The nonlinear processes responsible for frequency up- and down-conversion of resonant low-intensity laser radiation in Rb vapour have been evaluated from the spatial and temporal properties of blue and mid-IR light resulting from wave mixing.
Frequency up- and down-conversion in warm alkali vapours over a range of applied cw laser intensities has received much attention in recent years due to promising applications in the fields of remote detection, quantum information and computing, specifically in the implementation of correlated photon pairs, photon storage and quantum memory interface to name a few. The generation of directional laser-like blue and infrared light in alkali vapours has been extensively studied. However, important details of the interplay of parametric and nonparametric nonlinear processes, such as the four-wave mixing (FWM) and amplified spontaneous emission (ASE) responsible for the new field generation, are not entirely clear.
The parametric four-wave mixing (4WM) process responsible for the generation of coherent blue light in alkali vapors is a self-seeded process which starts by population inversion and amplified spontaneous emission (ASE). Lately, attention has been turned toward frequency up- and down-conversion in alkali vapors, using relatively low pump powers with CW diode lasers. In this Letter, we investigate the interplay between ASE and 4WM in rubidium (Rb) vapors by studying the mid-infrared (mid-IR) radiation emitted in the forward direction at 5.23 μm. We show that the ASE can be suppressed by 4WM in the CW regime. Thus, we demonstrate the generation of coherent mid-IR light at 5.23 μm in hot Rb vapors via parametric 4WM.
We demonstrate how the orientation of the Rb cell can significantly affect the intensity and spectral characteristics of both the frequency up- and downconverted fields generated by nonlinear processes in Rb vapor. The efficiency of parametric wave mixing in Rb vapor excited to the 5D5/2 level by two-color resonant laser light can be significantly increased by seeding the excitation region with coherent and directional radiation at 5.23 μm, which is resonant with the population inverted 6P3/2–5D5/2 transition. It is shown that the process of velocity-insensitive two-photon excitation is central to understanding the observed coherent blue and mid-IR light enhancements, and that velocity-insensitive and velocity-selective two-photon excitations could produce two co-existing but spectrally and spatially distinguishable mid-IR fields at 5.23 μm.