Progress towards a 100 000 finesse optical cavity in the mid-infrared

Description

Precision spectroscopy, time-frequency metrology, stable optical reference signal generation, interferometry and other uses of optical cavities generally benefit from mirror coatings with vanishingly low excess loss (absorption A and scatter S). In the near-infrared and visible wavelengths, A+S<1 ppm and cavity finesses F>1 000 000 have already been achieved. However, using traditional physical vapor deposition techniques, the lowest reported mid-infrared (MIR) A+S is ~30 ppm (with transmittance T of ~120 ppm), resulting in F≈20900 and on-resonance cavity transmission of ~64% near 4.5 μm.
We use a novel crystalline coating technique to produce distributed Bragg reflectors (DBR) at 4.54 μm with A+S<10 ppm. A refined fabrication procedure allows us to minimize A+S by growing the DBR as two “half mirrors”, which are later bonded to form a 34.5 period GaAs/AlGaAs mirror. This buries growth defects in the middle of the DBR, thereby reducing excess losses. These wafers are then processed to 12 mm discs and transfer-bonded to super-polished, concave silicon (Si) optical substrates.
We measured a total single-mirror loss L=151±3 ppm and T=144±2 ppm (where L=T+A+S=1-R, with reflectance R) via two independent cavity-ringdown experiments and two independent techniques for isolating T. As S is negligible for crystalline MIR DBRs, we infer A=7±4 ppm, which we confirmed by photothermal common-path interferometry. This results in a finesse F=20 805±413 and a cavity transmittance >92%. This is key for future optical resonators with F>100 000 in the MIR, which can now be achieved by an increase in DBR period count (reducing T).
Fabrication of such DBRs with more periods requires precise knowledge and control of many parameters, among them the MIR refractive index n of GaAs/AlGaAs. We used an as-grown half mirror to probe n of both materials simultaneously by means of a transfer-matrix model (TMM), based on precision data obtained from Fourier-transform spectrometry (FTS) and cross-sectional scanning electron microscopy (SEM). Modelling both indexes as single effective oscillators, uncertainty propagation via a Monte Carlo method suggests ~10E-3 uncertainties over a wide wavelength range.

Period	15 Sep 2022
Event title	FLAIR 2022 - Field Laser Applications in Industry and Research 2022
Event type	Conference
Location	Aix-les-Bains, FranceShow on map
Degree of Recognition	International