In terms of optical information processing, optical components are gradually replacing electrical components because of their large operating bandwidth, wavelength division multiplexing, and no-electromagnetic crosstalk. Thin-film lithium niobate on insulator (LNOI) has shown significant potential as a versatile platform for manipulating photons due to its excellent characteristics. Whispering-gallery-mode microresonators, as one of the most important components of photonic integrated circuits (PICs), have the ability to dramatically boost light-matter interactions. Sum-frequency generation (SFG) can effectively convert low frequency weak light down to one single photon to high frequency optical signal with the aid of a strong pump. Besides, efficient manipulation of the frequency and the pulse shape of single photon signals for interfacing optical flying qubits with narrowband atomic quantum memories have also been demonstrated by using SFG process, because it does not disturb the quantum state. Therefore, it is rather important to realize SFG process in WGM resonators to expand its application in PICs.

Here, we demonstrate effective sum-frequency generation via modal phase matching in a triply-resonant lithium niobate-on-insulator microdisk resonator through two individual continuous wave pumps working in the communication band. The diameter of our fabricated LNOI microdisk is 49 μm. According to the adjacent WGMs, the measured free spectral range (FSR) of the modes is 7.02 nm. Typical resonance wavelengths of the 1st radial orders of TE modes TE(1, 166) have been determined by comparison with a finite-element simulation.

Fig. 1.  (a) Top view scanning electron microscope (SEM) image of the LNOI microdisk after FIB processing. (b) Optical microscope image of the microdisk after hydrofluoric acid etching. (c) Normalized transmission spectrum of the LNOI microdisk (d) The Lorentz fitting (red curve) reveals a Q factor of 1.8×105.

Fig. 2.  The experimental results of SFG. (a) Spectrum of the generated SFG and SHG. (b) Schematic of SFG signal generated through modal phase matching process.

The pumps are excited from TE(1, 167) and TE(1, 165) modes, the SFG peak at the wavelength 768.37 nm can also be observed. In this case, the two modes of TE(1, 167) and TE(1, 165) possess wavevector mismatches of +Δk and -Δk with respect to TE(1, 166). The phase matching condition can be satisfied through counteracting such phase mismatch when the pumps are excited from TE(1, 167) and TE(1, 165) modes. Therefore, such SFG signal generated through this method satisfies the modal phase matching condition and generates much stronger signal than their individual SHG. The sum-frequency conversion efficiency is measured to be 2.22×10-6 mW-1. This work shows a high-efficiency frequency convertor and may have the application in on-chip integrated optics in the future.

This research was published in “Xiaona Ye, Shijie Liu, Yuping Chen, Yuanlin Zheng, and Xianfeng Chen, Sum-frequency generation in lithium-niobate-on-insulator microdisk via modal phase matching, Optics Letters, 45(2), 523-526 (2020).”