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Journal of Physics B Atomic Molecular and Optical Physics

Collimated blue light enhancement in velocity-selective pumped Rb vapour

A. M. Akulshin, A A Orel, Russell McLean

Published Dec 9, 2011• 44 citations
Publisher

Abstract

The effect of hyperfine optical pumping on the parametric four-wave mixing process responsible for the generation of collimated blue light (CBL) in alkali atoms is explored. Over a wide range of atomic densities (4.5 × 10 10 cm −3 ⩽ N ⩽ 1.4 × 10 11 cm −3 ), a more than ten-fold enhancement of CBL is obtained from optical pumping produced by an additional laser. Pulsed optical pumping has been applied to investigate the temporal characteristics of CBL generation. It is shown that optical pumping offers efficient control of the mixing process and the collimated light generation that may be useful for probing a small number of atoms.

DOI
10.1088/0953-4075/45/1/015401

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The effect of hyperfine optical pumping on the parametric four-wave mixing process responsible for the generation of collimated blue light (CBL) in alkali atoms is explored. Over a wide range of atomic densities (4.5 × 10 10 cm −3 ⩽ N ⩽ 1.4 × 10 11 cm −3 ), a more than ten-fold enhancement of CBL is obtained from optical pumping produced by an additional laser. Pulsed optical pumping has been applied to investigate the temporal characteristics of CBL generation. It is shown that optical pumping offers efficient control of the mixing process and the collimated light generation that may be useful for probing a small number of atoms.
The frequency evaluation of collimated blue light (CBL) generated by parametric four-wave mixing (FWM) in hot atomic vapor is presented. The cesium ( 133 Cs) atoms are excited from the 6S 1/2 ground state to 8S 1/2 excited state with the 852 and 795 nm pump lasers, producing an optical field at 4.2 µm through an amplified spontaneous emission via population inversion on the 8S 1/2 → 7P 3/2 transition and the 456 nm CBL via FWM on the 7P 3/2 → 6S 1/2 transition. The frequency shift of 456 nm CBL has been measured by velocity-selective optical pumping spectral technique when the frequency of any one of two pumping lasers is tuned. The ratio of the frequency shift of 456 nm CBL to the two-photon frequency detuning of two pumping lasers is 0.8912, implying that the generated 4.2 µm light frequency is also correspondingly shifted.
The four-wave mixing (FWM) process is an important method for generating collimated blue light from alkali vapor. In this paper, we introduced high-pressure He gas to broaden the absorption linewidth of Rb atoms for the pump laser. By using an optical parametric oscillator (OPO) that outputs tunable wavelength pump laser, we performed two-photon excitation of the Rb-He mixture and achieved redshifted and asymmetric broadened collimated light (CL) around 420 nm. We demonstrated the participation of Rb-He excimers in the FWM process for the first time, to the best of our knowledge, from both spectral and temporal dimensions, and revealed the underlying mechanism. Experiments showed that the peak of the ∼420 nm CL intensity appeared at higher heating temperatures, influenced by changes in the optical depth of the Rb-He mixture. This study provides both theoretical and experimental support for the promotion of a collimated blue light scheme in alkali metal -buffer gas systems, balancing the pump laser absorption and FWM efficiency.
We demonstrate the generation of coherent and collimated blue light (CBL) based on cesium (Cs) 6S1/2(F = 4)→6P3/2(F' = 5)→6D5/2(F'' = 6)→7P3/2(F' = 5)→6S1/2(F = 4) diamond-type atomic system in a heated vapor cell. Two infrared pumping lasers with wavelengths at 852 nm (6S1/2→6P3/2) and 917 nm (6P3/2→6D5/2), provide step-wise excitation to the 6D5/2 excited state, and induce strong two-photon coherence between the 6S1/2 state and 6D5/2 state. The atoms undergo a double cascade accompanied with the amplified spontaneous emission at 15.1 μm via the 7P3/2 intermediate excited state, and produce a beam of 455 nm (7P3/2→6S1/2) CBL with highly spatiotemporal coherence through a parametric four-wave mixing process. We investigate the influence of experimental parameters such as polarization combination of the two pumping lasers, and their power, and the temperature of Cs vapor cell on the CBL. Especially, we add a beam of 894 nm laser operating at the 6S1/2(F = 3) →6P1/2 transition as repumping laser, which can pump atoms back to the 6S1/2(F = 4) state from the 6S1/2(F = 3) state, thus significantly improving the power of CBL. This technique of the CBL enhancement via optical pumping is also useful for the other alkali metal atoms.
Pulsed collimated blue light at 420.3 nm is generated in hot Rb vapor by upconverting the 778.10 nm pumping beam through four wave mixing process. The energy conversion efficiency exceeds 1% when a 45 cm-long, 170°C heated Rb cell is used. The influence of cell temperature, wavelength, and energy of a pumping laser are fully examined. The efficiency of the photon conversion is found to be more sensitive to the blue detuning of the pump light and less sensitive to the red detuning of the pump light. This phenomenon can be explained by stimulated hyper-Raman scattering involved in the four-wave mixing process.
An evaluation of the absolute frequency and tunability of collimated blue light (CBL) generated in warm Rb vapour excited by low-power cw laser radiation at 780 nm and 776 nm has been performed using a Fabry–Perot interferometer and a blue diode laser. For the conditions of our experiments, the CBL tuning range is more than 250 MHz around the resonant frequency of the 85 Rb 5S 1/2 ( F = 3)→6P 3/2 ( F ′ = 4) transition. A simple technique for stabilizing the power and frequency of the CBL to within a few per cent and 10 MHz, respectively, is suggested and demonstrated.
Pulsed collimated blue light at 420.3 nm is generated in hot Rb vapor by upconverting the 778.10 nm pumping beam through four wave mixing process. The energy conversion efficiency exceeds 1% when a 45 cm-long, 170 degrees C heated Rb cell is used. The influence of cell temperature, wavelength, and energy of a pumping laser are fully examined. The efficiency of the photon conversion is found to be more sensitive to the blue detuning of the pump light and less sensitive to the red detuning of the pump light. This phenomenon can be explained by stimulated hyper-Raman scattering involved in the four-wave mixing process.