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Journal of the Physical Society of Japan

Optical Excitation of Zeeman Coherence in Ruby

Y. Takagi, Yukio Fukuda, Koichi Yamada, Tsuneo Hashi

Published Aug 1, 1981• 16 citations
Publisher

Abstract

Optically induced Zeeman coherences between various pairs of sublevels in the ground state 4 A 2 of a sample ruby (0.05 wt.%) were observed at room temperature by using impact excitation with circularly polarized resonant light pulses from a mode-locked ruby laser. The precessing magnetization associated with Zeeman coherence was detected magnetically in UHF, VHF and X-band microwave regions. The observed signals can be regarded as various types of ESR free induction decay signals and the decay times were of the order of nanosecond. Sustained precession signals were also observed. Characteristics of the signals were explained by numerical solutions of the Liouville equation.

DOI
10.1143/jpsj.50.2672

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Optically induced Zeeman coherences between various pairs of sublevels in the ground state 4 A 2 of a sample ruby (0.05 wt.%) were observed at room temperature by using impact excitation with circularly polarized resonant light pulses from a mode-locked ruby laser. The precessing magnetization associated with Zeeman coherence was detected magnetically in UHF, VHF and X-band microwave regions. The observed signals can be regarded as various types of ESR free induction decay signals and the decay times were of the order of nanosecond. Sustained precession signals were also observed. Characteristics of the signals were explained by numerical solutions of the Liouville equation.
Using excitation by ultrashort light pulses from a mode-locked ruby laser, we have observed optically induced magnetization precessing at a frequency corresponding to the energy separation near the level anticrossing points of the ground state 4 A 2 in ruby at room temperature. The motion of the magnetization is detected by a pick-up coil. The decay time of the signal is 5∼6 ns. The analysis is made by using the model of fictitious spin 1/2 and the agreement with the experiment is satisfactory.
A time varying macroscopic magnetization of ruby is observed at room temperature by selectively exciting the Cr 3+ ions with a right- and a left circularly polarized light of a Q -switched ruby laser from one of the ground 4 A 2 multiplets to a proper component of the upper \bar E ( 2 E ) multiplets. Dependence of such a magnetization in the Z -direction on the irradiation power, the ellipticity and the magnetic field was measured with a pick-up coil. The signal had a value of 1.7×10 -4 emu/g at the irradiation of 67 mJ. The relaxation time of the ground state was estimated from the decay to be about 180 ns in zero magnetic field. The signal changed with the ellipticity of the exciting light, and the amplitude of the signal was always proportional to the intensity of the excitation. These experimental results were quantitatively explained by a theoretical analysis for the time varying optically induced magnetization.
Sharp resonances in optical excitation of ruby by laser pulses are observed as Zeeman sublevels anticross in an external magnetic field. For the R1{R}_{1}R1​ optical line of ruby (694.3nm)(694.3\phantom{\rule{0.3em}{0ex}}\mathrm{nm})(694.3nm) excitation drops when the external magnetic field strength approaches 4.14kG4.14\phantom{\rule{0.3em}{0ex}}\mathrm{kG}4.14kG while for the R2{R}_{2}R2​ line (692.9nm)(692.9\phantom{\rule{0.3em}{0ex}}\mathrm{nm})(692.9nm) it increases in 4.144.144.14 and 2.07kG2.07\phantom{\rule{0.3em}{0ex}}\mathrm{kG}2.07kG fields. A simple theoretical description of the observed effects on the basis of level mixing explains both situations.
Coherent population trapping (CPT) in a three-level atomic medium pumped by two subsequent short optical pulses is considered under the condition of negligible population decay from the excited optical state. It is shown that the amount of atomic population transferred to the excited state by the combined action of the pulses strongly depends on the phase of the ground-state coherence excited by the first pulse at the arrival time of the second pulse. Oscillatory behavior of optical excitation efficiency on the time delay between the pulses is predicted. It is also shown that saturating optical pulses can produce population inversion in a resonantly pumped quasi-two-level system. A class of solid materials in which the predicted phenomena can be observed at room temperature is found. It includes some rare-earth and transition-metal doped dielectric crystals where Orbach relaxation between ground-state Zeeman states is suppressed: ruby, alexandrite, and several others. On the basis of the theoretical predictions, experimental observation of Ramsey fringes in CPT spectrum of ruby is reported.
We report detailed studies of freely evolving Zeeman coherence (Zeeman quantum beats) observed after optically pumping a gas of Na atoms and suddenly turning off the light field. This Zeeman coherence corresponds to a macroscopic magnetization that precesses in an external magnetic field; it is detected with a cw optical probe beam by using polarization-selective detection of the transmitted light. The amplitude and the phase of the signal show a pronounced variation with laser intensity, laser detuning, and the strength of the magnetic field. This dependence was measured on the 3s2S1/2 ground state of sodium, using the D1 line for optical excitation and detection. The experimental data are compared with theoretical predictions based on a Bloch-type equation of motion for an optically driven spin system in a J = 1/2 ground state.