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 ...

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We describe a fluctuating volume-current formulation of electromagnetic fluctuations that extends our recent work on heat exchange and Casimir interactions between arbitrarily shaped homogeneous bodies [Phys. Rev. B. 88, 054305] to situations involving incandescence and luminescence problems, including thermal radiation, heat transfer, Casimir forces, spontaneous emission, fluorescence, and Raman scattering, in inhomogeneous media. Unlike previous scattering formulations based on field and/or surface unknowns, our work exploits powerful techniques from the volume-integral equation (VIE) method, in which electromagnetic scattering is described in terms of volumetric, current unknowns throughout the bodies. The resulting trace formulas (boxed equations) involve products of well-studied VIE matrices and describe power and momentum transfer between objects with spatially varying material properties and fluctuation characteristics. We demonstrate that thanks to the low-rank properties of the associated matrices, these formulas are susceptible to fast-trace computations based on iterative methods, making practical calculations tractable. We apply our techniques to study thermal radiation, heat transfer, and fluorescence in complicated geometries, checking our method against established techniques best suited for homogeneous bodies as well as applying it to obtain predictions of radiation from complex bodies with spatially varying permittivities and/or temperature profiles.

Light can exert forces on objects, promising to propel a meter-scale lightsail to near the speed of light. The key to address many challenges in such an ambition hinges on the nanostructuring of lightsails to tailor their optical scattering properties. In this Letter, we present a comprehensive study of photonic design of lightsails by applying large-scale optimization techniques to a generic geometry based on stacked photonic crystal layers. The optimization is performed by rigorous coupled-wave analysis amended with automatic differentiation methods for adjoint-variable gradient evaluations. Employing these methods, the propulsion efficiency of a lightsail that involves a trade-off between high broadband reflectivity and mass reduction is optimized. Surprisingly, regardless of the material choice, the optimal structures turn out to be simply one-dimensional subwavelength gratings, exhibiting nearly 50% improvement in acceleration distance performance compared to prior studies. Our framework can be extended to address other lightsail challenges such as thermal management and propulsion stability and applications in integrated photonics such as compact mirrors.

Perspiration evaporation plays an indispensable role in human body heat dissipation. However, conventional textiles tend to focus on sweat removal and pay little attention to the basic thermoregulation function of sweat, showing limited evaporation ability and cooling efficiency in moderate/profuse perspiration scenarios. Here, we propose an integrated cooling (i-Cool) textile with unique functional structure design for personal perspiration management. By integrating heat conductive pathways and water transport channels decently, i-Cool exhibits enhanced evaporation ability and high sweat evaporative cooling efficiency, not merely liquid sweat wicking function. In the steady-state evaporation test, compared to cotton, up to over 100% reduction in water mass gain ratio, and 3 times higher skin power density increment for every unit of sweat evaporation are demonstrated. Besides, i-Cool shows about 3 °C cooling effect with greatly reduced sweat consumption than cotton in the artificial sweating skin test. The practical application feasibility of i-Cool design principles is well validated based on commercial fabrics. Owing to its exceptional personal perspiration management performance, we expect the i-Cool concept can provide promising design guidelines for next-generation perspiration management textiles.

A theoretical analysis of detection limits in swept-frequency whispering gallery mode biosensing modalities is presented based on application of the Cramér-Rao lower bound. Measurement acuity factors are derived assuming the presence of uncoloured and 1/ f Gaussian technical noise. Frequency fluctuations, for example arising from laser jitter or thermorefractive noise, are also considered. Determination of acuity factors for arbitrary coloured noise by means of the asymptotic Fisher information matrix is highlighted. Quantification and comparison of detection sensitivity for both resonance shift and broadening sensing modalities are subsequently given. Optimal cavity and coupling geometries are furthermore identified, whereby it is found that slightly under-coupled cavities outperform critically and over coupled ones.

Second harmonic conversion from 1550 nm to 775 nm with an efficiency of 400% W −1 is demonstrated in a gallium phosphide (GaP) on oxide integrated photonic platform. The platform consists of doubly-resonant, phase-matched ring resonators with quality factors Q ∼ 10 4 , low mode volumes V ∼ 30(λ/n) 3 , and high nonlinear mode overlaps. Measurements and simulations indicate that conversion efficiencies can be increased by a factor of 20 by improving the waveguide-cavity coupling to achieve critical coupling in current devices.

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