The compact and reliable ultraviolet (UV) source has attracted remarkable attention for its potential use in optical measurement systems, high-density optical storage, and biomedical applications. The UV region defined as the wavelength range from 10 nm 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 difficulties in fabrication. Therefore, on-chip compact and reliable UV coherent light realized by the small device size is of paramount importance.
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 um), which can be a useful supplement for the lithium niobate in achieving the UV coherent light. At present, there are few studies on lithium-tantalate-on-insulator (LTOI), and most of the researches are based on lithium tantalate bulk materials. The LTOI features a higher laser-radiation-induced damage threshold and a higher photorefractive damage threshold.
The research group led by Prof. Yuping Chen and Prof. Xianfeng Chen from Shanghai Jiao Tong University researched the on-chip 384.3 nm UV frequency doubling light in the lithium tantalate thin film microdisk via modal phase matching, which is the generation of the shortest UV light in the ferroelectric domain crystal thin film known so far, showing the great application prospect of the lithium tantalate thin film in the integrated UV light source platform. The research results are quickly published in Chinese Optics Letters , Volume 21, Issue 6, 2023 in just ten days from submission to acceptance (Miao Xue, et al., On-chip ultraviolet second-harmonic generation in lithium-tantalate thin film microdisk). For efficient SHG, it is necessary to satisfy energy and momentum conservations simultaneously, and the finite element method (FEM) is used to calculate the phase-matching condition. 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 2.74×105 in the visible band. On-chip UV SHG coherent light with a wavelength of 384.3 nm was achieved, with the normalized conversion efficiency of 5.74× 10−6/W. The researchers consider that one can obtain ultraviolet light to the medium-wave UV (UV-B, 280 to 320 nm) by optimizing the pump wavelength, power, and phase-matching conditions, and higher efficiencies can be realized by further optimization of the domain structure and experimental schemes.
Professor Yuping Chen from Shanghai Jiao Tong University pointed out that, although lithium tantalate belongs to ferroelectric crystal like lithium niobate, as a positive uniaxial crystal which has ferroelectric nature, it has completely different dispersion property. Lithium tantalate shows superiorities in resistance to laser damage and resistance to photorefraction compared with lithium niobate. Consequently, it's easier to observe some high-order nonlinear optical phenomenon for micro/nanophotonic device based on lithium tantalate membrane. In addition, its low UV cutoff wavelength predicts its unique application prospect. Although lithium tantalate is seldom applied into electro-optic modulator, its photoelastic coefficient in certain direction is equivalent to that of lithium niobate. Lithium tantalate shows universal application in acousto-optic modulator and surface acoustic wave (SAW) filter, and especially filters with high frequency and high bandwidth due to its good temperature stability.
Lithium tantalate thin film microdisk
