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1. Amplification of near field radiation at surfaces of pure dielectric domain with anti-reflection films and photonic crystal structures

   https://doi.org/10.1088/1361-6463/acc8e2  

   Wen, Sy-Bor; Jakkinapalli, Aravind (April 2023, Journal of Physics D: Applied Physics)

   Abstract With chemical stability under high temperatures, dielectric materials can be idealized thermal emitters for different energy applications. However, dielectric materials do not support surface waves at near-infrared ranges for longer-distance thermal photon tunneling, which limits their applications in near-field thermal radiation. It is demonstrated in this study that thermal field amplification at near-infrared wavelengths at dielectric surfaces could be achieved through asymmetric Fabry–Perot resonance with anti-reflection coatings or 1D photonic crystal type structures. ⩾100 nm near-infrared thermal photon tunneling can be achieved when these thin film structures are added to the emitter and the collector surfaces. Among these two thin film structures, 1D photonic crystal type periodic structures constructed with the same high refractive index material as the emitter/collector material allow near-field thermal photon tunneling at large parallel wavenumbers. Moreover, the field amplification can be increased by adding more 1D photonic crystal layers to achieve even longer distances near field thermal photon tunneling. 

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2. Large-area 1D selective emitter for thermophotovoltaic applications in the mid-infrared

   https://doi.org/10.1116/6.0002198  

   Oh, Minsu; Grossklaus, Kevin; Vandervelde, Thomas E. (January 2023, Journal of Vacuum Science & Technology B)

   Two- or three-dimensionally patterned subwavelength structures, also known as metamaterials, have the advantage of arbitrarily engineerable optical properties. In thermophotovoltaic (TPV) applications, metamaterials are commonly used to optimize the emitter’s radiation spectrum for various source temperatures. The output power of a TPV device is proportional to the photon flux, which is proportional to the emitter size. However, using 2D or 3D metamaterials imposes challenges to realizing large emitters since fabricating their subwavelength features typically involves complicated fabrication processes and is highly time-consuming. In this work, we demonstrate a large-area (78 cm 2 ) thermal emitter. This emitter is simply fabricated with one-dimensional layers of silicon (Si) and chromium (Cr), and therefore, it can be easily scaled up to even larger sizes. The emissivity spectrum of the emitter is measured at 802 K, targeting an emission peak in the mid-infrared. The emissivity peak is ∼0.84 at the wavelength of 3.75  μm with a 1.2  μm bandwidth. Moreover, the emission spectrum of our emitter can be tailored for various source temperatures by changing the Si thickness. Therefore, the results of this work can lead to enabling TPV applications with higher output power and lower fabrication cost. 

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3. Far-field thermal radiation from short-pitch silicon-carbide nanopillar arrays

   https://doi.org/10.1063/5.0109819  

   Pouria, Ramin; Chow, Philippe K.; Tiwald, Tom; Zare, Saman; Edalatpour, Sheila (September 2022, Applied Physics Letters)

   Silicon carbide (SiC) supports surface phonons in the infrared region of the electromagnetic spectrum where these modes can be thermally emitted. Additionally, the magnitude, spectrum, and direction of thermal radiation from SiC can be controlled by engineering this material at the sub-wavelength scale. For these reasons, SiC nanopillars are of high interest for thermal-radiation tuning. So far, theoretical and experimental studies of thermal emission from SiC nanopillars have been limited to long-pitch arrays with a microscale interpillar spacing. It is not clear how far-field thermal emission from SiC nanopillars is affected when the interparticle spacing reduces to the nanometer scale, where the near-field interaction between adjacent nanopillars arises and the array becomes zero order. In this Letter, we study physical mechanisms of far-field thermal radiation from zero-order arrays of silicon-carbide nanopillars with a nanoscale interpillar spacing. We show that the increased volume of thermal emitters and thermal radiation of the hybrid waveguide-surface-phonon-polariton mode from zero-order arrays increase the spectral emissivity of silicon carbide to values as large as 1 for a wide range of angles. The enhanced, dispersion-less thermal emission from a zero-order SiC array of nano-frustums with an optimized interspacing of 300 nm is experimentally demonstrated. Our study provides insight into thermal radiation from dense nanostructures and has significant implications for thermal management of electronic devices and energy harvesting applications. 

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4. Optical Dipole Structure and Orientation of GaN Defect Single-Photon Emitters

   https://doi.org/10.1021/acsphotonics.3c00917  

   Geng, Yifei; Jena, Debdeep; Fuchs, Gregory D.; Zipfel, Warren R.; Rana, Farhan (October 2023, ACS Photonics)

   GaN has recently been shown to host bright, photostable, defect single-photon emitters in the 600–700 nm wavelength range that are promising for quantum applications. The nature and origin of these defect emitters remain elusive. In this work, we study the optical dipole structures and orientations of these defect emitters using the defocused imaging technique. In this technique, the far-field radiation pattern of an emitter in the Fourier plane is imaged to obtain information about the structure of the optical dipole moment and its orientation in 3D. Our experimental results, backed by numerical simulations, show that these defect emitters in GaN exhibit a single dipole moment that is oriented almost perpendicular to the wurtzite crystal c-axis. Data collected from many different emitters show that the angular orientation of the dipole moment in the plane perpendicular to the c-axis exhibits a distribution that shows peaks centered at the angles corresponding to the nearest Ga–N bonds and also at the angles corresponding to the nearest Ga–Ga (or N–N) directions. Moreover, the in-plane angular distribution shows little difference among defect emitters with different emission wavelengths in the 600–700 nm range. Our work sheds light on the nature and origin of these GaN defect emitters. 

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5. Effects of Cascading Optical Processes: Part III. Impacts on Spectroscopic Measurements of Fluorescent Samples

   Wamsley, Max; Wathudura, Pathum; Nawalage, Samadhi; Zou, Shengli; Zhang, Dongmao (June 2023, Analytical chemistry)

   Cascading optical processes involve sequential photon–matter interactions triggered by the same individual excitation photons. Parts I and II of this series explored cascading optical processes in scattering-only solutions (Part I) and solutions with light scatterers and absorbers but no emitters (Part II). The current work (Part III) focuses on the effects of cascading optical processes on spectroscopic measurements of fluorescent samples. Four types of samples were examined: (1) eosin Y (EOY), an absorber and emitter; (2) EOY mixed with plain polystyrene nanoparticles (PSNPs), which are pure scatterers; (3) EOY mixed with dyed PSNPs, which scatter and absorb light but do not emit; and (4) fluorescent PSNPs that are simultaneous light absorbers, scatterers, and emitters. Interference from both forward scattered and emitted photons can cause nonlinearity and spectral distortion in UV–vis extinction measurements. Sample absorption by nonfluorogenic chromophores reduces fluorescence intensity, while the effect of scattering on fluorophore fluorescence is complicated by several competing factors. A revised first-principles model is developed for correlating the experimental fluorescence intensity with the sample absorbance in solutions containing both scatterers and absorbers. The optical properties of fluorescent PSNPs of three different sizes were systematically investigated by using integrating-sphere-assisted resonance synchronous spectroscopy, linearly polarized resonance synchronous spectroscopy, UV–vis, and fluorescence spectroscopy. The insights and methodology provided in this work should help improve the reliability of spectroscopic analyses of fluorescent samples, where the interplay among light absorption, scattering, and emission can be complex. 

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