Shawn-Yu Lin

In this paper, a new method is proposed to achieve wide-angle and efficient coupling of light into a thin film, holographic optical element (HOE). The coupling is based on diffraction of light (the probe beam) by a holographic element. Analytically, we find, when the probe beam follows a specific angular orbit, Bragg conditions for diffraction can be satisfied and the maximum diffraction efficiency expected. As an experimental test of the proposed idea, such a HOE device has been successfully realized. Our result indicates that a highly efficient diffraction (> 80%) can be achieved for a broad range of light incident angle when the angle is along the designed angular orbit. This finding would open up a new possibility for concentrated solar panel application that is both efficient, low cost, and for a broad incident angle of sunlight.

A recent discovery of super-Planckian radiation in the far-field has attracted a great deal of interest in thermal radiation control using photonic crystals (PC) architecture. The finding is based on a careful comparison of radiation intensity emitted from a PC nano-filament and a blackbody at the same temperature and same wavelength at λ∼1.7 μm. Extending that finding, we determined the absolute spectral radiance of such a PC filament at λ∼1.7 μm at elevated temperature, T = 445–723 ^° K, and compared its value to that predicted by Planck's blackbody radiation law. Our data confirmed that the PC spectral radiance surpassed that of a blackbody by an order-of-magnitude. We also determined the spectral emissivity of our PC filament and showed that its value far exceeded unity at resonance and fell below unity away from resonance. The observation of non-equality between absorptivity and emissivity suggests that a strong optical non-reciprocity takes place in our PC nano-filament.

A highly dispersive, diffractive optical element is designed and realized for an extremely high spectral resolution spectroscopy for exoplanet telescope application. Our design uses an annular Fresnel hologram to transform incident starlight directly into a spectrogram. The recording of the hologram is accomplished using two spherical waves of different radius of curvature. The resultant hologram consists of an annular grating structure with a gradually shrinking period as a function of increasing radius. The variable period not only could bring the incoming star-light into focus, but also exhibits a large on-axis chromatic behavior. We demonstrate a dispersion of wavelength 430–700 nm over 190 mm on-axis distance, leading to a super fine spectral resolution 0.0266 nm at wavelength 515 nm for a detector size of 20 µm.

The tantalizing possibility of 31% solar-to-electric power conversion efficiency in thin film crystalline silicon solar cell architectures relies essentially on solar absorption well beyond the Lambertian light trapping limit (Bhattacharya and John in Nat Sci Rep 9:12482, 2019). Up to now, no solar cell architecture has exhibited above-Lambertian solar absorption, integrated over the broad solar spectrum. In this work, we experimentally demonstrate two types of photonic crystal (PhC) solar cells architectures that exceed Lambertian light absorption, integrated over the entire 300–1,200 nm wavelength band. These measurements confirm theoretically predicted wave-interference-based optical resonances associated with long lifetime, slow-light modes and parallel-to-interface refraction. These phenomena are beyond the realm of ray optics. Using two types of 10-μm thick PhC’s, first an Inverted Pyramid PhC with lattice constant a = 2,500 nm and second a Teepee PhC with a = 1,200 nm, we observe solar absorption well beyond the Lambertian limit over λ = 950–1,200 nm. Our absorption measurements correspond to the maximum-achievable-photocurrent-density (MAPD), under AM1.5G illumination at 4-degree incident angle, 41.29 and 41.52 mA/cm² for the Inverted Pyramid and Teepee PhC, respectively, in agreement with wave-optics, numerical simulations. Both of these values exceed the MAPD (= 39.63 mA/cm²) corresponding to the Lambertian limit for a 10-μm thick silicon for solar absorption over the 300–1,200 nm band.

Planck’s law predicts the distribution of radiation energy, color and intensity, emitted from a hot object at thermal equilibrium. The Law also sets the upper limit of radiation intensity, the blackbody limit. Recent experiments reveal that micro-structured tungsten can exhibit significant deviation from the blackbody spectrum. However, whether thermal radiation with weak non-equilibrium pumping can exceed the blackbody limit in the far field remains un-answered experimentally. Here, we compare thermal radiation from a micro-cavity/tungsten photonic crystal (W-PC) and a blackbody, which are both measured from the same sample and alsoin-situ. We show that thermal radiation can exceed the blackbody limit by >8 times at λ = 1.7 μm resonant wavelength in the far-field. Our observation is consistent with a recent calculation by Wang and John performed for a 2D W-PC filament. This finding is attributed to non-equilibrium excitation of localized surface plasmon resonances coupled to nonlinear oscillators and the propagation of the electromagnetic waves through non-linear Bloch waves of the W-PC structure. This discovery could help create super-intense narrow band thermal light sources and even an infrared emitter with a laser-like input-output characteristic.

In this paper, we report a direct imaging of narrow-band super Planckian thermal radiation in the far field, emitted from a resonant-cavity/tungsten photonic crystal (cavity/W-PC). A spectroscopic study of the cavity/W-PC shows a distinct resonant peak at λ ~ 1.7 μm. Furthermore, an infrared CCD camera was used to record radiation image of the cavity/W-PC and a carbon-nanotube (CNT) black reference at λ ~ 1.7 μm emitted from the same sample. The recorded image displays a higher brightness emitted from the cavity/W-PC region than from the blackbody region for all temperatures tested, T = 530-650 K. This observation is in sharp contrast to the common understanding of equilibrium thermal radiation, namely, a blackbody has a unit absorptance, a unity emittance and should emits the strongest radiation. Since the image was taken from the same sample and the temperature difference across the W-PC/ CNT boundary is less than 0.1 K, the observed image contrast gives a truly convincing evidence of super Planckian behavior in our sample. The discovery of a super-intense, narrow band radiation from a heated W-PC could open up a new door for realizing narrow band infrared emitters. The W-PC filament could also be very useful for efficient energy applications such as thermo-photovoltaics, waste heat recycling and radiative cooling.

In this work, a novel lithographic method is proposed to prepare three-dimensional (3D) photonic crystal (PC) that is different from conventional top-down and bottom-up approaches. The method combines a 2D optical mask and off-the-axis double optical exposures to create a desirable 3D PC structure. Since the method uses only two optical exposures of a photo-resist layer, it is inherently a low-cost, high throughput and wafer-scale lithographic method. The method is implemented to make a slanted post 3D PC having the SP2 lattice symmetry. Three types of SP2 3D PC structures were successfully fabricated with a minimum feature size of d=1.5 μm over a large scale of 8x10 mm², without any observable fabrication defects. The SP2 PCs are: (i) SU8 posts in air background, (ii) air pores in CdS background and (iii) Pt coated on SU8 SP2 templates. A spectroscopic study of the SP2 PCs shows select spectral regions of high reflectance, indicating the existence of a photonic stop band. This low-cost and large-scale method could enable broader technological impacts of 3D PC materials in areas such as thermo-photovoltaics and above room-temperature Bose-Einstein Condensation. Furthermore, this off-axis method could lead to the creation of an entirely new class of slanted-rod based photonic crystals, such as topological photonic crystal in 3D.

A 900 nm thick TiO₂simple cubic photonic crystal with lattice constant 450 nm was fabricated and used to experimentally validate a newly-discovered mechanism for extreme light-bending. Absorption enhancement was observed extending 1–2 orders of magnitude over that of a reference TiO₂film. Several enhancement peaks in the region from 600–950 nm were identified, which far exceed both the ergodic fundamental limit and the limit based on surface-gratings, with some peaks exceeding 100 times enhancement. These results are attributed to radically sharp refraction where the optical path length approaches infinity due to the Poynting vector lying nearly parallel to the photonic crystal interface. The observed phenomena follow directly from the simple cubic symmetry of the photonic crystal, and can be achieved by integrating the light-trapping architecture into the absorbing volume. These results are not dependent on the material used, and can be applied to any future light trapping applications such as phosphor-converted white light generation, water-splitting, or thin-film solar cells, where increased response in areas of weak absorption is desired.

We report experimental observation of intrinsic Bloch-mode emission from a 3D tungsten photonic crystal at low thermal excitation. After the successful removal of conventional metallic emission (normal emission), it is possible to make an accurate comparison of the Bloch-mode and the normal emission. For all biases, we found that the emission intensity of the Bloch-mode is higher than that of the normal emission. The Bloch-mode emission also exhibits a slower dependence on than that of the normal emission. The observed higher emission intensity and a different T-dependence is attributed to Bloch-mode assisted emission where emitters have been located into a medium having local density of states different than the isotropic case. Furthermore, our finite-difference time-domain (FDTD) simulation shows the presence of localized spots at metal–air boundaries and corners, having intense electric field. The enhanced plasmonic field and local non-equilibrium could induce a strong thermally stimulated emission and may be the cause of our unusual observation.