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Journal of Physics B Atomic and Molecular Physics

Theory and observation of Faraday rotation obtained with strong light fields

I O G Davies, P. E. G. Baird, J L Nicol

Published Oct 28, 1987• 30 citations
Publisher

Abstract

The authors have observed dramatic changes in both the magnitude and the lineshape of Faraday rotation on an allowed E1 transition in Sm I (4f 6 6s 2 7 F 0 -4f 6 6s6p 7 G 1 ) as a function of laser intensity and buffer-gas pressure. The effect is ascribed to the development of optical and Zeeman coherences and a theoretical treatment, based on the solution of steady-state density matrix equations is given. Effects of optical pumping on the odd isotopes 147 Sm and 149 Sm, both with I=7/2, are also considered.

DOI
10.1088/0022-3700/20/20/015

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Results of a numerical study of the Faraday effect arising upon propagation of the light beams with the frequencies ω L1 (resonant to the nS 1/2-nP 1/2, 3/2 transitions) and ω L2 (resonant to the nP 1/2, 3/2-(n+2)S 1/2 transitions) through alkali-metal vapors are presented. Characteristics of the magneto-optical rotation spectra at each of the frequencies are strongly affected by the second intense radiation field resonant to the adjacent transition. When the atoms interact with two strong light waves, resonant to adjacent transitions, and with a magnetic field, the shape of the Faraday rotation spectra depends on the energy shifts of the atomic states that arise due to the dynamic Stark effect and the Zeeman effect (the Paschen-Back or an intermediate-type effect), as well as due to the difference of populations of these states caused by the interaction of the atoms with the fields. The results obtained show that in the frequency selection method, based on the resonance Faraday effect, the frequency of the generated narrow-band beam can be tuned by the intensity of the strong wave, resonant to the transition between the excited states.

Optics and Spectroscopy•Apr 1, 2002•1 citation
The authors have observed dramatic changes in both the magnitude and the lineshape of Faraday rotation on an allowed E1 transition in Sm I (4f 6 6s 2 7 F 0 -4f 6 6s6p 7 G 1 ) as a function of laser intensity and buffer-gas pressure. The effect is ascribed to the development of optical and Zeeman coherences and a theoretical treatment, based on the solution of steady-state density matrix equations is given. Effects of optical pumping on the odd isotopes 147 Sm and 149 Sm, both with I=7/2, are also considered.
New results demonstrating the dramatic changes to the form and magnitude of the resonant Faraday rotation with laser intensity, buffer gas pressure and magnetic field strength are presented for the transitions 4f 6 6s 2 7 F 0 -4f 6 6s6p 5 D 1 at 621 nm and 4f 6 6s 2 7 F 1 -4f 6 6s6p 5 D 0 at 654 nm. For both transitions, exact analytical solutions to the steady-state density matrix equations are possible and have been used. The experimental results show good qualitative agreement with the theoretical predictions. In the case of the 654 nm line Zeeman coherence occurs in the ground state and can be achieved at lower laser intensities than for the 621 nm line under comparable conditions.
The authors have used a sensitive laser polarimeter to measure the electric-field-induced dichroism on the transition 4f 6 6s 2 7 F 0 -4f 6 6s6p 7 G 1 of Sm I at lambda 639 nm. The dichroism gives an apparent (pseudo) optical rotation which can be sensitively detected using a Faraday modulator in the polarimeter. The results are interpreted in terms of the electric field coupling between the upper state of the transition and the nearby 4f 6 6s5d 7 G 1 level which lies only 322 GHz away. The value of the Stark matrix element mod (4f 6 6s5d 7 G 1 mod mod er mod mod 4f 6 6s6p 7 G 1 ) mod has been determined to be 0.85 (5) ea 0 , in broad agreement with theoretical estimates. Effects due to optical pumping and saturation caused by increased laser power have been observed and the theory of these phenomena is briefly reviewed. The transition 4f 6 6s 2 7 F 0 -4f 6 6s5d 7 G 1 has also been detected by laser polarimetry using electric field enhancement.
The Faraday rotation spectra were observed at 810.4 nm of the 5s 12–5p 22 transition in atomic krypton using a single mode diode laser. The lineshapes at the resonance were changed largely with laser intensity. At a low laser intensity the lineshape is a normal form of the differential of a dispersion curve. As the laser intensity increases, a Gaussian curve appears, overlapping the normal curve. At a sufficiently high laser intensity, the Gaussian component dominates the latter. The spectral change is ascribed to the development of optical and Zeeman coherence.
Theoretical and experimental studies of a gas laser amplifier are presented, assuming the amplifier is operating with a saturating optical frequency signal. The analysis is primarily concerned with the effects of the gas pressure and the presence of an axial magnetic field on the characteristics of the amplifying medium. Semiclassical radiation theory is used, along with a density matrix description of the atomic medium which relates the motion of single atoms to the macroscopic observables. A two-level description of the atom, using phenomenological source rates and decay rates, forms the basis of our analysis of the gas laser medium. Pressure effects are taken into account to a large extent through suitable choices of decay rate parameters. Two methods for calculating the induced polarization of the atomic medium are used. The first method utilizes a perturbation expansion which is valid for signal intensities which barely reach saturation strength, and it is quite general in applicability. The second method is valid for arbitrarily strong signals, but it yields tractable solutions only for zero magnetic field or for axial magnetic fields large enough such that the Zeeman splitting is much larger than the power broadened homogeneous linewidth of the laser transition. The effects of pressure broadening of the homogeneous spectral linewidth are included in both the weak-signal and strong-signal theories; however the effects of Zeeman sublevel-mixing collisions are taken into account only in the weak-signal theory. The behavior of a He-Ne gas laser amplifier in the presence of an axial magnetic field has been studied experimentally by measuring gain and Faraday rotation of linearly polarized resonant laser signals for various values of input signal intensity, and by measuring nonlinearity - induced anisotropy for elliptically polarized resonant laser signals of various input intensities. Two high-gain transitions in the 3.39-μ region were used for study: a J = 1 to J = 2 (3s2 → 3p4) transition and a J = 1 to J = 1 (3s2 → 3p2) transition. The input signals were tuned to the centers of their respective resonant gain lines. The experimental results agree quite well with corresponding theoretical expressions which have been developed to include the nonlinear effects of saturation strength signals. The experimental results clearly show saturation of Faraday rotation, and for the J = 1 t o J = 1 transition a Faraday rotation reversal and a traveling wave gain dip are seen for small values of axial magnetic field. The nonlinearity induced anisotropy shows a marked dependence on the gas pressure in the amplifier tube for the J = 1 to J = 2 transition; this dependence agrees with the predictions of the general perturbational or weak signal theory when allowances are made for the effects of Zeeman sublevel-mixing collisions. The results provide a method for measuring the upper (neon 3s2) level quadrupole moment decay rate, the dipole moment decay rates for the 3s2 → 3p4 and 3s2 → 3p2 transitions, and the effects of various types of collision processes on these decay rates.
The Faraday rotation spectra in the Rabi regime were studied in krypton to investigate the behavior of a Zeeman coherence effect. At a laser intensity of 2.5 mW and a magnetic field of 3.4 mT, we determined a single-peaked line profile having a Gaussian form. Using the density matrix formalism, we have obtained an analytical form for the line shape and compared it with the observed ones. It is found that the line profile at a low magnetic field is mainly the contribution of the Zeeman coherence effect.
Results of a numerical study of the Faraday effect arising upon propagation of the light beams with the frequencies ω L1 (resonant to the nS 1/2-nP 1/2, 3/2 transitions) and ω L2 (resonant to the nP 1/2, 3/2-(n+2)S 1/2 transitions) through alkali-metal vapors are presented. Characteristics of the magneto-optical rotation spectra at each of the frequencies are strongly affected by the second intense radiation field resonant to the adjacent transition. When the atoms interact with two strong light waves, resonant to adjacent transitions, and with a magnetic field, the shape of the Faraday rotation spectra depends on the energy shifts of the atomic states that arise due to the dynamic Stark effect and the Zeeman effect (the Paschen-Back or an intermediate-type effect), as well as due to the difference of populations of these states caused by the interaction of the atoms with the fields. The results obtained show that in the frequency selection method, based on the resonance Faraday effect, the frequency of the generated narrow-band beam can be tuned by the intensity of the strong wave, resonant to the transition between the excited states.