Doppler-free evanescent wave spectroscopy

Volume 59, number 2

OPTICS COMMUNICATIONS

15 August 1986

DOPPLER-FREE EVANESCENT WAVE SPECTROSCOPY P. S I M O N E A U , M. D U C L O Y

S. L E B O I T E A U X , C i d B. D E A R A U J O 1.2, D. B L O C H , J.R. Rios L E I T E 1.2 a n d

Laboratoire de Physique des Lasers ~, Universitb Paris-Nord, 93430 Villetaneuse, France

Received 28 April 1986

Nonlinear Doppler-free spectroscopy at the interface between a gas and a dielectric medium is experimentally demonstrated on Na vapor. Both saturated absorption and dispersion of sodium atoms near the surface are monitored, depending on the light beam incidence relatively to the critical angle for total internal reflections. Resonance line widths as narrow as 20 MHz have been observed on the Na D 1 line. This technique makes possible Doppler-free diagnostics of optically dense media and should find applications in the analysis of atom-surface interactions.

In conventional optics, the behavior o f em fields at the b o u n d a r y between two media is governed b y Snell-Descartes and Fresnel laws [1 ]. They are comm o n l y exploited for the determination o f refractive indices, in particular b y measuring the critical angle, 0c, for total internal reflection. In the same sense, linear gas spectroscopy can be performed either in selective reflection experiments at a gas-glass interface [2], or b y selective absorption in the evanescent wave [3]. On the other hand, generalization o f the above laws to nonlinear optics at interfaces has been performed b y Bloembergen et al. [4]. Attenuated total reflection (ATR) methods have been applied to surface nonlinear optics (e.g., surface plasmons) [5]. In this letter, we present the first observation, to our knowledge, o f Doppler-free nonlinear spectroscopy at an interface between a gas (here, Na vapor) and a dielectric medium. We demonstrate that it is possible to m o n i t o r saturated absorption (SA) and saturated dispersion (SD) o f atomic vapors near a surface. This technique o f evanescent wave (EW) Dopplerfree spectroscopy should provide a powerful tool to

1 Permanent address: Departamento de Fisica, Universidade Federal de Pernambuco, 50000, Recife, Brazil. 2 Partially supported by Conselho Nacional de Desenvolvimento Cientifico e Tecnologico - CNPq, Brazil. 3 Associ6 au Centre National de la Recherche Scientifique, no. 282.

monitor the behavior o f atoms in gas phase, at the vicinity o f surfaces (e.g., a t o m - w a l l collisions, modification o f the radiative behavior o f atoms, etc.). It also allows one to perform Doppler-free spectroscopy in optically dense media. The principle o f the experiment consists in a sample cell (here, a Na cell, heated to T ~ 2 0 0 - 2 5 0 ° C ) , ended with a glass prism (index n 1 ~ 1.45), irradiated b y two counter-propagating beams issued from a singlemode cw tunable dye laser (fig. 1). The two beams are incident on the interface with such an angle 0 i that they are totally reflected (0 i > 0 c = s i n - 1 ( 1 / n 1)]One o f the beams - " p u m p " - is amplitude-modulated at a high frequency (I2 = 1 MHz) with an acoustooptic modulator , 1 . The other (probe) beam is directed, after reflection on the interface, to a fast photodiode *1 HF pump modulation, and heterodyne detection techniques, allow us to monitor the reflectivity changes with high S/N ratios [6].

Fig. 1. Principle of evanescent wave Doppler-free spectroscopy. 103

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Fig. 2. Probe reflectivity change around the D2 line for 0 i 0 c = 1.4 mrd (1), 0.4 mrd (2) and -0.6 mrd (3). Resonance (a) corresponds to the 3S1/2 (F = 2) ~ 3Pa/2 (F = 1, 2, 3) transitions, and (b) to the 3S1/:z (F = 1) ~ 3Pa/2 (F = 0, 1, 2) transitions. (c) is a cross-over resonance induced by optical pumping between the F = 1 and F = 2 hyperf'me levels of the 3S ground state (Probe and pump intensities are respectively 3.4 mW and 6.5 mW. Beams are polarized in the incidence plane). -

(PD) and a lock-in detector, which monitor the reflection modulation. Induced at frequency ~2, when the laser frequency is scanned across the Na resonance lines (D2, X = 589 nm; D1,589.6 nm). To check the influence o f the angular incidence, the position o f the cell, relatively to the beams, can be adjusted by means of a rotating amount (accuracy ~1 '). As seen in fig. 2, recorded D 2 spectra exhibit Doppler-free absorption lineshapes for 0 i > 0 c, which originate in a (SA) process between the two evanescent waves (EW), for which the real components of the wave vectors are opposite. The EW pump modulates both absorption (6hi) and dispersion (6nR) indices of the vapor, as viewed by the counter-propagating EW probe (complex refractive index change, 8n = ~SnR + iSni). Like in ATR methods, due to energy balance between EW and reflected waves, the absorption process alters the probe reflection. For incidences far from the critical angles (ff = 0 i - 0 c >> lSng 1), the real index change 6n R does not modify the critical angle sufficiently to alter the total reflection process. The Doppler-free nature o f the resonances results from a velocity-selection process along the surface (and in the incidence plane), and is analogous to velocity selection in volume SA with freely-propagating waves. When the incidence angle comes closer to the critical angle, the probe reflectivity change increases sharply, to reach about 10 -3 . At the critical angle, the 104

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absorption lineshapes are dramatically altered, and suddenly turn into saturated dispersion (SD) lineshapes, for 0 i < 0 c (fig. 2). This remarkable change (which appears in less than 1 mrd) may be understood if one realizes that, in the transmission mode (0 i < 0c), the probe reflection is no longer total, and undergoes very fast variations with the angular detuning, q) = 0 i - 0 c [1]. Since 0 c is sensitive to the real part o f the refractive index, 5n, any change in 6n R (i.e., Na vapor dispersion) will affect the probe transmission, and thus its reflection. This viewpoint is substantiated by means o f Fresnel relations [1 ], which show that the probe reflectivity change is proportional to (in the case 16nl ~ I~1 "~ 1) ,2

8R ¢c 6nl/X/-~,

for ~ > 0,

cc 6nR/X/~-I, for ff < 0.

(1)

The sharp increase o f the signal for 0 i ~ 0 c is correlated to the simultaneous increase o f the EW amplitude (Indeed, the amplitude o f the transmitted field in the vapor has an absolute maximum for 0 i = 0 c [11.) Other physical phenomena affect both signal amplitude and lineshape: (i) In the total reflection mode (0 i > 0c), transit time in the EW is an important limitation to the SA linewidth. Indeed the EW penetration depth is of the order of l = (X/2rr)(n 2 Sin20i -- 1)-1/2 ~. (X/Zrr)((tan 0 c)/2 ~) 1/2

(2)

leading to a transit time broadening Au T ~ ku (2~/tan 0c) 1/2 (u, thermal velocity), which is in general larger than the natural width, I" = 10 MHz. This also affects the pump-induced population changes in the atomic levels, and reduces their amplitude by an extra factor ~'P/Au T. (ii) In the transmission mode, transit time is not a limitation, but the two beams are now transmitted into the Na vapor, with an angle X = (21~b[/tan 0c) 1/2 with the surface. This angular separation between *2 At the exact critical angle (~0 = 0), the signal is a complex admixture of absorption and dispersion lineshapes, which compared well with the theoretically predicted lineshape, Re (x/~--ff) [81.

V o l u m e 59, n u m b e r 2

OPTICS COMMUNICATIONS

gR (9 -4 )

~

t •

.

-6

o

mrd

11o

Fig. 3. A m p l i t u d e o f t h e r e f l e c t i v i t y c h a n g e versus ~0 = 0 i - 0 c.

transmitted pump and probe is responsible for a Doppler broadening of the resonance width, proportional to 2kux. Signal amplitudes and linewidths versus ~ for the D 2 line are shown in figs. 3, 4. In fig. 3, amplitude variations are compared with a model in I~ 1-1/2 on the transmission side [cf. eq. (1)], and in ~ - 1 on the total reflection side [in addition to (1), an extrafactor ~b-1/2 comes from the transit time limitation, l/u, eq. (2)]. As expected from the discussion above, the resonance linewidth is minimum at the critical angle, and increases sloiwy on both sides o f this angle (fig. 4). However, the narrowest linewidths (FWHM ~ 150 MHz) are still much larger than the radiative linewidth, because o f (i) the unresolved hyperfme structure of the 3P3/2 level, (li) the natural divergence of the incoming beams: a beam divergence, A0 = 1 mrad, in the prism, leads to a divergence, AX = (2A0/tan 0c) 1/2 ~ 46 mrd, in the transmitted beams, i.e. a residual Doppler broadening 2kuAx ~ 8 0 MHz ¢a

MHz •

•

oo

oo% oo

•

,,"

°

•

"."

I0( I

-Io

I

-5

o

16

crnrd •

Fig. 4. R e s o n a n c e l i n e w i d t h (full w i d t h a t h a l f m a x i m u m , FWHM) versus 0 i - 0 c.

o

15 A u g u s t 1986

b

50Mhz I

I

F=2 // - ~ / ~ } F=I G - - ~ 192 ~4hz F=2 c d F= 1 1772 Plhz Fig. 5. Partial spectrum of the D t line, with the fully resolved hyperfine structure: a(b) are the 3S1/2, F = 2 ~ 3Pt/2, F = 1 (F = 2) lines; c and d are two optical-pumping-induced crossovers between the F = 1 and F = 2 ground states (b-c separation ~790 MHz). By using well collimated beams (A0 ~ 0.1 mrd), we have been able to resolve completely the hyperfme structure o f the D 1 line, and to observe SA resonances with 40 MHz width (FWHM) for the main resonances, and only 20 MHz width, for the cross-over lines between the F = 1 and F = 2 ground states (fig. 5). These cross-overs are of opposite sign to the main resonances, because they are produced via an optical pumping cycle between, the two Na ground states. This optical pumping needs one fluorescence decay from the 3P1/2 excited state to the ground state, and may exist only for those atoms with transverse velocities, OT, small enough so that this radiative decay can occur during the atom transit time, l/oT. For such atoms, transit time broadening is reduced, making the cross-over linewidth smaller. In conclusion, we have observed narrow Dopplerfree resonances in selective reflection from a glassvapor interface, at the vicinity o f the critical angle. Both saturated absorption and dispersion can be monitored. This makes possible Doppler-free diagnostics o f optically thick media, and high-resolution spectroscopy without light propagation inside the gas sample. Since the probe reflectivity monitors the refractive index very close to the interface, this spectroscopic technique should provide a powerful tool to analyze

*a In the total reflection mode, the divergence/~× becomes imaginary, i.e. it describes a distribution of penetration depths, I [eq. (2)]. This leads to a residual transit time broadening comparable with the residual Doppler broadening. 105

Volume 59, number 2

OPTICS COMMUNICATIONS

the interactions between a surface and atoms or molecules in gas phase, at intermediate distances (100 n m 1/am). Two remarkable applications should be mentioned: (i) A tom-wall collisions could be analyzed: atom sticking time, atomic de-excitation (if the incoming atom is in an excited, or metastable, state), and angular re-distribution o f atomic velocities by the surface. For instance, by monitoring the modulation signal in quadrature with the narrow SA resonances, we have observed broad resonances, (Av ~ 500 MHz) which should be attributed to velocity-changing atom-wall collisions. The traosient response, to the em field, of atoms leaving the surface may be responsible for a non-local relationship between electric field and dipole polarization [2]. (ii) The presence of the dielectric medium alters the radiative behavior o f vapor atoms (e.g., modification o f the spontaneous emission rate, etc.) [7]. Dopplerfree EW spectroscopy should be a powerful tool to explore resonance line broadening, and frequency shifts induced in the vicinity of the surface. We have observed a dependence o f some cross-over signals on the beams polarization direction relatively to the interface, which could be a signature o f these long-range interac-

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tion effects [8]. These phenomena might be enhanced in three-level spectroscopy o f "large-volume" Rydberg states.

References [ 1] G. Bruhat and A. Kastler, Optique (6th edition, Masson, Paris, 1965) pp. 380-417. [2] J.L. Cojan, Ann. Phys. (Paris) 9 (1954) 385; A.L.J. Burgmans and J.P. Woerdman, J. Phys. (Paris) 37 (1976) 677; M.F.H. Schuurmans, J. Phys. (Paris) 37 (1976) 469. [3] P. Boissel and F. Kerherve, Optics Comm. 37 (1981) 397. [4] N. Bloembergen, J. Opt. Soc. Am. 70 (1980) 1429, and references therein. [5] See, e.g., Y.R. Shen, Nonlinear optics (Wiley, New York, 1984) pp. 479-504, and references therein. [6] R.K. Raj, D. Bloch, J.J. Snyder, G. Camy and M. Ducloy, Phys. Rev. Lett. 44 (1980) 1251; M. Ducloy, in: Revue du Cethedec, Ondes et Signal (Gauthier-Villars, Paris, 1983), Vol. NS 83-2, pp. 133163. [7] K.H. Drexhage, in: Progress in optics, Vol. 12, ed. E. Wolf (North-Holland, Amsterdam, 1974) p. 163. [8] P. Simoneau, Tb6se de Troisidme Cycle, Universit~ de Paris XI (1986); P. Simoneau et al., to be published.