Optical nutation under Raman excitation

Abstract

Optical nutation under Raman excitation accompanied by considerable (up to inverted) population changes was observed. The Raman-active 6P1/2 -6P3/2 transition of thallium atoms was excited by picosecond pulses of a dye and an Nd : YAG laser. Transition polarization and upper level population were probed by delayed pulses at a fixed time by means of coherent Raman scattering and third harmonic generation schemes, respectively. Optical nutation manifested itself in corresponding changes of anti-Stokes and third harmonic signal energies as functions of the product of the pump pulses energies. As a result of the a.c. Stark shift, the nutation picture strongly depended on the sign of wavenumber detuning from the transition wavenumber. Copyright 2000 John Wiley & Sons, Ltd. cross-sections in comparison with those of rotational or rotation-vibrational transitions. For the relatively high transition wavenumbers, typical for atoms, the Doppler dephasing time T 2 can be as low as1 ns. The coher- ence of excitation is secured, provided that pump pulses shorter than T1, T2 and T 2 are applied. By varying the energy of the pump pulses, the system can be transfered into different states, including a state with inverted popu- lation. This scheme was discussed in Ref. 5, where it was proposed to monitor the transition polarization by measur- ing the energy of the coherent Raman scattering signal as function of the product of the pump pulses energies with the probe pulse delayed at a fixed time relative to the pump pulses. At the same time, the third harmonic (TH) is much more effectively generated in thallium, when the upper level 6P3=2 of the Raman-active transition is populated. 6 This effect can be used to monitor the population of the excited level. The application of picosecond pulses ensures, as a rule, the fulfillment of the coherent excitation conditions in gases. Since the Raman cross-sections are small, it is necessary to use powerful picosecond pulses to obtain suf- ficient population changes. This makes the manifestation of the a.c. Stark effect real. In the present work, the optical nutation at a Raman- active transition was observed at resonant Raman excita- tion by picosecond pulses. Optical nutation was accompa- nied by considerable population changes. The a.c. Stark shift manifested itself in an 'asymmetry' of the nutation picture relative to the sign of the wavenumber detuning .!1 !2/ !R.