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.
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.
The magnetization of ruby in a magnetic field at 300\ifmmode^\circ\else\textdegree\fi{}K is changed when the ruby is optically pumped with linearly polarized radiation from a
Q-switched ruby laser. With a magnetic field parallel to the trigonal axis, laser light polarized parallel to this axis induces transitions from the spin \ifmmode\pm\else\textpm\fi{}\textonehalf{} levels of the
4A2 ground state to the levels of the
E(2E) excited state. The change in
Mz is linearly proportional to
H except near the anticrossing points of the
4A2 spin levels at 2.07 and 4.14 kG. At these field strengths, there is an enhancement of the magnetization caused by state mixing of the
4A2 wave functions. When
H has a small component perpendicular to the
z axis, a magnetization is detected parallel to this component in the vicinity of 4.14 kG. The effect requires a long ground-state relaxation time. In a separate experiment, the relaxation of
Mz was found to vary from 0.13 \ensuremath{\mu}sec in zero magnetic field to a constant value of 0.57 \ensuremath{\mu}sec for fields above 60 G.
Bichromatic excitation of the
R1 line in ruby at liquid-helium temperature is considered. The frequency difference of the spectral components of the driving field is resonant with the energy gap between the anticrossed spin sublevels
+1/2 and
+3/2 in the ground state
4A2. The condition of the spin coherence trapping in the nonabsorbing state is analyzed. Population trapping is revealed in the reduced spontaneous emission from the excited state and in large spin magnetization of the ground state
Cr3+ precessing with the frequency difference of the spectral components of the driving field. This magnetization can be detected by a pickup coil. The lifetime of the induced magnetization is strongly dependent on the ratio of the lifetime of the excited state $\ifmmode \bar{E}\else \={E}\fi{}$ and the spin coherence decay time
(T2)spin.Sharp resonances in optical excitation of ruby by laser pulses are observed as Zeeman sublevels anticross in an external magnetic field. For the
R1 optical line of ruby
(694.3nm) excitation drops when the external magnetic field strength approaches
4.14kG while for the
R2 line
(692.9nm) it increases in
4.14 and
2.07kG fields. A simple theoretical description of the observed effects on the basis of level mixing explains both situations.
The behavior of photon echoes near level anticrossings is described in terms of the magnetic-field-dependent mixing parameters. The general features of level-crossing behavior are obscured for most cases in dilute ruby and are dominated instead by strong spin-dependent decay resulting from Cr-Al interactions. However, new experiments at short time separation ($\ensuremath{\tau}=32$ nsec) between pulses have permitted observation of the predicted photon-echo anticrossing signals in ruby, and may be applied to other atomic systems.