Recent advancements in computational inverse design have begun to reshape the landscape of structures and techniques available to nanophotonics. Here, we outline a cross section of key developments at the intersection of these two fields: moving from a recap of foundational results to motivation of emerging applications in nonlinear, topological, near-field and on-chip optics.The development of devices in nanophotonics has historically relied on intuition-based approaches, the impetus for which develops from knowledge of some a priori known physical effect. The specific features of such devices are then typically calculated and matched to suitable applications by tuning small sets of characteristic parameters. This approach has had a long track record of success, giving rise to a rich and widely exploited library of templates that includes multilayer thin films 1 , Fabry-Perot 2 and microring resonators 3 , silicon waveguides 4,5 , photonic crystals 6 , plasmonic nanostructures 7 , and nanobeam cavities 8 , top of Fig. 1. Combining the principles of index guiding and bandgap engineering, along with material resonances, this collection of designs enables remarkable manipulation of light over bands of frequencies spanning from the ultra-violet to the mid infrared: group velocity can be slowed by more than two orders of magnitude 9 , light confined to volumes thousands of times smaller than its free-space wavelength 10 , and resonances made to persist in micron sized areas for tens of millions of cycles 11 .Yet, as the scope of nanophotonics broadens to include large bandwidth or multi-frequency applications, nonlinear phenomena, and dense integration, continuing with this prototypical approach poses a challenge of increasing complexity. For instance, consider the design of a wavelength-scale structure for enhancing nonlinear interactions 12 , discussed below. Even in the simplest case, several interdependent characteristics must be simultaneously optimized, among which are large quality factors at each individual wavelength and nonlinear overlaps, which must be controlled in as small a volume as possible. In such a situation, the templates of the aforementioned standard library offer no clear or best way to proceed; there is no definite reason to expect that an optimal design can be found in any of the traditional templates, or that such a design necessarily exists. Moreover, the performance of a given nonlinear device is likely to be highly dependent on the particular characteristics of the problem, and as greater demands are placed on functionality it becomes increasingly doubtful that any one class of structures will have the broad applicability of past devices. This lack of evident strategies for broadband applications also brings to attention the space of structures included in the standard photonic library. Predominately, traditional designs are repetitive mixtures and combinations of highly symmetric shapes described by a small collection of parameters. Since intuition-based optimization is then carried out ...

Engineering the optical properties using artificial nanostructured media known as metamaterials has led to breakthrough devices with capabilities from super-resolution imaging to invisibility. In this article, we review metamaterials for quantum nanophotonic applications, a recent development in the field. This seeks to address many challenges in the field of quantum optics using recent advances in nanophotonics and nanofabrication. We focus on the class of nanostructured media with hyperbolic dispersion that have emerged as one of the most promising metamaterials with a multitude of practical applications from subwavelength imaging, nanoscale waveguiding, biosensing to nonlinear switching. We present the various design and characterization principles of hyperbolic metamaterials and explain the most important property of such media: a broadband enhancement in the electromagnetic density of states. We review several recent experiments that have explored this phenomenon using spontaneous emission from dye molecules and quantum dots. We finally point to future applications of hyperbolic metamaterials of using the broadband enhancement in the spontaneous emission to construct single photon sources. arXiv:1204.5529v2 [physics.optics]

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Abstract:We develop the fluctuational electrodynamics of metamaterials with hyperbolic dispersion and show the existence of broadband thermal emission beyond the black body limit in the near field. This arises due to the thermal excitation of unique bulk metamaterial modes, which do not occur in conventional media. We consider a practical realization of the hyperbolic metamaterial and estimate that the effect will be observable using the characteristic dispersion (topological transitions) of the metamaterial states. Our work paves the way for engineering the near-field thermal emission using metamaterials.Engineering the black body thermal emission using artificial media promises to impact a variety of applications involving energy harvesting 1 , thermal management 2 and coherent thermal sources 3 . The usual upper limit to the black-body emission is not fundamental and arises since energy is carried to the far-field only by propagating waves emanating from the heated source. If one allows for energy transport in the near-field using evanescent waves, this limit can be overcome. Thus thermal emission beyond the black body limit is expected due to surface electromagnetic excitations 4 or at the edge of the bandgap in photonic crystals 5 where there is a large enhancement in the photonic density of states. Advances in near field scanning and probing techniques 6-8 have led to conclusive demonstrations of these effects.One limitation of the above mentioned approaches using photonic crystals or surface electromagnetic excitations is that the energy transfer beyond the black body limit (superplanckian 9,10 thermal emission) only occurs in a narrow bandwidth. In this paper, we show that artificial media (metamaterials) with engineered dielectric properties can overcome the limitation of super-planckian thermal emission at a single resonant frequency. Our work rests on the recently discovered singularity in the bulk density of states of metamaterials with hyperbolic dispersion [11][12][13] . The unique property which sets these hyperbolic metamaterials (HMMs) apart from conventional approaches of engineering the photonic density of states (PDOS) is the broad bandwidth in which the PDOS is enhanced.

Control of thermal radiation at high temperatures is vital for waste heat recovery and for high-efficiency thermophotovoltaic (TPV) conversion. Previously, structural resonances utilizing gratings, thin film resonances, metasurfaces and photonic crystals were used to spectrally control thermal emission, often requiring lithographic structuring of the surface and causing significant angle dependence. In contrast, here, we demonstrate a refractory W-HfO2 metamaterial, which controls thermal emission through an engineered dielectric response function. The epsilon-near-zero frequency of a metamaterial and the connected optical topological transition (OTT) are adjusted to selectively enhance and suppress the thermal emission in the near-infrared spectrum, crucial for improved TPV efficiency. The near-omnidirectional and spectrally selective emitter is obtained as the emission changes due to material properties and not due to resonances or interference effects, marking a paradigm shift in thermal engineering approaches. We experimentally demonstrate the OTT in a thermally stable metamaterial at high temperatures of 1,000 °C.