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Open Physics

Amplification of radiation in atomic vapor induced by a linearly polarized laser radiation

Marina Movsisyan, Svetlana Shmavonyan, A. Papoyan

Published Feb 21, 2011• 3 citations
Publisher

Abstract

Abstract We present the results of spectroscopic and polarization studies of dilute rubidium vapor exposed to a single-frequency linearly polarized diode laser radiation in a spectral range of atomic D2 line. We report the origin of a circularly polarized radiation on V-type transitions of 87Rb F g = 2 → F e = 3 and 85Rb F g = 3 → F e = 4, and amplification of this radiation in backward direction caused by a partial population inversion among magnetic sublevels of the ground and excited levels. This is confirmed experimentally by high directivity of backward radiation, absence in its spectrum of 85Rb F g = 2 → F e = 1 (Λ-type) radiation, as well as by different nature of intensity dependences of backward and fluorescence radiations.

DOI
10.2478/s11534-011-0011-x

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Abstract We present the results of spectroscopic and polarization studies of dilute rubidium vapor exposed to a single-frequency linearly polarized diode laser radiation in a spectral range of atomic D2 line. We report the origin of a circularly polarized radiation on V-type transitions of 87Rb F g = 2 → F e = 3 and 85Rb F g = 3 → F e = 4, and amplification of this radiation in backward direction caused by a partial population inversion among magnetic sublevels of the ground and excited levels. This is confirmed experimentally by high directivity of backward radiation, absence in its spectrum of 85Rb F g = 2 → F e = 1 (Λ-type) radiation, as well as by different nature of intensity dependences of backward and fluorescence radiations.
The resonant magneto-optic spectra of the Rb D2 transition in the Voigt configuration was measured by using a tunable, narrow-band, single-mode GaAlAs diode laser. The elliptically polarized laser beam with a small ellipticity Δα was incident upon Rb vapor in a transverse magnetic field of 0.75 T. The polarization change caused by magnetically induced optical anisotropy was analyzed with a birefringent prism placed behind the sample with a small offset angle Δθ from the perfectly crossed direction. The Voigt-effect signal had a Zeeman splitting of 50 GHz. The spectral intensity distribution of the signal, determined by the interference between the hyperfine components of 85Rb and 87Rb, was dependent on Δα, Δθ, and the number density of atoms. The theoretical prediction1 for the Voigt-effect line profiles for various Δα and Δθ agreed with the experimental result. The minimum detectable density of Rb atoms was limited by statistical fluctuations of photon counts.
Abstract We report experimental evidence of degenerate mirrorless lasing in the forward direction under excitation of a dilute atomic Rb vapor with a single linearly polarized cw laser light resonant with cycling F e > F g atomic D 2 transitions. Light polarized orthogonally to the laser light is generated for the input light intensity exceeding a threshold value of ∼3 mW cm −2 . Application of a transverse magnetic field directed along the input light polarization reveals a sharp ∼20 mG wide magnetic resonance centered at B = 0. Increasing the incident light intensity from 3 to 300 mW cm −2 , the generated light undergoes rapid amplitude increase followed by a decline and resonance broadening. Such nonlinear behavior of the observed magnetic resonance is attributed to the population inversion on optical transitions between magnetic sublevels established under linearly polarized excitation. We present observations that indicate that a combination of nonlinear-optical effects occurs in this system, including degenerate mirrorless lasing and four-wave mixing.
A new method based on optical pumping is proposed for increasing the parameters of dark resonance excitation on D 2 line in 87 Rb vapor. The pumping is provided by linearly polarized two-frequency laser radiation propagating in the direction perpendicular to the probing field directed along the cell with atomic rubidium vapor. This method significantly improves the dark resonance parameters as compared to the case of pumping by a circularly polarized field. Qualitative considerations are confirmed by the results of numerical calculations.
We report on the observation of a Zeeman shift (order of 100 MHz) of the Doppler-broadened D2 transition of both 85Rb and 87Rb isotopes via transmission through a 400 nm diameter optical nanofiber in the presence of a DC magnetic field. Linearly-polarized light propagating in the nanofiber is analyzed as a superposition of two orthogonally circularly polarized orientations, σ+ and σ-. In the absence of the magnetic field, the absorption of these polarizations by the atomic vapor, via the evanescent field at the waist of the nanofiber, is degenerate. When a weak magnetic field is applied parallel to the propagating light, this degeneracy is lifted and relative shifts in the resonance frequencies are detected. Typical linear shift rates of 1.6 MHz/G and -2.0 MHz/G were observed. We also demonstrate a dichroic atomic vapor laser lock line shape by monitoring the real-time subtraction of the two magnetically-shifted absorption spectra. This is particularly interesting for magneto-optical experiments as it could be directly implemented for diode laser frequencystabilization.
We investigated experimentally and theoretically angular momentum alignment-to-orientation conversion created by the joint interaction of laser radiation and an external magnetic field with atomic rubidium at room temperature. In particular, we were interested in alignment-to-orientation conversion in the atomic ground state. In the experiment the laser frequency was fixed to the hyperfine transitions of the D1{D}_{1}D1​ line of rubidium. To simulate the measured signals we used a theoretical model that takes into account all neighboring hyperfine levels, the mixing of magnetic sublevels in an external magnetic field, the coherence properties of the exciting laser radiation, and the Doppler effect. The experiments were carried out by exciting the atoms with linearly polarized laser radiation. Two orthogonal circularly polarized laser induced fluorescence (LIF) components were detected and afterward their difference was taken. The combined LIF signals originating from the hyperfine magnetic sublevel transitions of 85Rb^{85}\mathrm{Rb}85Rb and 87Rb^{87}\mathrm{Rb}87Rb rubidium isotopes were included. The alignment-to-orientation conversion can be incontrovertibly identified in the difference signals for various laser frequencies, and a change in signal shapes can be observed when the laser power density is increased. We studied the formation of and the underlying physical processes behind the observed signals of the LIF components and their difference by performing an analysis of the influence of incoherent and coherent effects. We performed simulations of theoretical signals that showed the influence of ground-state coherent effects on the LIF difference signal.
Measurements of stimulated Raman gain in rubidium vapor are made and compared with a detailed model. Amplification of a weak probe laser is observed in a \ensuremath{\Lambda}-type three-level system that is driven by a strong cw coupling laser. The intensity of the coupling laser is sufficiently strong that it both optically pumps the atomic levels, causing a population inversion in the ground-state hyperfine levels, and drives the stimulated Raman process producing gain. A theory that models all aspects of the gain mechanism is presented. The model includes all interactions between the external fields and the atomic transitions, including those processes that involve multiple fields interacting with the same transition. Numerical calculations are compared with gain measurements made at several coupling laser intensities and detunings from the D1D1D1 line in 85Rb.{}^{85}\mathrm{Rb}.85Rb.