arXiv subjects

Georgia T. Papadakis

Publications and source records attributed to Georgia T. Papadakis.

At least 19 recordsLinked to original sources

arXiv2026

Analytical Theory of Photon Tunneling and Near-Field Heat Transfer Between Dissimilar Materials

Near-field radiative heat transfer can exceed the blackbody limit through evanescent-mode coupling across nanoscale gaps. This enhancement underpins applications including thermophotovoltaic energy conversion, electroluminescent cooling, thermal rectification, and photon absorption in plasmon-assisted photodetection. These systems most often involve photon- or heat-exchange between dissimilar interfaces, particularly between a semiconductor and a metal. Despite the prevalence of this asymmetric configuration, no closed-form description of its near-field interaction exists. Here, we derive a closed-form analytical description of photon tunneling that clarifies the roles of material properties, namely the plasma frequency, optical loss, and semiconductor absorption, in the thermal exchange. We show that the dominant in-plane wave vector of the radiative heat transfer is an approximate average of the corresponding values for two symmetric reference systems: a plasmonic-plasmonic cavity and a semiconductor-semiconductor cavity. These results establish a compact analytical framework for near-field heat transfer between dissimilar materials.

physics.optics↗

arXiv2026

Far-field heat transfer and monochromatic thermal currents in a cylindrical nonreciprocal cavity

Breaking Kirchhoff's law of thermal radiation yields new opportunities in one-way radiative thermal transport and circuitry. We investigate its consequences in the far-field regime in cylindrical cavities, by employing a specular ray-tracing algorithm. At thermal equilibrium, we show that violation of Kirchhoff's law yields non-vanishing heat rectification coefficients within different sections of the cavity, which can be tuned for perfect rectification and circulation, while internal monochromatic currents vanish due to the intrinsic coupling between emission and absorption at specular surfaces. This constraint is lifted under nonequilibrium conditions, where rotational heat fluxes within the cavity can be precisely controlled by appropriately combining reciprocal and nonreciprocal materials. These findings open new avenues for thermal management and provide design principles for nonreciprocal photonic devices.

physics.optics↗

arXiv2025

The critical role of substrates in mitigating the power-efficiency trade-off in near-field thermophotovoltaics

Near-field thermophotovoltaic systems can achieve ultra-high power densities, however, this often comes at the cost of reduced efficiency. We show that this power-efficiency trade-off can be mitigated through substrate engineering. We exploit gradient-based optimization and show that thin lossless metallic films with plasma frequencies resonantly matched to the plasmonic emitter can yield high power and spectral efficiency by spectrally enhancing and confining radiative heat transfer to a narrow spectral range just above the photovoltaic bandgap. Compared to noble metals and air-bridged structures, designs deriving from such optimization yield more than an order-of-magnitude increase in radiative power density while maintaining high efficiency. Our results highlight the critical role of the substrate and the potential of substrate optimization for overcoming fundamental limitations of near-field thermophotovoltaic systems.

physics.optics↗

arXiv2025

Fundamental limitations of thermoradiative energy conversion

Understanding the fundamental limits of various energy conversion approaches is essential for assessing their efficiency and power output. In this work, we derive general performance bounds for thermoradiative heat engines that exchange heat radiatively with a cold sink, establishing power-versus-efficiency thermodynamic bounds for several configurations. We find that the performance of these engines is always bounded by that of radiative engines, which harness the thermal radiation emitted by a hot source, making thermoradiative engines inherently less favorable for energy conversion. By unifying the results of radiative and thermoradiative engines within a common thermodynamic framework, which also encompasses dual-engine configurations that combine both features, this work provides a comprehensive understanding of the thermodynamic limits of radiative energy conversion. Our framework offers general metrics for evaluating specific devices and raises critical questions regarding the relevance of thermoradiative cells for energy production.

physics.app-ph↗

arXiv2025

Mid-IR chirality and chiral thermal emission through twisting

Chirality in the mid-infrared spectral range plays a crucial role across physical, chemical, and biological sciences, yet sources of chiral infrared light do not currently exist. Their development, using principles from the mature field of metamaterials, requires complex three-dimensional architectures that call for high-resolution lithography. We leverage the natural optical anisotropy found in several van der Waals crystals, for example $α$-MoO$_3$, to demonstrate experimentally that its twisted bilayers break inversion-rotation symmetry and are thereby intrinsically chiral. Via direct thermal emission measurements of microscopic twisted bilayers, we demonstrate that these heterostructures generate chiral light through incandescence. Twisted configurations of van der Waals materials do not require any lithography, and offer a platform for large-scale chiral filters and thermal sources beyond conventional meta-architectures.

physics.optics↗

arXiv2024

Coherent Thermal Emission from Large-Scale Suspended Nanomechanical Membranes

Thermal radiation is an abundant form of incoherent light. Generating coherent infrared light through incandescence promises a cheap alternative to the costly and epitaxially complex quantum cascade laser, however it remains a fundamental challenge. Previous approaches leveraged the spatial coherence of polaritonic excitations that occur in the thermal near-field, by diffracting them into the far-field zone via patterned micro- or nano-scatterers. This approach requires high-resolution lithography, is difficult to scale-up, and yields limited outcoupled radiation due to the intrinsically polarized nature of polaritons. We overcome these limitations and report coherent thermal emission through simple wave interference. We show that unpatterned, millimeter-scale, suspended nanomechanical membranes of SiC operate for both linear polarizations and exhibit antenna-like directionality without relying on the excitation of near-field polaritons. The ability to generate polarization-insensitive, narrowband and spatially coherent incandescent light without lithography at large scales paves the way towards democratizing thermal infrared technologies.

physics.optics↗

arXiv2024

Design of narrowband infrared emitters by hybridizing guided-mode resonance structures with van der Waals materials

In this paper, narrowband emitters have been designed using particle swarm optimization (PSO) in the 10-20 μm infrared range. The device structure consists of an anisotropic α-MoO3 layer combined with the one- and two-dimensional guided-mode resonance structures. Well-defined absorption lines are present in the reflection spectrum for both TE and TM polarizations, thereby yielding narrowband emissivity at desired wavelengths. The band structure of the designed emitters under TM polarization demonstrates distinct features unlike its TE counterpart. These features are attributed to the interaction between guided-mode resonances and phonon polaritons. The results are relevant for applications in active and passive photonic elements in mid- and long-wave IR bands.

physics.optics↗

arXiv2023

Thermodynamic performance bounds for radiative heat engines

Heat engines cannot generally operate at maximum power and efficiency, imposing a trade-off between the two. Here, we highlight the exact nature of this trade-off for engines that exchange heat radiatively with a hot source. We derive simple analytical expressions for the performance bounds of reciprocal and nonreciprocal radiative heat engines. We also highlight that radiative engines can achieve a better power-efficiency trade-off than linear ones. These bounds are especially relevant for thermophotovoltaics, offering useful metrics against which to compare device performance.

physics.app-ph↗

arXiv2023

Hot-carrier thermophotovoltaic systems

A thermophotovoltaic (TPV) energy converter harnesses thermal photons emitted by a hot body and converts them to electricity. When the radiative heat exchange between the emitter and PV cell is spectrally monochromatic, the TPV system can approach the Carnot thermodynamic efficiency limit. Nonetheless, this occurs at the expense of vanishing extracted electrical power density. Conversely, a spectrally broadband radiative heat exchange between the emitter and the cell yields maximal TPV power density at the expense of low efficiency. By leveraging hot-carriers as a means to mitigate thermalization losses within the cell, we demonstrate that one can alleviate this trade-off between power density and efficiency. Via detailed balance analysis, we show analytically that one can reach near-Carnot conversion efficiencies close to the maximum power point, which is unattainable with conventional TPV systems. We derive analytical relations between intrinsic device parameters and performance metrics, which serve as design rules for hot-carrier-based TPV systems.

physics.app-ph↗

arXiv2023

Deep-subwavelength Phase Retarders at Mid-Infrared Frequencies with van der Waals Flakes

Phase retardation is a cornerstone of modern optics, yet, at mid-infrared (mid-IR) frequencies, it remains a major challenge due to the scarcity of simultaneously transparent and birefringent crystals. Most materials resonantly absorb due to lattice vibrations occurring at mid-IR frequencies, and natural birefringence is weak, calling for hundreds of microns to millimeters-thick phase retarders for sufficient polarization rotation. We demonstrate mid-IR phase retardation with flakes of $α$-molybdenum trioxide ($α$-MoO$_3$) that are more than ten times thinner than the operational wavelength, achieving 90 degrees polarization rotation within one micrometer of material. We report conversion ratios above 50% in reflection and transmission mode, and wavelength tunability by several micrometers. Our results showcase that exfoliated flakes of low-dimensional crystals can serve as a platform for mid-IR miniaturized integrated polarization control.

physics.optics↗

arXiv2023

Design rules for active control of narrowband thermal emission using phase-change materials

We propose an analytical framework to design actively tunable narrowband thermal emitters at infrared frequencies. We exemplify the proposed design rules using phase-change materials (PCM), considering dielectric-to-dielectric PCMs (e.g. GSST) and dielectric-to-metal PCMs (e.g. $\mathrm{VO_2}$). Based on these, we numerically illustrate near-unity ON-OFF switching and arbitrarily large spectral shifting between two emission wavelengths, respectively. The proposed systems are lithography-free and consist of one or several thin emitter layers, a spacer layer which includes the PCM, and a back reflector. Our model applies to normal incidence, though we show that the behavior is essentially angle-independent. The presented formalism is general and can be extended to \textit{any} mechanism that modifies the optical properties of a material, such as electrostatic gating or thermo-optical modulation.

physics.optics↗

arXiv2023

Dynamic modulation of thermal emission -- a Tutorial

Thermal emission is typically associated with a blackbody at a temperature above absolute zero, which exchanges energy with its environment in the form of radiation. Blackbody thermal emission is largely incoherent both spatially and temporally. Using principles in nanophotonics, thermal emission with characteristics that differ considerably from those of a blackbody have been demonstrated. In particular, by leveraging intrinsic properties of emerging materials or via nanostructuring at the wavelength or sub-wavelength scale, one can gain control over the directionality, temporal coherence, and other more exotic properties of thermal radiation. Typically, however, these are fixed at the time of fabrication. Gaining dynamic control of thermal emission requires exploiting external mechanisms that actively modulate radiative properties. Numerous applications can benefit from such thermal emission control, for example in solar energy harvesting, thermo-photovoltaic energy conversion, radiative cooling, sensing, spectroscopy, imaging and thermal camouflage. In this tutorial, we introduce thermal emission in two domains: the far-field, and the near-field, and we outline experimental approaches for probing thermal radiation in both ranges. We discuss ways for tailoring the spatial and temporal coherence of thermal emission and present available mechanisms to actively tune these characteristics.

physics.optics↗

arXiv2023

Perspective on Near-Field Radiative Heat Transfer

Although near-field radiative heat transfer was introduced in the 1950's, interest in the field has recently revived, as the effect promises improved performance in various applications where contactless temperature regulation in the small-scale is a requirement. With progress in computational electromagnetics as well as in nanoinstrumentation, it has become possible to simulate the effect in complex configurations and to measure it with high precision. In this Perspective, we highlight key theoretical and experimental advances in the field, and we discuss important developments in tailoring and enhancing near-field thermal emission and heat transfer. We discuss opportunities in heat-to-electricity energy conversion with thermophotovoltaic systems, as well as non-reciprocal heat transfer, as two of many recent focus topics in the field. Finally, we highlight key experimental challenges and opportunities with emerging materials, for probing near-field heat transfer for relevant technologies in the large-scale.

physics.optics↗

arXiv2023

Tight bounds and the role of optical loss in polariton-mediated near-field heat transfer

We introduce an analytical framework for near-field radiative heat transfer in bulk plasmonic and polar media. Considering material dispersion, we derive a closed-form expression for the radiative thermal conductance, which disentangles the role of optical loss from other material dispersion characteristics, such as the spectral width of the Reststrahlen band in polar dielectrics, as well as from the temperature. We provide a universal condition for maximizing heat transfer that defines the optimal interplay between a material's optical loss and polariton resonance frequency, based on which we introduce tight bounds to near-field heat transfer. With this formalism, one can quantitatively evaluate all polaritonic materials in terms of their performance as near-field thermal emitters.

physics.optics↗

arXiv2023

Maximal Violation of Kirchhoff's Law in Planar Heterostructures

Violating Kirchhoff's law has so far required nonreciprocal materials patterned in microstructures. In these configurations, the excitation of a guided or polaritonic mode that lies outside the light cone, often via gratings, was a requirement. Here, we describe how nonreciprocity manifests itself in pattern-free heterostructures. We demonstrate that a resonant mode in a dielectric spacer separating a nonreciprocal film from a back-reflector suffices to maximally violate Kirchhoff's law, and identify the minimal dielectric requirements for such functionality, which are satisfied by currently available materials.

physics.optics↗

arXiv2022

Lithography-free directional control of thermal emission

Blackbody thermal emission is spatially diffuse. Achieving highly directional thermal emission typically requires nanostructuring the surface of the thermally emissive medium. The most common configuration is a subwavelength grating that scatters surface polaritonic modes from the near-field to the far-field and produces antenna-like lobes of thermal emission. This concept, however, is typically limited to a particular linear polarization, and requires sophisticated lithography. Here, we revisit the simple motif of a planar Salisbury screen. We show analytically how the interplay between the real and imaginary part of the dielectric permittivity of the emitter layer defines the directional characteristics of emission, which can range from diffuse to highly directional. We propose a realistic configuration and show that hexagonal Boron Nitride thin films can enable grating-like thermal emission lobes in a lithography-free platform.

physics.optics↗

arXiv2021

Thermodynamics of light management in near-field thermophotovoltaics

We evaluate near-field thermophotovoltaic (TPV) energy conversion systems focusing in particular on their open-circuit voltage (Voc). Unlike previous analyses based largely on numerical simulations with fluctuational electrodynamics, here, we develop an analytic model that captures the physics of near-field TPV systems and can predict their performance metrics. Using our model, we identify two important opportunities of TPV systems operating in the near-field. First, we show analytically that enhancement of radiative recombination is a natural consequence of operating in the near-field. Second, we note that, owing to photon recycling and minimal radiation leakage in near-field operation, the PV cell used in near-field TPV systems can be much thinner compared to those used in solar PV systems. Since non-radiative recombination is a volumetric effect, use of a thinner cell reduces non-radiative losses per unit area. The combination of these two opportunities leads to increasingly large values of Voc as the TPV vacuum gap decreases. Hence, although operation in the near-field was previously perceived to be beneficial for electrical power density enhancement, here, we emphasize that thin-film near-field TPVs are also significantly advantageous in terms of Voc and consequently conversion efficiency as well as power density. We provide numerical results for an InAs-based thin-film TPV that exhibits efficiency > 50% at an emitter temperature as low as 1100 K.

physics.optics↗

arXiv2020

Deep subwavelength thermal switch via resonant coupling in monolayer hexagonal boron nitride

Unlike the electrical conductance that can be widely modulated within the same material even in deep nanoscale devices, tuning the thermal conductance within a single material system or nanostructure is extremely challenging and requires a large-scale device. This prohibits the realization of robust ON/OFF states in switching the flow of thermal currents. Here, we present the theory of a thermal switch based on resonant coupling of three photonic resonators, in analogy to the field-effect electronic transistor composed of a source, gate, and drain. As a material platform, we capitalize on the extreme tunability and low-loss resonances observed in the dielectric function of monolayer hexagonal boron nitride (hBN) under controlled strain. We derive the dielectric function of hBN from first principles, including the phonon-polariton linewidths computed by considering phonon isotope and anharmonic phonon-phonon scattering. Subsequently, we propose a strain-controlled hBN-based thermal switch that modulates thermal conductance by more than an order of magnitude, corresponding to an ON/OFF contrast ratio of 98%, in a deep subwavelength nanostructure.

cond-mat.mes-hall↗