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Register a new userContinuous-Wave Cavity Ring-Down Polarimetry
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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 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 measurement of the non-resonant Faraday effect in solid SiO$_2$ and CeF$_3$ 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.
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Cavity ring-down spectroscopy is a ubiquitous optical method used to study light-matter interactions with high resolution, sensitivity and accuracy. However, it has never been performed with the multiplexing advantages of direct frequency comb spectroscopy without sacrificing orders of magnitude of resolution. We present dual-comb cavity ring-down spectroscopy (DC-CRDS) based on the parallel heterodyne detection of ring-down signals with a local oscillator comb to yield absorption and dispersion spectra. These spectra are obtained from widths and positions of cavity modes. We present two approaches which leverage the dynamic cavity response to coherently or randomly driven changes in the amplitude or frequency of the probe field. Both techniques yield accurate spectra of methane - an important greenhouse gas and breath biomarker. The high sensitivity and accuracy of broadband DC-CRDS, shows promise for applications like studies of the structure and dynamics of large molecules, multispecies trace gas detection and isotopic composition.
Rotationally resolved spectra of the C$^2Sigma^+$-X$^2Pi$ electronic system of the CH radical were measured using cavity ring-down spectroscopy in supersonically expanding, planar hydrocarbon plasma. The experimental conditions allowed the study of highly excited rotational levels starting from vibrationally excited states. Here we present some 200+ new or more accurately recorded transitions in the 0-0, 1-1 and 2-2 vibronic bands in the ultraviolet between 30900-32400 cm$^{-1}$ (324-309 nm). The resulting data, compared to earlier measurements, allows for the determination of more precise molecular constants for each vibrational state and therefore more precise equilibrium values. From this an equilibrium bond length of 1.115798(17) r{A} for the C$^2{Sigma}^+$ state is determined. A comprehensive list with observed transitions for each band has been compiled from all available experimental studies and constraints are placed on the predissociation lifetimes.
A novel mid-infrared/near-infrared double resonant absorption setup for studying infrared-inactive vibrational states is presented. A strong vibrational transition in the mid-infrared region is excited using an idler beam from a singly resonant continuous-wave optical parametric oscillator, to populate an intermediate vibrational state. High output power of the optical parametric oscillator and the strength of the mid-infrared transition result in efficient population transfer to the intermediate state, which allows measuring secondary transitions from this state with a high signal-to-noise ratio. A secondary, near-infrared transition from the intermediate state is probed using cavity ring down spectroscopy, which provides high sensitivity in this wavelength region. Due to the narrow linewidths of the excitation sources, the rovibrational lines of the secondary transition are measured with sub-Doppler resolution. The setup is used to access a previously unreported symmetric vibrational state of acetylene, $ u_1+ u_2+ u_3+ u_4^1+ u_5^{-1}$ in the normal mode notation. Single-photon transitions to this state from the vibrational ground state are forbidden. Ten lines of the newly measured state are observed and fitted with the linear least-squares method to extract the band parameters. The vibrational term value was measured to be at 9775.0018(45) $text{cm}^{-1}$, the rotational parameter $B$ was 1.162222 $text{cm}^{-1}$, and the quartic centrifugal distortion parameter $D$ was 3.998(62)$times 10^{-6} text{cm}^{-1}$, where the numbers in the parenthesis are one-standard errors in the least significant digits.
Sensing chirality is of fundamental importance to many fields, including analytical and biological chemistry, pharmacology, and fundamental physics. Recent developments have extended optical chiral sensing using microwaves, fs pulses, superchiral light, and photoionization. The most widely used methods are the traditional methods of circular dichroism and optical rotation (OR). However, chiral signals are typically very weak, and their measurement is limited by larger time-dependent backgrounds and by imperfect and slow subtraction procedures. Here, we demonstrate a pulsed-laser bowtie-cavity-enhanced polarimeter with counter-propagating beams, which solves these background problems: the chiral signals are enhanced by the number of cavity passes; the effects of linear birefringence are suppressed by a large induced intracavity Faraday rotation; and rapid signal reversals are effected by reversing the Faraday rotation and subtracting signals from the counter-propagating beams. These advantages allow measurements of absolute chiral signals in environments where background subtractions are not feasible: we measure optical rotation from chiral vapour in open air, and from chiral liquids in the evanescent wave (EW) produced by total internal reflection at a prism surface. EW-OR of (+)-maltodextrin and (-)-fructose solutions confirm the Drude-Condon model for Maxwells equations in isotropic optically active media. In particular, the effective optical rotation path length, near index matching, is equal to the Goos-Hanchen shift of the EW. The limits of this polarimeter, when using a continuous-wave laser locked to a stable high-finesse cavity, should match sensitivity measurements for linear birefringence ($3times 10^{-13}$ rad), which is several orders of magnitude more sensitive than current chiral detection limits, transforming the power of chiral sensing in many fields.
A Doppler broadening thermometry (DBT) instrument is built based on cavity ring-down spectroscopy (CRDS) for precise determination of the Boltzmann constant. Compared with conventional direct absorption methods, the high-sensitivity of CRDS allows to reach a satisfied precision at lower sample pressures, which also reduces the influence due to collisions. By recording the spectrum of C$_2$H$_2$ at 787 nm, we demonstrate a statistical uncertainty of 6 ppm (part per million) in the determined linewidth values by several hours measurement at a sample pressure of 1.5 Pa. The influence on the spectroscopy-determined temperatures has been investigated, including the hidden weak lines overlapped with the selected transition for DBT measurements. The reproducibility has also been examined to be better than 10 ppm, and it indicates that the instrument is feasible for DBT measurement toward a precision at the ppm level.
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