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Physical Review A

Resonant third-harmonic generation in I2 with circularly polarized light

Chen-Yu Tai

Published Dec 1, 1983• 6 citations
Publisher

Abstract

Magnetic-field-induced resonant third-harmonic generation in ${\mathrm{I}}_{2}$ with circularly polarized light is studied. With a magnetic field perpendicular to the propagating direction of the light, the axial symmetry is broken and the angular momentum is no longer conserved in such a system, and the third harmonic may be generated under such conditions. Quantum beats in photon-echo-like third-harmonic generation with nonsynchronous laser pulses are also studied. The intensity of the harmonic generated may be interpreted in terms of the interference between a molecule and itself as well as the interference between different molecules. The gyromagnetic ratio of the resonant states is measured.

DOI
10.1103/physreva.28.3459

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The intensity of the resonant third-harmonic generation in ${\mathrm{I}}_{2}$ vapor is found to decrease with increasing strength of an external magnetic field even when the exciting light is polarized parallel to the magnetic field. This effect is explained with a theory based on the interference between different molecules. The gyromagnetic ratio of the excited state is determined in this work. This technique may also be applied to study the mechanism of the various processes of saturation in resonant wave mixing.

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Physical Review•Jan 7, 1966•51 citations
Magnetic-field-induced resonant third-harmonic generation in I2{\mathrm{I}}_{2}I2​ with circularly polarized light is studied. With a magnetic field perpendicular to the propagating direction of the light, the axial symmetry is broken and the angular momentum is no longer conserved in such a system, and the third harmonic may be generated under such conditions. Quantum beats in photon-echo-like third-harmonic generation with nonsynchronous laser pulses are also studied. The intensity of the harmonic generated may be interpreted in terms of the interference between a molecule and itself as well as the interference between different molecules. The gyromagnetic ratio of the resonant states is measured.
The intensity of the resonant third-harmonic generation in I2{\mathrm{I}}_{2}I2​ vapor is found to decrease with increasing strength of an external magnetic field even when the exciting light is polarized parallel to the magnetic field. This effect is explained with a theory based on the interference between different molecules. The gyromagnetic ratio of the excited state is determined in this work. This technique may also be applied to study the mechanism of the various processes of saturation in resonant wave mixing.
The gyromagnetic ratio of several states in I2{\mathrm{I}}_{2}I2​ is studied with the technique of polarization rotation and level crossing in doubly resonant third-harmonic generation. The level-crossing curve has a linewidth of approximately the Doppler linewidth of the corresponding non-coherent process divided by the angular momentum quantum number. In the polarization-rotation work, a rotated angle 80\ifmmode^\circ\else\textdegree\fi{} is observed with little depolarization. The ratio of the angle rotated to the magnetic field of some 20 resonances can be divided into two groups: large ($\frac{\ensuremath{\Delta}\ensuremath{\theta}}{\ensuremath{\Delta}H}\ensuremath{\sim}10$\ifmmode^\circ\else\textdegree\fi{}/kG) and small ($\frac{\ensuremath{\Delta}\ensuremath{\theta}}{\ensuremath{\Delta}H}<2$\ifmmode^\circ\else\textdegree\fi{}/kG). A theory based on the interference between the different molecules is presented to explain the experimental data. The observed differences in $\frac{\ensuremath{\Delta}\ensuremath{\theta}}{\ensuremath{\Delta}H}$ is interpreted in terms of the difference in the transition branch in the one- and two-quantum transition. The gyromagnetic ratio of the ground state as determined by this technique is approximately -6 and that of the two-quantum excited state is approximately -20.
A technique of third-harmonic generation (THG) with nonsynchronous pulses is applied to study the dynamics of resonant wave mixing. A circularly polarized laser pulse is focused into a cell containing I2{\mathrm{I}}_{2}I2​ vapor to create the coherent dipole moments. An oppositely circularly polarized pulse is then focused into the same region to generate the third harmonic. Since no THG is produced by any individual pulse, correlated THG can be easily measured without using any fast detection scheme. A theory is presented to explain the measured coherent lifetime of approximately 1 ns. The technique of wave mixing with two linearly polarized pulse is applied to study saturation in resonant THG and to measure the homogeneous relaxation rate of the molecular states. This technique is also applied to study the photon echo generating process.
The technique of harmonic generation with circularly polarized light is applied to study the Land\'e ggg factor of the two-quantum excited states of I2{\mathrm{I}}_{2}I2​ molecules. Systematic and abrupt changes in the ggg factor are observed. The large change in the ggg factor is interpreted in terms of perturbation by a level that has no bound state. Enhancement of harmonic generation in a high magnetic field with linearly polarized light is also observed in some resonances that have a small ggg factor in the two-quantum state. The enhancement is interpreted in terms of constructive interference of resonances originating from the ground state and the first thermally excited vibrational state.
We report the application of third-harmonic generation (THG) to study photon-echo-associated coherent molecular states. Two temporally separated circularly polarized laser pulses are focused into a cell containing I(2) vapor to create the coherent dipole moments. A delayed oppositely circularly polarized pulse is then focused into the same region to generate the third harmonic. Since no THG can be produced by any individual pulse, correlated THG may easily be measured without using any fast detection scheme.
The Lamb theory of the optical maser is applied to circularly polarized atomic transitions, and used to consider the beat frequencies and the coherence properties of such orthogonal fields when axial magnetic fields are applied to the gaseous laser. The beat frequency approaches zero in near-zero magnetic fields and synchronization can then occur between the right- and left-handed circularly polarized oscillations. For a resonator with no undue polarization constraint, such a strong coupling gives rise to a linearly polarized output, and to a rotation of the plane of polarization with increasing magnetic field. A self-consistent expression is derived for this rotation under steady-state conditions, and a maximum rotation of $\ifmmode\pm\else\textpm\fi{}\frac{1}{4}\ensuremath{\pi}$ with magnetic field is indicated before the synchronization breaks down and circularly polarized beat phenomena appear. The rotation with magnetic field depends on the laser intensity, on the anisotropy in the cavity losses, and on the position of the cavity resonance within the Doppler linewidth. Also, the angle of rotation is indeterminate unless such anisotropy is present. Other regions of such coherence can occur at higher magnetic fields, where the beat frequency again approaches zero. These depend on the detailed shape of the various dispersion curves of the laser medium. The results derived from the theory used are in general agreement with experimental observations on the 1.153-\ensuremath{\mu} He-Ne laser transition.