Hertzian coherence

In this chapter we shall be concerned mainly with the principles of the technique, the effect of relaxation processes, and magnetic resonance transitions between Zeeman sub-levels. We shall therefore initially describe the experiments in terms of the populations of the ground state sub-levels. The discussion of the effects of phase coherence (Hertzian coherence) and experiments involving transverse pumping is reserved until section 17.8. Moreover the application of optical pumping methods to the investigation of hyperfine intervals and the measurement of nuclear moments is postponed until Chapter 18, as are the applications of this technique in devices such as magnetometers, atomic clocks, and masers. 

In this contribution we present two laser spectroscopic methods that use coherent resonance Raman scattering to detect rf-or laser -induced Hertzian coherence phenomena in the gas phase these novel coherent double resonance techniques for optical heterodyne detection of sublevel coherence clearly extend the above mentioned previous methods using incoherent light sources. In the case of Doppler broadened optical transitions new signal features appear as a result of velocity-selective optical excitation caused by the narrow-bandwidth laser. We especially analyze the potential and the limitations of the new detection schemes for the study of collision effects in double resonance spectroscopy. In particular, the effect of collisional velocity changes on the Hertzian resonances will be investigated. 

In the first part of the paper we discuss the Raman heterodyne detection of rf-generated sublevel coherence in atomic Sm vapor in the presence of rare gas perturbers. In a second part we report on the observation of novel Ramsey-type resonances due to collisional velocity diffusion in Sm vapor here the experimental technique uses counterpropagating laser fields and relies on coherent Raman processes to optically excite and detect Hertzian coherence. 

We see that field-dependent terms appear in the denominator of equation (15.27) when m m, i.e. the Hanle effect signal is a direct result of the Hertzian coherence created in the excited state by excitation with coherently polarized light. We can describe the phenomenon as the result of a quantum-mechanical interference between the scattering amplitudes for the two possible routes from the initial ground level sub-state y > to the final sub-state... 

This calculation makes it clear that light beats are associated with the time evolution of the off-diagonal elements of the excited-state density matrix. Consequently they can only be observed in pulsed experiments if the light which excites the atoms of the sample is also polarization coherent. Only then is the necessary Hertzian coherence created in the excited-state density matrix. The theory predicts that modulation at the angular frequencies and will be detectable depending on the geometry and polarization used in the experiment. 

The physical phenomena associated with the Hertzian coherence of the density matrix are discussed by... 

Quantum mechanically the modulation of the fluorescent light is associated with the radio frequency coherence of the excited state density matrix. In a standard Brossel-Bitter double-resonance experiment Tt-polarized light excites the atoms initially to the m=0 state of the excited level. Fig. 16.13(b), and then interaction with the r.f. magnetic field transforms each atom into a coherent superposition of the m=0, l states. The relative phases of the probability amplitudes of these states are fixed by the phase of the r.f. field and are the same for every atom of the sample. Thus the r.f. field is able to generate substantial hertzian coherence in the excited state density matrix. The fluorescent light emitted in the direction of B is then a coherent... 

Modulation of light from incoherent sources. The light beats observed in resonance fluorescence experiments should be clearly distinguished from those observed by Forrester et aZ.(1955). In that experiment an exceedingly weak modulation was detected in the light emitted from a conventional mercury lamp. In this source the atoms are excited by random collisions and the ensemble density matrix possesses zero hertzian coherence. The beat frequencies observed were due to the mixing at the detector of the radiation frequencies emitted by different atoms and... 

---