Nonlinear optical techniques for ultrafast laser diagnostics : Development of femtosecond LIF, LIGS, CARS and backward lasing

The thesis work concerns development and application of four versatile nonlinear optical techniques, based on exploiting ultrashort laser pulses, for diagnostic purposes in gases and flames. The four techniques, all laser-based, are two-photon laser-induced fluorescence (TPLIF), hybrid femtosecond/nanosecond (fs/ns) rotational coherent anti-Stokes Raman scattering (fs/ns RCARS), fs-laser-induced grating spectroscopy (fs-LIGS), and backward lasing. Special characteristics of fs-laser pulses, such as short pulse duration, high peak power even at low pulse energy, and broad spectral bandwidth, advance the development of these techniques to a new level.In the TPLIF project, two-photon excited fluorescence of CO in CH_4/air flames was investigated and compared for ns, picosecond (ps), and fs-laser excitation. Moreover, based on fs-laser excitation, simultaneous interference-free two-photon excited fluorescence imaging of H and O atoms in turbulent flames was performed for the first time. In comparison with previous studies, it has been demonstrated that significantly larger measurement areas can be visualized in single-shot acquisitions.A concept for lasing in the backward direction, facilitated by fs-laser excitation, was developed and demonstrated for range-resolved detection of H atoms in flames. The technique shows great potential for stand-off measurements in devices with only one optical access. On the fundamental level, studies were performed to uncover the physical mechanism responsible for the lasing effect.With the developed hybrid fs/ns RCARS technique, the RCARS signal can be recorded with a high spectral and temporal resolution, simultaneously, allowing Raman linewidths to be measured on a single-shot basis. This capacity is of great importance for thermometry as it, in principle, eliminates the need for pre-knowledge about the chemical composition and availability of simulated linewidths. With increased fs-laser pulse energy, additional lines in the spectrum were observed due to Stark splitting.The pioneering work on fs-LIGS was performed in heated flows of N_2 gas with temperatures varying from room temperature to 750 K. The thermal grating was formed by resonant multi-photon absorption, based on 800-nm fs pulses, and the generated LIGS signals were detected time-resolved in single-shot acquisitions. The results show that the method works very well for single-shot thermometry in nitrogen, with a measurement uncertainty of ± 1 K for room temperatures and ± 14 K for 600 K, as an example. Increasing the laser pump energy above a certain threshold causes ionization and generates a plasma density grating.It is my hope and belief that the developed laser-based diagnostic techniques can contribute important tools for the larger research community in thermal energy conversion, and support the urgent transition to a sustainable energy system

Gain mechanism of femtosecond two-photon-excited lasing effect in atomic hydrogen

By aiming to establish single-ended standoff combustion diagnostics, bidirectional lasing emissions of atomic hydrogen at 656 nm wavelength have been generated via two-photon resonant excitation by focusing 205 nm femtosecond laser pulses into a premixed CH4/O2 flame. The forward lasing strength is approximately one order of magnitude stronger than that of the backward one, due to the geometry of traveling wave excitation over a 2-mm-long pencil-shaped gain volume and the short gain lifetime of 3.5 ps. The gain coefficient of hydrogen lasing was determined to approximate 52/cm. As for the underlying physics of hydrogen lasing, amplified spontaneous emission (ASE) occurs simultaneously with four-wave mixing (FWM), and ASE dominates in the forward direction, whereas the backward lasing is virtually only ASE

Single-shot coherent control of molecular rotation by fs/ns rotational coherent anti-Stokes Raman spectroscopy

We present a novel method, to our knowledge, to control the shape of the spectra using 2-beam hybrid femtosecond (fs)/nanosecond (ns) coherent anti-Stokes Raman scattering (RCARS). The method is demonstrated experimentally and theoretically by utilizing a species-selective excitation approach via a field-free molecular alignment as an illustrative example. Two non-resonant fs laser pulses with proper delay selectively create and then annihilate N2 resonances in a binary mixture with O2 molecules. The RCARS signal is simultaneously resolved in spectral and temporal domains within a single-shot acquisition. The method requires very low pulse energies for excitation, hence minimizing multiphoton ionization probability, allowing for coherent control at various temperatures and pressures, with spectroscopic applications in non-stationary and unpredictable reacting flows

Temporal dynamics of femtosecond-TALIF of atomic hydrogen and oxygen in a nanosecond repetitively pulsed discharge-assisted methane-air flame

The temporal dynamics of the spatial distribution of atomic hydrogen and oxygen in a lean methane-air flame, forced by a nanosecond repetitively pulsed discharge-induced plasma, are investigated via femtosecond two-photon absorption laser-induced fluorescence technique. Plasma luminescence that interferes with the fluorescence from H and O atoms was observed to decay completely within 15 ns, which is the minimum delay required for imaging measurements with respect to the discharge occurrence. During discharge, H atoms in the excited state rather than the ground state, produced by electron-impact dissociation processes, are detected at the flame front. It was found that the temporal evolution of H and O fluorescence intensity during a cycle of 100 µs between two discharge pulses remains constant. Finally, the decay time of O-atoms produced by the discharge in the fresh methane-air mixture was about 2 µs, which suggests a faster reaction between O-atoms and methane than in air

Laser-induced thermal grating spectroscopy based on femtosecond laser multi-photon absorption

Laser-induced grating spectroscopy (LIGS) is for the first time explored in a configuration based on the crossing of two focused femtosecond (fs) laser pulses (800-nm wavelength) and a focused continuous-wave (cw) laser beam (532-nm wavelength). A thermal grating was formed by multi-photon absorption of the fs-laser pulses by N 2 with a pulse energy around 700 μ J (∼ 45 TW/cm 2). The feasibility of this LIGS configuration was investigated for thermometry in heated nitrogen gas flows. The temperature was varied from room temperature up to 750 K, producing strong single-shot LIGS signals. A model based on the solution of the linearized hydrodynamic equations was used to extract temperature information from single-shot experimental data, and the results show excellent agreement with the thermocouple measurements. Furthermore, the fluorescence produced by the fs-laser pulses was investigated. This study indicates an 8-photon absorption pathway for N 2 in order to reach the B 3Π g state from the ground state, and 8 + 5 photon excitation to reach the B2Σu+ state of the N2+ ion. At pulse energies higher than 1 mJ, the LIGS signal was disturbed due to the generation of plasma. Additionally, measurements in argon gas and air were performed, where the LIGS signal for argon shows lower intensity compared to air and N 2

Femtosecond laser-induced quantum-beat superfluorescence of atomic oxygen in a flame

Among different approaches to generate mirrorless lasing, resonant multiphoton pumping of gas constituents by deep-UV laser pulses exhibits so far the highest efficiency and produces measurable lasing energies, but the underlying mechanism was not yet fully settled. Here, we report lasing generation from atomic oxygen in a methane-air flame via femtosecond two-photon excitation. Temporal profiles of the lasing pulses were measured for varying concentrations of atomic oxygen, which shows that the peak intensity and time delay of the lasing pulse approximately scales as N and 1/N, respectively, where N represents the concentration. These scaling laws match well with the prediction of oscillatory superfluorescence (SF), indicating that the lasing we observed is essentially SF rather than amplified spontaneous emission. In addition, the quantum-beating effect was also observed in the time-resolved lasing pulse. A theoretical simulation based on nonadiabatic Maxwell-Bloch equations well reproduces the experimental observations of the temporal dynamics of the lasing pulses. These results on fundamentals should be beneficial for the better design and applications of lasing-based techniques

Simultaneous single-shot imaging of H and O atoms in premixed turbulent flames using femtosecond two-photon laser-induced fluorescence

A method based on femtosecond two-photon excitation has been developed for simultaneous visualization of interference-free fluorescence of H and O atoms in turbulent flames. This work shows pioneering results on single-shot simultaneous imaging of these radicals under non-stationary flame conditions. The fluorescence signal, showing the distribution of H and O radicals in premixed CH4/O2 flames was investigated for equivalence ratios ranging from ϕ = 0.8 to ϕ = 1.3. The images have been quantified through calibration measurements and indicate single-shot detection limits on the order of a few percent. Experimental profiles have also been compared with profiles from flame simulations, showing similar trends