Diffractive Spectral-Splitting Optical Element Designed by Adjoint-Based Electromagnetic Optimization and Fabricated by Femtosecond 3D Direct Laser Writing

Abstract

The greatest source of loss in conventional single-junction photovoltaic cells is their inefficient utilization of the energy contained in the full spectrum of sunlight. To overcome this deficiency, we propose a multijunction system that laterally splits the solar spectrum onto a planar array of single-junction cells with different band gaps. As a first demonstration, we designed, fabricated, and characterized dispersive diffractive optics that spatially separated the visible (360–760 nm) and near-infrared (760–1100 nm) bands of sunlight in the far field. Inverse electromagnetic design was used to optimize the surface texture of the thin diffractive phase element. An optimized thin film fabricated by femtosecond two-photon absorption 3D direct laser writing shows an average splitting ratio of 69.5% between the visible and near-infrared light over the 380–970 nm range at normal incidence. The splitting efficiency is predicted to be 80.4% assuming a structure without fabrication errors. Spectral-splitting action is observed within an angular range of ±1° from normal incidence. Further design optimization and fabrication improvements can increase the splitting efficiency under direct sunlight, increase the tolerance to angular errors, allow for a more compact geometry, and ultimately incorporate a greater number of photovoltaic band gaps.

Octave bandwidth photonic fishnet-achromatic-metalens

“…This signifies that it is challenging to have perfect achromaticity and efficiency as phase-shift intercepts and slopes cannot be fully independently controlled in a planar design. The limitation confirms that this is intrinsically an optimization problem [32][33][34][35] . In metasurfaces, the spatial derivative of the slope controls the direction of incident rays to make them reach the focal point.…”

Nonlinear nanophotonic devices in the ultraviolet to visible wavelength range

“…This is in contrast to naïve approaches such as finite difference approximations which require N+1 simulations per gradient evaluation where N is the number of optimization parameters. The adjoint method has been applied to design many different devices as shown in Figure 10 including wavelength demultiplexers [254,255], polarization splitters [250], power splitters [251], photonic switches [256], nanophotonic resonators [257], and grating couplers [258]. Furthermore, thanks to the flexibility of the gradient-based optimization approach, arbitrary nonlinear objective functions with highly complex constraints can be handled, which has enabled incorporating fabrication [259] and robustness constraints at design time [256,260].…”

High-efficiency, large-area, topology-optimized metasurfaces

“…Topology optimization is an alternative design method that can overcome the efficiency limitations of conventional approaches 11 . Inverse design methods, including objective-first and adjoint-based topology optimization 12 , have produced photonic crystals 13,14 , optical demultiplexers 15 , and spectral splitters 16 with outstanding performance resulting from the devices’ unusual geometries and nonintuitive optical dynamics. More recently, topology optimization has led to high-performance metasurfaces that have a broad range of capabilities, such as high-efficiency light deflection with nearly arbitrary input and output angles 17 , spectral sorting of plane waves to distinct diffraction orders 18 , and light focusing with field-curvature correction 19 .…”