Volume 14, Issue 2

Nanophotonics, which explores significant light–matter interactions at the nanoscale, has facilitated significant advancements across numerous research fields. A key objective in this area is the design of ultra-compact, high-performance nanophotonic devices to pave the way for next-generation photonics. While conventional brute-force, intuition-based forward design methods have produced successful nanophotonic solutions over the past several decades, recent developments in optimization methods and artificial intelligence offer new potential to expand these capabilities. In this review, we delve into the latest progress in the inverse design of nanophotonic devices, where AI and optimization methods are leveraged to automate and enhance the design process. We discuss representative methods commonly employed in nanophotonic design, including various meta-heuristic algorithms such as trajectory-based, evolutionary, and swarm-based approaches, in addition to adjoint-based optimization. Furthermore, we explore state-of-the-art deep learning techniques, involving discriminative models, generative models, and reinforcement learning. We also introduce and categorize several notable inverse-designed nanophotonic devices and their respective design methodologies. Additionally, we summarize the open-source inverse design tools and commercial foundries. Finally, we provide our perspectives on the current challenges of inverse design, while offering insights into future directions that could further advance this rapidly evolving field.

Photons can be used to either monitor or induce catalysis by acting as photoexcited holes or quasi particles, which aid in water splitting reaction leading to a major step towards sustainable energy. However, the mechanism of catalysis using nanocatalysts under photo-illumination is not fully understood because of the complexity involved in three major steps during the oxygen evolution reaction: photoabsorption on nanocatalyst, hole transport to the surface, and the reaction kinetic barriers at the surface. In a photoelectrochemical cell used for water splitting, the surface states of optically and chemically dominant species affect the catalysts’ performance. For instance, the signature of the dominant absorption peak at 580 nm in the observed spectra of Fe 2 O 3 photoanode can shed light on the oxygen evolution reaction mechanism since each reaction intermediate affects the absorption spectrum, and the absorption coefficient in turn affects the photocurrent. In the recent decade, a combination of different theoretical methods starting from density functional theory up to Bethe–Salpeter equation accounting for excitonic effects helped to establish that the *O intermediate is the rate limiting step in agreement with experimental data. Therefore, this perspective focuses on the complexity and variety of fundamental phenomena involved in water splitting mechanism and various theoretical methods applied to address these and also suggests how the predictive capability of these methods can be used to understand mechanisms beyond water splitting, such as CO 2 reduction.

The utilization of metamaterials plays a pivotal role in integrated photonics. The precise design of metamaterials enables them to finely manipulate light, resulting in an ultra-compact footprint and exceptional performance that cannot be achieved by traditional structures. The conventional methods for metamaterial design, however, encounter challenges from intricate targets. Although attempts have been made to apply inverse design to metamaterials, there is still a need for a highly intelligent, low-computation method, and easy-to-fabricate metamaterial structure. Here, we present an efficient methodology that combines metamaterials, heuristic algorithms, and machine learning to facilitate the rapid development of intricate devices. The method is used to design 1 × N power splitters with arbitrary power ratios, as an application example. Specifically, 1 × 2, 1 × 3, 1 × 4 power splitters with arbitrary ratios are fabricated and experimentally demonstrated. The application of this method in arbitrary power splitter highlights its appropriateness for the design and optimization within integrated photonics devices.

Silicon photonics modulators based on a 2 × 1 Fabry–Perot (FP) cavity, which is circulator-free, are proposed and demonstrated by introducing two asymmetric multimode-waveguide grating (AMWG) reflectors and a short straight modulation section with interleaved PN junctions. In particular, the straight modulation section in the FP cavity is broadened to be far beyond the single-mode regime, alleviating the inherent sensitivity to the variations of waveguide dimensions and thus reducing stochastic resonance-wavelength variations. The Q factor of the FP cavity is manipulated by optimally manipulating the reflection of the AMWGs, and the modulation bandwidth is enhanced to be over 40 GHz by utilizing the optical peaking enhancement effect, which happens when operating at the wavelength slightly detuning to its resonance wavelength. Eye diagrams for high-speed modulation with 50 Gbps are also demonstrated in experiments. Finally, wafer-level measurement is conducted by characterizing the silicon photonic modulators based on the 2 × 1 FP cavity and a conventional microring fabricated on the same chip, experimentally revealing an average improvement of 43 % in minimizing the random resonance-wavelength variation, which is attributed to the implementation of broadening the straight modulation section in the FP cavity.

Simultaneous optical display and depth perception are crucial in many intelligent technologies but are usually realized by separate bulky systems unfriendly to integration. Metasurfaces, artificial two-dimensional optical surfaces with strong light–matter interaction capabilities at deep subwavelength scales, offer a promising approach for manufacturing highly integrated optical devices performing various complex functions. In this work, we report a polarization-multiplexed metasurface that can functionally switch between holographic display and Dammann gratings. By tailoring the incidence polarization, the metasurface can display high-quality holographic images in the Fresnel region or project a uniform spot cloud nearly covering the entire 180° × 180° transmissive space. For the latter, a projection and three-dimensional (3D) reconstruction experiment is conducted to elaborate the potential in retrieving 3D complex spatial information. The current results provide a prominent way to manufacture lightweight and highly-integrated comprehensive imaging systems especially vital for cutting-edge intelligent visual technologies.

When microspheres are illuminated by tightly focused vortex beams, they can be trapped in a non-equilibrium steady state where they orbit around the optical axis. By using the Mie–Debye theory for optical tweezers, we demonstrate that the orbital period strongly depends on the particle’s chirality index. Taking advantage of such sensitivity, we put forth a method to experimentally characterize with high precision the chiroptical response of individual optically trapped particles. The method allows for an enhanced precision at least one order of magnitude larger than that of similar existing enantioselective approaches. It is particularly suited to probe the chiroptical response of individual particles, for which light-chiral matter interactions are typically weak.

It is generally recognized that there is only a single optical potential-well near the focus in optical traps with a focused Gaussian beam. In this work, we show that this classic Gaussian-beam optical trap has additional optical potential-wells for optical manipulation at the subwavelength scale in the off-focus transverse plane. The additional optical potential-wells are formed by the synergy of both the gradient trapping force and the transverse scattering force, though in previous studies the scattering force usually has adverse effect such as reducing trapping stability. These potential-wells work for not only the metallic particles, but also the high refractive-index dielectric particles. By engineering the contribution of the gradient force and scattering force through the particle size, the particle material and the position of the manipulation transverse plane, the force field and trapping potential-well can be tailored to trap/manipulate nanoparticles at different off-axis distance at the subwavelength scale. Our work provides new insight into optical tweezers and promises applications in optical nanomanipulation, nanoparticle sorting/separation, particle patterning and micro-fabrication on substrates.

We introduce a novel technique for coherent control that employs resonant internally generated fields in CdTe quantum dot (QD) thin films at the L -point. The bulk band gap of CdTe at the L -point amounts to 3.6 eV, with the transition marked by strong Coulomb coupling. Third harmonic generation ( λ 3 = 343 nm, hν = 3.61 eV) for a fundamental wavelength of λ 1 = 1,030 nm is used to control quantum interference of three-photon resonant paths between the valence and conduction bands. Different thicknesses of the CdTe QDs are used to manipulate the phase relationship between the external fundamental and the internally generated third harmonic, resulting in either suppression or strong enhancement of the resonant third harmonic, while the nonresonant components remain nearly constant. This development could pave the way for new quantum interference–based applications in ultrafast switching of nanophotonic devices.

As a highly competitive display technology, the realization of pixelated full color quantum dot light emitting diodes (QLEDs) is an indispensable step for high resolution display. Meanwhile, with the rise of near eye display, a submicron pixel size is required for a high-resolution display within a small area less than 1 inch. However, the realization of submicron full color quantum dot pixels by direct patterning is still a big challenge. In this work, we propose a topological meta-mirror structure for the realization of submicron RGB QLEDs. The pixelated topological meta-mirror is introduced with a sufficient design freedom. A powerful light manipulation capability is offered by the topological meta-mirror even with limited period number, which enables the construction of RGB meta-cavities. The pure RGB emissions from meta-cavities can be realized with energy ratios larger than 88 % based on optimized topological meta-mirrors. For a subpixel size of 1 μm, the energy ratios for target color emission can still be larger than 85 %, which indicates a pure color emission. And a minimum subpixel size of 0.6 μm and an ultra-high pixel density of 21,666 pixel per inch can be realized with a 3 × 3 topological meta-mirror array. The proposed meta-cavity structure based on topological meta-mirror provides a new technique route for full color QLEDs especially for high pixel density required scenarios.

The integration of DNA origami structures with opto-plasmonic whispering gallery mode (WGM) sensors offers a significant advancement in label-free biosensing, overcoming the limitations of traditional fluorescence-based techniques, and providing enhanced sensitivity and specificity for detecting DNA hybridization events. In this study, DNA origami acts as a scaffold for the precise assembly of plasmonic dimers, composed of gold nanorods (AuNRs), which amplify detection sensitivity by generating strong near-field enhancements in the nanogap between the nanorods. By leveraging the strong electromagnetic fields generated within the nanogap of the plasmonic dimer, this platform enables the detection of transient hybridization events between DNA docking strands and freely diffusing complementary sequences. Our findings demonstrate that the salt concentration critically influences DNA hybridization kinetics. Higher ionic strengths reduce electrostatic repulsion between negatively charged DNA strands, thereby stabilizing duplex formation and prolonging interaction times. These effects are most pronounced at salt concentrations around 300–500 mM, where optimal conditions for duplex stability and reduced dissociation rates are achieved. By thoroughly investigating the hybridization kinetics under varying environmental conditions, this study contributes to a deeper understanding of DNA interactions and offers a robust tool for single-molecule detection with real-time capabilities.

Monolayer transition metals dichalcogenides (TMDs) have been coupled to bound-state in the continuum (BIC) hosted dielectric structures to attain high second harmonic generation (SHG). However, the transvers electric modes are strongly localized in the waveguides result in fairly weak exciton-photon coupling in monolayer TMD placed on the surface. To achieve SHG in few-layers TMDs based BIC-inspired structure is a challenge. Here, we report BIC in few-layers TMDs metasurface with high quality factor (Q-factor), tunability, and modes-upholding in different environments. The metasurface sustains BIC at different thickness of the meta-atoms, which is highly desired for maintaining the accuracy in fabrications. Next, we calculate the SHG efficiency from few-layers TMD metasurface around BIC wavelengths. The high conversion efficiency in this work is 1.47 × 10 −4 for 6 mW incident power. Moreover, our design is highly thin and can be used for various linear and non-linear applications in optics. This study will provide a new route to next generation post-silicon metasurfaces.