Objective Lithium niobate based integrated photonics has been receiving extensive attention over the past decades, and its development has gained significant momentum recently thanks to the emergence of lithium niobate on insulator (LNOI). While ensuring the high performance of the device, scalable fabrication and fiber compatibility are also particularly important for practical applications. Micrometer waveguides based on 3-mu m thick LNOI show excellent integration potential. For example, the lens fiber or high numerical aperture fiber can be directly coupled with the micro-waveguide with high efficiency, and the overall insertion loss of the device is small. The fabrication process of micro-waveguides by UV lithography and plasma dry etching features low fabrication cost and technical difficulty. In addition, the mode area of micro-waveguides is several times smaller than that of conventional proton-exchange or titanium indiffused waveguides, which ensures higher conversion efficiency. Here we propose to design and fabricate the LNOI micro-waveguide on 3-mu m thick LNOI, demonstrating its high performance in second harmonic generation (SHG) and sum-frequency generation (SFG) at the optical telecommunication band. The excellent frequency conversion capability, scalable fabrication, and fiber compatibility make the LNOI micro-waveguide highly appealing. We expect it to form a variety of functional devices in the future and promote the development of fundamental physics research and optical quantum information applications based on integrated photonics. Methods We fabricate the periodically poled LNOI (PPLNOI) micro-waveguide by direct electric poling followed by UV lithography and plasma dry etching techniques. We adopt the first-order quasi-phase matching (QPM) at 1550 nm, which determines the poling period. A 3-mu m z-cut magnesium-doped lithium niobate on insulator (MgO:LNOI) wafer is patterned via ultraviolet lithography and electron beam evaporation for electrodes. The electrodes are used for direct electric poling to achieve periodic domain reversal, and PPLNOI samples are obtained. Then we use UV lithography and plasma dry etching technology to transfer the designed micro-waveguide pattern to the PPLNOI layer. During the etching process, argon ion bombardment is used, and the etching depth reaches 3 mu m to completely penetrate the LNOI layer. Then, a protective layer of silica about 1 mu m thick is deposited on the surface by plasma-enhanced chemical vapor deposition ( PECVD). Finally, the end faces of the micro-waveguide are optically polished to obtain a buried PPLNOI micro-waveguide. Results and Discussions The processes of SHG and SFG are tested to evaluate the second-order nonlinear performance of the PPLNOI micro-waveguide. The normalized SHG efficiency of the micro-waveguide is investigated under small signal approximation (1. 12 mW pump). The quadratic relationship between the fundamental and the second harmonic powers is well established. The SHG conversion efficiency of the micro-waveguide is measured to be 335%/W at low pump powers. Considering the fiber coupling loss, the on-chip normalized conversion efficiency is 164%/( W center dot cm(2)). Compared with the theoretical prediction of 555%/(W center dot cm(2)), the obtained SHG normalized conversion efficiency still has a large room for improvement. As the pump power continues to increase, the quadratic relationship between them gradually degrades to linear and gradually converges to saturation. When the pump power reaches 1 W (the actual on-chip pump power is 750 mW), the second harmonic (SH) power reaches 429 mW, with an absolute on-chip conversion efficiency of 57.2%. In addition, the fiber coupling loss is only 1.2 dB/ facet. The overall absolute conversion efficiency of the device is close to 30% at 1 W pump light input and remains stable over one hour. The device can maintain stable frequency conversion under watt-scale power, which shows the great potential of MgO:PPLNOI micro-waveguide for realistic applications. Our PPLNOI waveguide also has excellent performance in the SFG process. Under 5-mW signal light, the conversion efficiency saturates when the input pumping exceeds 220 mW, and the absolute conversion efficiency is up to 139% in our experiment. This corresponds to the up-conversion of about 70% of the signal photons. The PPLNOI micro-waveguide has the advantages of low insertion loss, high scalability, and high performance. Its efficient frequency conversion provides a new choice for infrared light detection. Conclusions We demonstrate the fabrication process of PPLNOI micro-waveguides by UV lithography and dry etching on a 3-mu m thick LNOI platform. Results show that the micro-waveguide exhibits an SHG conversion efficiency of 335%/W at low powers and an absolute conversion efficiency of 57% under 1 W pump power. When the pump power reaches 300 mW, the absolute SFG conversion efficiency of the signal light reaches 139%, achieving efficient frequency upconversion. Our micro-waveguide is well compatible with optical fiber, featuring low insertion loss and better overall performance. It not only balances the normalized efficiency, coupling efficiency, and device length but also achieves highly efficient absolute frequency conversion at high power input, which makes LNOI micro-waveguides highly attractive for practical applications and advancing other areas of nonlinear optics.
