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Chinese Journal of Physics

Six-Wave Mixing Near-Infrared Emissions in Rubidium Vapor

Yao-Ju Chan, Wei‐Tzou Luh

Published Jun 1, 1995• 2 citations
OpenAlex

Abstract

During two-photon excitation studies of rubidium vapor, in addition to Rb near-infrared atomic transitions, we have observed several stimulated near-infrared emissions. For the 5S-5D two-photon allowed excitation we observed emissions near 1.325, 1.369, 1.373, 1.477, 1.533 and 1.537 pm, and for the Rb 5S-7s two-photon allowed excitation we observed emissions near 1.330, 1.395, 1.432, 1.484, 1.564 and 1.613 pm. The eight emissions, near 1.325, 1.330, 1.369, 1.395, 1.477, 1.484, 1.533 and 1.564 pm are interpreted as axially phase-matched six-wave mixing emissions, while the other four emissions, near 1.373, 1.432, 1.537 and 1.613 pm, are interpreted as angle phasematched six-wave mixing emissions. Temperature (or number density)-dependent evclution curves of some of the above six-wave mixing emissions have also been measured. The results reveal that different mixing processes occur at different rubidium number densities.

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Quantum beating in Rb at 18.3THz (608cm−1) detected by parametric six-wave mixing and sum-frequency generation in LiIO3

Changjun Zhu, Y. Xiao, Andrey A. Senin +4 more

Axially phase-matched parametric six-wave mixing (PSWM) has been observed in Rb vapor by two-photon excitation of the atom $(5s\ensuremath{\rightarrow}\ensuremath{\rightarrow}7s,5d)$ with $\ensuremath{\sim}150\phantom{\rule{0.3em}{0ex}}\mathrm{fs}$ pulses, centered at $\ensuremath{\lambda}\ensuremath{\sim}769\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ and generated by a $\mathrm{Ti}:{\mathrm{Al}}_{2}{\mathrm{O}}_{3}$ laser system. For a peak laser pulse intensity of $30\phantom{\rule{0.3em}{0ex}}\mathrm{GW}\phantom{\rule{0.2em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}2}$, PSWM is observed for Rb number densities $([\mathrm{Rb}])>2.5\ifmmode\times\else\texttimes\fi{}{10}^{15}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$ whereas, for $[\mathrm{Rb}]=2.2\ifmmode\times\else\texttimes\fi{}{10}^{16}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$, the PSWM process is detectable for pump intensities above $\ensuremath{\sim}3\phantom{\rule{0.3em}{0ex}}\mathrm{GW}\phantom{\rule{0.2em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}2}$. Upconversion of the $1.367\phantom{\rule{0.3em}{0ex}}\mathrm{\ensuremath{\mu}}\mathrm{m}$ and $1.323\phantom{\rule{0.3em}{0ex}}\mathrm{\ensuremath{\mu}}\mathrm{m}$ signal waves into the visible $(\ensuremath{\lambda}\ensuremath{\sim}494\phantom{\rule{0.3em}{0ex}}\mathrm{nm})$ has been accomplished by sum frequency generation in ${\mathrm{LiIO}}_{3}$, and quantum beating in Rb at $18.3\phantom{\rule{0.3em}{0ex}}\mathrm{THz}$ $(\ensuremath{\sim}608\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1})$ has been observed by monitoring the upconverted signal wave intensity produced in pump-probe experiments. These results broaden the scope of the wave packet detection scheme reported by Tran et al. [Opt. Lett. 23, 70 (1998)] in that a variety of coherent nonlinear optical processes $({\ensuremath{\chi}}^{(3)},{\ensuremath{\chi}}^{(5)},\dots{})$ appear to be suitable for observing the amplitude and phase of atomic (or molecular) wave packets.

Physical Review A•May 8, 2007•16 citations
During two-photon excitation studies of rubidium vapor, in addition to Rb near-infrared atomic transitions, we have observed several stimulated near-infrared emissions. For the 5S-5D two-photon allowed excitation we observed emissions near 1.325, 1.369, 1.373, 1.477, 1.533 and 1.537 pm, and for the Rb 5S-7s two-photon allowed excitation we observed emissions near 1.330, 1.395, 1.432, 1.484, 1.564 and 1.613 pm. The eight emissions, near 1.325, 1.330, 1.369, 1.395, 1.477, 1.484, 1.533 and 1.564 pm are interpreted as axially phase-matched six-wave mixing emissions, while the other four emissions, near 1.373, 1.432, 1.537 and 1.613 pm, are interpreted as angle phasematched six-wave mixing emissions. Temperature (or number density)-dependent evclution curves of some of the above six-wave mixing emissions have also been measured. The results reveal that different mixing processes occur at different rubidium number densities.
A dense sample of atomic rubidium vapor excited by a short (4 ps) transform-limited laser pulse two-photon resonant with the 5S-5D transition in a large Fresnel number geometry emits a delayed omnidirectional infrared superfluorescence pulse (coherent flash) on the 5D-6P transition. This superfluorescence emission involves a coherent population transfer to the 6P level and results in prompt directional yoked UV emission and coherent population transfer back to the ground 5S state. This emission completes the process of parametric four-wave mixing, selectively involving only those atoms on the 6P level whose 5D-6P emission satisfies the phase-matching condition. The unique feature of this four-wave mixing radiation is that it occurs after the termination of the pump pulse, as evidenced by direct measurement of emission timing. The multidirectional character of superfluorescent emission is confirmed by the observation of non-collinear parametric four-wave mixing when exciting the sample by two simultaneous overlapping pulses, for a wide range of beam angles. Also observed and investigated is superfluorescent transient diffraction that appears when the sample is excited by a sequence of three laser pulses.
Two processes of coupled difference-frequency axially phase-matched parametric six-wave mixing are carried out in Rb vapour by two-photon excitation using fs laser pulses, and parametric six-wave mixing signals in the infrared and near infrared regime are detected. The infrared parametric six-wave mixing signals are up-converted into the visible spectral range by sum-frequency mixing with the pump laser in a LHO 3 crystal. Moreover, quantum beating at 608 cm -1 , corresponding to the 7 s – 5d energy difference in Rb, is observed from the sum-frequency signal at 495 nm. As a result, we obtain modulated light signals in the visible, near infrared and infrared spectral ranges, and study the interference between 7s and 5d states of Rb.
The rubidium $5S\text{\ensuremath{-}}5D$ Doppler-free two-photon transition has been observed in a thermal vapor cell by detecting the infrared radiation emitted during the decay from the 5D5D5D excited level to the 5P5P5P intermediate level. Different to the usual detection scheme based on blue light emission this approach does not suffer from reabsorption and still works at high densities. This not only allows for a dramatically enhanced detectable emission rate but also for investigating collisional effects such as collisional broadening and collisional energy transfer. Furthermore, we observe a yet unknown temperature dependence of the resonance linewidth for transitions into the states of the 5D5/25{D}_{\text{5/2}}5D5/2​ level. Consequences for the design of an optical Rb-frequency standard are discussed.
The observation of an atomic wave packet by use of a coherent, nonlinear-optical process is reported. Wave packets formed in K or Rb vapor by two-photon excitation of ns and (n-2)dstates (n=8 for K; n=11 , 12 for Rb) with red (~620-nm) , 80-100-fs pulses were detected by four-wave mixing in pump-probe experiments. The temporal behavior of the wave packet is observed by monitoring the coherent UV radiation generated near the alkali mp(2)P? (2)S(1/2) (7</=m</=12 for Rb; 5</=m</=7 for K) resonance transitions when a probe pulse is scattered by the wave packet established by the earlier (identical) pump pulse. The spatial and spectral characteristics of the UV emission are well described by axially phase-matched four-wave mixing, and all the prominent frequency components of the wave packets are associated with energy differences between pairs of excited states for which Diota=0 or Diota=2 . These results demonstrate that the wave packet modulates chi((3))of the medium, thus rendering the wave packet detectable.
Recently we demonstrated lasing at 420 nm and 455 nm inside of rubidium and cesium vapor cells respectively, by non-resonant, two photon absorption at near infrared pump wavelengths. Two photon absorption from the ground S1/2 state to excited D3/2,5/2 or higher lying S1/2 state via virtual states and subsequent infrared stimulated emission pumps the upper laser levels, the second excited P1/2,3/2 states. To better understand the mechanism and phenomenology of this type of laser, this paper presents a method for determining the two-photon absorption (TPA) cross-section for the 5S1/2 −→ 5D5/2 transition in naturally occurring rubidium. For an incident pump intensity of 392 W cm2 and a rubidium concentration of 3.34 × 10 atoms cm3 , a peak absorbance of 0.10 is observed. The concentration of the Rb in the test cell is determined by one photon absorption spectra of the D2, indicating a vapor curve shifted by 14 C from the exterior wall temperature. The incident laser beam is focused to a waist of 75 μm. The beam quality is consistent with an M = 1.05 − 1.09. A model for the intensity as a function of propagation distance is developed to interpret the observed absorption spectrum and determine a cross-section for the two photon absorption.
Axially phase-matched parametric six-wave mixing (PSWM) has been observed in Rb vapor by two-photon excitation of the atom $(5s\ensuremath{\rightarrow}\ensuremath{\rightarrow}7s,5d)$ with $\ensuremath{\sim}150\phantom{\rule{0.3em}{0ex}}\mathrm{fs}$ pulses, centered at $\ensuremath{\lambda}\ensuremath{\sim}769\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ and generated by a Ti:Al2O3\mathrm{Ti}:{\mathrm{Al}}_{2}{\mathrm{O}}_{3}Ti:Al2​O3​ laser system. For a peak laser pulse intensity of $30\phantom{\rule{0.3em}{0ex}}\mathrm{GW}\phantom{\rule{0.2em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}2}$, PSWM is observed for Rb number densities $([\mathrm{Rb}])>2.5\ifmmode\times\else\texttimes\fi{}{10}^{15}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$ whereas, for $[\mathrm{Rb}]=2.2\ifmmode\times\else\texttimes\fi{}{10}^{16}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$, the PSWM process is detectable for pump intensities above $\ensuremath{\sim}3\phantom{\rule{0.3em}{0ex}}\mathrm{GW}\phantom{\rule{0.2em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}2}$. Upconversion of the $1.367\phantom{\rule{0.3em}{0ex}}\mathrm{\ensuremath{\mu}}\mathrm{m}$ and $1.323\phantom{\rule{0.3em}{0ex}}\mathrm{\ensuremath{\mu}}\mathrm{m}$ signal waves into the visible $(\ensuremath{\lambda}\ensuremath{\sim}494\phantom{\rule{0.3em}{0ex}}\mathrm{nm})$ has been accomplished by sum frequency generation in LiIO3{\mathrm{LiIO}}_{3}LiIO3​, and quantum beating in Rb at 18.3THz18.3\phantom{\rule{0.3em}{0ex}}\mathrm{THz}18.3THz $(\ensuremath{\sim}608\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1})$ has been observed by monitoring the upconverted signal wave intensity produced in pump-probe experiments. These results broaden the scope of the wave packet detection scheme reported by Tran et al. [Opt. Lett. 23, 70 (1998)] in that a variety of coherent nonlinear optical processes $({\ensuremath{\chi}}^{(3)},{\ensuremath{\chi}}^{(5)},\dots{})$ appear to be suitable for observing the amplitude and phase of atomic (or molecular) wave packets.