Quasi-phase-matched nonlinear optical frequency conversion in on-chip whispering galleries

Abstract

Chip-integrated whispering-gallery resonators enable compact and wavelength-agile nonlinear optical frequency synthesizers. So far, the most flexible phase-matching technique, i.e. quasi phase matching, has not been applied in this configuration. The reason is the lack of suitable thin films with alternating crystal structure on a low-refractive-index substrate. Here, we demonstrate an innovative method of realizing thin film substrates suitable for quasi phase matching by field-assisted domain engineering of lithium niobate, and subsequent direct bonding and polishing. We are able to fabricate high-Q on-chip WGRs with these substrates by using standard semiconductor manufacturing techniques. The Q-factors of the resonators are up to one million, which allows us to demonstrate quasi-phase-matched second-harmonic generation in on-chip WGRs for the first time. The normalized conversion efficiency is 9 × 10 −4 mW −1 . This method can also be transferred to other material systems. * These authors contributed equally to this work.

Periodically poled thin-film lithium niobate microring resonators with a second-harmonic generation efficiency of 250,000%/W

“…When the QPM condition m b − 2m a − M = 0 is satisfied, non-zero coupling strength g can then be accessed. The effective quadratic tensor element d eff of the LN material and the poling duty cycle D determine the effective χ (2) susceptibility of the PPLN microring with N-th order QPM, as denoted by χ (2) eff,N , through the relation [17,42] χ (2)…”

“…Leveraging the small mode volume and large power enhancement in high quality-factor (Q) optical microcavities [37], highefficiency SHG with low power consumption can be envisioned. In recent decades, cavity-enhanced SHG in LN integrated photonics platform has been reported through the techniques of modal (MPM) [18,38,39], cyclic (CPM) [34,40,41], and quasiphase matching (QPM) [17]. The latter predicts a higher SHG efficiency as it involves two fundamental modes for a larger modal overlap.…”

Efficient quasi-phase-matched frequency conversion in a lithium niobate racetrack microresonator

“…Here, we demonstrate for the first time, efficient second harmonic generation in a quasi-phasematched high-Q (Q 0 ≈ 5.3 × 10 5 ) racetrack microresonator with fundamental-modes matching. The recorded normalized conversion efficiency is 3.8% mW −1 , which is more than 40 times higher than recently reported quasiphase-matched lithium niobate microring [4].…”

Effective four-wave mixing in the lithium niobate on insulator microdisk by cascading quadratic processes

“…In LNOI microresonaotrs, quadratic nonlinear processes, such as second-harmonic generation (SHG) [16,17], sumfrequency generation (SFG) [18], parametric downconversion [19], and cascaded wave mixing [20,21], have been reported in different schemes to for various application demonstrations. Both modal phase matched SHG and difference frequency generation (DFG) processes have been demonstrated in the same LN microring resonator with a high conversion efficiency [22].…”