在高功率激光装置中，光学元件表面的污染物会降低光束质量甚诱导光学元件损伤。针对装置中受污染的镀有SiO₂溶胶-凝胶增透膜的大口径真空隔离片（430 mm×430 mm），使用波长为355 nm的Nd:YAG脉冲激光器模拟在线激光清洗实验。实验中采用了单发次激光干式清洗与气流置换系统辅助的激光清洗系统，研究了关键特征参数对激光在线清洗效果的影响规律，获得了可用于激光在线清洗的工艺参数。光学元件的处理采用光学显微镜、暗场成像法表征以及图像处理软件分析。实验结果表明，激光在线清洗光学元件存在最佳工艺窗口。通过气流置换辅助的激光清洗方法后，相较于单纯的单发干式激光清洗，激光清洗能力有大幅提升。因此，气流置换系统辅助单发激光清洗能有效提高其清洗能力，为高功率激光装置中大口径光学元件表面污染物在线去除提供了一种有效手段。

We propose and experimentally demonstrate a dielectric metasurface that allows monitoring of polarization deviations from an arbitrary elliptical input anchor state simply by tracking in real-time the output ratio between the powers of horizontal and vertical components after the metasurface. Importantly, this ratio can be enhanced corresponding to increased responsivity. Such nontrivial functionality is achieved by designing binary metasurfaces that realize tailored nonunitary and chiral polarization transformation. We experimentally demonstrate the operation at telecommunication wavelengths with enhanced responsivity up to 25 for various anchor states, including the strongly elliptical and circular. We also achieve the uncertainty of deviation measurement that is significantly better than the fundamental limit for nonchiral metasurfaces.

The two-photon state with spatial entanglement is an essential resource for testing fundamental laws of quantum mechanics and various quantum applications. Its creation typically relies on spontaneous parametric downconversion in bulky nonlinear crystals where the tunability of spatial entanglement is limited. Here, we predict that ultrathin nonlinear lithium niobate metasurfaces can generate and diversely tune spatially entangled photon pairs. The spatial properties of photons including the emission pattern, rate, and degree of spatial entanglement are analyzed theoretically with the coupled mode theory and Schmidt decomposition method. We show that by leveraging the strong angular dispersion of the metasurface, the degree of spatial entanglement quantified by the Schmidt number can be decreased or increased by changing the pump laser wavelength and a Gaussian beam size. This flexibility can facilitate diverse quantum applications of entangled photon states generated from nonlinear metasurfaces.

We propose an efficient and low-power second harmonic generation (SHG) process in a silicon-compatible hybrid plasmonic microring resonator. By making the microring resonator doubly resonant at the fundamental wavelength of 3.1 μm and second harmonic wavelength of 1.55 μm, the SHG efficiency is enhanced by almost two orders of magnitude when compared to the previous result induced in a straight plasmonic waveguide. A SHG efficiency of 13.71% is predicted for a low pump power of 20 mW in a ring with radius of 2.325 μm. This device provides a potential route for realizing efficient frequency conversion between mid-infrared and near-infrared wavebands on a chip.