Laser-Machined Microresonators Offer Efficient SHG

Femtosecond laser micromachining and other processes can create smaller optical resonators out of lithium niobate (LN), potentially allowing their integration into chip-scale sensors.

Whispering-gallery-mode (WGM) microresonators developed at the Chinese Academy of Sciences and Zhejiang University demonstrated high quality factors at IR wavelengths, as well as efficient second harmonic generation capabilities in the visible range.

A scanning electron microscope image of the microresonator. Courtesy of Science China Press.

Until recently, the fabrication of high-quality LN resonators typically relied on a mechanical polishing approach, which often limits the resonator diameter to the millimeter scale, meaning such resonators cannot be used in chip-level integrated applications.

High-quality WGM microresonators can efficiently confine light in a very small volume via total internal reflection. They have benefitted applications including nonlinear optics, cavity quantum electrodynamics, quantum optics and biosensing, the researchers said.

Featuring high nonlinear optical coefficients, low intrinsic absorption loss and large transparent window, crystalline resonators have recently attracted significant attention, showing promise as next-generation nonlinear sources of light.

An optical microscope image of second harmonic generation of violet light. Courtesy of Science China Press.

The researchers achieved second harmonic generation in their 82-μm-diameter microresonators using wavelength-tunable narrow linewidth CW lasers and broadband femtosecond laser sources. At a CW pump wavelength of 799.884 nm, conversion efficiency reached 1.35×10⁻⁵/mW. Efficiency reached 2.30 × 10⁻⁶/mW using the pulsed pump source.

The resonators were fabricated using a commercially available LN thin film 0.67 μm thick bonded to at 2-μm silica layer on an LN substrate. Laser machining was followed by focused ion beam milling and high-temperature annealing.

Funding came from China's National Basic Research Program, National Natural Science Foundation and the Fundamental Research Funds for the Central Universities.

The research was published in Science China Physics, Mechanics & Astronomy (doi: 10.1007/s11433-015-5728-x).