Broadband mirrors for thermophotovoltaics.

We present an innovative solution to improve the efficiency of thermophotovoltaic (TPV) devices by tackling the problem of sub-bandgap photon losses. We propose an optimized design for thin-film mirrors using inverse electromagnetic design principles, thereby enhancing the average reflectivity and photon re-use. Our method surpasses the traditional Bragg mirror by employing a gradient-descent based optimization over Bragg mirror geometrical parameters, leveraging the transfer matrix method for derivative calculations. The optimized structure, based on continuously chirped distributed Bragg reflectors proposed herein demonstrates a remarkable increase in reflectivity beyond 98%, over an almost three-octaves bandwidth (0.1eV-0.74eV). We show that the incident power loss in InGaAs TPV cells at an emitter temperature of 1200°C is significantly reduced. While our work shows considerable promise, further exploration is needed to ascertain the practicability and robustness of these designs under various operational conditions. This study thus provides a major step forward in TPV technology, highlighting a new route towards more effective energy conversion systems.

Adjoint optimization of polarization-splitting grating couplers.

We have designed a polarization-splitting grating coupler (PSGC) in silicon-on-insulator (SOI) that has 1.2 dB peak loss in numerical simulations, which is the best simulated performance of PSGCs without a bottom reflector to the best of our knowledge. Adjoint method-based shape optimization enables us to explore complex geometries that are intractable with conventional design approaches. Physics-based process-independent knowledge of PSGCs is extracted from the adjoint optimization and can be transferred to other platforms with a minimum of effort.

Wafer-level testing of inverse-designed and adjoint-inspired vertical grating coupler designs compatible with DUV lithography.

Perfectly vertical grating couplers have various applications in optical I/O such as connector design, coupling to multicore optical fibers and multilayer silicon photonics. However, it is challenging to achieve perfectly vertical coupling without simultaneously increasing reflection. In this paper, we use the adjoint method as well as an adjoint-inspired methodology to design devices that can be fabricated using only a single-etch step in a c-Si 193 nm DUV immersion lithography process, while maintaining good coupling and low reflection. Wafer-level testing of devices fabricated by a pilot line foundry confirms that both design paradigms result in state-of-the-art experimental insertion loss (<2 dB) and bandwidths (∼20 nm) while having only moderate in-band reflection (<-10 dB). Our best design has a (median) 1.82 dB insertion loss and 21.3 nm 1 dB-bandwidth.

Efficient spontaneous emission by metal-dielectric antennas; antenna Purcell factor explained.

The rate of spontaneous emission from an optical emitter can be greatly enhanced using a metallic optical antenna at the penalty of efficiency. In this paper we propose a metal-dielectric antenna that eliminates the tradeoff between spontaneous emission enhancement and radiative efficiency by using nanoscopic dielectric structures at the antenna tips. This tradeoff occurs due to Ohmic loss and is further exacerbated by electron surface collisions. We find that our metal-dielectric antenna can enhance spontaneous emission by a factor 5 × 105 with efficiency = 70%, greatly exceeding the radiative efficiency of a purely metallic antenna with similar enhancement. Moreover, the metal-dielectric antenna design strategy is naturally amenable to short-distance optical communications applications. We go on to discuss the Purcell effect within the context of antenna enhancement. Metallic optical antennas are best analyzed with conventional antenna circuit models, but if the Purcell enhancement were to be employed, we provide the effective mode volume, Veff = (3/4π2)2d2λ(λ/l)5, that would be needed.

Inverse design optimization for efficient coupling of an electrically injected optical antenna-LED to a single-mode waveguide.

Efficient high-speed nanoscale optical sources are required for low-power next-generation data communication. Here we propose an integrated antenna-LED on a single-mode optical waveguide. By leveraging inverse design optimization, we achieved a waveguide coupling efficiency of 94% and an antenna efficiency of 64%, while maintaining a high average enhancement of 144 - potentially enabling >100GHz direct modulation.