Cascaded Raman lasing in a lithium tetraborate (LB4) whispering gallery mode resonator

TL;DR

This paper demonstrates the first cascaded Raman lasing in a lithium tetraborate whispering gallery mode resonator, achieving high quality factors and multiple Raman peaks, paving the way for compact Raman laser sources.

Contribution

It reports the first implementation of a lithium tetraborate WGMR Raman laser with high Q-factor and cascaded SRS, expanding applications of LB4 in nonlinear optics.

Findings

Achieved a Q factor of 2.0 x 10^9 at 517 nm.

Observed four cascaded SRS peaks from 537 nm to 608 nm.

First demonstration of LB4 whispering gallery mode Raman laser.

Abstract

Lithium tetraborate (LB4) is a lithium borate compound and recently has shown renewed interest due to its exceptional linear and nonlinear optical properties. Its wide transparency range, spanning from 0.16$\mu m$ to 3.5$\mu m$, and low loss in the visible range make LB4 highly popular in applications of harmonics generation and deep ultraviolet radiation. Also, LB4 is a good Raman-active material due to its high Raman gain. Here, a millimeter sized LB4 whispering gallery mode resonator (WGMR) is machined using single point diamond cutting, which has, to the best of our knowledge, the highest reported quality ($Q$) factor of $2.0 \times 10^9$ at 517 nm. Then, stimulated Raman scattering (SRS) was investigated in this LB4 WGMR. When pumped with about 7 mW at 517 nm, four cascaded SRS peaks with wavelengths ranging from 537 nm to 608 nm are demonstrated, which can be clearly observed using an optical grating. Among them, the first order SRS is characterized and has a threshold of 0.69 mW with a slope efficiency of 7.2 %. This is the first implementation of a LB4 whispering gallery mode Raman laser, which will facilitate usages of LB4 WGMR as compact Raman lasing source in future.