Hans U. Stauffer

Development of simultaneous frequency- and time-resolved coherent anti-Stokes Raman scattering for ultrafast detection of molecular Raman spectra

The development of a time-resolved coherent anti-Stokes Raman scattering (CARS) variant for use as a probe of excited electronic state Raman-active modes following excitation with an ultrafast pump pulse is detailed. Application of this technique involves a combination of broadband fs-time scale pulses and a narrowband pulse of ps duration that allows multiplexed detection of the CARS signal, permitting direct observation of molecular Raman frequencies and intensities with time resolution dictated by the broadband pulses. Thus, this nonlinear optical probe, designated fs/ps CARS, is suitable for observation of Raman spectral evolution following excitation with a pump pulse. Because of the spatial separation of the CARS output signal relative to the three input beams inherent in a folded BOXCARS arrangement, this technique is particularly amenable to probing low-frequency vibrational modes, which play a significant role in accepting vibrational energy during intramolecular vibrational energy redistribution within electronically excited states. Additionally, this spatial separation allows discrimination against strong fluorescence signal, as demonstrated in the case of rhodamine 6G.

Communication: Hybrid femtosecond/picosecond rotational coherent anti-Stokes Raman scattering thermometry using a narrowband time-asymmetric probe pulse

A narrowband, time-asymmetric probe pulse is introduced into the hybrid femtosecond/picosecond rotational coherent anti-Stokes Raman scattering (fs/ps RCARS) technique to provide accurate and precise single-shot, high-repetition-rate gas-phase thermometric measurements. This narrowband pulse-generated by inserting a Fabry-Pérot étalon into the probe-pulse beam path-enables frequency-domain detection of pure-rotational transitions. The unique time-asymmetric nature of this pulse, in turn, allows for detection of resonant Raman-active rotational transitions free of signal contamination by nonresonant four-wave-mixing processes while still allowing detection at short probe-pulse delays, where collisional dephasing processes are negligible. We demonstrate that this approach provides excellent single-shot thermometric accuracy (<1% error) and precision (~2.5%) in gas-phase environments.

One-dimensional single-shot thermometry in flames using femtosecond-CARS line imaging

We report single-laser-shot one-dimensional thermometry in flames using femtosecond coherent anti-Stokes Raman scattering (fs-CARS) line imaging. Fs-CARS enables high-repetition-rate (1-10u2009kHz), nearly collision-free measurement of temperature and species concentration in reacting flows. Two high-power 800u2009nm beams are used as the pump and probe beams and a 983u2009nm beam is used as the Stokes beam for CARS signal generation from the N2Q-branch transitions at ∼2330u2009cm(-1). The probe beam is frequency-chirped for single-laser-shot imaging. All three laser beams are formed into sheets and crossed in a line which forms the probe region. The resulting 1D line-CARS signal at ∼675u2009nm is spatially and spectrally resolved and recorded as a two-dimensional (2D) image. Single-shot temperature measurements are demonstrated in flat-field flames up to temperatures exceeding 2000u2009K, demonstrating the potential of fs-CARS line imaging for high-repetition-rate thermometry in turbulent flames. Such measurements can provide valuable data to validate complex turbulent-combustion models as well as increase the understanding of the spatio-temporal instabilities in practical combustion devices such as modern gas-turbine combustors and augmentors.

Phase-tailoring molecular wave packets to time shift their dynamics

Abstract Time shifting (up to a global arbitrary phase) the dynamics of molecular wave packets, i.e, | Ψ ( t )〉→| Ψ ( t − t shift )〉, is demonstrated using a high degree of state selective coherent phase control with shaped femtosecond laser pulses. The benchmark system for the present work is the lithium dimer molecule. The phase-tailored Li 2 wave packets are composed of several rovibrational states of the electronic E 1 Σ g + shelf state excited from a single rovibrational level (selected using a cw laser) of the A 1 Σ u + state. The time-shifting operation has implications for the experimental implementation of coherent control, as well as for the use of the control ability to study coherent configurations and dynamics that otherwise would be difficult (sometimes impossible) to access experimentally. This is due, for example, to dephasing and/or depopulation of the wave packet in combination with long recurrence times. One such inaccessible coherent configuration of the present Li 2 wave packets corresponds to the global maximum of their ionization yield (as probed in the present experiment).

Optimization of wave packet coefficients in Li2 using an evolutionary algorithm: The role of resonant and nonresonant wavelengths

Using feedback and an evolutionary algorithm (EA), the weak field pump–probe photoionization signal at a single time delay is optimized in Li2. A single launch state is prepared via excitation with a cw laser, from which a pump pulse excites a superposition of two rotational states on an excited electronic potential energy curve: Eu200a1Σg+(vE=9,u2002JE=27 and 29). The EA modifies the phase pattern versus wavelength of the ultrafast pump pulses using a pulse shaper with a 128 pixel liquid crystal spatial light modulator. Limitations of frequency resolution for the pulse shaper create an effective temporal window in which pulses can be shaped. Optimization of the photoionization signal at pump–probe time delays outside of this temporal pulse shaping window involves phase shifts of only the two frequencies resonant with the transition of the wave packet states, effectively introducing a phase shift in the wave packet recurrences. For pump–probe time delays inside the pulse shaping window, optimization of the photoi...

Effect of Nonresonant Frequencies on the Enhancement of Quantum Beat Amplitudes in Rovibrational States of Li2: The Role of State Spacing

Optical phase manipulation of nonresonant frequencies is investigated as a method of achieving optimal population transfer during resonant impulsive stimulated Raman scattering. Wave packets containing quantum beats between an initially prepared rovibrational level in the A(1Σu+) electronic state of Li2 and states populated via a resonance-enhanced rotational Raman process are created using a shaped ultrafast pulse centered near 800 nm. Study of these wave packets allows a quantitative comparison of population transfer as a function of applied phases in the ultrafast pulse. Two cases are explored to determine the ability to enhance population transfer: one with a wide state spacing [A(νA=11, JA=28)-A(11,30) at 50.1 cm−1] and one with a narrow spacing [A(11,8)-A(11,10) at 16.6 cm−1]. In both cases, several different phase masks are applied to the wave packet preparation pulse to enhance the population transferred to the newly formed state of interest. One phase mask involves the application of a −90° phase...

Gas-phase thermometry using delayed-probe-pulse picosecond coherent anti-Stokes Raman scattering spectra of H 2

We report the development and application of a simple theoretical model for extracting temperatures from picosecond-laser-based coherent anti-Stokes Raman scattering (CARS) spectra of H2 obtained using time-delayed probe pulses. This approach addresses the challenges associated with the effects of rotational-level-dependent decay lifetimes on time-delayed probing for CARS thermometry. A simple procedure is presented for accurate temperature determination based on a Boltzmann distribution using delayed-probe-pulse vibrational CARS spectra of H2; this procedure requires measurement at only a select handful of probe-pulse delays and requires no assumptions about sample environment.

Laser-induced fluorescence detection of hydroxyl (OH) radical by femtosecond excitation.

The development of a laser-induced fluorescence detection scheme for probing combustion-relevant species using a high-repetition-rate ultrafast laser is described. A femtosecond laser system with a 1 kHz repetition rate is used to induce fluorescence, following two-photon excitation (TPE), from hydroxyl (OH) radicals that are present in premixed laminar flames. The experimental TPE and one-photon fluorescence spectra resulting from broadband excitation into the (0,0) band of the OH A(2)∑(+)-X(2)Π system are compared to simulated spectra. Additionally, the effects of non-transform-limited femtosecond pulses on TPE efficiency is investigated.

Simultaneous phase control of Li2 wave packets in two electronic states

State-selective phase control of rotational Li2 wave packets, prepared simultaneously in the E(1∑g+) electronic state by one photon absorption and the A(1∑u+) electronic state by resonant impulsive stimulated Raman scattering, is demonstrated. Following the initial population of a rovibrational launch state on the A electronic potential energy curve with a cw laser, a single sub-picosecond wave packet preparation pulse centered near 800 nm simultaneously creates a two-state rotational wave packet in the E state (νE=18, JE=23 and 25) and a three-state rotational wave packet in the A state (νA=15, JA=22, 24, and 26). A temporally delayed 800 nm probe pulse subsequently ionizes both electronic components of the wave packet to allow measurement of the time-dependent coherence in these two electronic states. Via phase manipulation of resonant transition frequencies contained within the preparation pulse, the phases of the E(18,25) and A(15,26) quantum states are either varied concurrently or individually contr...

Detailed calculation of hydroxyl (OH) radical two-photon absorption via broadband ultrafast excitation

The theoretical framework for calculation of two-photon absorption cross sections for intermediate Hund’s cases (a) and (b) diatomic species is described in detail and applied toward the hydroxyl (OH) radical. Analytical expressions are derived for the 20 rotational branches that are present in the two-photon AΣ+2←XΠ2 electronic transition. Calculation of the corresponding line strengths is necessary to permit accurate relative-concentration measurements obtained from the fluorescence induced by a broadband femtosecond excitation pulse. We demonstrate, in particular, that consideration of the temperature-dependent initial-state populations of OH is necessary to obtain accurate relative concentrations from observed two-photon-excitation based laser-induced-fluorescence measurements.