The ultimate goal of artificial intelligence (AI) is to mimic the human brain to perform decision-making and control directly from high-dimensional sensory input. Diffractive optical networks (DONs) provide a promising solution for implementing AI with high speed and low power-consumption. Most reported DONs focus on tasks that do not involve environmental interaction, such as object recognition and image classification. By contrast, the networks capable of decision-making and control have not been developed. Here, we propose using deep reinforcement learning to implement DONs that imitate human-level decision-making and control capability. Such networks, which take advantage of a residual architecture, allow finding optimal control policies through interaction with the environment and can be readily implemented with existing optical devices. The superior performance is verified using three types of classic games: tic-tac-toe, Super Mario Bros., and Car Racing. Finally, we present an experimental demonstration of playing tic-tac-toe using the network based on a spatial light modulator. Our work represents a solid step forward in advancing DONs, which promises a fundamental shift from simple recognition or classification tasks to the high-level sensory capability of AI. It may find exciting applications in autonomous driving, intelligent robots, and intelligent manufacturing.
Here, we experimentally demonstrate broadband nonlinear imaging by using Si resonant metasurfaces based on third-order nonlinear processes, including third harmonic generation (THG) and four-wave mixing (FWM). Particularly, the quadratic dependence of the FWM emission on the pump power allows to significantly reduce the dependence on high-power signal input in nonlinear imaging via designing a high-Q resonance at the pump wavelength based on bound states in the continuum (BICs). In the experiment, our metasurfaces support high-performance infrared imaging, including a large acceptance angle, a high frame rate, and a wide range of operating wavelengths (from 900 nm to 2500 nm).
we exploit the resonance inside the slab to boost the second-harmonic generation from the lithium niobate half-etching-slab metasurface system, which hosts two kinds of BIC states formed by TE and TM odd modes. We reveal the mechanisms of the formation of the two BIC states and three main processes that influence the conversion efficiency from the pump light into the SH radiation. This approach of constructing boosted second-harmonic generation can be applied in any low-index nonlinear materials, which can expand a range of the applications, such as nonlinear imaging, quantum light source, and high-resolution sensing.
High-index dielectric resonators support different types of resonant modes. However, it is challenging to achieve a high-Q factor in a single dielectric nanocavity due to the non-Hermitian property of the open system. We present a universal approach of finding out a series of high-Q resonant modes in a single nonspherical dielectric cavity with a rectangular cross section by exploring the quasi bound-state-in-the-continuum (QBIC). Unlike conventional methods relying on heavy brutal force computations (i.e., frequency scanning by the finite difference time domain method), our approach is built upon Mie mode engineering, through which many high-Q modes can be easily achieved by constructing avoid-crossing (or crossing) of the eigenvalue for pair-leaky modes. The calculated Q-factor of mode TE(5,7) can be up to Qtheory = 2.3 × 104 for a freestanding square nanowire (NW) (n = 4), which is 64 times larger than the highest Q-factor (Qtheory ≈ 360) reported so far in a single Si disk. Such high-Q modes can be attributed to suppressed radiation in the corresponding eigenchannels and simultaneously quenched electric (magnetic) field at momentum space. As a proof of concept, we experimentally demonstrate the emergence of the high-Q resonant modes [Q ≈ 211 for mode TE(3,4), Q ≈ 380 for mode TE(3,5), and Q ≈ 294 for mode TM(3,5)] in the scattering spectrum of a single silicon NW.
We propose an ultra-thin silicon metasurface supporting a high-quality leaky mode which is formed by partially breaking a bound-state-in-the-continuum (BIC) generated by the collective magnetic dipole (MD) resonance excited in the subdiffractive periodic systems. Such a quasi-BIC MD state leads to a robust near-field enhancement and a significant boost of the nonlinear process, resulting in measured 500-fold enhancement of third-harmonic emission in comparison to the conventional silicon disk metasurface. We further experimentally demonstrate the highly-efficient nonlinear image tuning via polarisation and wavelength control, opening the way for various applications in high-performance nonlinear metadevices including tunable laser, tunable displays, nonlinear imaging.
We investigate the high order anapole mode in single dielectric nanostructure with high refractive index from eigenmode perspective. We find that the anapole mode in both cylinder and sphere can only occur in the following two situations:(1) If only one mode is involved, the combined phase of intrinsic and extrinsic phase should be equal to 2π at certain frequency that is close to the resonance. (2) If two leaky modes are involved, the combined phase for each mode must be 2π at same frequency which is located between two resonances.
Switching the scattering direction of high-index dielectric nanoantennas between forward and backward, via Mie resonances in the linear regime, has been widely studied, recently. However, switching the harmonic emission of nanoantennas without applying any physical change to the antennas, such as geometry, or environment, is a chal- lenging task that has not been demonstrated yet. Here, we investigate multipolar second-harmonic switch from GaAs nanoantennas. Based on the peculiar nonlinearities of zinc-blende semiconductors, we demonstrate both theoretically and experimentally unidirectional nonlinear emission routing and switching via pump polarization control. Our results offer exciting opportunities for nonlinear nanophotonics technologies, such as nanoscale light routing elements, nonlinear light sources, nonlinear imaging, multifunctional flat optical elements.
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