Optical orientation of spin-polarized heavy and light holes followed by relaxation to other valence subband states has been observed unambiguously in undoped bulk GaAs in spite of the extremely short spin relaxation time. The measured relaxation time for the heavy holes is 110 fs +/-10%. The results are relevant for applications such as interpretation of spin-polarized transport in semiconductors as well as the assessment of feasibility of hole-based spin-transport devices which relies on precise knowledge of the hole-spin relaxation time.

The ultrafast relaxation in solution of two large organic molecules, malachite green and nile blue 690, has been investigated with the equal-pulse correlation technique. In addition to previously known picosecond processes, we have observed fast initial exponential decays (less than 100 fs) and damped sinusoidal oscillation over intermediate time delays. We believe that the latter behavior, the first such decay observed on a femtosecond time scale for any material, represents quantum beats.PACS numbers: 42.65.Re, 06.60JnThe initial relaxation of photoexcited large molecules, which occurs on a subpicosecond time scale, has been the subject of considerable current interest. Recent developments in experimental apparatus have significantly improved both the amplitude and time resolution relative to earlier experiments. Because of the absence of intermolecular collisions on the subpicosecond time scale, the ultrafast relaxation properties of isolated molecules can be effectively studied in condensed-phase experiments. We have employed the equal-pulse correlation technique to observe a saturation effect in the transmission of short pulses through a thin jet of organic dye in solution.The results reveal details of the relaxation dynamics which have not been observed before and which are considerably more complex than was previously thought to be the case. In particular, we believe that we have observed quantum beats in molecules for the first time on a femtosecond time scale. Quantum beats have been previously observed in the fluorescence decay of large molecules 1 ; however, the beat period in our experiment is 3 orders of magnitude shorter.We have studied the ultrafast relaxation of the triphenyl methane dye malachite green, which is dominated by a strong internal-conversion mechanism. Data from transmission correlation experiments performed on this dye exhibit a rapid oscillatory decay following photoexcitation as well as a previously identified process with a decay time of 5 ps. Measurements of the relaxation of nile blue 690 clearly reveal a fast component with a decay time of 80 fs as well as a slower process which has been observed by other workers. Although these results are not entirely understood, a possible mechanism for the observed features will be presented.A number of factors have enhanced our capability to observe and analyze ultrafast processes. The temporal resolution of this experiment (Fig. 1) can be attributed to the use of a modified colliding-pulse mode-locked laser as a source. This produces nearly transformlimited pulses as short as 40 fs with very clean profiles. 2 The emission is peaked at 630 nm (1.97 eV) and the pulse repetition frequency is 10 8 Hz. A multiple-prism arrangement external to the laser cavity permits the control of frequency chirp and compensates for dispersion in other optical elements. The prisms were adjusted to ensure that the experiments were performed with pulses of zero net chirp. We have verified that, for an unfocused beam, transmission through the dye jet does not af...

Measurement of the temporal, spatial, and saturation behavior of stimulated Brillouin scatteringAn analysis is given of the saturation effect in the stimulated Brillouin scattering of coherent light waves. It is shown that the three coupled nonlinear wave equations describing the complex amplitudes of the forward-traveling primary and acoustic waves and the backward-traveling Stokes wave can be replaced for room-temperature situations by two coupled first-order nonlinear equations for the intensities of the primary and Stokes waves. These resulting equations are solved exactly and the solutions describe completely the process of photoelastic amplification of the coherent Stokes wave via stimulated Brillouin scattering in both the linear and the nonlinear saturated regimes. The results also give a more realistic estimate of the intensity of the hypersonic wave generated in the process than that made on the basis of either the Manley-Rowe relationship, which does not take into account the losses, or the usual linear theories, which do not take into account the saturation effect. It is believed that the Stokes emission observed in the usual experiments on stimulated Brillouin scattering of ruby laser light is due to photoelastic amplification via the stimulated process of Stokes noise generated by the scattering of the laser light via the normal Brillouin process by the thermal phonons present throughout the electrostrictive medium. A detailed analysis of this situation is also given. Explicit formulas for the spectral characteristics and the total integrated intensities of the Stokes and acoustic waves are obtained. The results also describe the noise characteristics of the photoelastic backward-wave amplifier. As an illustration, all these results are applied to the case of stimulated Brillouin scattering of ruby laser light in quartz.

Experiments show that conventional solid-state lasers can go into oscillation simultaneously in many modes. This is somewhat surprising since it appears impossible to ``eat holes'' in temperature-broadened lines and thus only one or, at most, a few modes should be able to oscillate. However, the spatial variation in the field intensity of the various modes produces nonuniform distributions in the inverted population and one can show that there is little tendency for these distributions to smooth out due to spatial cross relaxation. Such nonuniform distributions could lead to simultaneous oscillation in many modes. Formulas which relate the number of unstable modes to the pump power and various other maser parameters are obtained. The results show that it is exceedingly difficult to obtain single mode operation in conventional masers at high pumping levels. Ways to avoid a nonuniform distribution density and methods to achieve high-power single-mode operation in practice are discussed. It is also possible to show the effect of slow spatial cross relaxation on the spiking behavior.

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