Mid-Infrared Optical Frequency Combs based on Crystalline Microresonators

TL;DR

This paper presents a novel mid-infrared frequency comb generated using a crystalline microresonator, enabling broad, high-power, and widely spaced spectral lines suitable for advanced molecular spectroscopy.

Contribution

The authors demonstrate a new approach to generate mid-infrared frequency combs with high power, large mode spacing, and broad bandwidth using magnesium fluoride microresonators.

Findings

Generated about 100 comb lines spaced by 100 GHz

Spanning 200 nm (~10 THz) at 2.5 μm wavelength

Achieved efficient conversion in a compact device

Abstract

The mid-infrared spectral range (2 to 20 \mu m) is of particular importance for chemistry, biology and physics as many molecules exhibit strong ro-vibrational fingerprints. Frequency combs - broad spectral bandwidth coherent light sources consisting of equally spaced sharp lines - are creating new opportunities for advanced spectroscopy. Mid-infrared frequency comb sources have recently emerged but are still facing technological challenges, like achieving high power per comb line and tens of GHz line spacing as required for e.g. direct comb spectroscopy. Here we demonstrate a novel approach to create such a frequency comb via four-wave mixing in a continuous-wave pumped ultra-high Q crystalline microresonator made of magnesium fluoride. Careful choice of the resonator material and design made it possible to generate a broad comb of narrow lines in the mid-infrared: a vast cascade of about 100 lines spaced by 100 GHz spanning 200 nm (~10 THz) at \lambda=2.5 \mu m. With its distinguishing features of compactness, efficient conversion, large mode spacing and high power per comb line, this novel frequency comb source holds promise for new approaches to molecular spectroscopy even deeper in the mid-infrared.