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Journal of the Optical Society of America B

Picosecond photon echo experiments using a superconducting accelerator-pumped free-electron laser

T. I. Smith, Yu Bai, Scott R. Greenfield, J. Frisch, Richard L. Swent, M. D. Fayer, H. Alan Schwettman

Published Aug 1, 1991• 4 citations

Abstract

A superconducting accelerator-pumped free-electron laser (SCA/FEL) is used to perform photon echo experiments on a near-IR dye molecule in a polymeric glass. The SCA/FEL produces Fourier-transform-limited picosecond pulses at a repetition rate of 11.8 MHz. Its wavelength is continuously tunable. The experiments are conducted at 0.776 μm by doubling the output of the SCA/FEL tuned to 1.55 μm. At this wavelength, the SCA/FEL is operated in a burst mode. The duration of the bursts (macropulses) is 2 ms with a repetition rate of 10 Hz. Each individual pulse (micropulse) has a length of 3.2 ps and is approximately 1 μJ in energy. In the experiment a number of single micropulses are selected from each macropulse. The doubling, stabilization, synchronization, single-pulse selection, and detection electronics are described. These are the first photon echo experiments and, to our knowledge, the first optical coherence experiments performed using a free-electron laser as a source.

DOI: 10.1364/josab.8.001652

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An account is given of the first infrared vibrational photon echo experiments conducted in liquids and a glasses. The experiments were performed on the CO stretching mode of the solute tungsten hexacarbonyl (W(CO)6) at approximately 5.05 micrometers (approximately 1980 cm-1) in the solvents 2-methytetrahydrofuran, and 2-methylpentane (2-MP). In 2-MP, it was possible to observe the photon echo decay at room temperature and follow the temperature dependence from room temperature to 10 K. The photon echo experiments were conducted using the Stanford superconducting-linear-accelerator-pumped Free Electron Laser as the source of tunable ps infrared pulses.

The first infrared vibrational photon echo experiments conducted in a liquid and a glass are reported. The experiments were performed on the CO stretching mode of tungsten hexacarbonyl at 5.1 micrometers (1960 cm-1) in 2-methyltetrahydrofuran over the temperature range 300 K to 16 K using picosecond pulses from the free electron laser at Stanford University. In addition, the first vibrational population relaxation measurements spanning a temperature range that takes a system from a liquid to a supercooled liquid to a glass are reported.

The first infrared vibrational photon echo experiments conducted in a liquid and a glass are reported. The experiments were performed on the CO stretching mode of tungsten hexacarbonyl at 5.1 \ensuremath{\mu}m (1960 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$) in 2-methyltetrahydrofuran over the temperature range 300 to 16 K using picosecond pulses from the free electron laser at Stanford University. In addition, the first vibrational population relaxation measurements spanning a temperature range that takes a system from a liquid to a supercooled liquid to a glass are reported.

A simple method for observing coherent optical-transient phenomena is described. Effects such as photon echoes and free induction decay (FID) should allow studies of the dynamic properties of atoms, molecules, and solids on a time scale ranging from milliseconds to \ensuremath{\sim} 50 ps. The frequency of a cw dye laser is switched with an intracavity electro-optic modulator, producing in an external sample coherent transients which are detected in the forward beam. Measurements of long dephasing times are possible, as in FID, because the laser can oscillate indefinitely at the new frequency following a step-function switching pulse. Measurements of short decay times are also practical because the switching time is not restricted by the laser gain or cavity ringing time. In addition, the dye's dephasing time of a few picoseconds or less is too rapid to interfere with these observations. Since the pulse sequence can be preselected, the entire class of coherent optical transients is accessible. The method thus incorporates all of the advantages inherent in Stark switching, including heterodyne detection and high sensitivity, without being restricted to Stark-tunable systems. Laser frequency switching is discussed also in terms of a theoretical model which exposes the time-dependent properties of a phase-modulated cavity mode. Dephasing studies have been carried out thus far in atomic and molecular gases and low-temperature solids containing inorganic impurity ions or large organic molecules. The utility of the FID effect is emphasized where precise dephasing times are obtained from time-frequency Fourier transformation of FID signals using a digital computer. The first quantitative and detailed test of FID theory is provided by these experimental techniques.

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.