arXiv subjects

Julien Legendre

Publications and source records attributed to Julien Legendre.

7 recordsLinked to original sources

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

Performances of far and near-field thermophotonic refrigeration devices from the detailed-balance approach

We study a near-field thermophotonic (NF-TPX) refrigerating device, consisting of a lightemitting diode and a photovoltaic cell in close proximity. Calculations are performed in the frame of the detailed-balance approach. We study how thermal radiation, separation distance and LED temperature can affect both cooling power and coefficient of performance. More specifically, we assess the impact of bandgap energy and external quantum efficiency for an artificial material on those cooling performances. For a particular device made of GaAs and/or AlGaAs we show that, in the near-field regime, the cooling power can be increased by one order of magnitude compared to far field. However, a 10% reduction of the external quantum efficiency can lead to a decrease of the cooling power by two orders of magnitude. Finally, we compare existing literature data on electroluminescent, TPX and thermoelectric cooling with our detailed balance prediction, which highlights design-rule requirements for NF-TPX cooling devices.

physics.app-ph↗

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

Operating conditions and thermodynamic bounds of dual radiative heat engines

We propose a unified description of dual radiative heat engines (RHEs), consisting of two facing optoelectronic components (diodes) and capable of generating electrical power from heat. They can operate in three regimes depending on the applied biases, namely in thermoradiative-negative electroluminescent (TRNEL), thermoradiative-photovoltaic (TRPV) or thermophotonic regimes (TPX, consisting of a light-emitting diode and a PV cell). They have access to operating conditions that are unachievable by single RHEs such as thermophotovoltaic systems: at the radiative limit, TRNEL devices can reach the Carnot efficiency for any bandgap, while TPX devices achieve large power outputs by means of electroluminescent enhancement. Expressions of the maximum power output and related efficiency achieved by dual engines are derived analytically, and reveal that the power output of TPX engines is not bounded. A comparison to usual efficiency bounds also highlights the impact of thermalisation losses, and the subsequent interest of spectral filtering to extend the operating region. The influence of nonradiative recombinations is also investigated. This work provides common framework and guidelines for the study of RHEs, which represent a promising solution for reliable and scalable energy conversion.

physics.app-ph↗

arXiv2023

Overcoming non-radiative losses with AlGaAs PIN junctions for near-field thermophotonic energy harvesting

In a thermophotonic device used in an energy-harvesting configuration, a hot light-emitting diode (LED) is coupled to a photovoltaic (PV) cell by means of electroluminescent radiation in order to produce electrical power. Using fluctuational electrodynamics and the drift-diffusion equations, we optimise a device made of an AlGaAs PIN LED and a GaAs PIN PV cell with matched bandgaps. We find that the LED can work as an efficient heat pump only in the near field, where radiative heat transfer is increased by wave tunnelling. A key reason is that non-radiative recombination rates are reduced compared to radiative ones in this regime. At 10 nm gap distance and for 100 cm.s --1 effective surface recombination velocity, the power output can reach 2.2 W.cm --2 for a 600 K LED, which highlights the potential for low-grade energy harvesting.

physics.class-ph↗

arXiv2022

GaAs-Based Near-Field Thermophotonic Devices: Approaching The Idealized Case With One-Dimensional PN Junctions

Thermophotonics (TPX) is a technology close to thermophotovoltaics (TPV), where a heated light-emitting diode (LED) is used as the active thermal emitter of the system. It allows to tune the heat flux, by means of electroluminescence, to a spectral range matching better the gap of a photovoltaic cell. The concept is extended to near-field thermophotonics (NF-TPX), where enhanced energy conversion is due to both electric control and wave tunneling. We perform a thorough numerical analysis of a GaAs-based NF-TPX device, by coupling a near-field radiative heat transfer solver based on fluctuational electrodynamics with an algorithm based on a simplified version of the drift-diffusion equations in 1D. This allows for the investigation of the emission and absorption profiles in the LED and the photovoltaic (PV) cell, and for the scrutiny of the impact of key parameters. We also demonstrate that the performance obtained with this algorithm can approach idealized cases for improved devices. For the considered simplified architecture and 300 K temperature difference, we find a power density output of 1 W.cm--2 , underlining the potential for waste heat harvesting close to ambient temperature.

physics.app-ph↗