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The exquisite time synchronicity between shock and diagnostics needed to unravel chemical events occurring in picoseconds has been achieved using a shaped ultrafast laser pulse to both drive the shocks and interrogate the sample via a multiplicity of optical diagnostics. The shaped laser drive pulse can produce well-controlled shock states of sub-ns duration with sub-10 ps rise times, sufficient for investigation of fast reactions or phase transformations in a thin layer with picosecond time resolution. The shock state is characterized using ultrafast dynamic ellipsometry (UDE) in either planar or Gaussian spatial geometries, the latter allowing measurements of the equation of state of materials at a range of stresses in a single laser pulse. Time-resolved processes in materials are being interrogated using UDE, ultrafast infrared absorption, ultrafast UV/visible absorption, and femtosecond stimulated Raman spectroscopy. Using these tools we showed that chemistry in an energetic thin film starts only after an induction time of a few tens of ps, an observation that allows differentiation between proposed shock-induced reaction mechanisms. These tools are presently being applied to a variety of energetic and reactive sample systems, from nitromethane and carbon disulfide, to micro-engineered interfaces in tunable energetic mixtures. [ABSTRACT FROM AUTHOR]

We present direct optical initiation (DOI) of energetic materials using coherent control of localized energy deposition. DOI requires depositing energy into the material to produce a critical size hot spot, which allows propagation of the reaction and thereby initiation. The hot spot characteristics needed for growth to initiation can be studied using quantum controlled initiation (QCI). Achieving QCI in condensed phase energetic materials requires optimally shaped ultrafast laser pulses to coherently guide the energy flow along desired paths. As a test of our quantum control capabilities we have successfully demonstrated our ability to control the reaction pathway of the chemical system stilbene. An acousto-optical modulator based pulse shaper was used at 266 nm, in a shaped pump/supercontinuum probe technique, to enhance and suppress the relative yields of the cis- to trans-stilbene isomerization. The quantum control techniques tested in the stilbene experiments are currently being used to investigate QCI of the explosive hexanitroazobenzene (HNAB). [ABSTRACT FROM AUTHOR]

We present the review of the recent experimental studies of fundamental mechanisms of femtosecond laser plasma formation and evolution performed in A. M. Prokhorov General Physics Institute of Russian Academy of Sciences. The main object we dealt with was the micro-sized plasma produced in gases with high intensity (up to 5×1017 W/cm2), tightly focused (to a few microns in diameter) IR and UV femtosecond laser pulses. The main attention in the experiments was paid to the initial stage of microplasma formation and evolution characterized by strong laser-plasma coupling resulting in efficient ionization and heating of the medium, distortion of laser beam, and nonlinear spectral conversion of laser radiation to the continuum and laser harmonics. Using a precise pump-probe micro-interferometric technique the dynamics of plasma was studied with femtosecond time-resolution in a wide density range—from a minimal detectable electron concentration (1019 cm-3) to the almost total (down to nuclei) ionization of ions occurred under femtosecond excitation. The obtained time-dependences of plasma density were analyzed. It was observed, for the first time, that a characteristic time of femtosecond laser plasma formation in gases considerably (in times) exceeds the duration of the pump laser pulse. This postionization process is attributed to impact ionization of plasma by hot electrons. We compare the results of the experiments with what the developed theory predicts. Using an ultrafast streak-camera-based spectroscopy technique the temporal evolution of microplasma emission and the dynamics of spectral lines formation in UV-visible range were studied with picosecond time-resolution. [ABSTRACT FROM AUTHOR]

Using an electro-optic sampling technique, we study the interaction of ultra-short midinfrared pulses with the active medium of a quantum cascade laser emitting at an 11.7 μm wavelength below and above the laser threshold. Gain clamping is observed above the lasing threshold and additional broadband resonant absorption is found at a low bias current. [ABSTRACT FROM AUTHOR]

In this work we report on two-color pump-probe measurements to investigate the intraminiband dynamics of doped GaAs/AlGaAs superlattices with different miniband widths smaller or larger than the optical phonon energy. For a miniband with a width larger than the optical phonon energy we found a fast relaxation, independent of the excitation intensity. For narrow minibands this relaxation takes longer and shows a strong temperature and intensity dependence. [ABSTRACT FROM AUTHOR]

We have used ultrafast pump-probe spectroscopy to coherently generate and study collective spin flip waves in a high mobility modulation doped Cd1-xMnxTe quantum well. The spin flip wave lifetime has a strong dependence on the magnetic field, temperature, and exciting photon energy. The dependence of the lifetime on excitation energy and magnetic field suggest that the optically-excited heavy holes and their spin orientation play an important role in the decay of the collective mode. The resonant behavior and other experimental data demonstrate the Raman nature of the generation process. [ABSTRACT FROM AUTHOR]

We present a study of the transition from the strong- to the weak-coupling regime in InGaAs microcavities by means of time-resolved photoluminescence. We have found that under circularly-polarized excitation, with increasing excitation power, the co-polarized polaritons undergo a transition to bare modes (exciton + photons) at a power significantly lower than that of the cross-polarized polaritons. This demonstrates that the appearance of the weak coupling depends mainly on the excitonic population of a given spin orientation. © 2007 American Institute of Physics [ABSTRACT FROM AUTHOR]

The exciton spin relaxation within the radiative doublet of the exciton ground state in InAs/GaAs self-assembled quantum dots is studied via an ultrafast spectral hole burning technique. In the case of cross-polarized pump and probe pulses a spectral “antihole” emerges due to relaxation of the exciton spin. The measured relaxation time decreases rapidly from 1.15 ns at T = 5 K to 90 ps at 90 K, suggesting exciton-acoustic phonon interaction as the underlying spin relaxation mechanism. © 2007 American Institute of Physics [ABSTRACT FROM AUTHOR]

We present ultrafast pump-probe studies on chemically prepared CdSe nanocrystals. Upon pulsed optical excitation we find pronounced oscillations, indicating strong coupling to a LO phonon mode. From the polarization dependence of the signal additional information about the very first coherent events of exciton dynamics is obtained. The observed anisotropy behaviour suggests strongly driven dynamics and rapid coherent motion. © 2007 American Institute of Physics [ABSTRACT FROM AUTHOR]

In this work we investigated the transient behavior of interminiband transitions in a doped GaAs/AlGaAs superlattice using the infrared picosecond pulses delivered by a free electron laser. We found a fast bleaching component, followed by relaxation and thermalization on a time scale of few ps. Thereafter the cooling of the electrons in the ground miniband is observed on a timescale of tens of ps. The cooling can show either positive of negative change in transmission, deriving from the temperature dependence of the absorption spectrum at the respective wavelength. © 2007 American Institute of Physics [ABSTRACT FROM AUTHOR]