266 1 EQEC'94 1 THURSDAY AFTERNOON

~ - ~~ -~

D d Measured Beatnote between

I I I I

Sum and Second Harmonlcs (filter-llmlted) 2L r,LIJ 0% I -

1 J 7 m 1 X 1 0 1

F n n m Y WI

QThM3 Fig. 3. Power spectral density of the down-converted "blue" beat signal. The width of the peak is solely due to the window function of the FFT spectrum analyzer. Its center of gravity coincides with the expected 0.5 Hz within our experimental uncertainty.

where the last two are degenerate if there are no shifts. With the abbreviations

Aw = w2 - w,

we can expand this for the case of small frequency differences up to first order:

w,* = 2Aw(1 + E)

wIs = Ao(1 + E)

wz = Aw(1 + E).

So our experiment is only sensitive to the frequency-dependent part of a shift.

In the experiment the down-converted signal appears shifted by 16 ~ H z , which is much less than our experimental un- certainty of 192 p H z . From that we ob- tain a limit on E:

-2.1 . io-" < E < 2.5 . io-13.

We expect to be able to reduce the experimental uncertainty by one or two more orders of magnitude. Then it should be possible to see shifts due to the distortion of the blue lieshape caused by a frequency-dependent doubling effi- ciency for the finite laser linewidth. 1. I. D. Abella, Proc. IRE 50, 1824

(1962). 2. H. S. Boyne, W. C. Martin, J. Opt.

Soc. Am. 52, 880 (1962).

QThM4 1745

Two-photon resonant second-harmonic generation from rubidium Rydberg atoms

Sandra S. Vianna, J. R. W. Tabosa, F. A. M. de Oliveira, Departamento de Ffsica, Universidade Federal de Pernambuco, 50670-901 Recife-PE, Brasif We report on the measurements of polar- ization and spatial beam profile depend- encies of the two-photon resonant sec- ond-harmonic generation (SHG) from

N r ~ r - r C ~r

QThM4 Fig. 1. SH beam cross section profiles for the 16 d + 5 s transition with the input beam polarizations (a) parallel and (b) perpendicular to the scan direction of the pinhole. I = 2.5 X 10' W/cm2, N = 5 X ~ m - ~ .

excited p and d Rydberg states of rubid- ium vapor.

An Nd:YAG laser-pumped dye laser (5 ns, 5 Hz) was linearly polarized and focused into a sealed cell containing ru- bidium at Torr pressure. The inten- sity dependence of the SH signal as a function of the pump-laser intensity I(w), and Rb atomic density are the same one as obtained for a heated pipe cell with a buffer gas.' For s, p, and d transitions, we found an intensity dependence greater than I', at lower intensities, which rap- idly reduces to I*, and then to linear, as I is increased.

The Rb atomic density, N, was varied between 3 X 10" and 6 X l O I 5 atoms/ cm3, and a quadratic dependence with N for the SHG originating from s and d states was obtained, while for the tran- sition np + 5 s a quartic dependence was observed.

For a cell temperature of 520 K (the highest value allowed before cell dam- age) the SHG measured efficiency was 2 x lo-' for the 16 d + 5 s transition and 3 X 10.' for the 17 p + 5 s transition. These measurements are unchanged when a buffer gas is introduced.

We also performed measurements of the SH beam cross sections for different polarizations of the input and output beams. For these measurements a pinhole with 1 mm of diameter was transversely displaced across the output beam just be- fore monocromator. The pinhole position and SH intensity were recorded by a computer.

The spatial profile for polarization of the input beam parallel and perpendicu- lar to the scan direction are shown in Fig. 1 for 16 d + 5 s and in Fig. 2 for 17 p + 5 s transitions. For the 16 d -) 5 s transi- tion, the output beam is composed nearly by two lobs, which are polarized and placed side by side parallel to the input beam polarization direction. In the 17 p + 5 s case, the output mode has a ring- like shape with radial polarization. Al- though binary collision models',' have been tried in order to describe these SHG experiments, they fail in explaining spa- tial nonuniformities.

The observed SH intensity profile can

1U3S TIOl, :~-1r-,:,

QThM4 Fig. 2. SH beam spatial profile for 17 p + 5 s transition. Same conditions as Fig. 1: (a) perpendicular and (b) parallel polarizations of the input beam.

be described using Bethune's theory? which attributes the SHG to the third-or- der susceptibility, x('), with a laser-in- duced static-electric-field involved. Fol- lowing this model, we calculated the ratios of SH intensities for polarization components parallel and perpendicular to that one of the input field, and ob- tained a reasonable agreement with the values measured directly from the spatial modes observed for 16 d + 5 s and 17 p + 5 s transitions, shown in Figs. 1, 2.

However, the two different behaviors with the atomic density for d and p lines and the unchanged doubling efficiency when an additional laser at 1.06 p is pres- ent remain to be explained, as well as the physical mechanism for the production of such a static-electric field. 1. S. S. Vianna, C. B. De Araujo, Phys.

Rev. A 44, 733 (1991). 2. S. Dinev, J. Phys. B 21, 1111 (1988);

21, 1681 (1988); A. El@, D. Depatie, Phys. Rev. Lett. 60, 688 (1988). D. S. Bethune, Phys. Rev. A 23,3139 (1981).

3.

QThM5 1800

News in investigation of radiation pressure force frequency properties

V. A. Grinchuk, I. A. Grishina, E. E Kuzin, M. L. Nagaeva, G . A. Ryabenko, V. E Yakovlev* Institute of General Physics, Russian Academy of Sciences, 38 Vavilov st., Moscow, 117924 Russia This paper is devoted to the investigation of mechanical action of light on neutral atoms. In our experiments' the properties of light pressure effect are investigated by the method of scattering of an atomic so- dium beam, intersecting perpendicularly an unhomogeneous strong pulse field of two counterpropagating waves (inczent +z and reflected from the mirror -k) of the resonant laser radiation (see Fig. 1). The short duration (-10 ns) of laser pulse (shorter than the spontaneous decay time of the resonant D,-line of sodium) gives us an opportunity to investigate the properties of the stimulated light pres- sure force, acting on atom in the unhom-