Photo-birefringent effects in crystalline AlGaAs mirror coatings

High-reflective crystalline $GaAs/Al_{0.92}Ga_{0.08}As$ coatings show reduced Brownian noise compared to conventional dielectric coatings. However, several ultra stable laser systems observed additional noise sources that hinder the realization of the expected improvements in frequency stability. These additional noise sources are related to the birefringence of the coatings and its modification by intracavity light. The origin of the birefringence is not yet well understood and its modification via illumination remains unexplained. Here we present an extensive study on the steady-state and transient modification of the birefringence by intracavity light and by uniform illumination at various wavelengths using an optical cavity at room temperature. We find a unified description that suggests a primary two-photon process for photon energies below the bandgap of GaAs, or a single-photon process at higher energies. Adding external illumination allows to reduce noise induced by laser power fluctuations by balancing the photo-thermal-optic response of the mirrors and the photo-birefringent effect at a more favorable low intracavity power.

💡 Research Summary

This paper investigates the photo‑birefringent behavior of crystalline Al₀.₉₂Ga₀.₀₈As/GaAs multilayer mirror coatings, which are known for their dramatically reduced Brownian thermal noise compared with conventional dielectric stacks. Despite this advantage, ultra‑stable laser cavities employing these coatings have exhibited excess frequency noise that limits the anticipated performance gains. The authors identify the source of this excess noise as a light‑induced modification of the coating’s birefringence, which couples laser power fluctuations to the cavity resonance frequencies.

The experimental platform consists of a 48 cm ultra‑low‑expansion (ULE) spacer with fused‑silica substrates coated with 38.5 pairs of Al₀.₉₂Ga₀.₀₈As/GaAs layers. Two independent 1542 nm lasers are locked to the slow and fast polarization eigen‑modes of the cavity, providing a direct measurement of the frequency splitting Δ_biref (≈104 kHz, corresponding to Δn≈4.5×10⁻⁴). Intracavity power is inferred from the transmitted power (P_trans) using the known mirror transmission (T≈15 ppm). In addition, broadband LEDs at 450, 535, 625, and 890 nm illuminate the rear side of the near‑end mirror through the substrate, delivering a uniform, diffuse optical flux (I_LED).

The steady‑state measurements reveal two distinct regimes. For photon energies below the GaAs bandgap (the 1542 nm intracavity field), the birefringence shift scales with the square of the intracavity intensity, i.e., Δ_biref ∝ (P_int)². This quadratic dependence is interpreted as a two‑photon absorption process that generates a photo‑induced electric field, which in turn modifies the linear electro‑optic coefficient of the material. Conversely, for photon energies above the bandgap (the visible LEDs), the shift scales linearly with the LED intensity, consistent with a single‑photon absorption mechanism. By defining a single dimensionless variable

x = I_LED / I₀(λ) + (P_int / P₀)²

the authors collapse all data onto a universal curve described by a Shockley‑diode‑like logarithmic function:

Δ_biref = Δ₀ + Δ_s ln