Inverse design of near-field thermal radiation and nonlinear optics

Abstract

Traditional photonic design relies on intuitive and established principles that perform extremely well when restricted to narrow-bandwidth applications. In this thesis, we show that recently developed inverse-design techniques can be tailored to systematically discover complex structures for multifrequency or multi-functional devices, including near-field radiative heat transfer (RHT) and nonlinear frequency conversion. We begin by drawing attention to far-field thermal radiation and spontaneous emission from wavelength-scale heterogeneous objects, captured by a fluctuating–volume current formulation of electromagnetic scattering, illustrating that even simple geometric designs can be used to tailor the emissive and scattering properties of compact bodies. Such tunablity can be important for RHT at subwavelength separations, which as we show next, can transfer heat in competition with thermal conduction in nanostructures, resulting in modified temperature gradients in plate-plate geometries and more significantly gratings.

Next, we demonstrate that optimization techniques can be exploited to systematically tune and enhance near-field RHT at desirable (selective) frequencies. We begin by looking into multilayer structures with arbitrary dielectric profiles along the gap directions, demonstrating that appropriately optimized, multilayer structures can lead to larger RHT compared to the best possible homogeneous thin films. We then investigate RHT between two-dimensional gratings having an enlarged two-dimensional design space. We demonstrate that RHT between lossy metals at infrared wavelengths such as tungsten, when appropriately structured, can approach that of a pair of ideal metal plates supporting surface plasmonic resonances. Moreover, we provide evidence of a previously predicted material scaling law in those extended structures, up to a threshold enhancement ratio.

Lastly, we discuss the application of inverse design techniques to the discovery of three-dimensional metasurfaces and wavelength-scale cavities that exhibit highly efficient nonlinear frequency-conversion. Large nonlinear enhancements are achieved by the creation of modes with large nonlinear overlap factors. Furthermore, as efficient nonlinear processes require simultaneous control of light coupling to and from a waveguide at widely separate frequencies, we apply inverse design techniques to demonstrate wavelength-scale waveguide–cavity couplers operating over tunable and broad frequency bands in both two and three dimensions.