Optical microcavity with a pair of suspended resonant mirrors

TL;DR

This work tackles achieving ultra-narrow optical linewidths in microcavities by employing two spectrally overlapping resonant (Fano) mirrors in a plane-parallel geometry. It introduces a resonant Fano mirror model and a planar Fabry–Perot framework to compare double-Fano cavities with broadband–broadband and single‑Fano configurations, showing that double-Fano cavities can yield narrower linewidths at short cavity lengths. Experimentally, it demonstrates two ultrathin SiN membranes patterned with 1D subwavelength gratings as Fano mirrors and constructs a double-Fano cavity, obtaining transmission spectra that agree with theory and observe the predicted linewidth reduction. The results point to routes for higher-Q cavities via improved Fano mirrors and spectral tuning, with potential impact on sensing and cavity optomechanics, including multi-membrane systems and bound-state-in-the-continuum resonances.

Abstract

We report on the realization of an optical microcavity consisting in the plane-plane arrangement of two suspended resonant mirrors possessing spectrally overlapping high-quality factor internal resonances. We first investigate its generic transmission spectra as the cavity length is varied on the basis of a simple linear Fabry-Perot model, compare them with those of broadband mirror cavities or Fano cavities possessing a single resoannt mirror, and then present an experimental realization using a pair of highly pretensioned, ultrathin silicon nitride films patterned with one-dimensional photonic crystal structures.

Figures (10)

• Figure 1: Broadband (left), single Fano (middle) and double Fano (right) mirror cavities.
• Figure 2: Transmission (red) and reflectivity (blue) spectra of the lossy resonant grating described in the text, which exhibits a high-reflectivity resonance around 951.2 nm with a quality factor of $\sim900$ and a peak reflectivity of 91.2%.
• Figure 3: Transmission spectra of single Fano (orange) and double Fano (magenta) mirror cavities for lengths $l=1$ mm (a) and $l=5.2$$\mu$m (b). The Fano mirror, whose reflectivity is shown as the dashed blue line, is that of Fig. \ref{['fig:spectra_grating_sim']}. For the single Fano mirror cavity, the reflectivity of the broadband mirror is chosen to be equal to the Fano mirror peak reflectivity (91.2%). (c) and (d): close ups of (a) and (b) around the Fano resonance.
• Figure 4: Simulated cavity linewidth (HWHM) as a function of its length for a broadband mirror cavity (black circles), single Fano mirror cavity (orange circles) and double Fano mirror cavity (magenta circles). In all cases, the total cavity losses at the Fano resonance wavelength are 17.6%. The points show the results of fits with Eq. (\ref{['eq:cavitylinewidth']}) of the numerically simulated spectra using Eq. (\ref{['eq:Tcav']}), while the black, orange and magenta dashed lines show the results of the analytical predictions of Eqs. (\ref{['eq:deltalambda_b']}), (\ref{['eq:deltalambda']}) and (\ref{['eq:deltalambda2']}), respectively.
• Figure 5: Double Fano mirror cavity transmission as a function of cavity length and wavelength for spectrally degenerate ($\Delta=0$, left) and spectrally non-degenerate ($\Delta=0.4$ nm (middle); $\Delta=1$ nm, right) Fano mirrors.