polarization nonreciprocal thermal emission at moderate magnetic fields

Conventional thermal emitters obey Kirchhoff's law of thermal radiation, which requires emissivity and absorptivity to be equal at each wavelength, angle, and polarization. This in turn places a fundamental limit on a range of radiative energy technologies as well as heat transfer. This constraint can however be overcome using magneto-optic semiconductors, which break Lorentz reciprocity under an applied magnetic field. However, in most prior experimental demonstrations, nonreciprocity has been restricted to TM polarization, since the electric field of TE-polarized light is parallel to the applied magnetic field. Here, we experimentally demonstrate nonreciprocal absorption in both polarizations at the same wavelengths and angles from a single structure, wherein a germanium Mie-resonant metasurface on a doped InAs layer scatters TE-polarized light into near fields with components perpendicular to the magnetic field, allowing the magneto-optic response of the InAs to act on them. Under a 0.48 T field from a permanent Halbach array, this structure exhibits a difference in absorptivity between $+B$ and $-B$ of 0.035 in both TE and TM polarization at 16.6 $μ$m. Our approach shows that the magneto-optic response of a planar semiconductor and the resonances of a dielectric metasurface can be designed together, offering a potential route to dual-polarization nonreciprocity at fields available from permanent magnets for emerging energy and heat transfer technologies.

physics.optics↗

arXiv2026

Nonreciprocal magneto-optic beam steering of thermal emission

Conventional thermal emitters obey Kirchhoff's law of thermal radiation, which ties absorptivity to emissivity and places a fundamental limit on a range of radiative energy technologies and sensing systems. This constraint can be overcome using magneto-optic materials, which exhibit nonreciprocal thermal emission under an applied magnetic field. While the spectral nonreciprocity of magneto-optic devices has been explored theoretically and experimentally, nonreciprocal control over the direction of thermal emission has received far less attention. Here, we propose and numerically demonstrate nonreciprocal beam steering of thermal emission, wherein an applied magnetic field lifts the degeneracy between guided modes at +/- kx, enabling asymmetric and tunable directional thermal emission. We highlight a III-V semiconductor photonic structure operating at 24.5 um, a wavelength with few laser sources. This structure exhibits a nonreciprocity greater than 0.5 between +/- 75 degrees at 50 mT, a peak emissivity asymmetry over 0.6 at 150 mT, and 22 degrees of beam steering between 0 and 1 T. Our approach provides a solid-state means of steering a thermal beam, offering a potential route to magnetically controlled beam scanning of incandescent sources.

physics.optics↗

arXiv2026

Magneto-optic perturbation theory for near-complete violation of Kirchhoff's law of thermal emission at low magnetic fields

Magneto-optic photonic systems can violate Kirchhoff's law of thermal emission by breaking Lorentz reciprocity. We develop a dispersive perturbation theory yielding an analytical expression for magneto-optical resonance frequency shifts in plasmonic semiconductors under applied magnetic fields. This expression shows the shift is governed by the overlap of the mode's optical spin density with the magneto-optical material. We use this expression to design a III-V metasurface that achieves nonreciprocal emissivity contrast of 0.8 at only 0.1 T, and demonstrate that the theory can explain order-of magnitude differences in magnetic field sensitivity between different photonic structures.

physics.optics↗

arXiv2026

Nonreciprocal Thermophotonic Cooling

Solid-state cooling via electroluminescent emission from light-emitting diodes is a promising alternative to thermoelectric and vapor-compression refrigeration, but practical performance remains limited by nonradiative losses and unfavorable tradeoffs between efficiency and cooling power. Thermophotonic (TPX) architectures partially address this by recycling PV-generated power back to the LED, improving the coefficient of performance (COP) but introducing a parasitic backward photon flux from the PV that reduces the cooling power density. Here we show that this tradeoff can be circumvented by inserting a nonreciprocal semi-transparent intermediate layer that violates Kirchhoff's law of thermal radiation. The layer permits unity transmission from the LED to the PV while fully absorbing the backward PV flux, functioning as a radiative heat shield that re-emits toward the LED at a lower intermediate temperature. In the idealized limit for $Δ$ T = 50 K between the hot and cold side, the nonreciprocal filter improves the cooling power density by nearly an order of magnitude over the unfiltered TPX case while preserving the COP benefit, while a reciprocal filter provides no improvement. Incorporating Shockley-Read-Hall and Auger recombination into GaAs and InP-based LED device models, we find enhancements of approximately 50% in both cooling power density and COP persisting across temperature differences from $Δ$ T = 50 K to 100 K. These results highlight the potential importance of electromagnetic nonreciprocity in improving the real-world performance of thermophotonic cooling devices.

physics.app-ph↗

arXiv2026

Quasi-bandgap behavior in non-Hermitian photonic crystals

We investigate non-Hermitian photonic crystals in which the lossy and lossless constituents share the same real permittivity and differ only in their imaginary part. We characterize the complex band structure and reflection response of both one-dimensional (1D) and two-dimensional (2D) systems, and show that introducing even a small amount of material loss opens a quasi bandgap at the Brillouin-zone boundary. This quasi bandgap, absent in the lossless limit of the same structure, gives rise to sharp reflectivity peaks whose origin we explain through second-order perturbation theory. As an application of this behavior, we demonstrate a selective reflector combining a conventional photonic-crystal waveguide with a non-Hermitian photonic crystal, achieving wavelength-selective reflection with broadband absorption.

physics.optics↗

arXiv2025

Loss-driven gain enhancements driven by topological singularities in non-Hermitian photonic crystals defects

We show that purely lossy defects in one- and two-dimensional non-Hermitian photonic crystals can induce transmission matrix singularities not accessible with lossless defects. These singularities in turn can enable dramatic enhancement in overall system gain not accessible through conventional means. We further show that the underlying mechanism behind the loss-induced gain enhancement is due to the resonances being located specifically at topological branch cut singularities in the reflection coefficient with nontrivial winding numbers. The resulting resonances can exhibit exceptionally high quality factors in excess of $\sim 10^4$. Our work highlights the counterintuitive role of loss in engineering singularities in the gain response in non-Hermitian systems and its connection to topological phenomena in photonic systems.

physics.optics↗

arXiv2025

Directional Thermal Emission Across Both Polarizations in Planar Photonic Architectures

Directional and spectral control of thermal emission is essential for applications in energy conversion, imaging, and sensing. Existing planar, lithography-free epsilon-near-zero (ENZ) films only support transverse-magnetic (TM) control of thermal emission via the Berreman mode and cannot address transverse-electric (TE) waves due to the absence of natural optical magnetism over optical and infrared wavelengths Here, we introduce a hyperbolic metamaterial comprising alternating layers of degenerately-doped and intrinsic InAs that exhibits an epsilon-and-mu-near-zero (EMNZ) response, enabling dual-polarized, directionally and spectrally selective thermal emission. We first theoretically demonstrate that a mu-near-zero (MNZ) film on a perfect magnetic conductor supports a magnetic Berreman mode, absorbing TE-polarized radiation in analogy to the conventional Berreman mode supported in TM polarization. Using genetic and gradient-descent optimization, we design a dual-polarized emitter with independently tunable spectral peaks and emission angles. Parameter retrieval via homogenization confirms simultaneous EMNZ points at the target wavelengths and angles. Finally, experimental measurement of a sample fabricated via molecular beam epitaxy exhibits high absorptivity peaks for both polarizations in close agreement with simulations. This work realizes lithography-free, dual-polarized, spectrally and directionally selective emitters, offering a versatile platform for advanced infrared thermal management and device integration.

cond-mat.mtrl-sci↗

arXiv2024

Chip-scale monolithic optoelectronic voltage boost conversion

Voltage conversion is a fundamental electronic process critical to engineered systems across a wide spectrum of applications and spanning many orders of magnitude in scale. Conventional approaches like transformers and charge pumps perform well in specific contexts but face fundamental limitations to miniaturization, electromagnetic interference, and voltage range. Here we present a chip-scale, fully integrated monolithic, non-switching optoelectronic voltage conversion platform capable of high gain, bootstrap-free boost-mode operation across several orders of magnitude in power density and voltage scale. Using the bidirectional coupling between LEDs and PV cells with identical active layer materials, our chip-scale, single-die strategy eliminates Stokes losses while improving key parameters like physical footprint, series resistance, and photon leakage by orders of magnitude over implementations using multiple packaged, discrete components. Moreover, by exploiting the large étendue of NIR-transparent semi-insulating InP substrates and the atomically smooth, void-free interface of lattice-matched epitaxial growth, simulations indicate that our InGaAsP architecture's photon transport simultaneously provides a > 60x increase in current density and > 50x reduction in non-radiative recombination losses compared with a multiple-die solution while simultaneously reducing fabrication complexity and improving mechanical robustness. We experimentally demonstrate a boost gain of 3.8x in an 8x8 mm$^2$ InGaAs-on-InP chip while validating key aspects of the voltage conversion platform.

physics.optics↗

arXiv2018

Optical image-based thickness characterization of atomically thin nanomaterials using computer vision techniques

The main objective of this study was to develop a novel method of characterizing nanomaterials based on the number of layers without the aid of state-of-the-art electron and force microscopes. While previous research groups have attempted to establish a correlation between optical image contrast and layer number for inferring layer numbers of nanomaterials with already well-known software such as ImageJ and Gwyddion SPM Analysis, the work for this study strived to automate the image contrast-based characterization of the layer numbers using computer vision algorithms. After acquiring the necessary data points using graphene samples from another study and nanoscale MoTe2 samples through an experimental method consisted of using both ImageJ and Gwyddion, curve fitting in RStudio was used to create quadratic models that were incorporated in a computer vision method composed of three algorithms. In total, 12 MoTe2 and 16 graphene samples were used in 28 test trials in order to determine the efficiencies of the algorithms in layer number characterization. Ultimately, a success rate of 89% was obtained with an average overall run time of 15 seconds for the computer vision method. As a consequence, this computational method may be faster, more effective, and more cost-effective than current widely used electron, atomic force, and optical microscopy techniques.

physics.app-ph↗