Nonlinear spectroscopy

84

XIIth International Quantum Electronics Conference 1982

6. N.Hata, K.Shimoda: Appl. Phys. 22, 1 (1980) 7. R.L.Barger, T.C.English, J.B, West: Proc. of the 29th Annual Frequency Control Syrup. 1975. (Electronics Industries Assoc. 2001 I St. N.W., Washington D.C. 20006) p. 316

A New Mechanism for the Production of Subnatural Optical Resonances W. Gawlik, J. Kowalski, F. Tr~iger, and M. VoUmer Physikalisches Institut der Universit~it, D-6900 Heidelberg, Fed. Rep. Germany PACS: 42.65, 07.65 A reduction of the width of optical resonances below the limit imposed by the natural lifetime has been achieved in several experiments like rf-optical double resonance [1], level crossing spectroscopy [2], or very recently by phase switching of a light field [3]. The idea of these methods is to produce a signal only with those atoms that are more long-lived than the average. This, however, goes along with a loss of signal-to-noise ratio which often can be outweighed only hardly by the improvement in the resolution. We report on a new mechanism which leads to resonances considerably narrower than the natural width but does not suffer from a reduction of the signal-to-noise ratio. Our method uses the technique of polarization spectroscopy [4], but in contrast to other experiments of this kind the probe beam perturbs the investigated medium about as strong as the pump beam. We consider the effect of this strong probe beam in a very simple s~gr~l

8. U.Klingbeil, J.Kowalski, F.Tr~iger, H.B.Wiegemann, G. zu Putlitz: Appl. Phys. 17, 199 (1978) 9. C.J.Bord6: Private communication 10. J. Helmcke, S. A.Lee, J.L. Hall : Private communication

atomic structure with a ground state Jg = 1 and an excited state Je=0. In such a situation new effects not considered in earlier works on polarization spectroscopy appear: (i) the components E+, E_ of the linearly polarized probe beam tend to equalize the populations of the m = + 1 sublevels altered by the circularly ,polarized pump beam E s ;(ii) the E+ and E_ components of the probe beam can create a coherence between the m = _+1 sublevels of atoms from all velocity classes; (iii) the E_ component of the probe beam and the pump beam E~ create a coherence between the m = ___1 sublevels of atoms with the projected velocity v = 0. The effects (ii) and (iii) are associated with the Zeeman coherence 0 + - and cannot be revealed as long as the sublevels are degenerate. However, even in the absence of external fields the degeneracy can be removed if light shifts of the appropriate energy levels are induced by the counter-propagating and differently polarized light beams. For an atom moving in such beams the energy shifts due to each light field are velocity dependent and have different signs because of opposite Doppler detunings of the frequencies seen by the moving atom. Thus, the light shifts remove the degeneracy of the Zeeman sublevels and reveal the coherence in a way similar to external magnetic field [5]. This is in fact a new velocity selective example of the optical Hanle effect [6], where the parameter which changes the "optical Larmor frequency" coL is the atomic velocity rather than the intensity of the light. s~jr~*

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I Fig. la-d. Sections of polarization spectra of the Na D1 line (a) weak probe beam, crossed polarizers; (b)-(d) spectra affected by the velocity selective optical Hanle effect with strong probe and pump fields and different rotations 0 of the polazizers from their crossed position

loser frequency

Applied Physics B 28, 2/3 Scanning of the laser frequency across the Doppler profile then picks up atoms from various velocity groups that bring different contributions of the Zeeman coherence to the probe beam signal. All population and coherence contributions interfere together if v = 0 and produce a narrow structure in the usual Doppler-free polarization spectroscopy signal if the detuning from the exact resonance frequency is 6=0. A theoretical treatment shows [7], that the 0 + -(6) dependence is resonant at ~5= 0 and n a r r o w e r than the natural width if the intensities of the probe beam Ip and the pump beam I s are not too large. In general the line shape is very complicated and composed of Lorentzian and dispersion signals, but also contains the subnatural, symmetrical and non-Lorentzian contribution. This complicated line shape, however, can be simplified to a large extent and made free from dispersion signals by properly adjusting the crossing angle 0 of the two polarizers. Experimental studies of the effects described above were performed on the N a D1 (589.6nm) resonance line. The experiments were carried out with a polarization spectroscopy setup and a frequency stabilized dye ring laser. The linearly polarized probe beam and the circularly polarized pump beam travel collinearly (in opposite directions). In order to eliminate any influence of external fields that could obscure the velocity selective optical Hanle effect the cell was placed in a threefold ~t-metal shielding which reduced the magnetic field to less than 10 .7 T. Figure l a represents part of a typical polarization spectrum of the Na D1 line, recorded with 0---0, weak pump and negligible weak probe beam ( I s = 5.6 mW/cm 2, Ip = 0.9 mW/cm2). If both beams are sufficiently strong (I~= 30mW/cm z, I p = 2 3 mW/cm 2) a line asymmetry appears caused by two dispersive contributions of different signs and amplitudes (Fig. lb and c). An appropriate angle 0 + 0 permits the observation of purely symmetrical signals (Fig. ld) resulting from the velocity selective optical Hanle effect. They appear as narrow dips in the power broadened Doppler-free signals. However, the conditions for producing dips as narrow as possible are competitive to the conditions for optimizing their amplitudes: on the one hand the intensities Ip and I s should be

Transit Time Linewidth Limitations in cw CARS Spectroscopy E. Gustafson and R. L. Byer Department of Applied Physics, Stanford University, Stanford, CA 94305, USA PACS: 42.65, 07.65 High resolution CARS spectroscopy linewidths are limited by Doppler broadening. To reduce Doppler broadening and at the same time improve signal levels by spectral simplification, Duncan and Byer [1] proposed CARS spectroscopy measurements in a supersonic expansion cooled molecular beam. Recent experiments using pulsed laser sources have verified the advantages of molecular beam CARS spectroscopy [2-4]. We report the first high resolution cw CARS measurements in a steady state supersonic jet expansion and the first observation of transit time broadening in a CARS spectrum. We have resolved the Q-branch structure of methane at rotational temperatures to 30K. The tight focusing required to maintain high cw CARS signal levels resulted in significant transit time broadening in the CARS linewidth. We have developed the theory of transit time broadening for CARS and present a comparison of theory and experiment.

85 signal

loser frequency Fig. 2. Narrowest subnatural dip obtained with a width of 2.6 MHz. The crossing angle of the polarizers was 0 = + 12mrad ; I, and Is were smaller than 10 mW/cm z small to reduce power broadening of the dips, at the same time they have to be large enough to crete the Zeeman coherence and to destroy it by the mechanism described before. Nevertheless, we have obtained signals as narrow as 2.6 MHz (26% of the natural width) with a contrast of about 10 % by reducing Ip and I s below 10 mW/cm 2 (see Fig. 2). These subnatural signals do not only have a smaller linewidth than achieved with other techniques [3], but also exhibit a much better signal-to-noise ratio. Furthermore, the method is extremely simple experimentally and should also permit the determination of frequency distances. 1. I.Ma, J.Mertens, G. zu Putlitz, G.Schfitte: Z. Phys. 208, 352 (1968) 2. J.S.Deech, P.Hannaford, G.W.Series: J. Phys. B7, 1131 (1974) and references therein 3. F.Shimizu, K.Umezu, H.Takuma: Phys. Rev. Lett. 47, 825 (1981) 4. C.Wieman, T.W.H~insch: Phys. Rev. Lett. 36, 1170 (1976) 5. W.Gawlik, J.Kowalski, R.Neumann, F.Tr~iger: Opt. Commun. 12, 400 (1974) 6. V.P.Kaftandjian, C.Delsart, J.C.Keller: Phys. Rev. A23, 1365 (1981) 7. W.Gawlik: Private communication

The Doppler and pressure broadening effects on the CARS lineshape were first presented by Henesian et al. [53. Since then others have considered the CARS lineshape theory. The inclusion of transit time broadening extends the previous treatments in a way that is important for future high resolution Raman measurements in supersonic molecular beam spectroscopy. The transit time theory proceeds by finding the time dependent driving polarization for the anti-Stokes field that includes the transverse flow velocity of the molecules. The driving polarization is calculated in the rest frame of the molecules and includes both pressure and Doppler broadening. A coordinate transformation to the laboratory frame is made and the Fourier Transform of the anti-Stokes field is calculated using a Greens Function approach. The spectrum is then found by applying Parseval's theorem. The integral must be done numerically to derive the final lineshape. Figure 1 shows a schematic of the cw CARS apparatus. The single axial mode argon ion laser operating at 514nm provides the pump wavelength. A Coherent model 599 cw dye laser, operating under computer control, is the tunable source tuned to the Stokes shifted wavelength. The beams are focussed with a 3.7 cm lens into the supersonic expansion, collimated and then spectrally and spatially filtered prior to the photon counting photomultiplier detector. Signal levels are near 1000 counts per second for 3 W of pump power and 100mW of dye laser power.