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Theory of Computing Systems

Characterization of a sub-picosecond dye laser for the study of atom-perturber collisions

Scott A. Fedorchak

Published Jun 1, 1991• 0 citations
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Abstract

This thesis covers an in-depth analysis of the pulsed laser system to be used in the developing a linear technique for the time domain study of the change in coherence of atomic superposition states for the Ytterbrium atom due to phase-interrupting collisions. The hybrid synchronously pumped dye laser yielded optimally shaped sub-picosecond pulses at the 555.6 nanometer wavelength. Comparison of the results for the experimental trials at the 576 and 555.6 nanometer wavelengths yielded an experimental value for the dephasing time, T sub 2 = 621 picoseconds, of the DMETCI dye used as a saturable absorber dye for the first time.

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This thesis covers an in-depth analysis of the pulsed laser system to be used in the developing a linear technique for the time domain study of the change in coherence of atomic superposition states for the Ytterbrium atom due to phase-interrupting collisions. The hybrid synchronously pumped dye laser yielded optimally shaped sub-picosecond pulses at the 555.6 nanometer wavelength. Comparison of the results for the experimental trials at the 576 and 555.6 nanometer wavelengths yielded an experimental value for the dephasing time, T sub 2 = 621 picoseconds, of the DMETCI dye used as a saturable absorber dye for the first time.
Synchronous mode locking of two dye lasers was employed to obtain pairs of independently tunable picosecond pulses that were precisely synchronized in time. Overlapped pulses in strontium vapor were used to generate tunable picosecond pulses in the vacuum uv. The tunability was employed to study resonant enhancement of a four-wave parametric process by the 4d6p autoionizing resonance. Mutually separated pulses were employed to measure the build-up and decay of coherence for a two-quantum excited state. The decay was found to be faster than theoretical predictions that assume only velocity dephasing.
I. A new technique utilizing the electro-optic frequency switching of a single mode ( 6 Mhz) dye laser was used to observe free induction decay and optical nutation in iodine gas and optical nutation in an organic mixed crystal (pentacene in p-terphenyl) at low temperature. A new technique, incoherent resonance decay (IRO) was also developed. These experiments allow one to determine T_1 and T_2, the longitudinal and transverse relaxation times of the excited ensemble, in analogy with magnetic resonance. For I_2(g) at O∙C; T_l = 2.3 x 10^(-7) sec, T_2 ≃ 1 x 10^(-7) sec. These techniques are limited to a time scale > 10 nsec. II. The construction and characterization of cw, mode-locked dye laser capable of putting out a continuous train of reproducible sub-picosecond pulses is described. Experiments utilizing this laser for the measurement of picosecond coherent transients and time resolved absorption spectroscopy are proposed.
Recent advances in the development of synchronously mode-locked dye-laser systems have permitted the first low-power measurement of the time-resolved dephasing dynamics of a Raman active molecular vibration. Excitation and delayed probing of the coherent ensemble of vibrators is accomplished with the help of synchronized picosecond pulse trains from two independently tunable mode-locked dye lasers. A new probe technique, based on a gain measurement, is employed that provides a signal that scales linearly with the coherent vibrational amplitude. We demonstrate this technique with the first measurement of the dephasing dynamics of the normal mode (656.5 cm−1) of CS2 in the liquid state. An exponential decay with time constant 20±1 psec is obtained for the isotropic component of the coherent scattering.
We perform time-resolved degenerated four-wave mixing experiments on thin CuCl platelets (thickness of about 30 μm) and epitaxially grown layers (thickness of 5 to 10 μm) at low temperature (2K). The light source is a tunable picosecond dye-laser of 20 ps duration (FWHM) and of 0.3 meV spectral width (FWHM). It is synchronously pumped by the third harmonics of a mode-locked Nd:YAG laser. The dye-laser beam is split into two parts (pump and test) with an intensity ratio of about 8 to 1. The optical delay between both pulses is varied.