Broadband highly efficient nonlinear optical processes in on-chip integrated lithium niobate microdisk resonators of Q-factor above 10⁸

Abstract

Microresonators of ultrahigh quality (Q) factors represent a crucial type of photonic devices aiming at ultra-high spectral resolution, ultra-high sensitivity to the environmental perturbations, and efficient nonlinear wavelength conversions at low threshold pump powers. Lithium niobate on insulator (LNOI) microdisks of high Q factors are particularly attractive due to its large second-order nonlinear coefficient and strong electro-optic property. In this Letter, we break through the long standing bottleneck in achieving the Q factors of LNOI micro-resonators beyond 108, which approaches the intrinsic material absorption limit of lithium niobate (LN). The ultra-high Q factors give rise to a rich family of nonlinear optical phenomena from optical parametric oscillation (OPO) to harmonics generation with unprecedented characteristics including ultra-low pump threshold, high wavelength conversion efficiency, and ultra-broad operation bandwidth. Specifically, the threshold of OPO is measured to be only 19.6 μW, and the absolute conversion efficiency observed in the second harmonic generation reaches 23%. The record-breaking performances of the on-chip ultra-high Q LNOI microresonators will have profound implication for both photonic research and industry.

Photon Conversion and Interaction on Chip

“…In our case, g = 9.1 × 10 6 and Q p,i = 7.1 × 10 5 , so that θ = 0.01, which is already appreciable from a fundamental standpoint. Increasing θ to π/2 would require improving Q p,i to 10 8 , which has been demonstrated in polished TFLN microdisks [18].…”

“…This table only includes those whose conversion efficiency is over 10%. Note: a recent arxiv preprint reported SHG of ηcon = 33% [18]. According to its reported normalized efficiency of 602%/mW, the required pump power shall be Pp = 16 66% 602%/mW = 1.75 mW [14,15], which is over 16 times larger than its claimed value.…”

“…Compared with third-order nonlinear (χ (3) ) materials, TFLN is centro-asymmetric and possesses exceptionally large second-order nonlinearity (χ (2) ) to produce orders of magnitude stronger effects, as desirable to optical nonlinearities at a single photon level. Thus far, a variety of TFLN microresonators have been demonstrated with impressive nonlinearities [13][14][15][16][17][18]. However, their performance has been capped by the use of relatively small χ (2) susceptibilities (e.g., d 31 ∼ 4.7 pm/V) [15], poor mode overlapping [16,18], and/or low photonextraction efficiency [13,[15][16][17].…”

“…Thus far, a variety of TFLN microresonators have been demonstrated with impressive nonlinearities [13][14][15][16][17][18]. However, their performance has been capped by the use of relatively small χ (2) susceptibilities (e.g., d 31 ∼ 4.7 pm/V) [15], poor mode overlapping [16,18], and/or low photonextraction efficiency [13,[15][16][17]. For photon conversion and interaction, while their high efficiency has been predicted from, e.g., second-harmonic generation (SHG) [15][16][17] or parametric downconversion experiments [13], there has been no direct demonstration.…”

“…By fur-ther improving the cavity quality, strong photon-photon interaction is within sight. Based on the current nonlinear parameters, a cavity qualify factor of 10 8 -which has been demonstrated in lithium niobate microdisks [18]will enable C-NOT gate between single photons. Meanwhile, the demonstrated photon conversion and interaction efficiency can already elevate the performance of nonlinear-optical devices for heralded entanglement generation, faithful entanglement swapping, and so on.…”