Nonlinear laser spectroscopy

Applied Physics B 29, 3, 1982

167

Resonant Raman Enhancement in Heterodyne Saturation Spectroscopy* E. Ktster, Q. F. Gao, R, K, Raj, D. Bloch, and M. Ducloy Laboratoire de Physique des Lasers, Associ6 au C.N.R.S., N ~ LA 282 - Universit6 Paris-Nord, Avenue J.-B. Cltment, F-93430 Villetaneuse, France PACS: 33 The recent development of high-frequency (HF) optical heterodyning has brought a considerable Improvement in laser spectroscopy. By means of H F detection, it is possible to overcome the main noise sources (amplitude noise of lasers) and to reach the ultimate (shot-noise limited) sensitivity in absorption techniques. Another distinct advantage of these methods is the possibility of observing as well resonant absorption as dispersion, depending on the detection phase. Here we briefly report on the observation of a Raman enhancement in a heterodyne saturation spectrum resonantly induced when the incident modulation frequency 6 is equal to a Raman frequency [1]. The basic set-up is essentially similar to the one used previously [2] in saturation (and two-photon) spectroscopy. A saturating beam (frequency cot) is frequency-shifted [at A) and 100% amplitudemodulated at 6 by an acousto-optic modulator, so that the pump beam consists of only the two-frequency side-bands coL+A+6/2. This pump and counter-propagating probe beam at col overlap in the sample. On resonance, a modulation at 6 is induced on the probe beam. and is detected through a phase-sensitive detection ("lock-in"). The experiments were performed on the resonance line -FtlJ = 1)~ 7F0(J = 0) of Samarium vapour (2 = 570 rim). A tunable magnetic field, parallel to the light propagation axis, was used to control the Zeeman splitting 2coz between the M - +_1 sublevels of the ground state 7F1. In these conditions, the Sm atom behaves like an ideal three-level system. Two physical processes can contribute to the induced probe modulation [3]. One is the well.known modulation of atomic populations, driven at 6 by the pump beam, and which is governed by a response factor (yj+it) -1 where 7J is the relaxation rate of an atomic population. The other one is a Doppler-free Raman process, also induced by the pump beam, and whose amplitude is governed by a factor [Tz+i(6__+2coz)]-~ where ?z is the relaxation rate of the Zeeman coherence. The sign in (3 + 2coz) depends on the ordering of the processes generated by the pump beam: either absorption at coL+A +3/2 and emission at coL+A--3~2, or viceversa. Hence, the Raman effect gives a resonant contribution for 6 - 2 m z (linewidth, 7z), while the population effect, maximum at zero frequency, decreases with increasing 6 (if 3> 7J) and has a phase-shift varying between 0 and ~ (this phase delay has been used to measure precisely atomic lifetimes) [2]. Figure 1 shows experimental spectra recorded for 6 - 1 . 7 MHz, and 6=34 MHz (Raman resonance, 6-2coz). The magnetic field is B - 8 . 1 G, corresponding to coz= 17 MHz. For a better understanding, we have represented the theoretical positions of the resonances on Fig. 2. To a single atomic transition, are associated two optical* Work supported in part by D.R.E.T.

i

20

MHz

I

Fig. la and b. Saturated absorption spectrum for (a) 6-1.7 MHz, (b) 6 - 3 4 MHz0 Raman resonance (magnetic field B= 8.1 G; the detection phase is chosen to yield absorption spectra)

aM u)0

1O_,

~o

6_,

(b)

6, 3~/Z

Fig. 2a and b. Theoretical positions of resonances (a) ~5~2coz, (b) 6 = 2coz= co- 1- co+ 1 (Raman resonance) resonance conditions (splitting 33/2) which correspond to a process where a field at coL+ 6 (respectively coL- 3) is created on the probe beam. The doublet centered at coo is a Doppler-crossover and is not affected by the Raman enhancement. The doublets centered at co-l=co0-i-coz ( M = - I ~ M = 0 transition) and c0+l=co0-coz are asymmetrical near the Raman resonance because, of the two components, oneis (Raman) resonant, the other one is antiresonant. Good agreement with theory is obtained [lJ. It is also noticeable that while the Raman-enhanced components become in phase (absorption) at the exact Raman resonance, the population components remain in quadrature (dispersion) due to the complex attenuation factor (TJ+ i6)- 1~ 1/i6. A striking feature is that the usual signal decrease in H F modulation is completely overcome by the Raman enhancement. Similar experiments can be performed by scanning 6 around the frequency of any atomic substructure. Such a method, purely based on optical techniques combines the advantages of RF spectroscopy and of optical frequency selectivity. In the case of intricate spectra, one should be able (i) to resolve (RF) substructures much smaller than the optical linewidth, (i/) to assign them to their respective optical structure, which is determined with a Doppler-free resolution and an optimum sensitivity (use of H F detection without signal attenuation). 1. A full account will be given elsewhere (E. Ktster et al., to be published) 2. R. K, Raj, D. Bloch, J. J. Snyder, G. Carny, M. Ducloy: Phys. Rev. Lett. 44, 1251 (1980) 3, M. Ducloy, D. Bloch: J. Phys. (Paris) 43, 57 (1982)