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Journal of Physics Conference Series

Analysis of high-efficiency widely-tunable N-resonances in Cs vapor

A. Krasteva, S. Gateva, Stefan Tzvetkov, Pradip Narayan Ghosh, Armen D. Sargsyan, S. Cartaleva

Published Mar 1, 2018• 0 citations
Publisher

Abstract

The narrow-band coherent N-type resonance, promising for the development of advanced atomic clocks, can be considered as a type of three-photon resonance, where a two-photon Raman excitation is combined with a resonant optical pumping field. In this communication, we present an experimental study and a theoretical analysis related to three-photon, bi-chromatic excitation of Cs atomic vapor contained in an 8-mm long cell with 20 Torr of neon. If a coupling laser is fixed at a frequency that is lower by several GHz than the position of the absorption profile of the F g = 4 set of transitions, and a probe laser is tuned over the D 2 line (λ = 852 nm), a narrow high-contrast enhanced absorption N-resonance will be observed in the probe light profile, superimposed on the absorption profile of the F g = 4 set of transitions. We present theoretical modeling aimed to clarify the processes behind the efficiency of the N-resonance preparation for different frequency positions of the coupling laser within the D 2 line of Cs.

DOI
10.1088/1742-6596/992/1/012017

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The narrow-band coherent N-type resonance, promising for the development of advanced atomic clocks, can be considered as a type of three-photon resonance, where a two-photon Raman excitation is combined with a resonant optical pumping field. In this communication, we present an experimental study and a theoretical analysis related to three-photon, bi-chromatic excitation of Cs atomic vapor contained in an 8-mm long cell with 20 Torr of neon. If a coupling laser is fixed at a frequency that is lower by several GHz than the position of the absorption profile of the F g = 4 set of transitions, and a probe laser is tuned over the D 2 line (λ = 852 nm), a narrow high-contrast enhanced absorption N-resonance will be observed in the probe light profile, superimposed on the absorption profile of the F g = 4 set of transitions. We present theoretical modeling aimed to clarify the processes behind the efficiency of the N-resonance preparation for different frequency positions of the coupling laser within the D 2 line of Cs.
Sub-natural-width N -type resonance, observed in the D 2 line of Cs vapor has been reported recently. Three-photon, bi-chromatic excitation of Cs atomic vapor with 20 Torr of Neon was performed and a sub-natural-width resonance was observed, superimposed on the absorption profile of a set of hyperfine transitions starting from single hyperfine ground level. Here we report the study of a different three-photon, couple-probe- excitation scheme that allows the observation of a sub-natural-width resonance situated on a flat background, well outside the D 2 line absorption spectrum of Cs. The lack of a noisy background could be advantageous for many applications related to the narrow N resonance. In addition, a new broader feature of enhanced transparency is demonstrated that is centered at the ground state level that does not interacting with the coupling beam. This enhanced transparency resonance can be attributed to the atomic population loss in a narrow spectral interval occurring due to excitation of energy levels situated above the 6 2 P levels.
Abstract N -type resonances in Cs D 2 lines are investigated in optical cell for different detuning of the fixed coupling transition frequency. Using two independent DFB lasers makes it possible to observe high contrast and subnatural-width N -resonances without magnetic field shielding of an optical buffer gas cell with sub-centimeter dimensions. According to the detuning of the coupling laser frequency, an N -resonance with frequency higher or lower than this of the coupling transition frequency is registered. In the first case the N -resonance is in absorption, but in the second case, an inverse N -resonance (enhanced transparency) is measured. A theoretical simulation of the line shape, which is based on solution of optical Bloch equations and considers the interaction of both coupling and probe radiations with the atomic system is done. The simulation leads to N -resonance peaks in agreement with the observed peaks for different positions of the coupling frequency. The presented approach is of importance for further study and optimization of the three-photon and magneto-optical resonances, as well as for miniaturization of optical sensors based on spatially restricted hot alkali vapor.
A new nano-cell with smoothly varying longitudinal thickness of the atomic vapour layer L in the range of 350 - 5100 nm allowing to study the resonant absorption of D1 and D2 lines of Cs atoms for thicknesses changing from L = Lambda/2 to L = 6 Lambda with the step of Lambda/2 (Lambda = 852 nm or 894 nm are the resonant laser wavelengths) and for different intensities is developed. It is revealed that for low laser intensities there is narrowing of the resonant absorption spectrum for the thicknesses L = (2n + 1)Lambda/2 (where n is an integer) up to L = 7Lambda /2 and broadening of the spectrum for L = n Lambda. For relatively high laser intensity (>1 mW/cm2), velocity selective optical pumping/saturated resonances of a reduced absorption (with the line-width close to the natural one), and centred on the hyperfine transitions occur when L = n Lambda. The possible application of these resonance peaks is given. The developed theoretical model describes well the experiment.
Summary form only given. We report on recent experiments to use dark line coherent population trapping (CPT) resonances in cesium vapor as the basis for a compact frequency reference. The resonance is excited by directly modulating the injection current of a VCSEL at one-half the ground state hyperfine transition of Cs (4.6 GHz) to produce RF sidebands on the optical carrier. The laser output is then passed through a room-temperature cell containing Cs at its vapor pressure together with 8.7 kPa of Neon buffer gas and the transmitted optical power is detected in a photodiode. When the laser carrier is tuned half way between the two hyperfine components of the D/sub 2/ line at 852 nm, and the RF modulation frequency is adjusted to be equal to one-half the hyperfine splitting, the absorption of the sidebands through the cell is slightly reduced due to the CPT effect. As the RF frequency is scanned over the resonance, lock-in detection is used to produce a /spl sim/1 kHz wide dispersive error signal which is in turn used to stabilize the modulation frequency. The narrow signal is the result of the buffer gas, which reduces the time-of-flight broadening of the transition.
Raman-Ramsey interference has proved a very effective technique to implement compact and high performing vapor cell frequency standards. In this paper, we theoretically characterize Raman-Ramsey resonances in an optically thick atomic vapor. Specifically, some parameters of interest for frequency standards applications, like contrast and linewidth of the central Raman-Ramsey fringe, are evaluated at different temperatures for 133Cs^{133}\mathrm{Cs}133Cs and 87Rb^{87}\mathrm{Rb}87Rb vapor cells with buffer gas. Density narrowing and broadening effects are described and explained in terms of a three-level theory where laser field propagation through the atomic medium is taken into account. Also, we investigate light shift both in low and high atomic density regimes. Light shift, which potentially degrades the medium- to long-term stability of Raman-Ramsey clocks, is composed of two contributions. The first is a pulling effect exerted by the wide Rabi profile enclosing the interference pattern on the central Raman-Ramsey fringe. The second light-shift term is strictly related to the detection time. Calculations derived from our model well describe already existing experimental results and new behaviors are predicted.
Based on a Λ-type three-level system consists of cesium (Cs) 6S1/2 (Fg = 3) and 6S1/2 (Fg = 4) long-lived ground states and 6P3/2 (Fe = 4) excited state, we have experimentally measured and theoretically analyzed the characters of coherent population trapping (CPT) resonances in Cs vapor cells with different partial pressure of neon (Ne) as buffer gas around room temperature. The impact of some experimental parameters, such as the relative intensity ratio between two phase-locked laser beams, the laser intensity, with or without buffer gas and the partial pressure of Ne, the temperature and the longitudinal magnetic field, on the CPT resonance was studied in details. With the optimized parameters, we got typical CPT signal with the full-width half-maximum (FWHM) linewidth as narrow as ~ 181 Hz in a Cs vapor cell filled with 30 Torr of Ne as buffer gas.