Resonance fluorescence using modulated excitation

Resonance fluorescence experiments using modulated excitation... 

Theory of resonance fluorescence excited by modulated light. Theoretical expressions for the intensity of light observed in resonance fluorescence experiments using modulated excitation can be obtained by a simple extension of equation (15.41). We now assume that the energy density of the incident radiation, U((o,t), is amplitude modulated at... 

Sequential double resonance excitation requires that both transitions fall within the range of suitable laser sources. However, it is often desirable to use population labelling to assign a single band system where this may not be possible. Figure 13b uses competitive excitation to seek pairs of transitions with a level in common. In this case it is necessary to partially saturate the pump transition in order that it may affect the fluorescence intensity of excitation via the probe laser. With an extended band system and c.w. lasers the pump beam may be chopped, with modulation detected on the fluorescence excited by the unchopped probe, a filter having been used to block the fluorescence from the pump. 

This optical-optical double-resonance technique has already been used for other Doppler-free techniques [10.25], such as polarization spectroscopy (see Sect.10.3). Its applications to molecular beams has, however, the following advantages compared to spectroscopy in gas cells. When the chopped pump laser periodically depletes the level E. and populates level Ej, there are two relaxation mechanisms in gas cells which may transfer the population modulation to other levels. These are collision processes and laser-induced fluorescence (see Fig.8.39). The neighboring levels therefore also show a modulation and the modulated excitation spectrum induced by the probe laser includes all lines which are excited from those levels. If several absorption lines overlap within their Doppler width, the pump laser simultaneously excites several upper states and also partly depletes several lower levels. 

Next we proceed to develop the theory o resonance fluorescence experiments using the ensemble density matrix to describe the system of atoms. The important concepts of optical and radio-frequency coherence and of the interference of atomic states are discussed in detail. As an illustration of this theory general expressions describing the Hanle effect experiments are obtained. These are evaluated in detail for the frequently employed example of atoms whose angular momentum quantum numbers in the ground and excited levels are J =0 and Jg=l respectively. Finally resonance fluorescence experiments using pulsed or modulated excitation are described. 

Introduction and experimental techniques. In previous sections we drew attention to the fact that, in both the classical and quantum theories, expressions derived for the intensity of resonance fluorescence from atoms subjected to an external magnetic field, equations (15,3) and (15.23) respectively, contain terms which may lead to a modulation of the intensity at the Larmor frequency or its second harmonic. This radio-frequency modulation has been observed in several different kinds of experiment, the simplest of which makes use of pulsed excitation and time-resolved detection of the fluorescent light. 

The first resonance fluorescence experiments using intensity-modulated excitation were performed by Aleksandrov (1963) and independently by Corney and Series (1964 a,b). 

Line sources are mostly used in atomic absorption and fluorescence because the energy which can be generated within the bandwidth of the absorption line is much higher than for any continuous source. In the ideal source only the resonance lines of the elements to be analyzed would be excited and these would be narrow, intense, highly stable and capable of selective modulation. Developments in sources have been directed toward this ideal and increasing the range of elements. These improvements will be of greatest benefit in trace-element analysis. 

Questions of linkage are posed and answered by asking the molecule to satisfy successively two resonance conditions. Schemes which accomplish this include Dispersed Fluorescence Spectroscopy (DF, Section 1.2.2.2 a laser is tuned to excite a single line and the spectrum of the resulting molecular fluorescence is recorded), Modulated Population Spectroscopy (MPS, Section 1.2.2.3) an intense, fixed frequency, amplitude modulated PUMP laser is used to modulate the population in the upper and lower levels connected by the laser excited transition the modulation is then detected by a frequency scanned PROBE laser), which is an example of Optical Optical Double Resonance (OODR, Section 1.2.2.3). 

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