**6. Acknowledgement**

282 Advanced Photonic Sciences

1000 2000 3000 4000 5000

1000 2000 3000 4000 5000

Cell thickness [nm]

 Absorption Fluorescence

Absorption Fluorescence

Cell thickness [nm]

Fig. 22. Contrast (a) and FWHM (b) of the dip observed in the fluorescence and absorption.

The application of the nanometric cell provides a new opportunity for significant enhancement of the resolution in laser spectroscopy of thermal cell, without application of complex atomic beam or laser cooling systems. This simple approach makes it possible to study also the dynamic processes in the absorption and fluorescence. It is shown that when the Cs vapor layer thickness is about the wavelength of the irradiating light, the fluorescence spectrum is significantly narrower than the absorption one and the absorption coefficient does not grow linearly with the thickness of the atomic layer. The observed in the optical domain coherent Dicke narrowing of atomic transitions is strongest at L = /2, while further expanding of vapor layer thickness results in periodical collapse and revival of this coherent effect. Demonstrated and analyzed is the significant difference between the

10

20

30

FWHM [MHz]

**5. Conclusion** 

40

0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50

Contrast

b)

a)

This chapter is dedicated to the memory of our colleague and friend Prof. Solomon Saltiel, great teacher in linear and non-linear spectroscopy.

The work is partially supported by the Bulgarian Fund for Scientific Research (grant No: DO 02-108/22.05.2009 and grant DMU 02/17 – 18.12.2009) and Indian -Bulgarian (BIn-2/07) bilateral contract**.**
