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Nanophotonic devices manipulate light at sub-wavelength scales, enabling tasks such as light concentration, routing, and filtering. Designing these devices to achieve precise light–matter interactions using structural parameters and materials is a challenging task. Traditionally, solving this problem has relied on computationally expensive, iterative methods. In recent years, deep learning techniques have emerged as promising tools for tackling the inverse design of nanophotonic devices. While several review articles have provided an overview of the progress in this rapidly evolving field, there is a need for a comprehensive tutorial that specifically targets newcomers without prior experience in deep learning. Our goal is to address this gap and provide practical guidance for applying deep learning to individual scientific problems. We introduce the fundamental concepts of deep learning and critically discuss the potential benefits it offers for various inverse design problems in nanophotonics. We present a suggested workflow and detailed, practical design guidelines to help newcomers navigate the challenges they may encounter. By following our guide, newcomers can avoid frustrating roadblocks commonly experienced when venturing into deep learning for the first time. In a second part, we explore different iterative and direct deep learning-based techniques for inverse design, and evaluate their respective advantages and limitations. To enhance understanding and facilitate implementation, we supplement the manuscript with detailed Python notebook examples, illustrating each step of the discussed processes. While our tutorial primarily focuses on researchers in (nano-)photonics, it is also relevant for those working with deep learning in other research domains. We aim at providing a solid starting point to empower researchers to leverage the potential of deep learning in their scientific pursuits.

Holography offers a vital platform for optical information storage and processing, which has a profound impact on many photonic applications, including 3D displays, LiDAR, optical encryption, and artificial intelligence. In this review, we provide a comprehensive overview of optical holography, moving from volume holography based on optically thick holograms to digital holography using ultrathin metasurface holograms in nanophotonics. We review the use of volume holograms for holographic multiplexing through the linear momentum selectivity and other approaches and highlight the emerging use of digital holograms that can be implemented by ultrathin metasurfaces. We will summarize the fabrication of different holographic recording media and digital holograms based on recent advances in flat meta-optics and nanotechnology. We highlight the rapidly developing field of metasurface holography, presenting the use of multi-functional metasurfaces for multiplexing holography in the use of polarization, wavelength, and incident angle of light. In the scope of holographic applications, we will focus on high bandwidth metasurface holograms that offer the strong sensitivity to the orbital angular momentum of light. At the end, we will provide a short summary of this review article and our perspectives on the future development of the vivid holography field.

In contrast to few-layer black phosphorus (BP) with a relatively larger area, BP quantum dots (BP-QDs) are expected to have distinctive electromagnetic response and carrier behaviors, especially in low-frequency range such as in the THz regime. Herein, we experimentally investigate the THz properties of BP-QDs as well as the optical control of these properties. It is demonstrated that the effects of weak carrier confinement, which is associated with diffusive restoring current in each BP-QD, contribute significantly to the effective THz conductivity of BP-QDs. Instead, spectral features of discretely spaced energy levels as shown for many kinds of semiconductor QDs in UV-visible range are not observed in the THz regime. This indicates an insignificant contribution of strong quantum confinement here. Based on the modified Drude–Smith formula, we show that the optical excitation/pump of a CW laser can induce photogenerated carriers and enhance the effects of weak carrier confinement in BP-QDs. Thus, a nonlinear enhancement of THz absorption can be observed by increasing the power of the excitation laser. These results not only deepen our understanding of the fundamental physics of BP nanomaterials but also provide an alternative approach to realize active control of BP-based THz devices.

Bound states in the continuum (BICs) in metasurfaces have lately attracted a great deal of attention stemming from their inherent (formally) divergent Q factors, which lead to an enhancement of light–matter interaction in two-dimensional geometries. However, the development of plausible means to actively manipulate them remains a major challenge. The use of graphene layers has recently been suggested, employed either as a substrate or a coating that modifies the dielectric environment of the metasurface. Here, instead, we propose to exploit graphene disk dimers supporting in-plane plasmons directly as active meta-atoms in a square array. We prove analytically that both the emergence of a BIC and its Q factor can be tuned in an active manner by applying a different external potential to each of the disks in the dimer, thus being formally equivalent to engineering the disk diameters in a passive, geometrically-dependent manner. Moreover, we propose an approach to mitigate the effect of the inherent losses of graphene plasmons based on exploiting the collective behavior of the array, which is achieved by adjusting the lattice parameter so that the wavelength of the BIC mode lies closer to the Rayleigh anomaly.

The significance of ultrafast laser-induced energy and mass transfer at interfaces has been growing in the field of nanoscience and technology. Nevertheless, the complexity arising from non-linear and non-equilibrium optical-thermal-mechanical interactions results in intricate transitional behaviors. This complexity presents challenges when attempting to analyze these phenomena exclusively through modeling or experimentation. In this study, we conduct time-resolved reflective pump-probe imaging and molecular-dynamics coupled two-temperature model (MD-TTM) simulations to investigate the ultrafast dynamics and ablation mechanism of Au/Ti bilayer systems. The calculated energy absorption curves indicate that Au film reduces the energy deposition in the underlying Ti layer, resulting in reduced melting and evaporation rate of Ti. The phase transition process induces different mechanical responses. The potential energy patterns indicate that the expansion of vapor Ti extrudes the surface Au layer outward. In simulated stress distribution images, the Au layer can hamper the expansion of the vapor-phase Ti and brings dynamic compressive stress to the residual Ti layer. When the compressive stress transforms into tensile stress, the material is removed through mechanical damage. Therefore, both Au and Ti in the 20 nm Au-covered Ti are completely removed. Our approach elucidates the ablation mechanism within the Au/Ti bilayer system and offers fresh insights into managing thermo-mechanical responses within analogous systems.

Mixed dimensional van der Waals heterostructure based on layered two-dimensional molybdenum disulfide (MoS 2 ) interfaced to gallium nitride (GaN) has attracted tremendous attention due to its unique properties and application in novel electronic, optoelectronic, and quantum devices. However, developing facile synthesis methods and insights into the exciton dynamics for this system still remains a major challenge. Here, a simple and cost-effective method is demonstrated for large-scale synthesis of monolayer MoS 2 on differently doped GaN substrates. A mixed aqueous solution of Na 2 MoO 4 and NaOH is spin-coated on GaN and sulfurated in one step by chemical vapor deposition (CVD). High quality monolayer MoS 2 nanosheets with side length over 400 μm and surface coverage ratio of more than 90 % are achieved on GaN. Furthermore, the PL intensity, excitonic transition ratios and ultrafast exciton dynamics of MoS 2 are observed to be largely modulated by the doping type of GaN, owing to substrate-induced doping, which is proved by Raman, PL and transient absorption spectroscopy. Notably, p-GaN can attract electrons from monolayer MoS 2 and weaken its intrinsic n-doping, thereby facilitating higher PL intensity as well as longer exciton lifetime, while n-GaN provides strong n-doping and generates opposite effect. This work hereby presents a pathway for large-scale synthesis of MoS 2 /GaN heterostructures and further understanding of their charge transfer properties and exciton dynamics, which should inspire their applications for optoelectronic devices.

Two-dimensional (2D) transition metal dichalcogenide (TMDC) monolayers are promising materials for light-emitting devices due to their excellent electric and optical properties. However, defects are inevitably introduced in the fabrication of TMDC monolayers, significantly influencing their emission properties. Although photoluminescence (PL) is considered as an effective tool for investigating the defects in TMDC monolayers. However, the PL from the defect-bound excitons is revealed only at low temperatures. Here, we show that the PL from the defect-bound excitons in a WS 2 monolayer can be effectively revealed at room temperature by exploiting the transverse electric polarized wave supported by a Si 3 N 4 /Ag heterostructure. It is revealed that the defect-bound excitons in all possible positions of the WS 2 monolayer can be effectively excited by the TE wave with significantly enhanced in-plane electric field localized on the surface of the Si 3 N 4 layer. In addition, the emission from defect-bound excitons can propagate to the collection point with small attenuation. More importantly, the exciton dynamics in the WS 2 monolayer can be modified by the Si 3 N 4 /Ag heterostructure, allowing the simultaneous excitation of neutral excitons, charge excitons (trions), and defect-bound excitons in the WS 2 monolayer attached on the Si 3 N 4 /Ag heterostructure. We inspect the PL spectra obtained at different positions and find that the relative intensity of defect-bound excitons depends on the collection position. We also examine the dependences of the PL intensity and bandwidth on the excitation power for the three types of excitons. It is found that they exhibit different behaviors from those observed in the optical measurements by using the traditional excitation method. Our findings suggest a new way for exciting and studying the dynamics of multi-excitons at room temperature and indicate the potential applications of the TE wave in probing the defects in TMDC monolayers.

Two-dimensional layered semiconductors have attracted intense interest in recent years. The van der Waals coupling between the layers tolerates stacking various materials and establishing heterostructures with new characteristics for a wide range of optoelectronic applications. The interlayer exciton dynamics at the interface within the heterostructure are vitally important for the performance of the photodetector and photovoltaic device. Here, a heterostructure comprising two-dimensional organic-inorganic Ruddlesden–Popper perovskites and transition metal dichalcogenide monolayer was fabricated and its ultrafast charge separation processes were systematically studied by using femtosecond time-resolved transient absorption spectroscopy. Significant hole and electron transfer processes in the ps and fs magnitude at the interface of the heterostructure were observed by tuning pump wavelengths of the pump-probe geometries. The results emphasize the realization of the exciton devices based on semiconductor heterostructures of two-dimensional perovskite and transition metal dichalcogenide.

Optical vortices (OVs), as eigenmodes of optical orbital angular momentum, have been widely used in particle micro-manipulation. Recently, perfect optical vortices (POVs), a subclass of OVs, are gaining increasing interest and becoming an indispensable tool in optical trapping due to their unique property of topological charge-independent vortex radius. Here, we expand the concept of POVs by proposing concentric ring optical traps (CROTs) and apply them to trapping and rotating particles. A CROT consists of a series of concentric rings, each being a vortex whose radius and topological charge can be controlled independently with respect to the other rings. Quantitative results show that the generated CROTs have weak sidelobes, good uniformity, and relatively high diffraction efficiency. In experiments, CROTs are observed to trap multiple dielectric particles simultaneously on different rings and rotate these particles with the direction and speed of rotation depending on the topological charge sign and value of each individual ring. In addition, gold particles are observed to be trapped and rotate in the dark region between two bright rings. As a novel tool, CROTs may find potential applications in fields like optical manipulation and microfluidic viscosity measurements.

The spin Hall effect of light (SHEL), a microscopic and transverse splitting of linearly polarized light into circularly polarized components during refraction and reflection, can be measured at subnanometer scales using weak measurements and has emerged as a powerful candidate for precision measurements. However, despite the strong demand for compact and miniaturized sensors and precision metrology, no efforts have downsized the weak measurements. Here I demonstrate that the location of the interface where the SHEL occurs does not impact the results of weak measurements and building on this observation, propose a modified setup called the compact weak measurement to reduce the form factor by replacing one convex lens with a concave one. The concept is theoretically validated and numerically confirmed across various setup parameters and interfaces. The compact weak measurement effectively reduces the required free space distance by twice the focal length and will facilitate the implementation of SHEL-based precision measurements in practical applications.