Gar-Wing Truong

Quantitative atomic spectroscopy for primary thermometry

Quantitative spectroscopy has been used to measure accurately the Doppler broadening of atomic transitions in {sup 85}Rb vapor. By using a conventional platinum resistance thermometer and the Doppler thermometry technique, we were able to determine k{sub B} with a relative uncertainty of 4.1x10{sup -4} and with a deviation of 2.7x10{sup -4} from the expected value. Our experiment, using an effusive vapor, departs significantly from other Doppler-broadened thermometry (DBT) techniques, which rely on weakly absorbing molecules in a diffusive regime. In these circumstances, very different systematic effects such as magnetic sensitivity and optical pumping are dominant. Using the model developed recently by Stace and Luiten, we estimate the perturbation due to optical pumping of the measured k{sub B} value was less than 4x10{sup -6}. The effects of optical pumping on atomic and molecular DBT experiments is mapped over a wide range of beam size and saturation intensity, indicating possible avenues for improvement. We also compare the line-broadening mechanisms, windows of operation and detection limits of some recent DBT experiments.

Accurate lineshape spectroscopy and the Boltzmann constant

Spectroscopy has an illustrious history delivering serendipitous discoveries and providing a stringent testbed for new physical predictions, including applications from trace materials detection, to understanding the atmospheres of stars and planets, and even constraining cosmological models. Reaching fundamental-noise limits permits optimal extraction of spectroscopic information from an absorption measurement. Here, we demonstrate a quantum-limited spectrometer that delivers high-precision measurements of the absorption lineshape. These measurements yield a very accurate measurement of the excited-state (6P1/2) hyperfine splitting in Cs, and reveals a breakdown in the well-known Voigt spectral profile. We develop a theoretical model that accounts for this breakdown, explaining the observations to within the shot-noise limit. Our model enables us to infer the thermal velocity dispersion of the Cs vapour with an uncertainty of 35 p.p.m. within an hour. This allows us to determine a value for Boltzmanns constant with a precision of 6 p.p.m., and an uncertainty of 71 p.p.m.

Open-path dual-comb spectroscopy to an airborne retroreflector

We demonstrate a new technique for spatial mapping of multiple atmospheric gas species. This system is based on high-precision dual-comb spectroscopy to a retroreflector mounted on a flying multi-copter. We measure the atmospheric absorption over long open-air paths to the multi-copter with comb-tooth resolution over 1.57 to 1.66 pm, covering absorption bands of CO2, Cm, H2O and isotopologues. When combined with GPS-based path length measurements, a fit of the absorption spectra retrieves the dry mixing ratios versus position. Under well-mixed atmospheric conditions, retrievals from both horizontal and vertical paths show stable mixing ratios as expected. This approach can support future boundary layer studies as well as plume detection and source location.

Atomic spectroscopy for primary thermometry

Spectroscopy has been a key driver and motivator of new understanding at the heart of physics. Here we describe high-precision measurements of the absorption lineshape of an atomic gas with an aim towards primary thermometry. We describe our progress in pushing this type of spectroscopy to the ultimate limit, in particular in describing experimental work with Rubidium and Cesium, although we also consider the potential for other elements in expanding the precision, accuracy and range of the approach. We describe the important technical and theoretical limits which need to be overcome in order to obtain accurate and precise results—these challenges are not unique to atomic spectroscopy but are likely to afflict all high precision spectroscopy measurements. We obtain a value for J K where the 71 ppm uncertainty arises with difficulties in defining the Lorentzian component of the lineshape.

Power-dependent line-shape corrections for quantitative spectroscopy

The Voigt profile—a convolution of a Gaussian and a Lorentzian—accurately describes the absorption lines of atomic and molecular gases at low probe powers. Fitting experimental absorption data to such a Voigt profile yields both the Lorentzian natural linewidth and the Gaussian Doppler broadening. However, as the probe power increases, saturation effects change the absorption line shape, such that it is no longer accurately described by a Voigt profile. Naively fitting a simple Voigt profile to the absorption line therefore introduces spurious power dependence into the extracted Doppler component. Using a simple atomic model, we calculate power-dependent corrections to the Voigt profile, which are parametrized by the Gaussian Doppler width, the Lorentzian natural linewidth, and the optical depth. We show numerically and experimentally that including the correction term substantially reduces the spurious power dependence in the fitted Gaussian width.

Absolute absorption line-shape measurements at the shot-noise limit

Spectroscopy has played the key role in revealing, and thereby understanding, the structure of atoms and molecules. A central drive in this field is the pursuit of higher precision and accuracy so that ever more subtle effects might be discovered. Here, we report on laser absorption spectroscopy that operates at the conventional quantum limit imposed by photon shot-noise. Furthermore, we achieve this limit without compromising the accuracy of the measurement. We demonstrate these properties by recording an absorption profile of cesium vapor at the 2 parts-per-million level. The extremely high signal-to-noise ratio allows us to directly observe the homogeneous lineshape component of the spectral profile, even while in the presence of Doppler broadening that is a factor of 100 times wider. We can do this because we can precisely measure the spectral profile at a frequency detuning more than 200 natural linewidths from the line center. We use the power of this tool to demonstrate direct measurements of a low-intensity optically-induced broadening process that is quite distinct from the well-known power broadening phenomenon.

Estimating vehicle carbon dioxide emissions from Boulder, Colorado using horizontal path-integrated column measurements

We performed 7.5 weeks of path-integrated concentration measurements of CO2, CH4, H2O, and HDO over the city of Boulder, Colorado. An open-path dual-comb spectrometer simultaneously measured time-resolved data across a reference path, located near the mountains to the west of the city, and across an over-city path that intersected two-thirds of the city, including two major commuter arteries. By comparing the measured concentrations over the two paths when the wind is primarily out of the west, we observe daytime CO2 enhancements over the city. Given the warm weather and the measurement footprint, the dominant contribution to the CO2 enhancement is from city vehicle traffic. We use a Gaussian plume model combined with reported city traffic patterns to estimate city emissions of on-road CO2 as (6.2 ± 2.2) × 105 metric tons (t) CO2 yr−1 after correcting for non-traffic sources. Within the uncertainty, this value agrees with the city’s bottom-up greenhouse gas inventory for the on-road vehicle sector of 4.5 × 105 t CO2 yr−1. Finally, we discuss experimental modifications that could lead to improved estimates from our path-integrated measurements.

Dual-Comb spectroscopy for GHG quantification

Near-infrared frequency-comb spectroscopy is a powerful tool with which to measure concentrations of gasses relevant to combustion and atmospheric monitoring (CO2, CH4, H2O, HDO) over meter and kilometer scale paths.

Real-time Phase Correction for High-SNR Fieldable Dual-Comb Spectroscopy

We discuss the significance of coherent averaging and real-time phase correction to dual-comb spectroscopy. A digital real-time phase correction implementation for high-SNR averaging is presented.

Dual Comb Outdoor Spectroscopy for Complex Molecular Response Retrieval

Trace gas concentration is measured across a turbulent open-air path. Using the appropriate dual-comb configuration, phase and absorbance spectra are simultaneously obtained. Decoherence caused by turbulence is estimated and corrected for.