Observation of Raman Self-Conversion of fs-Pulse Frequency due to Impulsive Excitation of Molecular Vibrations

Abstract

We report on the observation of a Raman frequency conversion mechanism different from the usual Stokes (anti-Stokes) stimulated process. A continuous down-shift of laser pulse frequency was measured for sufficiently short ͑#40 fs͒ pulses propagating in gaseous SF 6 . We attribute this behavior to the extremely nonstationary nuclear response of SF 6 molecules to the light field with a duration shorter than the period of the Raman-active vibration. [S0031-9007(98)06773-8] The purpose of this Letter is to present some new experimental and theoretical characteristics of stimulated Raman scattering of light in the region of fs-laser pulse durations. Stimulated Raman scattering (SRS) was extensively studied during the past three decades, and nonlinear properties of this phenomenon are well established in the literature: The effect occurs for laser field intensities exceeding some threshold, and the frequency spectrum of generated coherent radiation generally contains discrete Stokes (anti-Stokes) frequency components whose number increases with an increase of the pump laser intensity and the interaction length [1]. In the ps-pulse regime the physical picture is more complicated, and such factors as nonstationary molecule response, group velocity dispersion, self-phase modulation due to electronic Kerr effect, etc., come into play [2]. Considerable progress in the understanding of short-pulse mechanisms of SRS has been made in nonlinear optics of fibers where SRS is involved in the processes of pulse nonlinear self-action and soliton formation [3].The transient properties of SRS are usually discussed in the connection with the vibrational dephasing time T 2 . For pulses shorter than T 2 the amplitude of resonantly driven molecular vibrations has no time to settle within the interaction time t p which gives rise to a number of Raman-type coherent propagation effects observed in gases and liquids. Delayed Stokes generation, shortening of the Stokes pulse, an oscillating structure of the pump, and Stokes field were reported and discussed by many authors [2,4,5]. Despite the different interaction regimes and different parameters of the Raman media used, the main features of the SRS process established in the first experiments are normally observed down to fs-pulse durations.We draw attention to the fact that a very specific behavior of the SRS process could be expected for laser pulse durations comparable or shorter than the molecular vibrational period: t p , T y 2pV 21 y .(1) The nature of nonstationarity here originates from the transient character of the buildup of the vibrational response itself. If one remembers that the vibrational frequencies of many Raman-active materials lie in the region of several hundreds of cm 21 , it can be seen that the discussed extreme regime of SRS is rather a regular trend for nonlinear optical interactions with pulses of sub-100-fs duration. Within the short time (1), a quantum oscillator has no time to exhibit its vibrational properties, and therefore it cannot be resonantly driven by...

Four-Wave Optical Parametric Amplification in a Raman-Active Gas

“…When the pulse duration of the pump pulse is shorter than a molecular motional period, the dynamics changes to impulsive stimulated Raman scattering that continuously shifts the frequency of the short pump pulse to a lower frequency. This was observed in previous researches [13,14,19] and in the present research as shown in Figure 4. This process does not require any phase matching.…”

“…Such coherent motion induces the ultrafast variation of the refractive index of the medium, modulating the frequency of a laser pulse with an arbitrary wavelength copropagating with the pump pulse [13] as well as that of the pump pulse [13][14][15][16]. The coherent molecular motion is also expected to have effects on the amplification characteristics of the FWOPA.…”

“…A pump pulse propagating through a Raman-active medium impulsively excites coherent molecular motion when the duration of the pump pulse is shorter than the period of the motion [12,13] …”

“…In both cases of molecular vibration and rotation, the frequency of the pump pulse redshifts during the excitation of coherent molecular vibration [13,14] or rotation [19]. This arises from the delayed nuclear response 2,nuc () nt  originating from the coherent molecular vibration or rotation induced by the pump pulse.…”

“…When the pulse energy of the pump pulse increased from 150 J to 310 J, a part of the pump spectrum continuously shifted to long wavelengths to form a spectral peak at 822 nm when the energy reached 310 J or the center of gravity of the spectrum of the pump pulse shifted to the longer wavelength. The spectral redshift of the pump pulse indicates the generation of coherent motion [13,14]. Since the pump pulse duration was shorter than the periods of molecular rotations of the nitrogen and oxygen contained in air, the coherent rotational wavepackets of these molecules would have been excited.…”