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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE

Coherent population trapping resonances in potassium with amplitude-modulated light

Silvia Gozzini, Luca Marmugi, Dimitar G. Slavov, A. Lucchesini, S. Cartaleva

Published Oct 8, 2010• 3 citations
Publisher

Abstract

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.

DOI
10.1117/12.885314

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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.
Coherent Population Trapping (CPT) is observed on the first and second resonance lines of potassium, by irradiating the vapor by 770.1 nm and 404.5 nm diode laser light, respectively. With the 770.1 nm excitation, narrow CPT resonance is observed demonstrating 80 times larger contrast than in case of Cs vapor, in coated optical cell. A theoretical model is proposed to explain strong resonance contrast enhancement. Irradiating K vapor with the 404.5 nm laser, the CPT resonance is observed both on the 404.5 nm and 770.1 nm lines, which makes the study of various cascade transitions in potassium possible.
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.
The characteristics of coherent population trapping (CPT) signal were investigated in small-size glass vapor cells containing [Formula: see text]Rb and Ne buffer gas with narrow line-width laser pumping on D2 line. The parameters of CPT signals were measured using small-size vapor cells with Ne buffer gas pressure in the range of 200–400 Torr, cell temperature in the range of 65–120 [Formula: see text]C and the values of laser pumping power of 30–400 [Formula: see text]W/cm2. Optimal conditions, under which the minimal value of short-term instability of resonance line is achieved, were obtained in experiments. Orientation frequency shifts of CPT resonance using glass [Formula: see text]Rb vapor cells containing buffer gas and anti-relaxation coating were compared. CPT signals using vapor cells based on integrated technologies containing [Formula: see text]Rb in atmosphere of Ne were also investigated. The CPT signals with typical line widths of 2–3 kHz and signal-to-noise ratio of 1500 in 1 Hz bandwidth are observed, which allows one to provide relative frequency instability of 10[Formula: see text] at 100 s.
Characteristics of CPT (coherent population trapping) signal are investigated in small-size glass vapor cells containing 87 Rb and Ne buffer gas with narrow linewidth laser pumping on D 2 line. Parameters of CPT absorption signals are measured using small-size vapor cells with Ne buffer gas pressure in range of 200-400 Torr, cell temperature in range of 65-120 °C and values of laser pumping power of 30-400 μW/cm 2 . Optimum conditions, under which the minimal value of short-term instability of resonance line is achieved, are obtained in experiments. CPT signals using vapor cells based on integrated technologies containing 87 Rb in atmosphere of Ne are also investigated. The CPT signals with typical linewidths of 2-3 kHz and signal-to-noise ratio of 1500 in 1 Hz bandwidth are observed, which allows one to provide relative frequency instability of 10 -11 at 100 s.
We report the first observation of sub-natural-width dark resonances of enhanced transparency on the D2 line of potassium, contained in an optical cell with reduced (8mm) dimensions and buffered by 30Torr of Ne. The Potassium vapours are irradiated by a frequency modulated DFB diode laser light (with λ = 766.7nm and 2MHz bandwidth), and are placed in orthogonal to the light magnetic field scanned around zero value. Very good signal-to-noise ratio and narrow (∼20kHz) dark resonances are observed and studied in unshielded laboratory environment. The dark resonance formation is related to coherent superposition of the ground-state Zeeman sublevels by respective components of the frequency-modulated light.