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Physica Scripta

Polarisation properties of the stimulated zero-field level crossing

Peter Jungner, Birger Stahlberg, Markus Lindberg

Published Oct 1, 1988• 5 citations
Publisher

Abstract

The polarisation properties of the single mode linearly polarised laser beam interaction with a Doppler broadened absorber has been investigated. The absorber is placed in a longitudinal tunable magnetic field. The laser beam is analysed with a polariser after it has traversed the absorber. With various analyser rotation angles the lineshapes of the nonlinear resonances known as the population effect and the Zeeman coherence effect have been studied near zero magnetic field tuning. Theoretical predictions derived from the semi-classical laser theory are compared with experimental results. The experiment has been carried out with a low power thermally stabilised 633 nm He-He laser. The measured lineshapes of the resonances of the 2 p 4 -3 s 2 transition in neon agree well with the theoretical predictions. By using an analyser in the beam an improved signal-to-noise ratio is shown to result.

DOI
10.1088/0031-8949/38/4/007

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AIP conference proceedings•Jan 1, 1997•0 citations
The polarisation properties of the single mode linearly polarised laser beam interaction with a Doppler broadened absorber has been investigated. The absorber is placed in a longitudinal tunable magnetic field. The laser beam is analysed with a polariser after it has traversed the absorber. With various analyser rotation angles the lineshapes of the nonlinear resonances known as the population effect and the Zeeman coherence effect have been studied near zero magnetic field tuning. Theoretical predictions derived from the semi-classical laser theory are compared with experimental results. The experiment has been carried out with a low power thermally stabilised 633 nm He-He laser. The measured lineshapes of the resonances of the 2 p 4 -3 s 2 transition in neon agree well with the theoretical predictions. By using an analyser in the beam an improved signal-to-noise ratio is shown to result.
The authors have investigated the zero-field level crossing lineshape in resonance absorption. A single-mode HeNe laser beam at 633 nm is applied to an external neon-gas absorption cell in a longitudinal magnetic field. They compare the measured lineshapes with theoretical predictions. Two superposed nonlinear Lorentzian-shaped resonances, known as the population effect of the 2p 4 -3s 2 transition and the Zeeman coherence effect of the lower 2p 4 level, appear on the background caused by the inhomogeneous broadening. Taking into account the background and the power broadening the authors obtain the linewidths at zero pressure gamma (2p 4 ,3s 2 )=(7.7+or-2.1) MHz and gamma (2p 4 )=(8.4+or-1.2) MHz. For the neon pressure broadening the authors get for pressures below 200 Pa (1.5 Torr) d gamma (2p 4 ,3s 2 )/dp=0.20+or-0.02 MHz Pa -1 and d(2p 4 )/dp=0.040+or-0.007 MHz Pa -1 . Their values are in acceptable agreement with the values obtained with other spectroscopic methods.
Level-crossing effects have been detected non-optically with the use of a Rabi-type atomic-beam method. The sodium beam was excited by intense linearly polarised radiation from a CW narrow-band dye laser. Several superimposed resonances were observed in the atomic-beam signal when scanning the magnetic field around zero. The two narrower resonances are interpreted as high-order non-linear effects. They arise from the coupling of laser-induced Zeeman coherences in the ground state (including a hexa-decapole moment) with the populations of the Zeeman sublevels.
We present analytical predictions based on a semiclassical theory concerning a single-mode cw laser interaction with longitudinally Zeeman tuned J=1--J=0 atomic systems. In the theoretical model, the laser frequency detuning from the inhomogeneously broadened absorber line center is considered. Equation for the interaction signal are derived for all rotations of a polarizating analyzer transmisson axis with respect to an original laser beam linear polarization. Results are presented for the forward-scattering case when the analyzer transmission axis is orthogonal to the beam polarization. Calculated results on bright and dark ring-shaped light structures are compared with experimental ones originating from a Gaussian beam interaction with a neon 1s4{\mathit{s}}_{4}s4​(J=1, g=1.464)--2p3{\mathit{p}}_{3}p3​(J=0) transition. Calculated results on forward-scattering spectra are presented, i.e., the forward-scattering light intensity as a function of the longitudinal magnetic tuning when the laser frequency is fixed to selected detunings from the absorber line center. Spectra are presented both for the case when the inhomogeneously broadened line originates from a single absorbing isotope and for the case when it originates from overlapping contributions of two isotopes. The results show that laser frequency detunings of tens of megahertz from the absorber line center(s) show up in the forward-scattering spectra. The conclusion is that reliable data on atomic parameters of a nonlinearly absorbing sample can be extracted from experimental spectra only when the sample is monoisotopic and when the laser frequency detuning from the absorber line center is known.
Conventional three level probe spectroscopy is used to investigate magnetically induced level crossings. Pump laser induced signals in the probe laser response are investigated when the atomic systems are tuned magnetically. This has been carried out in both a co-propagating and a counter-propagating configuration. The level scheme considered is a simplified J = 1 → J = 2 → J = 1 system. The theoretical calculations are based on the semiclassical laser theory and the equations obtained are solved numerically. Experimental verification of the theoretical lineshapes is carried out in a neon gas absorber. The agreement between the calculated results and the measurements is good.
We measure and simulate numerically the Hanle effect and non-zero field level crossing signals in 85 Rb and 87 Rb atoms in a magnetic field at room temperature. Diode laser radiation from 4 mW cm −2 to 3.3 W cm −2 tuned to the D 2 absorption line of each isotope excites atoms into all the excited-state hyperfine levels simultaneously inside the unresolved Doppler profile. Polarization fluorescence detection is used to observe dark and bright resonances, as well as non-zero field level crossing resonances, for several excitation lines. A broad spectral line excitation model is applied to analyse the measured signals. The non-linear Zeeman effect is included in the model for both ground and excited states. Although the applied magnetic field does not exceed 80 G, several hyperfine levels of the excited state show a substantial deviation from the linear Zeeman effect.
We have investigated atomic coherence effects in the ladder scheme of the triplet transitions 2S-2P-3S of helium. A powerful dressing laser at 706 nm in resonance with the 2P-3S transition induces a transparency on the 2S-2P transition at 1083 nm probed by a weak laser. We have characterised this induced transparency also known as Electromagnetically Induced Transparency (EIT), demonstrating the possibility of reaching 100% of bleaching. Also, we have evidenced the presence of the two photon effect on the EIT process by studying its dependence on the mutual probe-dress polarisation angle. With such a simple level scheme a quantitative comparison with the theory has been made possible and many of the predicted effects have been investigated. Furthermore, polarisation of the dressing laser field has allowed for the first time the study of the electromagnetically induced birefringence which causes a change in the probe laser polarization. The experiments were performed not only in the clean environment of a metastable atomic beam but also in a weak radiofrequency discharge; this configuration could be suggestive for non inverted gain phenomena through incoherent population of excited levels.