Subpicosecond Relaxation Research Articles

Kinetics of high-density plasmas generated in Si by 1.06- and 0.53- microm picosecond laser pulses.

The dynamics of high-density (${10}^{19}$${10}^{21}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$), high-temperature (300${T}_{e}$3000 K) plasmas, generated in silicon by single 1.06- and 0.53-\ensuremath{\mu}m picosecond laser pulses, is simulated with a self-consistent model based on the Boltzmann transport equation. Balance equations, which include possible plasma degeneracy effects, are used to obtain the spatial and temporal evolution of the lattice temperature as well as the carrier density and temperature. We compare predictions of the model, which employs parameters extracted from near-equilibrium experiments, with data from recent experiments which measured the melt threshold or time-dependent infrared reflectivity. The model is able to account quantitatively for the weak dependence of the melt threshold on pulse width at 0.53 \ensuremath{\mu}m as well as the strong dependence at 1.06 \ensuremath{\mu}m. In the former case, direct absorption and lattice parameters dominate energy evolution, but at 1.06 \ensuremath{\mu}m two-photon absorption and plasma dynamics dominate. The infrared reflectivity measurements are found to be consistent with a simple Drude model of the dielectric constant and the calculated plasma kinetics when one accounts for nonzero probe pulse widths and plasma spatial inhomogeneity. When this is done there is no need to introduce ultrashort momentum relaxation times or plasmon assisted recombination as speculated on in the recent literature. Finally, contrary to earlier suppositions, it is found that the carrier temperature can exceed the lattice temperature by more than ${10}^{3}$ K during pulses as long as 100 psec even for subpicosecond carrier-phonon relaxation times. It is also found that the carrier temperature exhibits two peaks as a function of time, one due to direct laser heating of the carriers and the other due to Auger heating.

Subpicosecond relaxation study of malachite green using a three-laser frequency-domain technique

We report measurements of the ground-state-recovery dynamics of malachite green in water, made using a new three-laser tunable-induced-grating technique. We observe a recovery consistent with either the sum of two decaying exponentials (0.78±0.10 and 7.4±3.0 ps) or with a uniform range of exponentials (0.34±0.04 to 7.2±3.0 ps). Our three-frequency induced-grating method provides a new and useful approach for studying picosecond and femtosecond atomic and molecular processes, working in the frequency rather than the time domain.

Third-order optical nonlinearity induced by effective mass gradient in heterostructures

In semiconductor heterostructures the electron effective mass can vary spatially, giving rise to a nonlinear force parallel to the mass gradient. In an optical field this nonlinear force modulates the electron mass through its position dependence, leading to a novel third order optical nonlinearity. The susceptibility of this process is calculated through the kinetic equations for the electrons. It is found that large nonlinear refractive indices can be obtained with subpicosecond relaxation times. Possible device configurations are discussed.

A picosecond kinetic study of the excited-state properties of some osmium octaethylporphyrins

Four osmium porphyrin complexes, Os(OEP)LL'((OEP) = octaethylporphyrin), have been investigated by picosecond laser flash photolysis using a double-beam, mode-locked Nd:glass system delivering 6-ps (fwhm) pulses at 527 nm with 1-2 mJ/pulse. Time-resolved excited-state spectra were recorded from the time of photolysis to 5 ns after photolysis. For the four compounds studied (Os(OEP)(P(OMe)/sub 3/)/sub 2/, Os(OEP)CO(py), Os(OEP)NO(OMe), and Os(OEP)O/sub 2/((Me = methyl, py = pyridine, and O/sub 2/ = dioxo)), the initial excited state, S/sub 1/, decayed in less than or equal to8, approx.50, approx.36, and approx.13 ps, respectively. The yields of the later excited state, T/sub 1/, were approx.0.8, unmeasurable, approx.0.6, and approx.1.0, respectively. The T/sub 1/ states lived for times 6.2, 16, 5.5, and 6 ns, respectively. A third relaxation time of approx.15 ps was found for Os(OEP)NO(OMe). The lowest energy excited states for the first two complexes were previously identified as (d,..pi..*) and for the second two as (..pi..,..pi..*) and a red shift of the excited-state absorption of the former with respect to the latter is observed. For the two molecules with lowest energy (..pi..,..pi..*) states, S/sub 1/ and T/sub 1/ are identified as /sup 1/(..pi..,..pi..*) and /sup 3/(..pi..,..pi..*), i.e., singlet and triplet states. For the two moleculesmore » with lowest energy (d,..pi..*) states, spin should not be a good quantum number, and S/sub 1/ is identified as a limited set of (d,..pi..*) states produced after subpicosecond relaxation of the initial photoexcited /sup 1/(..pi..,..pi..*) state.« less

Overtone spectroscopy and sub-picosecond vibrational relaxation in supercold polyatomic molecules

Overtone spectroscopy and sub-picosecond vibrational relaxation in supercold polyatomic molecules

A theory of two-wave saturation spectroscopy with subpicosecond relaxation time in p-Ge

Time evolution of the nonequilibrium hole distribution function is followed by numerical integration of Boltzmann's equations in p-Ge taking into account anisotropic band structure. Both the hole-burning and light-light modulation experiments of Keilmann can be reasonably explained by a simple model. Subpicosecond relaxation time for these processes is obtained in good agreement with Keilmann's estimate.

Subpicosecond relaxation and hot energy transfer accompanying excitation of the higher electronic states of organic molecules

Subpicosecond relaxation and hot energy transfer accompanying excitation of the higher electronic states of organic molecules

Femtosecond relaxation of an iron porphyrin observed with polarization spectroscopy in a three-level system

An experimental and theoretical study is presented of the nonlinear response of a dilute solution being pumped by two laser fields having different frequencies omega(1) and omega(2) that are both in resonance with electronic transitions of the solute. Experimental results were obtained for the dispersion of the third-order susceptibility chi((3))(omega(2))(2) for a dilute solution of iron(III) tetraphenylporphyrin chloride in chloroform with a fixed intense field (omega(1)) resonant with the Q(o) transition and a tunable weaker field (omega(2)) resonant with the Soret transition. The observed Lorentzian dispersion is slightly narrower than the linear absorption spectrum. The theoretical model for the third-order polarization incorporates three levels coupled to two electromagnetic fields and a population reservoir. The equations of motion for the density operator were solved in third order for the appropriate Fourier component. The resulting dispersion curve depends on both total dephasing and population decay rates, thereby demonstrating a general method for the determination of subpicosecond relaxation processes in systems to which the model is applicable. Theoretical fits to the results for iron(III) tetraphenylporphyrin chloride imply that the Soret transition is essentially homogeneously broadened with a total dephasing width of 900 +/- 200 cm(-1), and it is argued that the population decay time in this case is approximately 3.5 fsec. The broad linear absorption spectrum of the Soret band is interpreted as resulting from intramolecular perturbations between the Soret and lower energy excited states of iron(III) tetraphenylporphyrin chloride.

Tunable-laser-induced grating dip for measuring sub-picosecond relaxation

This paper describes the experimental verification of a laser-induced double-dip resonance predicted theoretically for two-wave saturation spectroscopy. Fine structure in the form of a central dip with a width (FWHM) near 20 cm−1 is found in the broad hole-burning feature induced by 942 cm−1 wavelength radiation inp-type germanium. The central dip is ascribed to Bragg scattering caused by, and thus measuring the decay of, the excited-state population grating set up by interference of the saturating and the probing waves. From the composite spectrum, therefore, apopulation difference lifetimeT 1=0.6 ps can be directly obtained as the inverse width of the central dip. Likewise the broad background feature yields the homogeneous ordipole lifetimeT 2=0.1 ps.

Sub-picosecond relaxation of large organic molecules in solution

Using sub-picosecond ultraviolet and visible pulses we have studied relaxations from highly-excited molecular vibronic states with a resolution of 2 × 10 −13 s.

Subpicosecond relaxation of localized electrons in liquid ammonia

We report the results of an experimental study of the optical bleaching of the absorption spectrum of the solvated electron in dilute Na–NH3 and Na–ND3(2×10−4 M) solutions by a single 1.06 μ psec pulse (pulse duration 6 psec, pulse energy 30 mJ). The absorption spectrum of the solvated electron was monitored in the spectral range 1100–850 nm with the time resolution 20–3.3 psec. The line broadening of the solvated electron optical band originates from homogeneous broadening effects. The relaxation time of the excited state of the solvated electron was determined to be τ?2×10−13 sec. This ultrashort relaxation time favors relaxation between bound states of the solvated electron center.