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Optica

Broadband, background-free, single-laser-shot absorption

Hans U. Stauffer, Patrick S. Walsh, Stephen A. Schumaker, Sukesh Roy

Published Jun 26, 2020• 15 citations
Publisher

Abstract

A robust approach for acquiring background-free, multitransition absorption spectra under single-laser-shot conditions is demonstrated using broadband, ultrashort laser pulses. This technique—referred to as time-resolved optically gated absorption (TOGA)—exploits the inherent differences in timescales between broadband, femtosecond-duration light sources and the longer-duration responses of narrowband atomic or molecular absorption features. An optical temporal gate, based on frequency mixing via sum-frequency generation or difference-frequency generation, is used to isolate these long-lived time-domain absorption features from the ultrashort component associated with the broadband absorption light source. A proof-of-principle demonstration of TOGA is provided using atomic Rb as an absorbing medium. Application of this technique toward single-laser-shot simultaneous detection of hydroxyl radical concentration and the corresponding local temperature is also demonstrated in a reacting flow. These results indicate that TOGA can provide spectrally resolved, broadband, background-free absorption measurements at laser-source repetition rates.

DOI
10.1364/optica.394469

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A robust approach for acquiring background-free, multitransition absorption spectra under single-laser-shot conditions is demonstrated using broadband, ultrashort laser pulses. This technique—referred to as time-resolved optically gated absorption (TOGA)—exploits the inherent differences in timescales between broadband, femtosecond-duration light sources and the longer-duration responses of narrowband atomic or molecular absorption features. An optical temporal gate, based on frequency mixing via sum-frequency generation or difference-frequency generation, is used to isolate these long-lived time-domain absorption features from the ultrashort component associated with the broadband absorption light source. A proof-of-principle demonstration of TOGA is provided using atomic Rb as an absorbing medium. Application of this technique toward single-laser-shot simultaneous detection of hydroxyl radical concentration and the corresponding local temperature is also demonstrated in a reacting flow. These results indicate that TOGA can provide spectrally resolved, broadband, background-free absorption measurements at laser-source repetition rates.
We propose and demonstrate a new method for single-shot multiplex absorption spectroscopy that permits enhanced sensitivity in the simultaneous measurement of multiple spectral lines in rapidly changing gas-phase media, such as turbulent flames. It uses an ultrashort laser pulse that propagates through the absorbing medium, for which the relevant absorption information resides in the free-induction decay that is trailing behind the transmitted pulse. Time gating out most of the transmitted pulse, but not the free-induction decay, enhances the relative fraction of light that contains absorption information when the spectrum is measured. This procedure reduces the background associated with the input light, thus enhancing detection sensitivity.
Atomic spectroscopy using broadband ultrashort laser pulses with durations in the 100-fs range seems nearly impossible because the large bandwidth of the pulses exceeds the line-width of atomic transitions by several orders of magnitude. Therefore the energy loss of ultrashort pulses crossing a narrow-linewidth medium is very low, even if the center frequency of the pulses is tuned to the transition frequency. For instance, the total absorption of a 100-fs pulse centered at the D/sub 2/-absorption line of Rb near 780 nm is well below 10/sup -3/ if the pulse is sent through a 75-mm-long Rb vapor cell at room temperature. Much more information can be obtained by measurement of the change of the pulse phase rather than the pulse energy after the pulse has crossed the medium. We found that a measurement technique similar to previously published second harmonic generation (SHG) frequency-resolved optical gating (FROG) is very convenient for that purpose. In our first experiments we send light pulses with a duration of 100 fs, a Fourier-limited bandwidth of 10 nm, and a pulse energy of 1 nJ through a 75-mm long Rb vapor cell. The pulses were analyzed with use of a SHG-FROG system composed on a single shot correlator and dispersive optics that depicted temporal and spectral information as two-dimensional images on a CCD sensor. The SHG-FROG signals depended strongly on the Rb vapor pressure inside the cell, which was controlled by the cell temperature. While these experiments analyzed steady states, the new technique principally allows the monitoring of transient states as well. We present first results on the investigation of optical pumped material.
Traditional absorption spectroscopy has a fundamental difficulty in resolving small absorbance from a strong background due to the instability of laser sources. Existing background-free methods in broadband vibrational spectroscopy help to alleviate this problem but face challenges in realizing either low extinction ratios or time-resolved field measurements. Here, we introduce optical-parametric-amplification-enhanced background-free spectroscopy, in which the excitation background is first suppressed by an interferometer, and then the free-induction decay that carries molecular signatures is selectively amplified. We show that this method can improve the limit of detection in linear interferometry by order(s) of magnitude without requiring lower extinction ratios or a time-resolved measurement, which can benefit sensing applications in detecting trace species.
The goal of this work is to study and understand dynamics of atomic systems interacting with strong fields. To this end, a method referred to as strong-field spectroscopy is used, which measures the absorption spectrum of atoms interacting with such fields. In the scope of this work, a technique which allows the simultaneous measurement of absorption spectra of attosecond pulses and their reference signal is developed, which significantly increases the sensitivity to absorption changes. Additionally, a reconstruction method is introduced which retrieves the time-dependent evolution of the dipole response of an atomic system directly from a single absorption spectrum. Strong-field spectroscopy combines these techniques to examine the nonlinear strong-field dynamics in atomic and molecular systems and is applied to study the strongly correlated doubly excited states in helium. Furthermore, a measurement approach is introduced which uses the interaction with a high-intensity laser pulse to impose a time gate on the evolution of the coherent response of a system, which has previously been excited by an ultrashort laser pulse. With this approach the buildup of resonance phenomena in a wide range of physical systems can be studied. As a first application, the time-dependent buildup of a Fano resonance as well as the buildup of a whole Rydberg series are investigated.
Summary form only given.Frequency-selective materials, based on spectral hole-burning allow recording holograms and interferograms of spectrally broad-band, temporally incoherent light signals. An important task for information technology as well as spectroscopic applications is simultaneous recording of the temporal, spectral, and spatial characteristics of ultrashort optical signals. Single-shot recording with a spectral hole-burning material will allow us to measure individual time-varying optical amplitudes without involving any temporal or spatial averaging, thus maintaining complete information about an optical signal. We have performed experiments on recording and playback of time- and space-domain holograms using single femtosecond pulses. We have studied the dependence of the hologram recording and playback on both the energy and the number of the applied pulses as well as on photon gating mechanism by an additional laser pulse at a different frequency.
High-peak-power ultrafast lasers are widely used in nonlinear spectroscopy, but often limit its spectral resolution because of the broad frequency bandwidth of ultrashort laser pulses. Improving the resolution by achieving spectrally narrow excitation of, or emission from, the resonant medium by means of multiphoton interferences has been the focus of many recent developments in ultrafast spectroscopy. We demonstrate an alternative approach in which high resolution is exercised by detecting narrow spectral correlations between broadband excitation and emission optical fields. All-optical correlation analysis, easily incorporated into a traditional spectroscopic setup, enables direct, robust, and simultaneous detection of multiple narrow resonances in a single measurement.