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Optics Express

Observation of large group index enhancement in Doppler-broadened rubidium vapor

Bo-Xun Wang, Chih-Yuan Liu, D. J. Han

Published Jul 13, 2015• 5 citations
Publisher

Abstract

We report experimental observation of large group index across the Lamb dips of ground hyperfine states in Doppler-broadened 87Rb vapor. By sweeping the laser frequency through each hyperfine transition we measure the saturated absorption and optical phase shift using a phase-locked Mach-Zehnder interferometer. Our measurements provide a direct demonstration of the theoretical prediction by Agarwal et al. [G. S. Agarwal and T. N. Dey, Phys. Rev. A 68, 063816, (2003)] for the first time. An enhancement factor as large as 1005 in group index was observed for Rb vapor at temperature of 85 °C. The experimental data are in good agreement with the theory.

DOI
10.1364/oe.23.018792

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We report experimental observation of large group index across the Lamb dips of ground hyperfine states in Doppler-broadened 87Rb vapor. By sweeping the laser frequency through each hyperfine transition we measure the saturated absorption and optical phase shift using a phase-locked Mach-Zehnder interferometer. Our measurements provide a direct demonstration of the theoretical prediction by Agarwal et al. [G. S. Agarwal and T. N. Dey, Phys. Rev. A 68, 063816, (2003)] for the first time. An enhancement factor as large as 1005 in group index was observed for Rb vapor at temperature of 85 °C. The experimental data are in good agreement with the theory.
We have experimentally demonstrated a general method to perform high-precision refractive index spectrum measurement using phase-shifting interferometry in a phase-tunable Mach–Zehnder interferometer. We observed coherent amplification on the refractive spectrum, across the Lamb dips of ground hyperfine states in a Doppler-broadened Rb87 vapor. Most importantly, the spectrum still has a large signal-to-noise ratio when the probe power is in the dark light level under which the method using a single interferogram fails. This experimental scheme is efficient for refractive spectrum measurements and could be extended for 2D phase imaging. It provides many potential applications for optical phase measurements in linear, nonlinear, and quantum optics.
We report the experimental observation of steady-state electromagnetically induced transparency and sub-Doppler linewidth in a Doppler-broadened \ensuremath{\Lambda}-type Rb atomic system formed on the 87Rb^{87}\mathrm{Rb}87Rb D2{\mathit{D}}_{2}D2​ transitions. The \ensuremath{\Lambda} system is coupled on one transition by a strong laser and probed on the other transition by a weak laser. The observed sub-Doppler linewidth decreases with the increasing detuning of the strong-coupling laser from the atomic transition frequency and is in good agreement with the theoretical calculations of G. Vemuri, G. S. Agarwal, and B. D. N. Rao [Phys. Rev. A 53, 2842 (1996)]. \textcopyright{} 1996 The American Physical Society.
The commonly quoted saturation intensity of rubidium (Rb) for D2 transitions is 16.7W.m−2\mathrm{W}.\mathrm{m}^{-2}W.m−2. This value, however, is only accurate for the cycling transition, which renders the Rb atom to resemble a two-level system. In practice, the effective saturation intensity is influenced by various factors, such as the polarization of light, optical pumping, and atom collision which can redistribute the population. As a result, it is crucial to experimentally determine the effective saturation intensity. In this context, we demonstrate a novel yet simple experimental method to measure the effective saturation intensities of Doppler-broadened transitions in atomic vapours. Our method only requires on-resonance absorbance data for various input laser intensities, which can be obtained with a tunable CW laser without the need for precise frequency locking. We applied this method to measure the effective saturation intensities of all four Doppler-broadened D2 transitions in natural rubidium (Rb) vapour (27.8%87Rb and 72.2 %85Rb{}^{\text{8}} {}^{\text{5}}Rb85Rb) atr=35∘Cr={35}^{\circ} \mathrm{C}r=35∘C. The results show that the effective saturation intensities for Rb range from 0.752 to 16.8W.m−2\mathrm{W}.\mathrm{m}^{-2}W.m−2. Notably, for certain transitions (specifically, repump transitions), the effective saturation intensities were smaller than 16.7 W.m−2\mathrm{m}^{-2}m−2(up to an order of magnitude), with potentially important implications for optical pumping in Doppler-broadened transitions.
We report an experimental measurement of giant group index in the 85 Rb atomic medium at room temperature via a four-level V-type scheme on the D 2 line. Our experiment uses co-propagation configuration of probe and coupling lasers through an atomic sample. In this configuration, three sharp EIT windows with significant transparency depths are observed on the probe absorption spectrum. By establishing a Mach–Zehnder interferometer, we measure the dispersive curve and hence obtain the group index curve with three enhanced positive peaks at the locations of the EIT windows, interspersed with negative peaks. The amplitude of the group index curve is increased as the temperature decreases and is decreased as the temperature increases. We estimate from our experimental results the good values of the group index to be n g = 5.8 × 10 4 (slow light regime) and n g = −4.0 × 10 4 (fast light regime). We also show that the experimental measurements are in good agreement with the theoretical results.
This paper describes experiments performed by undergraduate physics majors to measure the temperature dependence of Doppler-broadening in Rb absorption lines over the interval 290–500 K. We report measurements made with a laser-diode spectrometer on the resolved hyperfine transitions of Rb87 (5 2S1/2 F=1 to 5 2P1/2, F=1 and 2) near 795 nm, and on the unresolved envelope of the transitions of Rb85 (5 2S1/2 F=2 to 5 2P3/2, F=1, 2 and 3) near 780 nm. Agreement between measured and predicted linewidths is very good.
We propose and demonstrate pump-probe spectroscopy of rubidium absorption that reveals the sub-Doppler hyperfine structure of the ${}^{5}{\mathrm{S}}_{1/2}\ensuremath{\leftrightarrow}{}^{5}{\mathrm{P}}_{3/2}$ (D2{\mathrm{D}}_{2}D2​) transitions. The counterpropagating pump and probe lasers are independently tunable in frequency, with the probe operating at the single-photon level. The two-dimensional spectrum measured as the laser frequencies are scanned shows fluorescence, Doppler-broadened absorption dips, and sub-Doppler features. The detuning between the pump and probe lasers allows compensation of the Doppler shift for all atomic velocities in the room-temperature vapor, meaning we observe sub-Doppler features for all atoms in the beam. We detail a theoretical model of the system that incorporates fluorescence, saturation effects, and optical pumping and compare this with the measured spectrum, finding a mean absolute percentage error of 3.11%. This high level of agreement allows us to extract the number density and temperature of the Rb\mathrm{Rb}Rb vapor from the two-dimensional spectrum. In the future this technique could be used as an atomic frequency standard for calibrating and characterising a single-photon or low-light-level source.