Collimated blue and infrared beams generated by two-photon excitation in Rb vapor

Abstract

Utilizing two-photon excitation in hot Rb vapor we demonstrate the generation of collimated optical fields at 420 and 1324 nm. Input laser beams at 780 and 776 nm enter a heated Rb vapor cell collinear and circularly polarized, driving Rb atoms to the 5D 5∕2 state. Under phase-matching conditions coherence among the 5S 1∕2 → 5P 3∕2 → 5D 5∕2 → 6P 3∕2 transitions produces a blue (420 nm) beam by four-wave mixing. We also observe a forward and backward propagating IR (1324 nm) beam, due to cascading decays through the 6S 1∕2 → 5P 1∕2 states. Power saturation of the generated beams is investigated by scaling the input powers to greater than 200 mW, resulting in a coherent blue beam of 9.1 mW power, almost an order of magnitude larger than previously achieved. We measure the dependences of both beams in relation to the Rb density, the frequency detuning between Rb ground-state hyperfine levels, and the input laser intensities. A wide range of phenomena can be created by exploiting nonlinear optical processes in a dense atomic vapor. Large enhancements of these processes are possible through the generation of quantum coherences among atomic states and include effects, such as electromagnetically induced transparency, fast and slow light propagation, four-wave mixing (FWM), and lasing without inversion. FWM in particular has been shown to produce both efficient frequency upconversion [1-3] and downconversion [4] using low-power continuous-wave (cw) lasers. The newly created optical fields are narrowband tunable coherent light sources [5], with wavelengths from the IR to approaching the UV depending upon the atomic states involved. Frequency upconversion by FWM has most often been studied in Rb vapor, first demonstrated using low-power cw lasers by Zibrov et al. in 2002 [6] where 15 μW of coherent radiation at 420 nm was achieved. The method relies upon input lasers at 780 and 776 nm [see Fig. 1(a)] driving Rb atoms from the 5S 1∕2 ground state to the 5D 5∕2 state by two-photon excitation, with the 5P 3∕2 level as an intermediate state. A third optical field between the 5D 5∕2 → 6P 3∕2 levels at 5.23 μm is initiated through spontaneous emission. Strong atomic coherences are thus formed in a diamond-type energy level structure, creating coherent blue light at 420 nm (6P 3∕2 → 5S 1∕2 ) by FWM. Recent experiments achieved first 40 μW of 420 nm light through the additional coupling of both 5S 1∕2 hyperfine ground-state levels [1], and subsequently 1.1 mW by further optimization of input laser polarizations and frequencies [3]. The generated blue beam exhibits a high degree of spatial coherence [2], with a spectral linewidth typically limited by the linewidths of the applied laser fields [7]. The absolute frequency of the blue light has been found to be centered on the 6P 3∕2 → 5S 1∕2 transition, with tunability of ≥100 MHz possible by adjustment of the input laser frequencies [5]. Incorporating an additional laser at 795 nm has been shown to both enhance and suppress the FWM process through control of optical pumping [8]. ...

Parametric wave mixing enhanced by velocity-insensitive two-photon excitation in Rb vapor

“…It thus represents the CBL generated in the second region with IR seeding. This approximately six-fold CBL intensity increase produced by injecting extra radiation at 5.23 µm emphasizes the importance of the process of amplified spontaneous emission that is responsible for frequency down-conversion, as the enhanced mid-IR light is directly involved in preparing nonlinear polarization for the 5S1/2 -6P3/2 transition (P ~ χ (3) E1 E2 EIR, where χ (3) is nonlinear susceptibility and Ei is the amplitude of the optical field at 780, 776 and 5 320 nm, respectively) and subsequent CBL generation. However, the mechanism of forward-directed mid-IR enhancement shown in Figure 2 remains to be clarified.…”

“…To date, studies of the FWM process in diamond-type energy-level systems in alkali atoms, mostly in Rb, have focused almost exclusively on detecting the CBL and inferring details of the generation process from the blue light properties [1][2][3][4][5][6][7][8][9][10][11][12][13]. The primary reason for this is that the mid-IR radiation at 5.23 µm generated by the process of amplified spontaneous emission (ASE) is completely absorbed by the glass windows of vapour cells usually used in such experiments.…”

Collimated UV light generation by two-photon excitation to a Rydberg state in Rb vapor

“…A generated 5.2 µm infrared radiation corresponding to the transition 5D 5/2 → 6P 3/2 , enabling the FWM process, has been observed in some of the experiments [14]. The difference between our scheme and the works in [9][10][11][12][13][14][15][16] is that, instead of the 5D 5/2 state, a lowlying Rydberg state is being excited by pump lasers. As a result, two very different frequencies outside the typical optical regime are generated, namely a THz field on transition 3 and a UV field on transition 4.…”

Distinguishing nonlinear processes in atomic media via orbital angular momentum transfer

“…At relatively low Rb density (N ≤ 6 × 10 12 cm −3 ), only forwarddirected collimated light at 1.37 μm is generated [13]. However, at higher N collimated emission at 1.32 μm dominates, having been observed in both the co-and counter-propagating directions and attributed to nonparametric ASE on the 6S 1∕2 → 5P 1∕2 transition [14].Appearance of the CBL and near-IR emission at 1.37 μm in only the co-propagating direction suggests a common parametric origin. The lifetimes of the 5D 5∕2 , 6P 3∕2 , and 6S 1∕2 levels (240, 120, and 46 ns, respectively) mean that ASE at 5.23 and 2.73 μm can occur along the pencil-shaped region defined by the applied laser beams.…”

“…At relatively low Rb density (N ≤ 6 × 10 12 cm −3 ), only forwarddirected collimated light at 1.37 μm is generated [13]. However, at higher N collimated emission at 1.32 μm dominates, having been observed in both the co-and counter-propagating directions and attributed to nonparametric ASE on the 6S 1∕2 → 5P 1∕2 transition [14].…”