**2. Unique optical cells for confinement of Cs atomic layers with nanometric thickness**

## **2.1 Main characteristics of atomic confinement**

In this chapter, the high resolution laser spectroscopy is concerned of alkali vapor confined in unique optical cell with nanometric thickness [Sarkisyan, 2001], further on called Extremely Thin Cell (ETC). The transversal and longitudinal dimensions of such cell (Fig.1) differ significantly. The distance between the high-quality ETC windows L varies from 100 nm to (1-3) μm. At the same time, the cell window diameter is about 2 cm (Fig.1b). Therefore, a strong spatial anisotropy is present for the time of interaction between atoms confined in the ETC and the laser radiation used for spectroscopy performed with such optical cell.

Let us consider Cs atoms flying orthogonally to the cell windows (Fig.1a, atoms denoted by v┴), which average thermal velocity at room temperature is about 200 m/s. Those atoms

High Resolution Laser Spectroscopy of Cesium Vapor Layers with Nanometric Thickness 257

In this chapter, the spectral properties of Cs atoms confined in ETC are particularly concerned. The diagram of 133Cs energy levels with the hyperfine transitions on the D2 line is

Fig. 2. Energy-level diagram for D2 line of 133Cs. Fg → Fe ≤ Fg transitions (solid line) are distinguished from Fg → Fe > Fg transitions (dashed line). The wavelength of the D2 line is

Cesium D2 line consists of two sets of hyperfine transitions, forming two absorption

**9192 MHz** 

**151 MHz** 

**201 MHz** 

**251 MHz** 

**Fe=5** 

**Fe=3** 

**Fe=4** 

**Fe=2** 

**Fg=3** 

**Fg=4** 

Fg = 3 set, involving three hyperfine transitions starting from the Fg = 3 ground level to

In the widely used optical cells with centimeter dimensions (further on called conventional cells), the hyperfine transitions starting from single ground level completely overlap due to the Doppler broadening (the spectral width ~ 400 MHz, for individual hyperfine transition), which is larger than the separation between the excited state hyperfine levels. Thus, the two types of transitions (Fg → Fe ≤ Fg or Fg → Fe > Fg) involved in the absorption line can not be resolved. However, they have different properties. The first type (Fg → Fe ≤ Fg transition) can suffer population loss from the excited by the light ground level due to hyperfine optical pumping to the other ground hyperfine level, which does not interact with the light. When this type of transition is closed for hyperfine optical pumping, it intrinsically will exhibit Zeeman optical pumping to ground-state Zeeman sublevels non-interacting with the light. In opposite, if the transition of the second type (Fg → Fe > Fg transition) is closed for hyperfine optical pumping, it can be considered as a completely closed one [Andreeva, 2007,

Fg = 4 set of transitions - the Fg = 4 Fe = 3, 4, 5 hyperfine transitions.

**2.2 Hyperfine transitions on the D2 line of Cs atoms** 

shown in Fig.2.

**6p 2 P3/2** 

λ = 852nm.

**6s 2 S1/2** 

(fluorescence) lines:

the respective Fe = 2, 3, 4 excited levels and

will pass the L = 1 μm distance for 5 ns. Hence the time of flight of atoms is much shorter than the lifetime of the excited atomic state. Such a limit is not imposed on the atoms (Fig.1a, denoted by vII), moving parallel to the windows of the ETC. The second group of atoms will interact with the laser radiation for a time determined by the diameter of the laser beam D (D >> L). When the ETC is irradiated by a laser beam propagating in direction orthogonal to the ETC window surfaces, the atoms with velocity direction close to parallel to the window surface can be considered as "slow" atoms, i.e. atoms with very small velocity projection on the laser beam direction. Hence two groups of atoms can be mainly distinguished – "slow" (moving parallel to the windows) and "fast" (moving parallel to the laser beam propagation direction, in the extremely narrow space between the two windows of the ETC). As a first result of the light interaction with those atomic groups, a strong reduction of the Doppler effect influence occurs and of the related Doppler broadening of spectral lines as well.

Fig. 1. (a) Atomic movement in cell of nanometric thickness L: vII – velocity component parallel to the cell windows; v┴ - atomic velocity component orthogonal to the windows and along the laser beam, (b) Practical realization of nanometric cell.

## **2.2 Hyperfine transitions on the D2 line of Cs atoms**

256 Advanced Photonic Sciences

will pass the L = 1 μm distance for 5 ns. Hence the time of flight of atoms is much shorter than the lifetime of the excited atomic state. Such a limit is not imposed on the atoms (Fig.1a, denoted by vII), moving parallel to the windows of the ETC. The second group of atoms will interact with the laser radiation for a time determined by the diameter of the laser beam D (D >> L). When the ETC is irradiated by a laser beam propagating in direction orthogonal to the ETC window surfaces, the atoms with velocity direction close to parallel to the window surface can be considered as "slow" atoms, i.e. atoms with very small velocity projection on the laser beam direction. Hence two groups of atoms can be mainly distinguished – "slow" (moving parallel to the windows) and "fast" (moving parallel to the laser beam propagation direction, in the extremely narrow space between the two windows of the ETC). As a first result of the light interaction with those atomic groups, a strong reduction of the Doppler effect influence occurs and of the related Doppler broadening of spectral lines as well.

L

II

D

b)

a)

Fig. 1. (a) Atomic movement in cell of nanometric thickness L: vII – velocity component parallel to the cell windows; v┴ - atomic velocity component orthogonal to the windows

and along the laser beam, (b) Practical realization of nanometric cell.

In this chapter, the spectral properties of Cs atoms confined in ETC are particularly concerned. The diagram of 133Cs energy levels with the hyperfine transitions on the D2 line is shown in Fig.2.

Fig. 2. Energy-level diagram for D2 line of 133Cs. Fg → Fe ≤ Fg transitions (solid line) are distinguished from Fg → Fe > Fg transitions (dashed line). The wavelength of the D2 line is λ = 852nm.

Cesium D2 line consists of two sets of hyperfine transitions, forming two absorption (fluorescence) lines:


In the widely used optical cells with centimeter dimensions (further on called conventional cells), the hyperfine transitions starting from single ground level completely overlap due to the Doppler broadening (the spectral width ~ 400 MHz, for individual hyperfine transition), which is larger than the separation between the excited state hyperfine levels. Thus, the two types of transitions (Fg → Fe ≤ Fg or Fg → Fe > Fg) involved in the absorption line can not be resolved. However, they have different properties. The first type (Fg → Fe ≤ Fg transition) can suffer population loss from the excited by the light ground level due to hyperfine optical pumping to the other ground hyperfine level, which does not interact with the light. When this type of transition is closed for hyperfine optical pumping, it intrinsically will exhibit Zeeman optical pumping to ground-state Zeeman sublevels non-interacting with the light. In opposite, if the transition of the second type (Fg → Fe > Fg transition) is closed for hyperfine optical pumping, it can be considered as a completely closed one [Andreeva, 2007,

High Resolution Laser Spectroscopy of Cesium Vapor Layers with Nanometric Thickness 259

As above mentioned, in the conventional thermal cells the different hyperfine Cs transitions starting from single ground level Fg are completely overlapped and cannot be resolved at all. Consequently, 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. Moreover, this simple tool makes it possible to study also the dynamic processes in the absorption and fluorescence, as

A schematic drawing of the experimental set up is presented in Fig.4. Three different radiation sources emitting at = 852 nm were used in the experiments presented here:

a cw Extended Cavity Diode Laser (ECDL) operating at single-frequency mode with

a Distributed Feedback Laser (DFB) with very low-noise current controller and

well as in the electromagnetically induced transparency and absorption.

**3.1 Sub-Doppler resonances in the absorption (transmission) of confined in** 

**3. Main properties of the experimental fluorescence and absorption spectra** 

**of Cs atoms confined in nanometric cells** 

**nanometric cell atoms – Experimental observations** 

a free running diode laser with linewidth of about 15 MHz,

**3.1.1 Experimental set up** 

linewidth of about 3 MHz, and

linewidth of about 2 MHz.

Fig. 4. Experimental set up

a; Andreeva, 2007, b; Andreeva, 2002], i.e. the atomic population is cycling only between the energy levels involved in the atomic transition, with all Zeeman sublevels of the ground level excited by the light.

The situation is different in the case of ETC. Here the hyperfine transitions within a single absorption (fluorescence) line can be resolved [Andreeva, 2007, a; Andreeva, 2007, b] (see Fig.3). Hence we can investigate separately the two types of hyperfine transitions, which are involved in the absorption (fluorescence) line by means of very simple single beam spectroscopy. The Fg = 4 → Fe = 3, 4, 5 set of transitions involves the Fg = 4 Fe = 5 transition, which is the only completely closed one on the D2 line.

The presence of the two main groups of atoms, determined by the ETC anisotropy, leads also to the observation of a significant difference between the fluorescence and transmission spectra (Fig. 3). In the ETC, the basic contribution to the creation of fluorescence signal belongs to the atoms moving parallel to the windows, i.e. "slow" atoms. In fact, the "fast" atoms moving along the laser beam direction do not have enough time to perform a complete absorption-emission cycle, i.e. to fluoresce. Hence, strong narrowing of the fluorescence profiles occur and the hyperfine optical transitions are completely resolved in the fluorescence case.

The situation with the absorption is different. Here, together with the "slow" atoms, the "fast" atoms are also able to contribute to the absorption spectrum of the nanometric atomic vapor layer, because the absorption of a quantum of light is a process a lot faster than the fluorescence. In this way, the absorption profile "suffers" much more from the Doppler effect. Hence, because different velocity groups of atoms contribute to the fluorescence and absorption profile formation, a big difference in the width of the corresponding optical transitions occurs (Fig.3).

Fig. 3. Difference between the spectral widths of fluorescence and absorption profiles; the illustration is on the D2 line of Cs, for optical transitions starting from the ground level Fg = 4; the 4-3, 4-4 and 4-5 are the respective hyperfine transitions. λ = 852nm.
