Three-dimensional (3D) nonlinear photonic crystals have received intensive interest as an ideal platform to study nonlinear wave interactions and explore their applications. Periodic fork-shaped gratings are extremely important in this context because they are capable of generating second-harmonic vortex beams from a fundamental Gaussian wave, which has versatile applications in optical trapping and materials engineering. However, previous studies mainly focused on the normal incidence of the fundamental Gaussian beam, resulting in symmetric emissions of the second-harmonic vortices. Here we present an experimental study on second-harmonic vortex generation in periodic fork-shaped gratings at oblique incidence, in comparison with the case of normal incidence. More quasi-phase-matching resonant wavelengths have been observed at oblique incidence, and the second-harmonic emissions become asymmetric against the incident beam. These results agree well with theoretic explanations. The oblique incidence of the fundamental wave is also used for the generation of second-harmonic Bessel beams with uniform azimuthal intensity distributions. Our study is important for a deeper understanding of nonlinear interactions in a 3D periodic medium. It also paves the way toward achieving high-quality structured beams at new frequencies, which is important for manipulation of the orbital angular momentum of light.

基于面向对象的C++语言研制三维柱坐标共形网格生成程序，对束-场互作用器件作共形网格剖分，为粒子模拟算法提供积分线元、面元。通过定义三维柱坐标网格体系如网格步长、网格索引、守护网格层、包围盒等数据，使模型的空间信息能转换成柱坐标网格信息。将轴上网格单元作特殊处理，使粒子模拟算法形式在轴上网格和在非轴上网格上保持一致。利用射线跟踪法得到属于模型子面、模型棱边的离散边界点，接着通过拓扑关系获得模型的顶点，保存上述三类离散边界点的拓扑信息和网格信息。将构建的基础网格元与边界点信息耦合，在离散网格体系中重构模型。用该共形网格剖分技术对相对论磁控管进行剖分，能够识别该磁控管的透明阴极、阳极和谐振腔等结构。

We demonstrate the generation, spectral broadening and post-compression of second harmonic pulses using a thin beta barium borate (BBO) crystal on a fused-silica substrate as the nonlinear interaction medium. By combining second harmonic generation in the BBO crystal with self-phase modulation in the fused-silica substrate, we efficiently generate millijoule-level broadband violet pulses from a single optical component. The second harmonic spectrum covers a range from long wave ultraviolet (down to 310 nm) to visible (up to 550 nm) with a bandwidth of 65 nm. Subsequently, we compress the second harmonic beam to a duration of 4.8 fs with a pulse energy of 0.64 mJ (5 fs with a pulse energy of 1.05 mJ) using chirped mirrors. The all-solid free-space apparatus is compact, robust and pulse energy scalable, making it highly advantageous for generating intense second harmonic pulses from near-infrared femtosecond lasers in the sub-5 fs regime.

Whispering-gallery-mode (WGM) microresonators can greatly enhance light–matter interaction, making them indispensable units for frequency conversion in nonlinear optics. Efficient nonlinear wave mixing in microresonators requires stringent simultaneous optical resonance and phase-matching conditions. Thus, it is challenging to achieve efficient frequency conversion over a broad bandwidth. Here, we demonstrate broadband second-harmonic generation (SHG) in the x-cut thin-film lithium niobate (TFLN) microdisk with a quality factor above 10⁷ by applying the cyclic quasi-phase-matching (CQPM) mechanism, which is intrinsically applicable for broadband operation. Broadband SHG of continuous-wave laser with a maximum normalized conversion efficiency of ∼15%/mW is achieved with a bandwidth spanning over 100 nm in the telecommunication band. Furthermore, broadband SHG of femtosecond lasers, supercontinuum lasers, and amplified spontaneous emission in the telecommunication band is also experimentally observed. The work is beneficial for integrated nonlinear photonics devices like frequency converters and optical frequency comb generator based on second-order nonlinearity on the TFLN platform.

The design of nonlinear photonic Vogel’s spiral based on quasi-crystal theory was demonstrated. Two main parameters of Vogel’s spiral were arranged to obtain multi-reciprocal circles. Typical structure was fabricated by the near-infrared femtosecond laser poling technique, forming a nonlinear photonic structure, and multiple ring-like nonlinear Raman–Nath second-harmonic generation processes were realized and analyzed in detail. The structure for the cascaded third-harmonic generation process was predicted. The results could help deepen the understanding of Vogel’s spiral and quasi-crystal and pave the way for the combination of quasi-crystal theory with more aperiodic structures.

This article presents an 8-element dual-polarized phased-array transceiver (TRX) front-end IC for millimeter-wave (mm-Wave) 5G new radio (NR). Power enhancement technologies for power amplifiers (PA) in mm-Wave 5G phased-array TRX are discussed. A four-stage wideband high-power class-AB PA with distributed-active-transformer (DAT) power combining and multi-stage second-harmonic traps is proposed, ensuring the mitigated amplitude-to-phase (AM-PM) distortions across wide carrier frequencies without degrading transmitting (TX) power, gain and efficiency. TX and receiving (RX) switching is achieved by a matching network co-designed on-chip T/R switch. In each TRX element, 6-bit 360° phase shifting and 6-bit 31.5-dB gain tuning are respectively achieved by the digital-controlled vector-modulated phase shifter (VMPS) and differential attenuator (ATT). Fabricated in 65-nm bulk complementary metal oxide semiconductor (CMOS), the proposed TRX demonstrates the measured peak TX/RX gains of 25.5/21.3 dB, covering the 24?29.5 GHz band. The measured peak TX OP1dB and power-added efficiency (PAE) are 20.8 dBm and 21.1%, respectively. The measured minimum RX NF is 4.1 dB. The TRX achieves an output power of 11.0?12.4 dBm and error vector magnitude (EVM) of 5% with 400-MHz 5G NR FR2 OFDM 64-QAM signals across 24?29.5 GHz, covering 3GPP 5G NR FR2 operating bands of n257, n258, and n261.

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.