Diffractive optical elements with a large diffraction angle require feature sizes down to sub-wavelength dimensions, which require a rigorous electromagnetic computational model for calculation. However, the computational optimization of these diffractive elements is often limited by the large number of design parameters, making parametric optimization practically impossible due to large computation times. The adjoint method allows calculating the gradient of the target function with respect to all design variables with only two electromagnetic simulations, thus enabling gradient optimization. Here, we present the adjoint method for modeling wide-angle diffractive optical elements like 7×7 beam splitters with a maximum 53° diffraction angle and a non-square 5×7 array generating beam splitter. After optimization we obtained beam splitter designs with a uniformity error of 16:35% (7×7) and 6:98% (5×7), respectively. After reviewing the experimental results obtained from fabricated elements based on our designs, we found that the adjoint optimization method is an excellent and fast method to design wide-angle diffractive fan-out beam-splitters.
Nowadays, diffractive optical elements are used for a variety of applications because of their high design flexibility, compact size, and mass productivity. At the same time, they require having high and complex optical functionalities such as a large number of diffraction orders and a wide diffraction angle, which is beyond the limits of scalar paraxial diffraction domain. We propose a stable and fast gradient-based optimization algorithm based on step-transition perturbation approach applied to design binary diffractive elements with small and many features for being performed in a large number of diffraction orders and wide diffraction angles. Using our optimization, we obtained high-performance elements than using optimization based on purely scalar theory. In addition, it needs much less calculation time than parametric optimization based on rigorous diffraction theory. Upon verification with the experimental results, we observed that our gradient-based optimization method is valid for 1-by-117 fan-out grating with some small features (on the order of the illumination wavelength) and about 22° full pattern diffraction angle.
Improvement in high-temperature stable spectrally selective absorbers and emitters is integral for the further development of thermophotovoltaic (TPV), lighting and solar thermal applications. However, the high operational temperatures (T>1000oC) required for efﬁcient energy conversion, along with application specific criteria such as the operational range of low bandgap semiconductors, greatly restrict what can be accomplished with natural materials.
Motivated by this challenge, we demonstrate the first example of high temperature thermal radiation engineering with metamaterials. By employing the naturally selective thermal excitation of radiative modes that occurs near topological transitions, we show that thermally stable highly selective emissivity features are achieved for temperatures up to 1000°C with low angular dependence in a sub-micron thick refractory tungsten/hafnium dioxide epsilon-near-zero (ENZ) metamaterial. We also investigate the main mechanisms of thermal degradation of the fabricated refractory metamaterial both in terms of optical performance and structural stability using spectral analysis and energy-dispersive X-ray spectroscopy (EDS) techniques. Importantly, we observe chemical stability of the constituent materials for temperatures up to 1000°C and structural stability beyond 1100°C.
The scalable fabrication, requiring magnetron sputtering, and thermally robust optical properties of this metamaterial approach are ideally suited to high temperature emitter applications such as lighting or TPV. Our findings provide a first concrete proof of radiative engineering with high temperature topological transition in ENZ metamaterials, and establish a clear path for implementation in TPV energy harvesting applications.
We present a self-assembled refractory absorber/emitter without the necessity to structure the metallic surface itself, still
retaining the feature of tailored optical properties for visible light emission and thermophotovoltaic (TPV) applications.
We have demonstrated theoretically and experimentally that monolayers of zirconium dioxide (ZrO2) microparticles on a
tungsten layer can be used as large area, efficient and thermally stable selective absorbers/emitters. The band edge of the
absorption is based on critically coupled microsphere resonances. It can be tuned from visible to near-infrared range by
varying the diameter of the microparticles. We demonstrated the optical functionality of the structure after annealing up
to temperatures of 1000°C under vacuum conditions. In particular it opens up the route towards high efficiency TPV
systems with emission matched to the photovoltaic cell.
We report on the fabrication of metallodielectric photonic crystals by means of interference (or holographic) lithography
and subsequent coating by gold nanoparticles. The grating is realized in a SU-8 photoresist using a He-Cd laser of
wavelength 442 nm. The use of the wavelength found within the photoresist low absorption band enables fabricating
structures that are uniform in depth. Parameters of the photoresist exposure and development for obtaining a porous
structure corresponding to an orthorhombic lattice are determined. Coating of photonic crystals by gold nanoparticles is
realized by reduction of chloroauric acid by a number of reductants in a water solution. This research shows that the
combination of interference lithography and chemical coating by metal is attractive for the fabrication of
metallodielectric photonic crystals.
For the first time, the band structure of three-dimensional cubic photonic approximants of quasicrystals is studied
theoretically. The approximants of different orders are found to have large, near-isotropic band gaps in a wide range of
permittivity values. The effect of atom coordination on the size and threshold of the photonic band gap is explored. The
existence of a complete band gap in the cubic photonic quasicrystal with a body-centered six-dimensional lattice is
The most interesting phenomena in photonic crystals stipulate the presence of omnidirectional band gap, i.e. overlapping of stop bands in all directions. Higher rotational symmetry and isotropy of quasicrystals in comparison with ordinary crystal give a hope to achieve a gap opening at lower dielectric contrasts. But nonperiodic nature of quasicrystals makes the size of stop bands lower than in the case of ordinary periodic crystal. We study transition from periodic structure to nonperiodic one by considering pseudoquasicrystals - quasicrystal approximants with growing period to weigh advantages and disadvantages of quasicrystals. We consider the structures that can be obtained by multiple exposure holographic lithography for the case of 2, 3, 4, and 6-fold exposure by two wave interference pattern, corresponding square, hexagonal, 8-fold and 12-fold symmetry lattice.