High-resolution spectroscopy of liquid water with dispersive atomic vapor prism cell.

This article presents an experimental demonstration of a spectroscopic method based on the dispersion of the scattering spectrum from laser-illuminated liquid water collected through a rubidium atomic vapor prism cell. Resonant absorption at 780 nm suppresses Mie/Rayleigh scattering and the steep gradients in refractive index near the 780 nm absorption lines separate Brillouin scattering from Raman scattering in liquid water. The opposing spatial displacements of the Stokes and Anti-Stokes shifted Brillouin peaks yield a measurement of their spectral shifts and thus the temperature or salinity of the water. Performance of the prism cell was mapped with a frequency tunable laser for frequency offsets from the center of the rubidium absorption feature of between -15 GHz and 15 GHz and at rubidium cell temperatures between 148 °C and 177 °C. The experimental results are compared with a numerical model and show good agreement with the scattering peak displacements within experimental uncertainties of probe frequency and cell temperature. In the present configuration, the minimum detectable frequency shift is estimated to be 15.5 MHz. Experiments were conducted in water demonstrating the utility of this method for the measurement of water temperature. Liquid water LiDAR was suggested as one of the possible applications for this method and several ways to improve the experimental setup and cell temperature stability were identified.

Thermometry of liquid water through slow light imaging spectroscopy.

This work presents the first, to the best of our knowledge, experimental demonstration of slow light imaging spectroscopy for thermometry of liquid water. This novel technique for measuring temperature relies on detecting the spectral shift of Brillouin peaks in water using the temporal delay through a cell containing an atomic vapor. Stand-off sensing capabilities are achieved by time-domain measurements of Brillouin scattering tuned to be near a rubidium atomic resonance and passed through a cell filled with rubidium vapor. An injection seeded optical parametric oscillator (OPO) is demonstrated to be a versatile light source for slow light imaging spectroscopy applications. The narrow OPO pulse spectrum allows for a precise profiling of slow light features of rubidium and accurate tracking of the temperature dependence of Brillouin scattering spectral shift. A comparison between the experimental data and numerical simulation over a temperature range of 20 to 99 degrees Celsius shows a good agreement for both qualitative and quantitative results.

Simultaneous single-shot rotation-vibration non-equilibrium thermometry using pure rotational fs/ps CARS coherence beating.

We report the development of a simple and sensitive two-beam hybrid femtosecond/picosecond pure rotational coherent anti-Stokes Raman scattering (fs/ps CARS) method to simultaneously measure the rotational and vibrational temperatures of diatomic molecules. Rotation-vibration non-equilibrium plays a key role in the chemistry and thermalization in low-temperature plasmas as well as thermal loading of hypersonic vehicles. This approach uses time-domain interferences between ground state and vibrationally excited N2 molecules to intentionally induce coherence beating that leads to apparent non-Boltzmann distributions in the pure rotational spectra. These distortions enable simultaneous inference of both the rotational and vibrational temperatures. Coherence beating effects were observed in single-shot fs/ps CARS measurements of a 75 Torr N2 DC glow discharge and were successfully modeled for rotational and vibrational temperature extraction. We show that this method can be more sensitive than a pure rotational fs/ps CARS approach using a spectrally narrow probe pulse. Lastly, we experimentally measured the beat frequencies via Fourier transform of the time-domain response and obtained excellent agreement with the model.

1D time evolving electric field profile measurements with sub-ns resolution using the E-FISH method.

We present an approach for the measurement of time evolving electric field profiles in atmospheric pressure plasma discharges using electric field induced second harmonic generation (E-FISH). While the E-FISH effect has been known of for some time, recent advances in laser and detection technology have allowed the method to be utilized for spatial measurements of an arbitrarily applied electric field. A cylindrical lens is used to focus the femtosecond laser light to a line and an intensified charge coupled device is used for detection, allowing for one-dimensional (1D) spatial resolution on the order of ∼50 µm. Measurements have been carried out verifying the spatial resolution using a spatially periodic, localized electric field. Calibrated 1D electric field measurements have been completed with a time resolution of 500 ps in a laminar cold atmospheric pressure plasma jet with argon core flow and N2 co-flow powered by a nanosecond (ns) pulse dielectric barrier discharge. The field was shown to propagate as an ionization wave, with a velocity of ∼0.3 mm/ns.

Three-photon femtosecond pumped backwards lasing in argon.

We demonstrate backwards lasing in atomic argon directly excited via a three-photon pumping in air mixtures with argon mole fractions down to 10%. We achieve well collimated, narrowband coherent emission at 1327nm by using both broadband femtosecond excitation and narrow linewidth picosecond excitation in the vicinity of 261nm. This approach shows promise for standoff trace detection in the atmosphere.

New diagnostic methods for laser plasma- and microwave-enhanced combustion.

The study of pulsed laser- and microwave-induced plasma interactions with atmospheric and higher pressure combusting gases requires rapid diagnostic methods that are capable of determining the mechanisms by which these interactions are taking place. New rapid diagnostics are presented here extending the capabilities of Rayleigh and Thomson scattering and resonance-enhanced multi-photon ionization (REMPI) detection and introducing femtosecond laser-induced velocity and temperature profile imaging. Spectrally filtered Rayleigh scattering provides a method for the planar imaging of temperature fields for constant pressure interactions and line imaging of velocity, temperature and density profiles. Depolarization of Rayleigh scattering provides a measure of the dissociation fraction, and multi-wavelength line imaging enables the separation of Thomson scattering from Rayleigh scattering. Radar REMPI takes advantage of high-frequency microwave scattering from the region of laser-selected species ionization to extend REMPI to atmospheric pressures and implement it as a stand-off detection method for atomic and molecular species in combusting environments. Femtosecond laser electronic excitation tagging (FLEET) generates highly excited molecular species and dissociation through the focal zone of the laser. The prompt fluorescence from excited molecular species yields temperature profiles, and the delayed fluorescence from recombining atomic fragments yields velocity profiles.

Femtosecond laser electronic excitation tagging for quantitative velocity imaging in air.

Time-accurate velocity measurements in unseeded air are made by tagging nitrogen with a femtosecond-duration laser pulse and monitoring the displacement of the molecules with a time-delayed, fast-gated camera. Centimeter-long lines are written through the focal region of a ∼1 mJ, 810 nm laser and are produced by nonlinear excitation and dissociation of nitrogen. Negligible heating is associated with this interaction. The emission arises from recombining nitrogen atoms and lasts for tens of microseconds in natural air. It falls into the 560 to 660 nm spectral region and consists of multiple spectral lines associated with first positive nitrogen transitions. The feasibility of this concept is demonstrated with lines written across a free jet, yielding instantaneous and averaged velocity profiles. The use of high-intensity femtosecond pulses for flow tagging allows the accurate determination of velocity profiles with a single laser system and camera.

Control of nitromethane photoionization efficiency with shaped femtosecond pulses.

The applicability of adaptive femtosecond pulse shaping is studied for achieving selectivity in the photoionization of low-density polyatomic targets. In particular, optimal dynamic discrimination (ODD) techniques exploit intermediate molecular electronic resonances that allow a significant increase in the photoionization efficiency of nitromethane with shaped near-infrared femtosecond pulses. The intensity bias typical of high-photon number, nonresonant ionization is accounted for by reference to a strictly intensity-dependent process. Closed-loop adaptive learning is then able to discover a pulse form that increases the ionization efficiency of nitromethane by ∼150%. The optimally induced molecular dynamics result from entry into a region of parameter space inaccessible with intensity-only control. Finally, the discovered pulse shape is demonstrated to interact with the molecular system in a coherent fashion as assessed from the asymmetry between the response to the optimal field and its time-reversed counterpart.

Detecting localized trace species in air using radar resonance-enhanced multi-photon ionization.

A microwave-scattering-based resonance-enhanced multi-photon ionization technique is used to detect molecular species such as NO, CO, Xe, and Ar in pure form, and for standoff detection of trace species in atmospheric pressure air. In this paper,the spectra, dynamics, and the detection limits of trace species in air are studied. We demonstrate 10 m scale standoff detection of NO, and show that the system has a linear response down to the parts in 10(9) NO levels in ambient air.

High-gain backward lasing in air.

The compelling need for standoff detection of hazardous gases and vapor indicators of explosives has motivated the development of a remotely pumped, high-gain air laser that produces lasing in the backward direction and can sample the air as the beam returns. We demonstrate that high gain can be achieved in the near-infrared region by pumping with a focused ultraviolet laser. The pumping mechanism is simultaneous resonant two-photon dissociation of molecular oxygen and resonant two-photon pumping of the atomic oxygen fragments. The high gain from the millimeter-length focal zone leads to equally strong lasing in the forward and backward directions. Further backward amplification is achieved with the use of earlier laser spark dissociation. Low-divergence backward air lasing provides possibilities for remote detection.

Real-time monitoring of blood using coherent anti-Stokes Raman spectroscopy.

We demonstrate a real-time method of measuring the vibrational Raman spectrum of whole blood. Using a novel coherent Raman technique, we record the vibrational spectrum of the red blood cells from picoliters in blood in milliseconds. This method will allow real-time in vivo blood monitoring.

Single-shot detection of bacterial endospores via coherent Raman spectroscopy.

Recent advances in coherent Raman spectroscopy hold exciting promise for many potential applications. For example, a technique, mitigating the nonresonant four-wave-mixing noise while maximizing the Raman-resonant signal, has been developed and applied to the problem of real-time detection of bacterial endospores. After a brief review of the technique essentials, we show how extensions of our earlier experimental work [Pestov D, et al. (2007) Science 316:265-268] yield single-shot identification of a small sample of Bacillus subtilis endospores (approximately 10(4) spores). The results convey the utility of the technique and its potential for "on-the-fly" detection of biohazards, such as Bacillus anthracis. The application of optimized coherent anti-Stokes Raman scattering scheme to problems requiring chemical specificity and short signal acquisition times is demonstrated.

Optimizing the laser-pulse configuration for coherent Raman spectroscopy.

We introduce a hybrid technique that combines the robustness of frequency-resolved coherent anti-Stokes Raman scattering (CARS) with the advantages of time-resolved CARS spectroscopy. Instantaneous coherent broadband excitation of several characteristic molecular vibrations and the subsequent probing of these vibrations by an optimally shaped time-delayed narrowband laser pulse help to suppress the nonresonant background and to retrieve the species-specific signal. We used this technique for coherent Raman spectroscopy of sodium dipicolinate powder, which is similar to calcium dipicolinate (a marker molecule for bacterial endospores, such as Bacillus subtilis and Bacillus anthracis), and we demonstrated a rapid and highly specific detection scheme that works even in the presence of multiple scattering.

Sensitive femtosecond coherent anti-Stokes Raman spectroscopy discrimination between dipicolinic acid and dinicotinic acid.

We demonstrate that femtosecond ultraviolet and visible coherent anti-Stokes Raman spectroscopy provides the sensitivity and specificity needed to distinguish between two similar molecules of pyridinedicarboxylic acid. The Fourier transforms of the temporal measurements provide the energy difference between the ground state vibrational modes. Quantum chemical calculations provide theoretical predictions that agree well with the measurements. The present technique allows us to distinguish 10 cm(-1) frequency shifts by using pulses ten times broader than the shifts.

Application of adaptive feedback loop for ultra-violet femtosecond pulse shaper control.

We apply an adaptive feedback loop to control a ultra-violet (UV) femtosecond pulse shaping apparatus. The adaptive feedback control is implemented by a continuous parameter genetic algorithm. We use the adaptive shaper to compensate for the pulse chirp. The genetic algorithm produces a pulse with a width of 115 fs, identical to that of the transformlimited pulse. We then apply the adaptive shaper to the Stokes pulse in a femtosecond coherent anti-Stokes Raman scattering (CARS) experiment on dipicolinic acid solution. The algorithm maximizes the first CARS beat signal at the probe pulse delay of 650 fs. We confirm that a transformlimited Stokes pulse achieves the best detection sensitivity.