Ultraviolet (UV) light generation devices have further advantages in many applications such as optical data storage, optical information processing, and optical sensors. The UV region defined as the wavelength range from 10nm to 400 nm, which is further subdivided into four distinct regions: UV-A or long-wave UV (320–400 nm); UV-B or mid-wave UV (290–320 nm); UV-C or short-wave UV (200–290 nm); and vacuum UV (10–200 nm). Second-harmonic generation both in bulk and waveguide devices via quasi-phase-matching has been studied in crystals such as KTiOPO4, LiNbO3，and LiTaO3. However, UV light generation from quasi-phase-matching materials has several limitations, such as lower transmittance in the UV wavelength, optical damage, and diffculties in fabrication. To solve these problems, in this work we experimentally obtained on-chip long-wave UV in lithium-tantalate-on-insulator (LTOI) microdisk via modal-phase-matching frequency doubling, and the high optical Q of our LTOI microdisk in the visible band implies its great potential for cavity-enhanced nonlinear optics.

Fig. 1. LTOI experimental setup and optical characterization. (a) Experimental setup for nonlinear processes generating in the LTOI microdisk, VOA: variable optical attenuator. (b) and (c) are the transmission spectrum and Lorentzian fitting of a measured mode around 768.53 nm, respectively.

Lithium tantalate is a positive uniaxial crystal with ferroelectric properties, widely used in acousto-optic, electro-optic, integrated optics, nonlinear optics. Comparing to lithium niobate, the lithium tantalite is transparent in a larger wavelength range (0.28-5.5 µm), which can be a useful supplement for the lithium niobate in achieving the UV coherent light. The 50-µm-diameter microdisk was milled by focused ion beam (FIB) and followed by chemo-mechanical polishing (CMP) to smooth the disk surface and edge, and the Q-factor reaches 2.74×10^5 in the visible band. On-chip UV SHG coherent light with a wavelength of 384.3 nm was achieved, with the normalized conversion effciency of 5.74× 10^−6/W.

Fig. 2. Ultraviolet (UV) second harmonic generation in a LTOI microdisk. (a) The recorded spectrum of the UV SHG signal (around 384.3 nm) is produced by pump light in the visible band with different power. (b) Power dependence of the UV SHG signal on the visible fundamental pump. (c) Effective indices as functions of wavelength. Insets show the simulated mode profile of the FW and UV SH waves. (d) Intracavity pump power dependency of the generated SH power.

This is the first time to reach on-chip UV band in a LTOI microdisk to the best of our knowledge, which shows the great promise of LTOI using in the integrated ultraviolet source platform.

The work was published in “Miao Xue, Xiongshuo Yan, Jiangwei Wu, Rui Ge, Tingge Yuan, Yuping Chen, Xianfeng Chen, On-chip ultraviolet second-harmonic generation in lithium-tantalate thin film microdisk, Chinese Optics Letters, 21(6), 61902 (2023)”.

Link: https://doi.org/10.3788/COL202321.061902