Applied Physics B 29, 3, 1982 Nonlinear Optical Processes Sub-Doppler Measurements of Predissoeiative Broadening Y. Prior, D. Brenner, and M. Shapiro Department of Chemical Physics, Weizmann Institute of Science, 76100 Rehovot, Israel PACS: 33 Predissociation is a process where a bound molecular level a is coupled to an unbound level (or set of levels) b. This coupling is manifested by spectral broadening of transitions into and out of level a (in the frequency domain), and by shortening of the total lifetime of this level (in the time domain), In principle, both approaches should give the same physical information, but in practice, due to experimental limitations, the methods have been limited to non-overlapping ranges. Halogen and interhalogen diatomic molecules are a prime candidate for the study of this process, since their spectra are well known, and relatively simple to analyze. Clyne and Heaven performed many time domain experiments [1] on these molecules. The range of measurement has been limited by the laser pulse width- approx. 10 ns, or broadening of less than 30 MHz, In the frequency domain many works have been reported [2] - all of them limited to broadening larger than the Doppler width or about 1 GHz. Saturation spectroscopy is by now a well established means for obtaining sub-Doppler spectral information [3], and its application for the extension of the spectral measurement of predissociative broadening is discussed here. A strong "pump" laser beam at frequency v saturates the transition between a ground state level g and the level of interest a in a gaseous sample in a cell. Because of the Maxwellian distribution of velocities, only molecules with velocity v~-= c(v- v0)/v0 (here hvo = E~- Eo) will be on resonance with the laser. A pump of intensity Iv will decrease the population difference between levels g and a, and will Weak-Wave Retardation and Phase-Conjugate Self-Defocusing in Si E. W. Van Stryland, Arthur L, Smirl, Thomas F. Boggess, M. J. Soileau, and B. S. Wherrett Center for Applied Quantum Electronics, North Texas State University, Denton, TX 76203, USA F. A. Hopf Optical Sciences Center, University of Arizona, Tucson, AZ 85721, USA PACS: 42.10 We describe and measure the effects of self-defocusing on the various coupling effects produced when two coherent, noncollinear, picosecond optical pulses (strong pump and weak probe) are both spatially and temporally coincident in a thin silicon wafer. We observe that the weak probe beam experiences considerably more defocusing than the pump beam. We believe this is the first direct confirmation of weak-wave retardation in light-by-light-scattering experiments. We also demonstrate the effects of this defocusing on the quality of the forward-traveling conjugate wave. The observed defocusing is caused by the accumulation of free carriers created by linear absorption of 1.06 gm light. The addi- tional increase in refractive index experienced by the weak probe 159 burn a hole of spectral shape (N,,- No) (v;)=ANo _/Av~2+(z/tsp) CZAvlv z ' (1) iT) ~ +(~-~') where ANo is the population difference in the absence of a saturating field, Av is the homogeneous linewidth, tsp is the lifetime for spontaneous emission, and r is the total (not necessarily radiative) lifetime of level a. A weak probe now interacts with the same molecules and scans the hole described by (1). The physical information we are after is the rate of tunnelling across the pre- dissociative barrier or the actual lifetime of level a. This parameter enters the formula in z and in the homogeneous linewidth Av. Thus it can be observed both from the width of the hole at low saturating power or from the saturation parameter I~ as given by Yariv [3] 2~ z n2hvaA v t~= (~/s 2 The experimental system consists of two counter propagating beams from a ring dye laser (10-20 MHz) - a modulated pump and a weak probe - and phase sensitive detection. Several halogens and interhalogens were measured, and pressure broadening by a buffer gas [4] was used for calibration of the sensitivity. The question of what constitutes a homogeneous linewidth in molecular dissociation processes is briefly discussed. 1. M. A. A. Clyne, M. C. Heaven: J. C. S. Faraday II 76, 49 (1980) and references therein 2. H. Kn6ckel, E. Tiemann, D. Zoglowek: J. MoL Spectrosc. 85, 225 (1981) 3, A. Yariv: Quantum Electronics, 2nd ed. (Wiley, New York 1975) Chap. 8 4. D. Brenner, Y. Prior: Appl. Phys. B28 (in press, 1982) was named weak-wave retardation by Chia and co-workers [1], who first predicted this effect. These workers later observed light- by-light scattering, but they did not verify weak-wave retardation [2]. We measure the degree of self-defocusing by observing the transmitted beam profiles with a vidicon detector. The self- defocusing of the transmitted probe and conjugate in Si has been studied recently by Hopf et al, [3] using nonlinear interferometers. They observed a substantial self-defocusing of the conjugate, but they were unable to detect weak wave retardation. For their work, the pulse width was comparable to the grating lifetime due to carrier diffusion. Figure 1 illustrates the distortion of the pump and probe beam profiles during these selfdiffraction studies. The fiuence of the pump pulse was 46 mJ/cm 2, and the fluence of the probe was a factor of 500 smaller. Figure 1A shows scans of the probe profile (in the far field) when the pump was blocked - the profile is reasonably Gaussian. Figure 1B and C show profiles of the transmitted probe and pump, respectively, when both were simultaneously present. The broadening of the pump caused by self-defocusing by the optically-created free carriers in the Si is evident, and the additional defocusing of the probe (weak-wave retardation) is clear. Transmitted beam profiles and energies for all three beams (probe, pump, and conjugate) were measured for various excitation levels