Cavity buildup dispersion spectroscopy

Abstract

Measurements of ultrahigh-fidelity absorption spectra can help validate quantum theory, engineer ultracold chemistry, and remotely sense atmospheres. Recent achievements in cavity-enhanced spectroscopy using either frequency-based dispersion or time-based absorption approaches have set new records for accuracy with uncertainties at the sub-per-mil level. However, laser scanning or susceptibility to nonlinearities limits their ultimate performance. Here we present cavity buildup dispersion spectroscopy (CBDS), probing the CO molecule as an example, in which the dispersive frequency shift of a cavity resonance is encoded in the cavity’s transient response to a phase-locked non-resonant laser excitation. Beating between optical frequencies during buildup exactly localizes detuning from mode center, and thus enables single-shot dispersion measurements. CBDS can yield an accuracy limited by the chosen frequency standard and measurement duration and is currently 50 times less susceptible to detection nonlinearity compared to intensity-based methods. Moreover, CBDS is significantly faster than previous frequency-based cavity-enhanced methods. The generality of CBDS shows promise for improving fundamental research into a variety of light–matter interactions.

Dual-comb cavity ring-down spectroscopy

“…The corresponding field for the case of instantaneous switching off the probe comb at , is where the first term describes fields of conventional ring-down signals of unique modes excited by the probe comb and are the conventional intensity-based ring-down time constant. It is clear that the same transient cavity responses, , can be observed in both situations—when the probe signal is switched on and off—if the cavity response can be spectrally separated from the probe comb excitation field 24 and from the responses of other cavity modes. For this purpose, a heterodyne beat signal between the comb-like transmission spectrum of the cavity and the LO comb can be observed where the difference in repetition frequencies between the probe and LO combs, , must be large enough to resolve the mode widths .…”

“…Consider an optical cavity excited by an optical frequency comb (probe comb) switched on instantaneously at time whose transmitted fields beat with another frequency comb (local oscillator) bypassing the cavity. The total output electric field as a function of time can be described as a sum of fields of individual cavity modes with corresponding teeth of the probe comb 23 , 24 and the local oscillator (LO) comb where and are field amplitudes of the probe comb at the output of the cavity and LO comb teeth; and are angular frequencies of the probe and LO comb teeth; and are angular frequencies and spectral widths (HWHM) of the cavity modes, and are phase shifts between the probe and LO comb teeth. The term represents the transient response of the cavity (Green’s function) induced by the step-change in the probe field amplitude.…”

“…Similar to the derivation of the cavity buildup spectrum given in Ref. 24 , the intensity of the DC-CRDS field given by Eq. ( 1 ), is .…”

“…( 6 ) and ( 7 ) can be used to calculate the real-valued quantity, , to model the power spectral density of the measured signal for each time interval. Evaluation of accounting for cross-terms and wings from non-local resonances can be approximated by Lorentzian shapes with asymmetric, dispersive components 24 that are dependent on the phase .…”

“…In CRDS, the modulation of the probe light induces transient fields that oscillate only at the cavity resonant frequencies 23 , 24 . Consequently, CRDS decay rates are immune to the probe-cavity frequency mismatch, and signal frequencies arising from parallel cavity mode excitation encode both mirror and sample dispersion.…”