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.
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.
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.
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.
Cavity-enhanced absorption spectroscopy is now widely used as an ultrasensitive technique in observing weak spectroscopic absorptions. Photons inside the cavity are reflected back and forth between the mirrors with reflectivities R close to one and thus (on average) exploit an absorption pathlength L that is 1/(1 − R ) longer than a single pass measurement. As suggested by the Beer-Lambert law, this increase in L results in enhanced absorbance A (given by αL with α being the absorption coefficient) which in turn favours the detection of weak absorptions. At the same time, however, only (1 − R ) of the incident light can enter the cavity [assuming that mirror transmission T is equal to (1 − R )], so that the reduction in transmitted light intensity Δ I caused by molecular absorption equates to that would be obtained if in fact no cavity were present . The enhancement in A = Δ I / I , where I is the total transmitted light intensity, achievable from CEAS therefore comes not from an increase in Δ I , but a sharp decrease in I. In this paper, we calculate the magnitudes of these two terms before and after a cavity is introduced, and aim at interpreting the sensitivity improvement offered by cavity-enhanced absorption spectroscopy from this observable-oriented (i.e. Δ I and I ) perspective. It is first shown that photon energy stored in the cavity is at best as intense as the input light source, implying that any absorbing sample within the cavity is exposed to the same or even lower light intensity after the cavity is formed. As a consequence, the intensity of the light absorbed or scattered by the sample, which corresponds to the Δ I term aforementioned, is never greater than would be the case in a single pass measurement. It is then shown that while this “numerator” term is not improved, the “denominator” term, I , is reduced considerably; therefore, the increase in contrast ratio Δ I / I is solely contributed by the attenuation of transmitted background light I and is ultimately down to the suppression of any measurement noise that is associated with it. The noise component that is most effectively suppressed is the type whose magnitude scales linearly with light intensity I , as is typical of noise caused by environmental instabilities, followed by the shot noise which scales as square root of I . No suppression is achievable for noise sources that are independent of I , a notable example being the thermal noise of a detector or of detection electronics. The usefulness of this “noise suppression” argument is that it links the sensitivity gain offered by a cavity with the property of measurement noise present in the system, and clearly suggests that the achievable sensitivity is dependent on how efficient the various noise components are “suppressed” by the cavity.
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.
The concept of resonant perfect absorption, enabled by the combined action of pulse propagation and an auxiliary gate pulse, was recently proposed and demonstrated in a group of two-level systems [Y. He , ]. Here we exploit this method in a more realistic scenario by solving the coupled time-dependent Schrödinger equation and the Maxwell wave equation in helium. Through emptying the population of the 1s2p excited state after its excitation, we explore the evolution of the spectral profile with time delay and propagation distance and link the observations to the controlled interference between the original field and the gated new field. We find that resonant absorption for higher-lying states can also be strongly enhanced, in spite of the congestion of multiple resonances and the presence of complex laser-induced couplings. Our results show that interferometric control of absorption using intense laser fields can be applied selectively in both the temporal and spatial domains. Published by the American Physical Society 2024