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

Author(s)

Lukas W. Perner, Gar-Wing Truong, Georg Winkler, Gang Zhao, David Follman, D. Michelle Bailey, Catherine Nguyen, Maximilian Prinz, Adam J. Fleisher, Garrett D. Cole, Oliver H. Heckl

Abstract

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 [1] 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) [2], resulting in F≈20900 and on-resonance cavity transmission of ~64% near 4.5 μm.
We use a novel crystalline coating technique [3] to produce distributed Bragg reflectors (DBR) at 4.54 μm with A+S<10 ppm [4]. 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.
As summarized in Fig. 1(a), 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 [4]. 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 [5], uncertainty propagation via a Monte Carlo method suggests ~10-3 uncertainties over a wide wavelength range, see Fig. 1(b).
Figure 1:(a) Total loss 1-R as measured in two CRD setups at NIST and CDL, fitted with polynomials; design transmittance T (dashed line); T via modelling pinned to measured coating parameters (red line); direct T measurement (triangles) (b) GaAs/AlGaAs refractive indices, via TMM fit based on precision FTS and SEM.
References:
[1] G. Rempe et al., Measurement of ultralow losses in an optical interferometer. Opt. Lett. 17, 363 (1992).
[2] M. G. Delli Santi et al., Biogenic fraction determination in fuel blends by laser-based 14CO2 detection. Adv. Phot. Res. 2, 2000069 (2021).
[3] G. D. Cole et al., High-performance near- and mid-infrared crystalline coatings. Optica 3, 647–656 (2016).
[4] G. Winkler et al., Mid-infrared interference coatings with excess optical loss below 10 ppm. Optica 8, 686–696 (2021).
[5] M. A. Afromowitz, Refractive Index of Ga1-xAlxAs. Solid State Comm. 15, 59–63 (1974)

Organisation(s)

Faculty Center for Nano Structure Research

External organisation(s)

Thorlabs Inc., National Institute of Standards and Technology, Gaithersburg

Pages

130-130

No. of pages

1

Publication date

2022

Peer reviewed

Yes

Austrian Fields of Science 2012

103021 Optics, 103040 Photonics, 205002 Coating technology

Portal url

https://ucrisportal.univie.ac.at/en/publications/c8f42b18-ebca-4942-9731-4d9d020ccdb3