Continuous-wave cavity ring-down polarimetry

Abstract

We present a new cavity-based polarimetric scheme for highly sensitive and time-resolved measurements of birefringence and dichroism, linear and circular, that employs rapidly pulsed single-frequency continuous wave (CW) laser sources and extends current cavity-based spectropolarimetric techniques. We demonstrate how the use of a CW laser source allows for gains in spectral resolution, signal intensity, and data acquisition rate compared to traditional pulsed-based cavity ring-down polarimetry (CRDP). We discuss a particular CW-CRDP modality that is different from intensity-based cavity-enhanced polarimetric schemes as it relies on the determination of the polarization rotation frequency during a ring-down event generated by large intracavity polarization anisotropies. We present the principles of CW-CRDP and validate the applicability of this technique for the measurement of the non-resonant Faraday effect in solid SiO2 and CeF3 and gaseous butane. We give a general analysis of the fundamental sensitivity limits for CRDP techniques and show how the presented frequency-based methodology alleviates the requirement for high finesse cavities to achieve high polarimetric sensitivities and, thus, allows for the extension of cavity-based polarimetric schemes into different spectral regimes, but most importantly renders the CW-CRDP methodology particularly suitable for robust portable polarimetric instrumentations.

Chiral sensing with achiral isotropic metasurfaces

“…By taking their mean value, i.e., 2∆θ rev ≡ θ + + θ − and 2∆η rev ≡ η + + η − , any signal originating from the metasurface is cancelled, as well as any other potential achiral backgrounds, while the pure chiroptical signal, which is even under this polarization reversal, doubles. Thus, the importance of the signal reversal becomes apparent: under realistic experimental conditions, one can appropriately tune the frequency of the probing radiation around the resonance of the metasurface and apply this reversal (e.g., with the use of polar-ization modulators), isolating the chiral signal even under the presence of high-noise environments and other achiral effects (generally, signal reversals have been crucial for enabling sensitive measurements of circular birefringence in conditions where traditional polarimetry fails to perform [30,31,35]).…”

Absolute optical chiral analysis using cavity-enhanced polarimetry

“…Gaseous mixtures (chiral vapours) are introduced into the gas cell via PEEK tubing. We use a 6.35mm thick, AR-coated, SiO2 window as an intracavity magneto-optic crystal to generate Faraday-related circular birefringence, θ Q , with the use of permanent magnets attached directly to the mount holder of the substrate (in our case we obtain θ Q ≈ 2°) 67 . This intracavity Faraday rotation symmetrically splits the cavity resonances into two (orthogonal) circularly polarized cavity modes, i.e., one resonant for right circularly polarized light and the other for left circularly polarized light and, hence, denoted hereafter as R and L modes.…”

Rapid parameter determination of discrete damped sinusoidal oscillations

“…In Ref. [15], the authors demonstrate that the CRLB limit is the appropriate estimator of the fundamental sensitivity of frequency-based measurements within the context of CRDP, as the frequency measurements are directly translated into polarimetric results. However, one needs to carefully investigate whether different signal processing techniques can approach the CRLB, and if yes, under what conditions this is possible.…”

“…Several different research fields rely on the precise and accurate extraction of the time constants and frequencies of damped sinusoidal signals. Prominent examples include: nuclear magnetic resonance (NMR) [1], where information on the structure and the spin environment of a target molecule is extracted from precise determination of the frequency and decay constant of a damped sinusoidal signal; free-induction-decay (FID) optical magnetometry [2][3][4][5][6][7], where the magnetometric sensitivities depend on the precision of the measurement of the oscillating frequency; and pulsed/continuous-wave cavity ring-down polarimetry (CRDP) [8][9][10][11][12][13][14][15] and ellipsometry (CRDE) [16][17][18][19], where polarization-dependent absorption and refraction/reflection through/by an optical medium is extracted with high sensitivity through the precise measurement of the signal-decay time and its polarization beat frequency.…”