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NONRESONANT ECHO SPECTROSCOPY OF INHOMO- GENEOUSLY BROADENED TWO-LEVEL SYSTEMS

Vladimir Kuz, Г. Г. Федорук

Published Jan 1, 1994• 0 citations
OpenAlex

Abstract

effects, due to the propagation of laser light in optically dense media, have been observed and studied. The use of the method of Stark switching [6] has been of much assistance in overcoming experimental difficulties. In that method, time-varying coherent effects are observed through the use of cw lasers, and the interaction between molecules and laser light can be switched on and off rapidly by changing the energy interval between levels through the use of the Stark effect. The idea of changing the energy separation was subsequently implemented in ESR [7], where the use of cw microwave radiation and pulsed changes in a polarizing magnetic field (Zeeman switching) have also made it possible to detect the basic time-varying effects. Many of the new properties of time-varying phenomena which have been studied in the field of optical resonances stem from the partial excitation of a spectral line (~o 1 a, this signal is reflected by a sinusoidal signal with a frequency specified by the generalized Rabi frequency (Wl 2 + b2)1/2.

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effects, due to the propagation of laser light in optically dense media, have been observed and studied. The use of the method of Stark switching [6] has been of much assistance in overcoming experimental difficulties. In that method, time-varying coherent effects are observed through the use of cw lasers, and the interaction between molecules and laser light can be switched on and off rapidly by changing the energy interval between levels through the use of the Stark effect. The idea of changing the energy separation was subsequently implemented in ESR [7], where the use of cw microwave radiation and pulsed changes in a polarizing magnetic field (Zeeman switching) have also made it possible to detect the basic time-varying effects. Many of the new properties of time-varying phenomena which have been studied in the field of optical resonances stem from the partial excitation of a spectral line (~o 1 a, this signal is reflected by a sinusoidal signal with a frequency specified by the generalized Rabi frequency (Wl 2 + b2)1/2.
Recently we have reported[1] on the measurements of electronic dephasing (T_2) and radiationless decay (T_1) in molecules using emission spectroscopy as a probe for the detection of the photon echo. This right-angle photon echo utilizes a single mode of a narrow-band (~MHz) cw dye laser and an intracavity electro-optical (EO) element that modulates the frequency of the laser.[2] Applying voltage pulses into the EO crystal switches the laser frequency from ω_0 (the peak frequency of the transition resonance) into another frequency ω. This switching procedure introduced by Brewer et al. for the observation of transients in the forward direction of the laser allows one to observe coherent transients in gases,[1,2] solids[3-5] and molecular beams.[6] Typically, the dispersion of the nonlinear EO crystal (e.g., AD*P) gives a 60 MHz frequency shift within the inhomogeneous resonance of the optical transition (width ~GHz or larger). Therefore, to switch the laser frequency further, one must use high-voltage signals that are usually hard to obtain especially for multiple-pulse experiments. Secondly, there is a limit on the frequency shift which is conditioned by the mode spacing of the cavity. Thirdly, because the laser is switched into a new group of molecules some complications may arise in interpreting the transient spectra especially in large molecules. This is because in large molecules the density of excited states is high and thus the buildup of transients of w molecules (which could be complex) will be superimposed on the decay of ω_0 molecules. Finally, off-resonance effects (e.g., beats) between the molecules at frequency ω and ω_0 cannot be avoided.
Coherent transient phenomena can be generated by the abrupt changes of parameters either of the electromagnetic field or of the matter. Comparative analysis of these two approaches, made on the base of experimental research in molecular gases SF6, 15NH3, HFCO and 13CH3F, excited by the radiation of CW CO2 and 13CO2 lasers, is presented. The first approach of photon echo (and its modifications) generation, based on electro-optic pulses formation from the resonant CW exciting radiation, has proven to be successful for intense infrared molecular transitions. The coherent responses are sufficiently powerful to be applied in high resolution spectroscopy, the spectral markers creation ,and molecular collisions investigations. However, this technique is not very effective for the free polarization decay investigations. Another approach based on the Stark switching molecular levels of polar gas, irradiated by CW laser, was demonstrated for free polarization decay and photon echo generation in 13CH3F. Free polarization decay has shown the pronounced frequency shift because of dynamic Stark effect in the CW radiation field. This specific feature of coherent transients, created by Stark switching, limits the high resolution spectroscopy by the low intensity range of exciting radiation. From other viewpoint, it gives unique possibility to investigate coherent transients generated at the dressed molecular levels.
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.