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

Influence of plasma parameters on optical coherence in a forward scattering signal

Emilia A. Alipieva, Svetlana Karabasheva

Published Dec 27, 1996• 0 citations
Publisher

Abstract

We report magneto-optical rotation due to the interaction of atoms with a resonance laser beam. Our experiment is performed on the excited He states, 23P-33D transition, (lambda) equals 587,56nm. Two processes are responsible for the magneto-optical activity: the Zeeman coherences, induced in the absorbing level, and the optical pumping processes, each of them due to the laser beam excitation. The Zeeman coherence creates peaks with a width determined by the coherence relaxation rate, while the optical pumping creates a structure with a width in the order of the natural width of the transition. Due to the difference between the relaxation rates of the 2P and 3D He levels, the two parts of signal are well resolved and depend on the He pressure in different ways. The narrow coherence signal disappears at high He pressure, unless the amplitude of the optical pumping signal increases. The optical pumping signals saturates as He pressure increased and changes its sign. At high gas pressures, such that only the optical pumping signal exists, an increase of the He pressure leads to the narrowing of both the 'classical parabolic dependence' and the optical pumping signal.

DOI
10.1117/12.262933

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Experimental investigation of hidden alignment in Ne plasma using linear laser magneto-optical spectroscopy

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Summary form only given, as follows. We investigated the hidden alignment of Ne plasma in a magnetic field observing the optical characteristics of the medium-absorption and refraction. A tunable single frequency laser beam was used as a probe beam. Depending on the direction of the applied magnetic field and the geometry of observation, the dependence of the plasma birefringence or dichroism on the magnetic field was registered. The dye laser was tuned to /spl lambda/=607.4 nm (2p/sub 3/-1s/sub 4/ Ne transition). The dependence on the external magnetic field of the laser beam intensity passed through the Ne discharge cell was registered, for various laser frequencies in the limits of the absorption line. At the edge of the absorption line, the signal of the dichroism consists of two Lorentzian components while at the center of the absorption line the wide component is absent. The wide component width depends on neon pressure as: H(P)=(1.64/spl plusmn/0.03) gauss + P(torr) (0.33/spl plusmn/0.08) gauss/torr. This value is in a good agreement with other results concerning the lifetime of the 1s/sub 4/ state. The mechanism of alignment formation in this state is not clear. This state could not be aligned by light absorption following a quantitative calculation. The irradiation of atoms is isotropic within the limits of all the absorption line because of the high magnitude of absorption for the 1s/sub 4/-1s/sub 0/ transition. We consider the optical pumping as the most probable mechanism for alignment of this state. The magnitude of absorption on the s-p neon transitions are not high. The intensity of the spontaneous emission beam along the tube is greater than in the perpendicular direction. If the quantization axis coincides with the tube axis, this beam empties the magnetic sublevels with M=/spl plusmn/1, thus creating longitudinal alignment. The absence of a signal at the center of the absorption line can be explained by the high selfabsorption on the s-p transitions. The width of the narrow component of the signal is approximately 0.7 gauss. Its pressure dependence is small. We have established experimentally the absence of signals of such width in the spontaneous emission from 2p levels. Hence, all optical mechanisms of alignment transmission from higher or from lower states to the 1s/sub 4/ state are excluded. Apparently this signal arises from alignment of metastable state 1s/sub 5/. In our opinion the collisions are the most probable mechanism of alignment transmission.

Trans Black Sea Region Symposium on Applied Electromagnetism•Nov 11, 2002•0 citations

Saturation resonances by magnetic mode crossing in optical pumping with a multimode gas laser

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Magneto-optical control of coherent nonlinear processes

Paul S. Hsu

Laser-atom interactions create atomic coherence and large nonlinear atomic polarization.\nWe investigate resonant laser-atom interactions to generate large nonlinearities\nand control them using magneto-optical fields. Coherent control of high-order\nsusceptibilities and magneto-optical rotation are demonstrated. Experiments are supported\nby theoretical studies that effectively describe the observed phenomena.\nIt is shown that a new coherent field, with polarization orthogonal to a weak\nsignal field, can be parametrically generated via an all-resonant four-wave-mixing\nprocess. This is demonstrated in a double-ladder system having two intermediate\nstates between a ground and an excited state. It is shown that the parametricgeneration\nprocess can be coherently controlled by coupling lasers and magnetic fields.\nIt is theoretically established that the underlying physics is a resonant three-photon\nprocess with a wide domain of control parameters.\nElectromagnetically induced transparency (EIT), where absorption of a weak\nprobe is suppressed via quantum interference, is demonstrated in a usual three-level\nladder system. It is observed that in contrast with EIT in a usual ladder system,\naddition of a second channel helps to suppress the absorption of two weak probe\nfields in the double-ladder system. The resulting enhancement of transmission in two\ndifferent channels is due to gain caused by three-photon processes.\nCoherent control is strongly limited by coherence lifetime, which is the inverse of\nthe dephasing rate. A lambda-system, having two ground states coupled to a common\nexcited state by lasers, can generate a new eigen (dark)-state that is transparent to incoming fields and hence suppresses fluorescence. However, ground-state dephasing\nperturbs the dark state. A new method for measuring the ground-state dephasing\nrate from fluorescence signals is proposed and a proof-of-principle experiment demonstrated.\nWhile two laser fields in a lambda-system are resonant with their respective\ntransitions, the atomic polarizations are very sensitive to an applied magnetic field.\nThis effect can be used for optical magnetometry. The degree of sensitivity of the\nmagnetometer is determined by two competing parameters–atomic density and laser\nintensity. It is shown experimentally that the optimal sensitivity reaches saturation,\nwhich is contrary to the idea that sensitivity increases indefinitely with an increase\nin the above parameters.

•Jan 1, 2008•0 citations
We report magneto-optical rotation due to the interaction of atoms with a resonance laser beam. Our experiment is performed on the excited He states, 23P-33D transition, (lambda) equals 587,56nm. Two processes are responsible for the magneto-optical activity: the Zeeman coherences, induced in the absorbing level, and the optical pumping processes, each of them due to the laser beam excitation. The Zeeman coherence creates peaks with a width determined by the coherence relaxation rate, while the optical pumping creates a structure with a width in the order of the natural width of the transition. Due to the difference between the relaxation rates of the 2P and 3D He levels, the two parts of signal are well resolved and depend on the He pressure in different ways. The narrow coherence signal disappears at high He pressure, unless the amplitude of the optical pumping signal increases. The optical pumping signals saturates as He pressure increased and changes its sign. At high gas pressures, such that only the optical pumping signal exists, an increase of the He pressure leads to the narrowing of both the 'classical parabolic dependence' and the optical pumping signal.
Summary form only given. The resonance laser interaction with an atomic system with degenerate levels creates coherent properties in the medium. Depending on the laser polarization and the experimental geometry it could be alignment, orientation or optical pumping. This coherence could be destroyed by application of a magnetic field, which removes the level degeneracy. This process has an effect on the optical characteristics of the medium, such as absorption and fluorescence from the levels involved. They show the resonance dependence on the applied magnetic field with widths, depending on /spl gamma//sub a/, /spl gamma//sub b/, and /spl gamma//sub ab/, where /spl gamma//sub I/ is the width of the upper or lower state of the transition, resonant to the laser beam, /spl gamma//sub ab/ is the width of the optical transition a/spl rarr/b. Using different experimental geometry, /spl gamma//sub a/, /spl gamma//sub b/, and /spl gamma//sub ab/ could be measured, as well as some rate constants of these levels. We investigated the coherent properties of the 2/sup 3/ P/spl rarr/3/sup 3/ D transition of He I under resonant laser radiation. Due to the peculiarities of the coherent signals, registered in the lateral and forward direction the cross-section for velocity selective collisions for the 3/sup 3/ D He I level were obtained. The influence of the strong laser field on the widths and shapes of the signals was determined. The cross-section, for depolarizing collisions of the 3/sup 3/ D level was measured, too.
We study numerically the nonlinear magneto-optical rotation of polarization (NMOR) of the laser pulse during its propagation through a cold Rb cloud with induced Zeeman coherences and electromagnetically induced transparency. Evolution of NMOR is calculated by solving the Maxwell–Bloch equations. We consider a linearly polarized Gaussian pulse with different pulse peak amplitudes and widths. For an intensity peak of 5 mW cm−2 and full-width at half-maximum of 10 μs, transient behaviour of NMOR does not follow the pulse intensity variation: NMOR begins to decrease as the pulse's intensity nears its peak value. When the pulse has a smaller peak intensity, NMOR behaves qualitatively differently: the angle of rotation constantly increases during the pulse propagation. Observed differences are explained by the optical pumping into the dark state and the behaviour of the ground-state coherences subjected to coherent population trapping. The same pulse intensity, from different sides of the pulse, during rising and falling sides, produces qualitatively different NMOR shapes, its amplitudes and widths which result can be explained by the successive excitation of atoms during the pulse propagation. It was shown that increasing the relaxation rates of the ground-state coherences, shifts the maximum of the NMOR to higher magnetic field, while the atomic density strongly influences the magnitude of the NMOR.
We present, experimentally and theoretically, nonlinear magneto-optical rotation (NMOR) using two spatially separated laser beams, the pump laser beam for creation and the probe laser beam for detection of the coherence between ground Zeeman sublevels. Both pump and probe lasers are tuned to ${F}_{g}=2\ensuremath{\rightarrow}{F}_{e}=1$ transition in 87Rb^{87}\text{R}\text{b}87Rb. With the specially designed spatial configuration of the pump and the probe beams we were able to obtain dispersively shaped probe laser NMOR resonances in the Rb vacuum gas cell. The theory indicates that the obtained line shapes are due to the interference between prepared atomic states and the probe laser field. The interference nature of the resonances is supported by narrowing of NMOR resonances with increased separation between laser beams, and by the shift of the central fringe from the zero magnetic field with increased angle between initial orientation of electric vectors of linearly polarized pump and probe beams.
Summary form only given, as follows. We investigated the hidden alignment of Ne plasma in a magnetic field observing the optical characteristics of the medium-absorption and refraction. A tunable single frequency laser beam was used as a probe beam. Depending on the direction of the applied magnetic field and the geometry of observation, the dependence of the plasma birefringence or dichroism on the magnetic field was registered. The dye laser was tuned to /spl lambda/=607.4 nm (2p/sub 3/-1s/sub 4/ Ne transition). The dependence on the external magnetic field of the laser beam intensity passed through the Ne discharge cell was registered, for various laser frequencies in the limits of the absorption line. At the edge of the absorption line, the signal of the dichroism consists of two Lorentzian components while at the center of the absorption line the wide component is absent. The wide component width depends on neon pressure as: H(P)=(1.64/spl plusmn/0.03) gauss + P(torr) (0.33/spl plusmn/0.08) gauss/torr. This value is in a good agreement with other results concerning the lifetime of the 1s/sub 4/ state. The mechanism of alignment formation in this state is not clear. This state could not be aligned by light absorption following a quantitative calculation. The irradiation of atoms is isotropic within the limits of all the absorption line because of the high magnitude of absorption for the 1s/sub 4/-1s/sub 0/ transition. We consider the optical pumping as the most probable mechanism for alignment of this state. The magnitude of absorption on the s-p neon transitions are not high. The intensity of the spontaneous emission beam along the tube is greater than in the perpendicular direction. If the quantization axis coincides with the tube axis, this beam empties the magnetic sublevels with M=/spl plusmn/1, thus creating longitudinal alignment. The absence of a signal at the center of the absorption line can be explained by the high selfabsorption on the s-p transitions. The width of the narrow component of the signal is approximately 0.7 gauss. Its pressure dependence is small. We have established experimentally the absence of signals of such width in the spontaneous emission from 2p levels. Hence, all optical mechanisms of alignment transmission from higher or from lower states to the 1s/sub 4/ state are excluded. Apparently this signal arises from alignment of metastable state 1s/sub 5/. In our opinion the collisions are the most probable mechanism of alignment transmission.
The fluorescence light emitted by atoms (Ne) optically pumped with a multimode laser beam exhibits a resonance each time the Zeeman splitting is equal to the frequency difference between two modes. The experimental study shows that these resonances are very sensitive to the relative phase of modes and that their width is approximately equal to the Hanle effect width. A theoretical interpretation is given with the help of a perturbation calculation. Two phenomena produce resonances : 1) A Population Effect (PE) which is resonant when the σ + component of one mode and the σ- component of another interact with atoms of same velocity (hole crossing). The width of these resonances is 2 Γ'ab (natural width broadened by collisions). 2) A Zeeman Coherence Effect (ZCE) : the action of a pair of modes (ν, σ +) and (μ, σ-) produces (at 2nd order) transverse alignment modulated at the beat frequency ων — ω μ and resonant when the Zeeman splitting is equal to that frequency ; by interaction with a second pair of modes (λ, σ-) and (κ, σ+) (ων —ω μ + ωλ — ωκ = 0), this alignment produces unmodulated longitudinal quantities (4th order) which exhibit, as the magnetic field is scanned, a resonance whose width is close to that of the Hanle curve (Γ b(2)). In our experimental conditions (close modes : Γ'ab≳ Δω > Γ b(2)), the resonances of the first kind are not resolved and the observed resonances are only of the second type's which arevery sensitive to the phases of modes. We also discuss the possibility of using these resonances to measure Landé g factors.
Laser-atom interactions create atomic coherence and large nonlinear atomic polarization.\nWe investigate resonant laser-atom interactions to generate large nonlinearities\nand control them using magneto-optical fields. Coherent control of high-order\nsusceptibilities and magneto-optical rotation are demonstrated. Experiments are supported\nby theoretical studies that effectively describe the observed phenomena.\nIt is shown that a new coherent field, with polarization orthogonal to a weak\nsignal field, can be parametrically generated via an all-resonant four-wave-mixing\nprocess. This is demonstrated in a double-ladder system having two intermediate\nstates between a ground and an excited state. It is shown that the parametricgeneration\nprocess can be coherently controlled by coupling lasers and magnetic fields.\nIt is theoretically established that the underlying physics is a resonant three-photon\nprocess with a wide domain of control parameters.\nElectromagnetically induced transparency (EIT), where absorption of a weak\nprobe is suppressed via quantum interference, is demonstrated in a usual three-level\nladder system. It is observed that in contrast with EIT in a usual ladder system,\naddition of a second channel helps to suppress the absorption of two weak probe\nfields in the double-ladder system. The resulting enhancement of transmission in two\ndifferent channels is due to gain caused by three-photon processes.\nCoherent control is strongly limited by coherence lifetime, which is the inverse of\nthe dephasing rate. A lambda-system, having two ground states coupled to a common\nexcited state by lasers, can generate a new eigen (dark)-state that is transparent to incoming fields and hence suppresses fluorescence. However, ground-state dephasing\nperturbs the dark state. A new method for measuring the ground-state dephasing\nrate from fluorescence signals is proposed and a proof-of-principle experiment demonstrated.\nWhile two laser fields in a lambda-system are resonant with their respective\ntransitions, the atomic polarizations are very sensitive to an applied magnetic field.\nThis effect can be used for optical magnetometry. The degree of sensitivity of the\nmagnetometer is determined by two competing parameters–atomic density and laser\nintensity. It is shown experimentally that the optimal sensitivity reaches saturation,\nwhich is contrary to the idea that sensitivity increases indefinitely with an increase\nin the above parameters.