Chiral sum-frequency generation (SFG) has proven to be a versatile spectroscopic and imaging tool for probing chirality. However, due to polarization restriction, the conventional chiral SFG microscopes have mostly adopted noncollinear beam configurations, which only partially cover the aperture of microscope and strongly spoil the spatial resolution. In this study, we report the first experimental demonstration of collinear chiral SFG microscopy, which fundamentally supports diffraction-limited resolution. This advancement is attributed to the collinear focus of a radially polarized vectorial beam and a linearly polarized (LP) beam. The tightly focused vectorial beam has a very strong longitudinal component, which interacts with the LP beam and produces the chiral SFG. The collinear configuration can utilize the full aperture and thus push the spatial resolution close to the diffraction limit. This technique can potentially boost the understanding of chiral systems.

通过小角度V形腔外腔光谱合束将两个808 nm半导体激光器合束，提高半导体激光器的输出功率及光束质量。两个合束单元分别工作在795.8 nm和800.5 nm，将所获光束通过非线性光学方法进行频率转换。外腔光谱合束实现输出功率为6.5 W快慢轴光束质量M²=2.2×18.5的光束输出，所获光束慢轴M²因子相较于自由运转单管激光器提高了30%，外腔光谱合束效率为83%。基于所获光源，实现了半导体激光器小角度V形腔外腔光谱合束和频，获得输出功率为18.3 mW波长为401.0 nm的蓝光输出，和频效率为0.28%。

As a newly discovered type of structured light, a spatiotemporal optical vortex (STOV), which is remarkable for its space–time spiral phase and transverse orbital angular momentum (OAM), has garnered substantial interest. Most previous studies have focused on the generation, characterization, and propagation of STOVs, but their nonlinear frequency conversion remains largely unexplored. Here, we experimentally demonstrate the generation of green and ultraviolet (UV) STOVs by frequency upconversion of a STOV carried near-infrared (NIR) pulse emitted by a high repetition rate Yb-doped fiber laser amplifier system. First, we verify that the topological charge of spatiotemporal OAM (ST-OAM) is doubled along with the optical frequency in the second-harmonic generation (SHG) process, which is visualized by the diffraction patterns of the STOVs in the fundamental and second-harmonic field. Second, the space–time characteristic of NIR STOV is successfully mapped to UV STOV by sum-frequency mixing STOV at 1037 nm and Gaussian beams in the green band. Furthermore, we observe the topological charges of the ST-OAM could be degraded owing to strong space–time coupling and complex spatiotemporal astigmatism of such beams. Our results not only deepen our understanding of nonlinear manipulation of ST-OAM spectra and the generation of STOVs at a new shorter wavelength, but also may promote new applications in both classical and quantum optics.

The phase summation effect in sum-frequency mixing process is utilized to avoid a nonlinearity obstacle in the power scaling of single-frequency visible or ultraviolet lasers. Two single-frequency fundamental lasers are spectrally broadened by phase modulation to suppress stimulated Brillouin scattering in fiber amplifier and achieve higher power. After sum-frequency mixing in a nonlinear optical crystal, the upconverted laser returns to single frequency due to phase summation, when the phase modulations on two fundamental lasers have a similar amplitude but opposite sign. The method was experimentally proved in a Raman fiber amplifier-based laser system, which generated a power-scalable sideband-free single-frequency 590 nm laser. The proposal manifests the importance of phase operation in wave-mixing processes for precision laser technology.

The control of thermal emission is of great importance for emerging applications in energy conversion and thermometric sensing. Usually, thermal emission at ambient temperature is limited to the mid- to far-infrared, according to the linear theory of Planck’s law. We experimentally demonstrate a broadband nonlinear thermal emission in the visible-NIR spectrum within a quadradic nonlinear medium, which emits visible thermal radiation through a pump-driven nonlinear upconversion from its mid-IR components even at room temperature, unlike its linear counterpart which requires ultrahigh temperature. The broadband emission is enabled by the crucial random quasi-phase-matching condition in our nonlinear nanocrystal powders. Moreover, nonlinear thermal emission also permits visible thermometry using traditional optical cameras instead of thermal ones. This scheme paves the way to understand thermal radiation dynamics with nonlinearity in many fields, such as nonlinear heat transfer and nonlinear thermodynamics.

介绍了一种LD泵浦Nd：YVO₄和Nd：YAG的单纵模589 nm激光器。1 064 nm与1 319 nm的基频光通过“L”型复合谐振腔获得同时振荡，通过Ⅱ类相位匹配切割的KTP晶体腔内和频以及由布鲁斯特片和KTP组成的双折射滤波器的选频，获得了单纵模589 nm连续激光器。利用琼斯矩阵方法分析了两个基频光的S偏振和P偏振光通过双折射滤波器时的损耗。结果表明：1 064 nm和1 319 nm次纵模的损耗分别比峰值透射率纵模的损耗大0.5%和2%以上。以此为基础，在实验上实现了一种单纵模589 nm激光器，其最大输出功率为58 mW，功率稳定性优于0.36%，线宽约为30 MHz。双折射滤波器方法对双波长振荡及和频激光器实现单纵模输出是一种有效的方法。