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Bulletin of the Lebedev Physics Institute

Coherent Population Trapping Resonances under Multifrequency Optical Pumping

Савинов Константин Николаевич, Nickolai. N. Golovin, А. К. Дмитриев

Published Aug 1, 2023• 2 citations
Publisher

Abstract

Abstract Coherent population trapping resonances are detected in a cell with Rb-87 exposed to the multifrequency radiation from an external-cavity diode laser under the combined action of microwave and VHF modulation of the injection current. The relative amplitudes of the resonances and the frequency spacing between them are measured.
DOI
10.3103/s1068335623140129

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Abstract The paper presents the results of studying the coherent population trapping (CPT) resonances observed in a cell with 87Rb vapor with addition of a buffer gas under multifrequency excitation by radiation from an external cavity diode laser (ECDL). The laser diode injection current is modulated simultaneously by microwave (MW) and very high-frequency (VHF) signals. It has been revealed that, for a certain band of VHF modulation frequencies, the amplitude of a CPT resonance is proportional to the product of the electrical field strengths of the spectral components contributing to resonance excitation. The proposed model of calculating the resonance amplitude is in good agreement with experimental data.
For the first time, a series of CPT resonances under the simultaneous exposure of microwave and VHF modulation of the injection current of a diode laser with an external cavity, represented by diffraction gratting, was observed. The frequency spacing between resonances was coincided with half of the VHF modulation frequency. The contribution to pumping in this case comes from pairs of VHF components located in different spectral bands and separated from each other by the frequency of hyperfine splitting of ground state of rubidium-87 atom (about 6.8 GHz). The use of multifrequency pumping will make it possible to reduce the light shifts of the observed CPT resonances due to averaging, and the use of a semiconductor laser with an external cavity opens up the possibility of creating a combined optical and radio frequency standard. The dependences of the CPT parameters of resonances on the intensity of the incident radiation are also studied in this work, and the threshold observed in this case is consistent with the results described in the literature. The dependences of the amplitude of CPT resonances on the deviation of the applied VHF modulation have also been studied. The VHF modulation frequency remained unchanged at 34 MHz (1/200 of the clock transition frequency).
We present the results of an experimental study of Coherent Population Trapping (CPT) in potassium, obtained by means of modulation of laser light amplitude with kHz frequency. The radiation from an external cavity diode laser, matching the D1 line of K, is modulated by an acousto-optical modulator. In the cell containing buffer gas, the CPT resonance width is reduced more than three orders of magnitude as compared to the cell containing pure potassium vapor. In K this resonance narrowing occurs with high resonance contrast; such behavior is not observed in buffered cells containing Rb or Cs, where the optical pumping to the non-interacting with the light ground level is very effective and depletes the population of the working ground Zeeman sublevels. The narrow CPT resonance of reduced fluorescence transforms to the one of enhanced fluorescence with the cell temperature rising. The transformed resonance exhibits higher contrast and lower width than those of the reduced fluorescence resonance. Hence, beside its scientific importance the resonance sign reversal can be used for the improvement of the CPT resonance parameters.
We present the first experimental observation of Coherent Population Trapping (CPT) in Potassium, obtained with kHz-frequency modulation of the laser light amplitude. It is performed by acousto-optical amplitude modulation of the radiation from an external cavity diode laser, matching the D1 line of K. The CPT resonances are detected both through K absorption and fluorescence. The resonances are studied in three kinds of K cells: i) pure-evacuated, ii) polydimethylsiloxane (PDMS)-coated-evacuated and iii) Ne-gas buffered. In all cases CPT-resonance narrowing with cell temperature is observed. In the pure-evacuated cell we registered the lowest contrast and the highest width of the resonance, while in buffered/coated cells a strong enhancement of the CPT resonance contrast up to 15% is observed. This behavior is the opposite to the one exhibited by Cs and Rb. The observed contrast enhancement in K is accompanied by more than two orders of magnitude reduction of the resonance width. The results here presented prove the advantage of using Potassium in CPT-based applications.
This letter presents for the first time a systematic study of an atomic frequency standard based on coherent population trapping (CPT) effect in87Rb vapour optically pumped with a multifrequency laser radiation formed with sidebands of a single-frequency laser whose injection current is modulated at a subharmonic frequency (1/2-1/6) relative to the frequency of hyperfine ground state splitting. Measured are the CPT resonance parameters and the atomic clock short-term stability at different subharmonic frequencies of microwave modulation. The possibility of CPT resonance light shift suppression is experimentally demonstrated at modulation frequencies between 1/3 and 1/5 of the resonant frequency for continuous wave optical pumping of the87Rb vapour D1 line. Analyzed are the capabilities and potential of reduced energy consumption of atomic clocks when CPT resonance in rubidium vapour is excited at subharmonic modulation frequencies.