Recently, the emerge of the thin-film lithium niobate (TFLN) platform has propelled the development of integrated nonlinear photonics as well as LN based devices to a new height. Among these, whispering-gallery-mode (WGM) microresonators are indispensable units for frequency conversion, as they can confine light in a small volume for a long time and greatly enhance the light-matter interaction. In particular, TFLN is also an ideal platform for optical frequency comb (OFC) generation, promising for direct self-referencing. Generally, there are two different methods for on-chip frequency comb generation. The most common method is based on Kerr nonlinearity for spectral broadening. The other method is quadratic frequency comb generation which arises from a back-to-back OPO cascaded with SHG or SFG. Broadband SHG for quadratic frequency combs has always been difficult since efficient wavelength conversion is required across a wide wavelength with strong dispersion.

In this work, we achieve broadband SHG in the x-cut TFLN microdisk with a high Q factor of 3.32×10^7 by utilizing the scheme of cyclic quasi-phase matching (CQPM). The broadband SHG is observed using different sources including continuous-wave (cw) laser, femtosecond (fs) laser, supercontinuum (SC) laser and amplified spontaneous emission (ASE) in the telecommunication band. A high normalized conversion efficiency of 15.2%/mW is achieved using cw pump, and the efficient conversion spans a wide range of over 100 nm. These results show the strong capability of TFLN microdisks for integrated nonlinear.

The TFLN microdisk is fabricated utilizing ultraviolet lithography and chemo-mechanical polishing (CMP) with an extremely smooth sidewall and a small wedge angle of approximately 4°. The scanning electronic microscopy (SEM) topography of the microdisk sample is shown in Fig. 1. The thickness of the LN film is reduced to 550 nm after a second CMP procedure. The diameter of our x-cut LN microdisk is approximately 100 μm and that of the silica pedestal is about 50 μm. Large microdisks have high mode density, as they are supportive for higher-order WGM modes, which can facilitate simultaneous resonance for different waves during nonlinear mixing.

Fig. 1. False-colored SEM images of the TFLN microdisk viewed at different angles.

The experimental setup for the nonlinear optical experiments is schematically illustrated in Fig. 2(a). The pump is, respectively, launched from a continuous-wave (cw), femtosecond (fs), supercontinuum (SC) laser, and amplified spontaneous emission (ASE) in the communication band. The transmission spectrum of the microdisk is conducted by scanning the cw laser wavelength without thermal broadening at low input power of 0.5 μW, as shown in Fig. 2(b). The loaded Q factor of our TFLN microdisk is determined to be 3.32×10^7 by Lorentz fittings of the resonance at the wavelength of 1563.3 nm as shown in Fig. 2(c). The Q factor at the SH wavelength of at 781.7 nm is measured to be 2.83×10^6, as shown in Fig. 2(d).

Fig. 2. (a) Schematic of the experimental setup. (b) Transmission spectrum of the TFLN microdisk. (c)-(d) Lorentzian fitting of the resonances at 1563.3 nm and 781.7 nm, revealing a loaded Q factor of 3.32×10^7 and 2.83×10^6, respectively.

Figure 3(a) shows the observed spectrum of the SHG signal at the wavelength of 782.2 nm (The pump wavelength is 1564.3 nm). Cascaded third-harmonic generation (cTHG), i.e, simultaneous SHG and SFG of FW and the generated SH, is also observed in the experiment. By quadratic fitting of data points below 0.8 mW, the normalized SHG conversion efficiency of our microdisk is calculated to be 15.2%/mW.

Fig. 3. Experimental results of SHG in the TFLN microdisk. (a) Spectrum of the SHG and cTHG. (b) SH power as a function of the cw pump.

The SHG process in the TFLN microdisk is observed to be phase matched in a broad bandwidth, which is a direct result of the CQPM scheme. Figure 4(a) shows the recorded SHG spectra as the wavelength of cw pump is scanned from 1510 to 1630 nm (the whole tunable range of the laser) with an interval of ~1 nm. Figure 4(b) shows the SHG spectra as the pump wavelength is finely tuned with an interval of ~ 0.1 nm from 1550 to 1552 nm. According to the results, our high-Q TFLN microdisk shows a strong ability of efficient nonlinear conversion, which provides significant promise for direct SHG of broadband light.

Fig. 4. Experimental results of broadband SHG in the TFLN microdisk using tunable cw pump. (a)-(b) Spectra of broadband SHG with an wavelength interval of 1 nm and 0.1 nm, respectively.

Remarkably, broadband SHG could still be observed using fs, SC laser and ASE in our experiment. The spectra of the fs laser before (blue line) and after (red line) coupling with the microdisk are shown in Fig. 5(a). Figure 5(b) shows the spectrum of broadband SHG generated by the fs pump with a bandwidth of ~ 10 nm. The SHG as a function of the input fs power is shown in Fig. 5(c) which shows a quadratic relation with a normalized conversion efficiency of 3.5 ×10-5/mW. Similarly, Figures 5 (d) - (f) show the experimental results using SC laser, and (g) - (i) show the experimental results using ASE laser.

Fig. 5. Experimental results of the broadband SHG pumped by fs, SC laser and ASE in the TFLN microdisk. (a) Spectra of the fs laser before and after coupling into the microdisk. (b) Broadband SHG of fs laser. (c) SH power varies with respect to the incident fs pump. (d) Spectra of the SC laser before and after coupling into the microdisk. (e) Broadband SHG of the SC laser. (f) SH power varies with respect to the incident SC pump. (g) Spectra of the ASE before and after coupling into the microdisk. (h) Broadband SHG of ASE. (i) SH power varies with respect to the incident ASE pump.

It can be envisioned that the current scheme holds significant promise for frequency doubling of OFC if it is realized in the TFLN microdisk. And if OFC is directly generated in the microdisk the conversion efficiency can be much higher.

This work is published in “Zhu J, Ding T, Sun X, et al. Broadband second-harmonic generation in thin-film lithium niobate microdisk via cyclic quasi-phase matching[J]. Chinese Optics Letters, 2024, 22(3): 031903”.