arXiv subjects

Adam J. Fleisher

Publications and source records attributed to Adam J. Fleisher.

14 recordsLinked to original sources

arXiv2026

Bridging Theory and Experiment in Virtually Imaged Phased Array (VIPA) Spectrometers

Virtually imaged phased array (VIPA) spectrometers provide high resolution and fast acquisition in a compact design, but their performance is sensitive to fabrication tolerances, component dimensions, and alignment. Here, leveraging numerical simulations validated by experimental data, we present a framework to identify the parameters that limit VIPA spectrometer resolution. This framework is applied to the construction of a new mid-infrared VIPA spectrometer, tested at wavelengths near $\lambda$ = 4.6 $\mu$m with both continuous-wave and frequency-comb laser sources, with a resolving power predicted by analytical expressions to be as high as $RP$ = 830 000 (corresponding to a resolution of $\delta\nu$ = 78 MHz). Validated numerical simulations, however, provided a more realistic estimate that captures limits set by all the optical components. By minimizing aberrations and optimizing alignment, a resolving power of $RP$ = 440 000 ($\delta\nu$ = 150 MHz) was experimentally achieved, corresponding to 80 % of the value predicted by numerical simulation of the entire spectrometer. The results bridge the gap between analytical expressions and experimental results for compact, high-resolution VIPA spectrometers to enable more efficient fabrication and advanced design across critical areas like space optics, line-by-line pulse shaping, and broadband spectral sensors.

physics.optics↗

arXiv2023

Dual-comb Spectroscopy of Ammonia Formation in Non-thermal Plasmas

Plasma-activated chemical transformations promise the efficient synthesis of salient chemical products. However, the reaction pathways that lead to desirable products are often unknown, and key quantum-state-resolved information regarding the involved molecular species is lacking. Here we use quantum cascade laser dual-comb spectroscopy (QCL-DCS) to probe plasma-activated NH$_3$ generation with rotational and vibrational state resolution, quantifying state-specific number densities via broadband spectral analysis. The measurements reveal unique translational, rotational and vibrational temperatures for NH$_3$ products, indicative of a highly reactive, non-thermal environment. Ultimately, we postulate on the energy transfer mechanisms that explain trends in temperatures and number densities observed for NH$_3$ generated in low-pressure nitrogen-hydrogen (N$_2$-H$_2$) plasmas.

physics.chem-ph↗

arXiv2022

Dual electro-optic frequency comb photonic thermometry

We report a precision realization of photonic thermometry using dual-comb spectroscopy to interrogate a $\pi$-phase-shifted fiber Bragg grating. We achieve read-out stability of 7.5 mK at 1 s and resolve temperature changes of similar magnitude--sufficient for most industrial applications. Our dual-comb approach enables rapid sensing of dynamic temperature, and our scalable and reconfigurable electro-optic generation scheme enables a broad sensing range without laser tuning. Reproducibility on the International Temperature Scale of 1990 is tested, and ultimately limited by the frequency reference and check-thermometer stability. Our demonstration opens the door for a universal interrogator deployable to multiple photonic devices in parallel. Applications include on-chip measurements to simultaneously evaluate quantities like temperature, pressure, humidity, magnetic field and radiation dose.

physics.optics↗

arXiv2021

Frequency stabilization of a quantum cascade laser by weak resonant feedback from a Fabry-Pérot cavity

Frequency-stabilized mid-infrared lasers are valuable tools for precision molecular spectroscopy. However, their implementation remains limited by complicated stabilization schemes. Here we achieve optical self-locking of a quantum cascade laser to the resonant leak-out field of a highly mode-matched two-mirror cavity. The result is a simple approach to achieving ultra-pure frequencies from high-powered mid-infrared lasers. For short time scales (<0.1 ms), we report a linewidth reduction factor of $3\times10^{-6}$ to a linewidth of 12 Hz. Furthermore, we demonstrate two-photon cavity-enhanced absorption spectroscopy of an N$_{2}$O overtone transition near a wavelength of 4.53 $μ$m.

physics.optics↗

arXiv2020

Cavity ring-down spectroscopy of CO$_2$ near $λ$ = 2.06 $μ$m: Accurate transition intensities for the Orbiting Carbon Observatory-2 (OCO-2) "strong band"

The $λ$ = 2.06 $μ$m absorption band of CO$_2$ is widely used for the remote sensing of atmospheric carbon dioxide, making it relevant to many important top-down measurements of carbon flux. The forward models used in the retrieval algorithms employed in these measurements require increasingly accurate line intensity and line shape data from which absorption cross-sections can be computed. To overcome accuracy limitations of existing line lists, we used frequency-stabilized cavity ring-down spectroscopy to measure 39 transitions in the $^{12}$C$^{16}$O$_2$ absorption band. The line intensities were measured with an estimated relative combined standard uncertainty of $u_r$ = 0.08 %. We predict the $J$-dependence of the measured intensities using two theoretical modesl: a one-dimensional spectroscopic model with Herman-Wallis rotation-vibration corrections, and a line-by-line ab initio dipole moment surface model [Zak et al. JQSRT 2016;177:31-42]. For the second approach, we fit only a single factor to rescale the theoretical integrated band intensity to be consistent with the measured intensities. We find that the latter approach yields an equally adequate representation of the fitted $J$-dependent intensity data and provides the most physically general representation of the results. Our recommended value for the integrated band intensity equal to 7.183$\times$10$^{-21}$ cm molecule$^{-1}$ $\pm$ 6$\times$10$^{-24}$ cm molecule$^{-1}$ is based on the rescaled ab initio model and corresponds to a fitted scale factor of 1.0069 $\pm$ 0.0002. Comparisons of literature intensity values to our results reveal systematic deviations ranging from $-$1.16 % to +0.33 %.

physics.atom-ph↗

arXiv2020

Precision Spectroscopy of Nitrous Oxide Isotopocules with a Cross-Dispersed Spectrometer and Mid-Infrared Frequency Comb

As a potent greenhouse gas and ozone depleting agent, nitrous oxide (N$_2$O) plays a critical role in the global climate. Effective mitigation relies on understanding global sources and sinks which can be supported through isotopic analysis. We present a cross-dispersed spectrometer, coupled with a mid-infrared frequency comb, capable of simultaneously monitoring all singly substituted, stable isotopic variants of N$_2$O. Rigorous evaluation of the instrument lineshape function and data treatment are discussed. Laboratory characterization of the spectrometer demonstrates sub-GHz spectral resolution and an average precision of $7.4\times10^{-6}$ for fractional isotopic abundance retrievals in 1 s.

physics.ins-det↗

arXiv2020

Doppler-Free Two-Photon Cavity Ring-Down Spectroscopy of a Nitrous Oxide (N$_2$O) Vibrational Overtone Transition

We report Doppler-free two-photon absorption of N$_2$O at $λ$ = 4.53 $μ$m, measured by cavity ring-down spectroscopy. High power was achieved by optical self-locking of a quantum cascade laser to a linear resonator of finesse $F$ = 22730, and accurate laser detuning over a 400 MHz range was measured relative to an optical frequency comb. At a sample pressure of p = 0.13 kPa, we report a large two-photon cross-section of $σ_{13}^{(2)}$ = 8.0 $\times$ 10$^{-41}$ cm$^4$ s molecule$^{-1}$ for the $Q$(18) rovibrational transition at a resonant frequency of $ν_0$ = 66179400.8 MHz.

physics.chem-ph↗

arXiv2019

High-resolution cavity ring-down spectroscopy of the $ν_1 + ν_6$ combination band of methanol at 2.0 $μ$m

Reported here are portions of the infrared absorption cross-section for methanol (CH$_3$OH) as measured by frequency-stabilized cavity ring-down spectroscopy (FS-CRDS) at wavelengths near $λ$ = 2.0 $μ$m. High-resolution spectra of two gravimetric mixtures of CH$_3$OH-in-air with nominal mole fractions of 202.2 $μ$mol/mol and 45.89 $μ$mol/mol, respectively, were recorded at pressures between 0.8 kPa and 102 kPa and at a temperature of 298 K. Covering the experimental wavenumber range of 4990 cm$^{-1}$ to 5010 cm$^{-1}$ in increments of 0.0067 cm$^{-1}$ and with an instrument linewidth of 30 kHz, we observed an evolution in the CH$_3$OH spectrum from resolved absorption lines at a low pressure (0.833 kPa) to a pseudo-continuum of absorption at a near-atmospheric pressure (101.575 kPa). An analysis of resolvable features at the lowest recorded pressure yielded a minimum intramolecular vibrational energy redistribution (IVR) lifetime for the OH-stretch ($ν_1$) plus OH-bend ($ν_6$) combination of $τ_{IVR} \geq$ 232 ps - long compared to other methanol overtones and combinations. Consequently, we show that high-resolution FS-CRDS of this relatively weak CH$_3$OH combination band provided an additional avenue by which to study the intramolecular dynamics of this simplest organic molecule with hindered internal rotation.

physics.chem-ph↗

arXiv2018

Direct frequency comb saturation spectroscopy with an ultradense tooth spacing of 100 Hz

Electro-optic frequency combs with tooth spacings as low as 100 Hz were employed to probe electromagnetically induced transparency (EIT) and hyperfine pumping in rubidium and potassium vapor cells. From the potassium EIT transition we were able to determine the ground state hyperfine splitting with a fit uncertainty of 8 Hz. Importantly, because of the mutual coherence between the control and probe beams, which originate from a single laser, features with linewidths several orders-of-magnitude narrower than the laser linewidth could be observed in a multiplexed fashion. This approach removes the need for slow scanning of either a single laser or a traditional mode-locked-laser-based optical frequency comb.

physics.atom-ph↗

arXiv2018

Quantitative modeling of complex molecular response in coherent cavity-enhanced dual-comb spectroscopy

We present a complex-valued electric field model for experimentally observed cavity transmission in coherent cavity-enhanced (CE) multiplexed spectroscopy (i.e., dual-comb spectroscopy, DCS). The transmission model for CE-DCS differs from that previously derived for Fourier-transform CE direct frequency comb spectroscopy [Foltynowicz et al., Appl. Phys. B 110, 163-175 (2013)] by the treatment of the local oscillator which, in the case of CE-DCS, does not interact with the enhancement cavity. Validation is performed by measurements of complex-valued near-infrared spectra of CO and CO$_2$ by an electro-optic frequency comb coherently coupled to an enhancement cavity of finesse $F=19600$. Following validation, we measure the $30012\leftarrow00001$ $^{12}$C$^{16}$O$_2$ vibrational band origin with a combined standard uncertainty of 770 kHz (fractional uncertainty of $4\times10^{-9}$).

physics.ins-det↗

arXiv2016

Multiplexed sub-Doppler spectroscopy with an optical frequency comb

An optical frequency comb generated with an electro-optic phase modulator and a chirped radiofrequency waveform is used to perform saturation and pump-probe spectroscopy on the $D_1$ and $D_2$ transitions of atomic potassium. With a comb tooth spacing of 200 kHz and an optical bandwidth of 2 GHz the hyperfine transitions can be simultaneously observed. Interferograms are recorded in as little as 5 $μ$s (a timescale corresponding to the inverse of the comb tooth spacing). Importantly, the sub-Doppler features can be measured as long as the laser carrier frequency lies within the Doppler profile, thus removing the need for slow scanning or a priori knowledge of the frequencies of the sub-Doppler features. Sub-Doppler optical frequency comb spectroscopy has the potential to dramatically reduce acquisition times and allow for rapid and accurate assignment of complex molecular and atomic spectra which are presently intractable.

physics.atom-ph↗

arXiv2016

Coherent cavity-enhanced dual-comb spectroscopy

Dual-comb spectroscopy allows for the rapid, multiplexed acquisition of high-resolution spectra without the need for moving parts or low-resolution dispersive optics. This method of broadband spectroscopy is most often accomplished via tight phase locking of two mode-locked lasers or via sophisticated signal processing algorithms, and therefore, long integration times of phase coherent signals are difficult to achieve. Here we demonstrate an alternative approach to dual-comb spectroscopy using two phase modulator combs originating from a single continuous-wave laser capable of > 2 hours of coherent real-time averaging. The dual combs were generated by driving the phase modulators with step-recovery diodes where each comb consisted of > 250 teeth with 203 MHz spacing and spanned > 50 GHz region in the near-infrared. The step-recovery diodes are passive devices that provide low-phase-noise harmonics for efficient coupling into an enhancement cavity at picowatt optical powers. With this approach, we demonstrate the sensitivity to simultaneously monitor ambient levels of CO2, CO, HDO, and H2O in a single spectral region at a maximum acquisition rate of 150 kHz. Robust, compact, low-cost and widely tunable dual-comb systems could enable a network of distributed multiplexed optical sensors.

physics.optics↗

arXiv2012

Mid-infrared VIPA Spectrometer for Rapid and Broadband Trace Gas Detection

We present and characterize a 2-D imaging spectrometer based on a virtually-imaged phased array (VIPA) disperser for rapid, high-resolution molecular detection using mid-infrared (MIR) frequency combs at 3.1 and 3.8 μm. We demonstrate detection of CH4 at 3.1 μm with >3750 resolution elements spanning >80 nm with ~600 MHz resolution in a <10 μs acquisition time. In addition to broadband detection, rapid, time-resolved single-image detection is demonstrated by capturing dynamic concentration changes of CH4 at a rate of ~375 frames per second. Changes in absorption above the noise floor of 5\times 10-4 are readily detected on the millisecond time scale, leading to important future applications such as real time monitoring of trace gas concentrations and detection of reactive intermediates.

physics.optics↗

arXiv2012

Cavity-enhanced optical frequency comb spectroscopy in the mid-infrared - application to trace detection of H2O2

We demonstrate the first cavity-enhanced optical frequency comb spectroscopy in the mid-infrared wavelength region and report the sensitive real-time trace detection of hydrogen peroxide in the presence of a large amount of water. The experimental apparatus is based on a mid-infrared optical parametric oscillator synchronously pumped by a high power Yb:fiber laser, a high finesse broadband cavity, and a fast-scanning Fourier transform spectrometer with autobalancing detection. The comb spectrum with a bandwidth of 200 nm centered around 3.75 μm is simultaneously coupled to the cavity and both degrees of freedom of the comb, i.e., the repetition rate and carrier envelope offset frequency, are locked to the cavity to ensure stable transmission. The autobalancing detection scheme reduces the intensity noise by a factor of 300, and a sensitivity of 5.4 {\times} 10^-9 cm^-1 Hz^-1/2 with a resolution of 800 MHz is achieved (corresponding to 6.9 {\times} 10^-11 cm^-1 Hz^-1/2 per spectral element for 6000 resolved elements). This yields a noise equivalent detection limit for hydrogen peroxide of 8 parts-per-billion (ppb); in the presence of 2.8% of water the detection limit is 130 ppb. Spectra of acetylene, methane and nitrous oxide at atmospheric pressure are also presented, and a line shape model is developed to simulate the experimental data.

physics.optics↗