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Journal of Experimental and Theoretical Physics

Coherent Dicke Narrowing of Absorption Spectra, Collapse and Revival: Second-Derivative Processing

Armen D. Sargsyan, A. Amiryan, David G. Sarkisyan

Published Aug 1, 2020• 3 citations
Publisher

Abstract

Abstract The coherent Dicke narrowing (CDN) of the absorption spectrum in an Rb, Cs, K, or Na atomic vapor column of thickness L = (2n + 1)λ/2 (when n is an integer) and its collapse (absence of narrowing), which occurs at an atomic vapor column thickness L = nλ (where λ is the wavelength of the laser radiation in resonance with an atomic transition), are experimentally and theoretically studied. The application of second-derivative (SD) processing is shown give additional important information about CDN and its collapse. Optical nanocells with a wedge gap thickness of 30—3000 nm between the windows inner surfaces are used in experiments. At thicknesses λ/2, 1.5λ, and 2.5λ, all atomic transitions, which are even masked by a large Doppler broadening of 1000–1500 MHz, are fully spectrally resolved in SD spectra, and the frequency spacings (30–50 MHz) between the atomic transitions of a hyperfine structure and the relative probabilities of the atomic transitions are correctly displayed. CDN is shown to be applied in practice. Additional narrow velocity selective optical pumping (VSOP) resonances appear in the SD spectra of the resonance fluorescence of the Rb vapors. A developed theoretical model well describes the experimental results.

DOI
10.1134/s1063776120050179

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Abstract The coherent Dicke narrowing (CDN) of the absorption spectrum in an Rb, Cs, K, or Na atomic vapor column of thickness L = (2n + 1)λ/2 (when n is an integer) and its collapse (absence of narrowing), which occurs at an atomic vapor column thickness L = nλ (where λ is the wavelength of the laser radiation in resonance with an atomic transition), are experimentally and theoretically studied. The application of second-derivative (SD) processing is shown give additional important information about CDN and its collapse. Optical nanocells with a wedge gap thickness of 30—3000 nm between the windows inner surfaces are used in experiments. At thicknesses λ/2, 1.5λ, and 2.5λ, all atomic transitions, which are even masked by a large Doppler broadening of 1000–1500 MHz, are fully spectrally resolved in SD spectra, and the frequency spacings (30–50 MHz) between the atomic transitions of a hyperfine structure and the relative probabilities of the atomic transitions are correctly displayed. CDN is shown to be applied in practice. Additional narrow velocity selective optical pumping (VSOP) resonances appear in the SD spectra of the resonance fluorescence of the Rb vapors. A developed theoretical model well describes the experimental results.
Comparison of the resonant absorption and fluorescence in the extremely thin cell (ETC) filled with pure Rb with another one filled with Rb and Neon gas with the pressures 6 Torr and 20 Torr is provided. The effect of the collapse and revival of Dicke-type narrowing is still observable for the thickness L=λ/2, and L=λ, where λ is a resonant laser wavelength 794 nm (D1 line of the Rb), i.e. the spectra of the resonant absorption and fluorescence demonstrate the sub-Doppler narrowing for the thickness L=λ/2 and broadening for the thickness L=λ. It is important to note, that in an ordinary Rb cell with the thickness L in the range of 1mm- 10 cm filled with buffer gas, with a pressure higher than 1 Torr in the Saturation Absorption scheme, any sub-Doppler features are absent. For L= λ the so called velocity selective optical pumping (VSOP) resonance located at the atomic transition is still observable when Ne pressure is 6 Torr , while the linewidth is increased by 4 times in comparison with that obtained in the pure Rb ETC. This VSOP resonance is wash out when Ne pressure is 20 Torr. It is demonstrated that use of the fluorescence spectra at L=λ/2 allows one to use the ETC for the buffer gas pressure measurement. Comparison with the theory is presented.
Abstract A nanometer-thin cell (in the direction of laser beam propagation) has been elaborated with the thickness of the atomic vapor column varying smoothly in the range of . The cell allows one to study the behavior of the resonance absorption over the D 1 line of potassium atoms by varying the laser intensity and the cell thickness (from to with the step where is the resonant wavelength of the laser). It is shown that despite the huge Doppler broadening ( at the cell temperature ), at low laser intensities a narrowing of the resonance absorption spectrum is observed for ( at FWHM) and , whereas for and the spectrum broadens. At moderate laser intensities narrowband velocity selective optical pumping (VSOP) resonances appear at and with the linewidth close to the natural one. A comparison with saturated absorption spectra obtained in a 1.4 cm-sized K cell is presented. The developed theoretical model well describes the experiment.
The features of resonance fluorescence and absorption of resonance laser radiation in atomic vapors in a superthin cell with thickness L of the order of a wavelength lambda are investigated theoretically. Taking into account the Fabry-Perot interference effect, it is found that the dependence of the fluorescence and absorption spectra on the ratio L/lambda is substantially different. Coherent spectral narrowing - the Dicke effect - when L=(2n+1)lambda/2 and broadening of the spectral line when L = n lambda (n is a whole number) are observed in the absorption spectra. This effect is absent in the spector of resonance fluorescence in extended media. It is also shown that the processes of saturation and optical pumping significantly alter the spectral line shape as the laser intensity increases; in particular, additional sub-Doppler dips appear at the frequency of the atomic transitions. Calculated curves are given for the D2 lines of ^85Rb atoms. © 2004 Optical Society of America
Comparison of absorption and fluorescence in a nano-cell containing Rb vapor with other Rb nano-cells with addition of neon gas is presented. It is shown that the effect of collapse and revival of Dicke-type narrowing occurs for Rb nanocells containing N2 as buffer gas under 6 and 20 Torr pressure for the thickness L = λ /2 and L = where λ is the resonant λ, laser wavelength 794 nm (D1 line). Particularly for 6 Torr the line-width of the transmission spectrum for the thickness L =λ/2 is 2 times narrower than that for L = λ. For an ordinary Rb cell with L = 0.1 - 10 cm with addition of buffer gas, the velocity selective optical pumping/saturation (VSOP) resonances in saturated absorption spectra are fully suppressed when the buffer gas pressure > 0.5 Torr. A spectacular difference is that for L = λ, VSOP resonances located at the atomic transitions are still observable even when Ne pressure is ≥ 6 Torr. Narrowband fluorescence spectra of a nano-cell with L = λ/2 can be used as a convenient tool for online buffer gas pressure monitoring for the conditions when ordinary pressure gauges are unusable. Comparison of electromagnetically induced transparency (EIT) effect in a nano-cell filled with pure (without a buffer gas) Rb with another nano-cell, where buffer gas nitrogen is added, is presented. The use of N2 gas inside Rb nano-cells strongly extends the range of coupling laser detunings in which it is still possible to form EIT resonance.
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.
Abstract A cell with a submicron thickness was used to measure the broadening and shift of the D1 and D2 lines of Rb atoms by neon. The resolution of hypefine components is achieved by a combination of two techniques. First, the Rb vapor column thickness in the direction of propagation of the laser radiation was chosen equal to half of its wavelength λ under conditions of resonance with the atomic transition frequency. For rubidium atoms λ/2 ~ 400 nm. At a nanocell thickness L ~ λ/2, in the transmission spectrum A(ν), the spectral lines of atomic transitions are narrowed due to the exclusion of Doppler broadening. Second, further narrowing of the detected signals was achieved by the double differentiation of the transmission spectrum, A''(ν). The transmission spectra of pure rubidium vapor and rubidium vapor with the addition of neon have been measured at different pressures. The measured values of the shift coefficients of the D1 and D2 lines of rubidium in the presence of Ne were –1.1 ± 0.2 MHz/Torr and –2.1 ± 0.2 MHz/Torr, respectively. The broadening coefficients of the D1 and D2 lines coincide and are equal to 10 ± 1 MHz/Torr. Due to its high spectral resolution, this technique allows separate measurements for each individual transition.