We systematically investigate the power distribution characteristics of microjets generated by prismatic scatterers with different shapes at sub-THz region (λ = 8.57 mm). Among these prismatic scatterers, the hexagonal-type one shows better focusing feature than the others. Aiming at the hexagonal-type one, we propose a double-layer scatterer composed of a Teflon hexagonal prism as an outer layer and a semiconductor cuboid as an inner layer. Aiming at the double-layer scatterer, we further study the effects of refractive index, size, and shape of the inner cuboid on microjet’s features. The study allows us to present an optimized double-layer scatterer, which has a side length λ/2 (λ) and a refractive index 2.0 (1.4) for the inner (outer) layer. We show that the optimized scatterer can produce an ultra-strong, ultra-narrow microjet with a power enhancement of ∼ 30 and a full width at half maximum (FWHM) of ∼ 0.26λ, and the microjet is just located at the output face. The microjet keeps compact within the distance range of λ from the output face. These features and effects are explained from the viewpoint of ray optics theory. According to the optimized double-layer scatterer, we further study the multi-frequency focusing features of the microjets, and find that the microjet remains good features at harmonic frequencies 2f ₀ and 3f ₀. In addition, we investigate the effect of an Au sphere presence in the center of the microjet on the power distribution. The results show that a spherical dark spot with a size similar to that of the Au sphere emerges in the area where the Au sphere is placed. The feature can be used to measure the size of a metallic particle.

A mid-infrared difference frequency generator (DFG) based on a periodically poled thin-film lithium niobate rib waveguide on a sapphire substrate is theoretically studied. A mode analysis is carried out at the mid-infrared region, and the analysis focuses on the effects of waveguide geometry on effective refractive indices of a few lower-order modes. A complete theory suitable for modeling a DFG based on a waveguide structure is described. Its validity is confirmed by comparing the theoretical results with previously reported experimental data. Explicit expressions are presented for nonlinear conversion efficiency, thermal tunability and quasi-phase matching (QPM) bandwidth. The effects of waveguide geometry and mode hybridization on the effective mode field area and mode overlap factor, which are either inversely or linearly proportional to nonlinear conversion efficiency, are studied in detail. In this article, an optimized mid-infrared DFG with improved geometry that exhibits excellent performance, including a higher nonlinear conversion efficiency of 230–273% W⁻¹cm⁻² in the temperature range of 20–120 °C; a larger temperature tunability of 2.2 nm/°C; a larger QPM bandwidth of ~130 nm; and a higher idler wave output power, as much as −2 dBm when P_p = 20 dBm and P_s = 11.5 dBm, is suggested.

Highly integrated and stable devices are appealing in optical communication and sensing. This appeal arises from the presence of high refractive index contrast and high-quality waveguides. In this study, we improved the vapor proton exchange (VPE) process, enabling large-scale waveguide fabrication and addressing the issue of liquid exchange during cooling. Additionally, we have prepared and characterized planar waveguides on X-cut lithium niobate (LN) crystals. The exchanged samples exhibit α and k1 phases, refractive index contrasts as high as 0.082, and exceptional refractive index uniformity. Furthermore, we utilized the same process to fabricate channel waveguides and Y-branch waveguides. We achieved low propagation losses in channel waveguides, accompanied by small mode sizes, and low-loss Y-branch waveguides with a highly uniform beam splitting ratio. All waveguides exhibited consistent performance across multiple preparations and tests, remaining free from aging effects for three months. Our results underscore the promising potential of VPE for creating Y-branch splitters and modulators in LN crystals.

A theoretical study was performed on waveguiding characteristics of a special Ti:LiNbO₃ structure that an array of strip waveguides is embedded in a planar waveguide. First, a special Ti:LiNbO₃ structure was fabricated by diffusion of homogeneously coated Ti film at first and then photolithographically patterned Ti-strips with different widths of 4, 5, 6, 7, and 8 μ m, and the transverse magnetic (TM) mode guided in the strip waveguides of the special structure was characterized by near-field measurements. Second, a 2-D refractive index profile is supposed for the special structure. Third, based upon this supposed index profile model and the experimentally determined mode field profile type, the mode sizes of the strip waveguides were calculated using variational method and compared with the experimental results to verify the validity of the index model supposed and the numerical method adopted. Finally, the mode sizes and effective indexes of several lowest order TM modes guided in the strip waveguides were calculated as a function of the initial Ti-strip width for different waveguide parameters that include the surface index increment and depth (width) of both the planar and strip waveguides. The numerical results show that the field profile in the width direction is influenced by many factors such as the initial Ti-strip width, waveguide width and depth, and the surface index increment of the strip waveguide. Instead, the mode field in the depth direction is only influenced by three factors that include the depth of strip waveguide and the surface index increments of both planar and strip waveguides. The single-mode condition depends on all of these parameters.