Exploiting Zone-Folding Induced Quasi-Bound Modes to Achieve Highly Coherent Thermal Emissions

Abstract

Thermal emissions with high coherence, although not as high as those of lasers, still play a crucial role in many practical applications. In this work, by exploiting the geometric perturbation-induced optical lattice tripling and the associated Brillion zone folding effect, we propose and investigate thermal emissions in the mid-infrared with simultaneous high temporal and spatial coherence. In contrast with the case of period-doubling perturbation in our previous work, the steeper part of the guided mode dispersion band will be folded to the high-symmetry Γ point in the ternary grating. In this case, a specific emission wavelength corresponds to only a very small range of wavevectors. Consequently, apart from the high temporal coherence characterized by an experimental bandwidth around 30 nm, the achieved thermal emissions also feature ultrahigh spatial coherence. Calculations show that at the thermal emission wavelengths in the mid-infrared, the spatial coherence length can easily reach up to mm scale.

Ultra-narrowband and rainbow-free mid-infrared thermal emitters enabled by a flat band design in distorted photonic lattices

“…In general, both intrinsic (by breaking the structural symmetry) and extrinsic (by changing the excitation angle) perturbations can transform BICs into quasi-BICs (QBICs) 31 , enabling the realization of ultra-narrow resonance linewidths for practical applications. QGMs originate from folding of the dispersion diagram of broadband infinite-Q guided modes (GMs) to the continuum when the period-increasing perturbation is introduced into the photonic lattice 27 , 32 . As an exceptional type of guided-mode resonance (GMR), the QGMs not only exhibit perturbation-dependent Q-factors but also advantageously feature high robustness of the Q-factor against the frequency/wavenumber over the QBICs 33 .…”

“…If one aspires for a high spatial coherence of the thermal emission, a steep dispersion is desired. In that case, one frequency only corresponds to few spatial/Fourier components, which will work in phase to offer a long spatial coherence length 32 . One can even manipulate the FBZ folding to make the thermal emitter operating at a steeper part of the dispersion band in order to achieve a higher spatial coherence.…”