Angle-Selective Reflective Filters for Exclusion of Background Thermal Emission

Abstract

Selective filtering of spectral and angular optical transmission has recently attracted a great deal of interest. While optical passband and stopband spectral filters are already widely used, angular selective transmission and reflection filtering represents a less than fully explored alternative. Nonetheless, this approach could be promising for several applications, including stray radiation minimization and background emission exclusion. In this work, a concept for angle-selective reflection filtering using guided mode resonance coupling is proposed. Although guided mode resonance structures are already used for spectral filtering, in this work, a novel variation on angle-selective reflection filtering using guided mode resonance coupling is proposed. We investigate angle-dependent properties of such structures for potential use as angularly selective reflection filters. We utilize interference between diffraction modes to provide tunable selectivity with a sufficient angular width. Combining these structures with thermal emitters can exclude selected emission angles for spatially selective thermal emissivity reduction toward sensitive targets, as well as directionally selective emissivity exclusion for suppression of solar heating. We show a very large selective reduction of heat exchange by 99.77% between an engineered emitter and a distant receiver, using just a single groove grating and an emitting substrate in the emitter's side. Also, we show a selective reduction of heat exchange by approximately 77% between an emitter covered by engineered sets of angular selective reflection filters and a nearby sensitive target. The suggested angle-selective structure may have applications in excluding background thermal radiation: in particular, thermal emission reduction for daytime radiative cooling, sensitive IR telescope detectors, and high-fidelity thermoluminescent spectroscopy. I. INTRODUCTION Controlling the angular selectivity of optical transmission is a recently emerging branch of photonics, which has recently attracted a great deal of interest [1-5]. With recent advances in nanophotonics, broadband angular selectivity has recently been achieved in the laboratory. Some examples include microscale compound parabolic concentrators to limit the emission angle for solar cells [1,6], non-resonant Brewster modes in metallic gratings for angle-selective broadband absorption and selective thermal emission [7] and 1D photonic crystal heterostructures [8,9]. This approach can also allow for significant reduction of unwanted optical noise over a wide frequency range [4]. These examples show that selective angular transmission is well-established. However, a tunable angle-selective reflection peak has not been demonstrated yet. In fact, Babinet's principle indicates that it should generally be possible to achieve such a goal, through processes such as inversion [10]. Such an approach could be uniquely useful for elimination of unwanted optical components from a certain direction, for example to mitigate optical noise...

Nanophotonic engineering of far-field thermal emitters

“…For example, by patterning an array of antennas with a non-uniform scattering phase distribution on a SiC surface, free-space focusing of TE has been demonstrated [ Fig. 2(e)] 45 . Guided by a similar principle, a non-uniform metasurface composed of thermal emitters with varying angular emissivity patterns was shown to suppress TE into a certain area, forming a "dark spot" 51 . A recent theoretical study demonstrated TE from objects comprising an epsilon-near-zero material, enabled by the enhanced spatial coherence resulting from the stretching of the wavelength inside the epsilon-near-zero material 52 .…”

Thermophotovoltaics with spectral and angular selective doped-oxide thermal emitters

“…7 is a filter-dependent property, caused by Bragg reflection at different incident angles. In contrast to emissivity plots in [26], Fig. 7 illustrates a complementary response, where the high-emissivity is restricted to a specific range of angles and frequency, with frequency selectivity mainly provided by the filter.…”

“…7 illustrates a complementary response, where the high-emissivity is restricted to a specific range of angles and frequency, with frequency selectivity mainly provided by the filter. However, in [26] a low-emissivity response is restricted to a specific range of angles and frequencies, with the frequency selectivity provided by material absorption. Table. 1 and applying (6), (7) and (8).…”

“…For directional control of thermal emission, it can be modified using engineered structures like metallic gratings and metasurfaces [21,22]. Furthermore, combining spectral selectivity and angular selectivity has been also demonstrated using highly coherent plasmonic bandgap structures [23], or by coupling spectrally-selective emitters to angular selective surfaces [24][25][26]. For example, by coupling quantum dot emission modes to photonic crystals [24], coupling gap plasmon resonances to periodic metallic gratings [25], or coupling naturally-selective emitters to guided mode resonant filters [26].…”

Manipulating thermal emission with spatially static fluctuating fields in arbitrarily shaped epsilon-near-zero bodies

“…Thermal fields are inherently broadband and stochastic (8,9), making them challenging to manipulate via traditional optical techniques, which ultimately resort to interference phenomena. However, different photonic nanostructures have been successfully utilized to address this challenge, including corrugated surfaces supporting surface plasmons (1,9,10), photonic crystals (4,(11)(12)(13), resonators (14,15), metamaterials (16)(17)(18)(19), graphene (20), transformation optics (21), and angle-selective filters (22,23). In essence, these methodologies utilize structural degrees of freedom to either suppress (e.g., photonic bandgaps, angle-selective filters) or enhance (e.g., resonant cavities, corrugated surfaces) specific modes, thus leading to an enhanced spatial coherence that allows for a partial control of thermal fields.…”