Abstract
The recent advancement in lithium-niobite-on-insulator (LNOI) technology is opening up new opportunities in optoelectronics, as devices with better performance, lower power consumption and a smaller footprint can be realised due to the high optical confinement in the structures. The LNOI platform offers both large χ(2) and χ(3) nonlinearities along with the power of dispersion engineering, enabling brand new nonlinear photonic devices and applications for the next generation of integrated photonic circuits. However, Raman scattering and its interaction with other nonlinear processes have not been extensively studied in dispersion-engineered LNOI nanodevices. In this work, we characterise the Raman radiation spectra in a monolithic lithium niobate (LN) microresonator via selective excitation of Raman-active phonon modes. The dominant mode for the Raman oscillation is observed in the backward direction for a continuous-wave pump threshold power of 20 mW with a high differential quantum efficiency of 46%. We explore the effects of Raman scattering on Kerr optical frequency comb generation. We achieve mode-locked states in an X-cut LNOI chip through sufficient suppression of the Raman effect via cavity geometry control. Our analysis of the Raman effect provides guidance for the development of future chip-based photonic devices on the LNOI platform.
Highlights
We demonstrate multi-wavelength Raman lasing in an X-cut high-Q lithium niobate (LN) microresonator with Raman frequency shifts of 250 cm−1, 628 cm−1, and 875 cm−1 via pumping with the transverse electric (TE) polarisation and a shift of 238 cm−1 with the traverse magnetic (TM) polarisation
LN devices are fabricated on an X-cut thin-film wafer, where the x-axis is normal to the wafer plane
LN has two Raman-active phonon symmetries: the A symmetry polarised along the z-axis and the E symmetry polarised in the degenerate x–y plane[19,21] due to the atomic vibration
Summary
LN has two Raman-active phonon symmetries: the A symmetry polarised along the z-axis and the E symmetry polarised in the degenerate x–y plane[19,21] due to the atomic vibration Both transverse (TO) and longitudinal (LO) optical phonon modes exist. The efficiency of the Raman effect is higher in the backward direction, which is phase-matched[44]; this is true for microscale waveguides that feature a non-negligible longitudinal electric field component[45]. This asymmetric gain can be attributed to strong polaritonic effects that affect the phase-matching conditions in the forward direction[24,46].
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