Jason G. Mance

Seedless velocimetry at 100 kHz with picosecond-laser electronic-excitation tagging

Picosecond-laser electronic-excitation tagging (PLEET), a seedless picosecond-laser-based velocimetry technique, is demonstrated in non-reactive flows at a repetition rate of 100 kHz with a 1064 nm, 100 ps burst-mode laser. The fluorescence lifetime of the PLEET signal was measured in nitrogen, and the laser heating effects were analyzed. PLEET experiments with a free jet of nitrogen show the ability to measure multi-point flow velocity fluctuations at a 100 kHz detection rate or higher. Both spectral and dynamic mode decomposition analyses of velocity on a Ma=0.8 free jet show two dominant Strouhal numbers around 0.24 and 0.48, respectively, well within the shear-layer flapping frequencies of the free jets. This technique increases the laser-tagging repetition rate for velocimetry to hundreds of kilohertz. PLEET is suitable for subsonic through supersonic laminar- and turbulent-flow velocity measurements.

Femtosecond dynamics of exciton localization: self-trapping from the small to the large polaron limit

We use femtosecond vibrational wavepacket techniques to time-resolve the coupled electronic and vibrational dynamics of exciton self-trapping in a series of materials in which the relative strength of the electron-phonon coupling can be compositionally tuned from the small to the large polaron limit. Transient absorption experiments are carried out in the quasi-one-dimensional halide-bridged mixed-valence transition metal linear chain complexes [Pt(en)2][Pt(en)2X2]⋅(ClO4)4 (en=ethylenediamine, C2H8N2) with X=Cl, Br and I. In each complex, we detect the formation of the self-trapped exciton through the appearance of its characteristic red-shifted optical absorption, and find that self-trapping occurs on a time scale of the order of a single vibrational period of the optical phonon mode that dominates the self-trapping dynamics. The associated optical phonon response, detected as wavepacket oscillations that modulate the exciton absorption, shows a significant softening of the optical phonon frequency compared to that of the unexcited system. The degree of softening is found to vary significantly with coupling strength, ranging from more than 40% in the strongly coupled chloride-bridged complex to less than 20% in the weakly coupled iodide-bridged complex. We relate these results to the extent of electronic delocalization by comparison with the electronic properties of the ground states of the materials and with the properties of their equilibrated self-trapped electronic states predicted by theoretical modeling.

Development of a diode-pumped 100-ms quasi-continuous burst-mode laser for high-speed combustion diagnostics

An all-diode-pumped quasi-continuous burst-mode laser with burst period of 100 ms and pulse energy up to 225 mJ at 1064 nm has been developed. The high-speed laser incorporates a fiber amplifier and six diode-pumped amplifiers in a transportable 2.5-foot × 4-foot system. The repetition rate can be varied from 10–100 kHz resulting in pulse sequences of 1,000–10,000 pulses, a nearly ten-fold increase in record length over current state-of-the-art systems. Individual-pulse shaping allows temporal tailoring of the burst envelope, reducing pulse-to-pulse standard deviation two-fold and producing nearly flat-top bursts. Finally, the generation of pulse pairs with 1-μs spacing is enabled by acousto-optic modulation, allowing straightforward implementation of particle-image velocimetry with a single laser.

Burst-Mode Two-Dimensional Coherent Anti-Stokes Raman Scattering (2D-CARS) at 1 kHz

Two-dimensional gas-phase coherent anti-Stokes Raman scattering (2D-CARS) thermometry is demonstrated at 1 kHz using a femtosecond pump/Stokes pulse from a regenerative amplifier synchronized to a picosecond probe pulse from a burst-mode laser.

100-kHz burst-mode particle image velocimetry: space-time correlations and considerations for spatial and temporal resolution

100-kHz particle image velocimetry is demonstrated using a double-pulsed burst-mode laser with record length of 100 ms. Pulse doublets with inter-pulse spacing of 1–5 μs and repetition rate of 100 kHz are generated using a fiber-based oscillator and amplified through an all-diode pumped burst-mode amplifier. Pre-compensation of oscillator pulse width and pulse intensity enable control of output pulse energy even for unequally spaced pulse sequences. Spatial and temporal resolution of the velocity measurements is quantified and demonstrated to be sufficient for investigating space–time correlations in highly turbulent diffusion jets with jet-diameter Reynold’s Numbers from 14,000–43,000. Quantitative assesments of standard integral time and length scale definitions are made along with highly resolved space–time correlations and compared with Taylor’s Frozen Turbulence Hypothesis for turbulent flows.

Observation of structural relaxation during exciton self-trapping via excited-state resonant impulsive stimulated Raman spectroscopy

We detect the change in vibrational frequency associated with the transition from a delocalized to a localized electronic state using femtosecond vibrational wavepacket techniques. The experiments are carried out in the mixed-valence linear chain material [Pt(en)2][Pt(en)2Cl2]⋅(ClO4)4 (en = ethylenediamine, C2H8N2), a quasi-one-dimensional system with strong electron-phonon coupling. Vibrational spectroscopy of the equilibrated self-trapped exciton is carried out using a multiple pulse excitation technique: an initial pump pulse creates a population of delocalized excitons that self-trap and equilibrate, and a time-delayed second pump pulse tuned to the red-shifted absorption band of the self-trapped exciton impulsively excites vibrational wavepacket oscillations at the characteristic vibrational frequencies of the equilibrated self-trapped exciton state by the resonant impulsive stimulated Raman mechanism, acting on the excited state. The measurements yield oscillations at a frequency of 160 cm(-1) corresponding to a Raman-active mode of the equilibrated self-trapped exciton with Pt-Cl stretching character. The 160 cm(-1) frequency is shifted from the previously observed wavepacket frequency of 185 cm(-1) associated with the initially generated exciton and from the 312 cm(-1) Raman-active symmetric stretching mode of the ground electronic state. We relate the frequency shifts to the changes in charge distribution and local structure that create the potential that stabilizes the self-trapped state.

Exciton localization probed via excited-state resonant impulsive stimulated Raman spectroscopy

We probe the transition from a delocalized to a localized electronic state in a quasi-one-dimensional system by the change in vibrational frequency detected by resonant impulsive Raman excitation of the excited state in a pump-pump-probe measurement.

Ultrafast Dispersive Transport in a-SiGe:H

Carrier dynamics in a-SiGe:H are studied using femtosecond optical techniques. The response reveals dispersive transport with a time-dependent mobility that varies systematically with temperature and with the degree of disorder in the material.

Coherent phonon dynamics in exciton self-trapping in the strong coupling limit

We probe the coupled electronic and lattice dynamics of exciton self-trapping in a strongly coupled quasi-one-dimensional system. The coherent phonon response reveals both optical and acoustic phonon contributions to the localization dynamics.