Coherent and incoherent laser spectroscopy of spatial and temporal fluctuations

Dipankar Bhattacharyya, Jyotirmoy Guha

Takayoshi Kobayashi, Toshiaki Hattori, Akira Terasaki

Until now all reported studies of transient spectroscopy with incoherent light, including photon echo and pump-probe spectroscopy, were concerned with two-level systems where quasi-monochromatic light of only one frequency couples the two levels.[1-4] However, there exist various coherent transient phenomena where light of two or more frequencies are concerned, and time resolution of the transient spectroscopies using these phenomena are also expected to be limited not by the duration of used light pulses but by the correlation time of the radiation fields.[5]

Kai J. Drühl

We address the problem of pumping Stokes radiation by broadband incoherent laser beams in contrast to aberrated beams. While for aberrated beams the transversal phase variations in any given plane of reference are constant in time, they will fluctuate in time for incoherent beams. As is well known for Raman scattering by broadband lasers, the frequency mode structure of the amplified Stokes beam will be modified by the pump laser, leading to an increase in temporal correlation between pump and Stokes. For collinear pumping by an incoherent pump this implies that the amplified Stokes beam will also be incoherent, even if seeded initially with a coherent beam. For transversal pumping geometries Stokes beam coherence will be affected by the reduced temporal correlation between the pump fields at different transversal locations. In the case of resonant scattering (linewidth much smaller than mode spacing), we show that the transversal Stokes coherence length is limited by the longitudinal coherence length of the pump. In addition, a reduction in gain occurs. We present analytical and numerical results which indicate corresponding limitations for intermediate scattering (linewidth comparable to mode spacing), which is the case of practical interest in gases.

N. C. Wong

Mikhail D. Lukin

Michael G. Raymer, Ian A. Walmsley, Jan Mostowski +1 more

A quantum theory of stimulated Raman scattering is presented that takes into account three-dimensional propagation and collisional dephasing, allowing the study of the spatial and temporal coherence properties of the generated Stokes light. Maxwell-Bloch equations for the Stokes field operator and the collective atomic operators are solved analytically under low-signal-gain conditions, where the laser field and the atomic ground states remain undepleted. The intensity and the space-time autocorrelation function of the Stokes field are calculated. The Stokes field is expanded into a set of statistically independent ``coherence modes,'' which are determined explicitly for the case of a cylindrically shaped pumped volume. The Stokes pulse energy W is found to fluctuate from pulse to pulse. The probability distribution function for pulse energies P(W) is calculated for a range of Fresnel numbers of the excited volume and collisional dephasing rates. For small values of Fresnel number and dephasing rate, P(W) is a negative exponential distribution. For large values of either, P(W) narrows and approaches a Gaussian-shaped distribution. This occurs because many independent modes contribute to the Stokes emission, making it spatially and/or temporally incoherent.