Control of nonlinear processes in two-photon excited rubidium vapour by resonant laser light

摘要

Frequency up- and down-conversion in warm alkali vapours over a range of applied cw laser intensities has receivedmuch attention in recent years due to promising applications in the fields of remote detection, quantum information andcomputing, specifically in the implementation of correlated photon pairs, photon storage and quantum memoryinterface to name a few. The generation of directional laser-like blue and infrared light in alkali vapours has beenextensively 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 fieldgeneration, are not entirely clear.Excitation to the 5D_(5/2) level of Rb has been widely used, particularly in the contexts of frequency up- and downconversion,conservation of orbital angular momentum and amplified spontaneous emission. However, excitation to the5D_(3/2) level has not been as well studied, largely because it is less efficient since at least one of the excitation steps mustinvolve an open transition. Selection rules allow more than one decay channel from the 5D_(3/2) level, providing anopportunity to study competition between the different channels. Two-photon excitation of Rb atoms to the 5D_(3/2) levelprovides an interesting possibility of studying the processes involved under different conditions.The 5D_(3/2) level has a longer lifetime (240 ns) than the levels to which it decays, so that a population inversion can beestablished on each transition. If the rate of excitation to the 5D_(3/2) state and the atomic number density are sufficientlyhigh, then ASE may occur on the 5.03 μm transition. Polarization induced between the 6P_(1/2) state and the ground state bythe applied laser fields and the forward directed ASE can lead to the generation of a highly coherent and collimated beamas a result of parametric four wave mixing on the 5S_(1/2)→ 5P_(1/2)→ 5D_(3/2)→ 6P_(1/2)→ 5S_(1/2) loop provided the phase matchingcondition is satisfied. Under appropriate experimental conditions mid-IR directional light and collimated blue light(CBL) are observed.Here we report the effective control of both the FWM and ASE processes by additional resonant laser fields. Forexample, when atoms are excited to the 5D_(3/2) level by laser light at 795 and 762 nm, an additional field at 776 nmdepopulates the 5D_(3/2) level to the 5P_(3/2) level, reducing the inversion on the 5D_(3/2)→ 6P_(1/2) transition (Fig. 1a). This leads tosuppression of the directional mid-IR light at 5.03 μm and even complete suppression when the inversion becomes toosmall to support ASE, confirming its threshold nature (Fig 2b). The CBL at 422 nm is also suppressed (Figs.1b and 2a),while fluorescence at 422 nm and 762 nm reduces but is never suppressed completely because spontaneous decay can always occur via these pathways even when the threshold conditions for the non-linear processes are not met, (Fig.2a andb). As well, the control light triggers stimulated emission generating gain at 776 nm.In the time domain, the control beam acts to switch the non-linear processes and CBL on or off. As shown in figure 2c,the CBL responds quickly to the control light. The rise time of the control light is limited by the switching time of theacousto-optic modulator used.In a similar way, the non-linear processes can be controlled following excitation to the 5D_(3/2) level via 5P_(3/2) using 780 and776 nm lasers, with the 762 nm laser acting as the control light. These Rb experiments can provide valuable insight inidentifying the optimal excitation scheme in mesospheric sodium atoms for generating backward-directed emission thatcould dramatically enhance the efficiency of laser guide stars. They may also inform experiments in which Cs vapouris excited to highly excited states via single step transition by UV radiation at 320 nm1, where the balance between thenonlinear processes like ASE and FWM happening on various cascading transitions may be controlled.