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Physical Review A

Level crossing of resonant third-harmonic generation in I2

C. Tai

Published Jul 1, 1981• 4 citations
Publisher

Abstract

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.

DOI
10.1103/physreva.24.294

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Journal of Physics B Atomic Molecular and Optical Physics•Sep 25, 2001•3 citations
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.
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 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.
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.
We analyze second harmonic generation in sodium vapor in a transverse magnetic field induced by the two-photon resonant excitation of the 3S-3D transition. The phenomenon is explained by the rotation of the two-photon induced quadrupole moment in the magnetic field. The analysis explains well the saturation of the intensity and rotation of the polarization of the second harmonic radiation in the higher magnetic field ≳0.5{\gtrsim}0.5≳0.5 kG. The collisional decay rate of the coherence of the E1 forbidden transition has been measured as an application of this SHG.
Application of a steady magnetic field allows sum-frequency radiation generated via dipole-forbidden mixing of two-colour fields in an atomic medium to be emitted in the (otherwise forbidden) forward direction. As a result of resonance tuning at low-field strength (not exceeding 0.1 T) a fourth-order magnetic-dipole-induced amplitude for the sum-frequency process can be made comparable to the third-order field-free amplitude. We show that the interference between different terms of the field-dependent amplitude may cause the circular dichroism of the forward-scattering cross section for one of the two incident waves or the sum-frequency wave when the other two waves are linearly polarized. The dichroism effect is due to the dissipation processes determined by the resonant level width Γ and depends strongly both on Γ and the resonance detuning Δ. The effect may be used to control the frequency conversion process or to make possible a frequency mixing laser spectroscopy of excited atomic levels.