We present an adjoint-based optimization for electromagnetic design. It embeds commercial Maxwell solvers within a steepest-descent inverse-design optimization algorithm. The adjoint approach calculates shape derivatives at all points in space, but requires only two "forward" simulations. Geometrical shape parameterization is by the level set method. Our adjoint design optimization is applied to a Silicon photonics Y-junction splitter that had previously been investigated by stochastic methods. Owing to the speed of calculating shape derivatives within the adjoint method, convergence is much faster, within a larger design space. This is an extremely efficient method for the design of complex electromagnetic components.

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.

Heat-assisted magnetic recording (HAMR) allows for data writing in hard disks beyond 1 Tb/in 2 areal density, by temporarily heating the area of a single bit to its Curie temperature. The metallic optical antenna or near-field transducer (NFT), used to apply the nanoscale heating to the media, may self-heat by several hundreds of degrees. With the NFT reaching such extreme temperatures, demonstrations of HAMR technology experience write-head lifetimes that are orders of magnitude less than that required for a commercial product. Hence, heating of the NFT is of upmost importance. In this paper, we first derive fundamental limits on the temperature ratio NFT/Media to drive NFT design choices for low-temperature operation. Next, we employ inverse electromagnetic design software, which solves for unexpected geometries of the NFT and waveguide. We present computationally generated designs for the waveguide feeding the NFT that offer a 50% reduction in NFT self-heating (∼220°C) compared with typical industry designs.

Traditionally, aberration correction in extreme ultraviolet (EUV) projection optics requires the use of multiple lossy mirrors, which results in prohibitively high source power requirements. We analyze a single spherical mirror projection optical system where aberration correction is built into the mask itself, through Inverse Lithography Technology (ILT). By having fewer mirrors, this would reduce the power requirements for EUV lithography. We model a single spherical mirror system with orders of magnitude more spherical aberration than would ever be tolerated in a traditional multiple mirror system. By using ILT, (implemented by an adjoint-based gradient descent optimization algorithm), we design photomasks that successfully print test patterns, in spite of these enormous aberrations. This mathematical method was tested with a 6 plane wave illumination source. Nonetheless, it would have poor power throughput from a totally incoherent source.

No abstract