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.
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.
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.
Direct relaxation within the optically excited
E(2E) level of 700-ppm ruby is investigated in magnetic fields up to 65 kG making an angle to the
c axis. At low pump intensities, the experimental relaxation rates are found to be in excellent agreement with the theory by Blume et al., predicting a fifth-power dependence on field and a ${cos}^{3}\ensuremath{\theta}{sin}^{2}\ensuremath{\theta}$ dependence on orientation. The observed proportionality constant of 4.7 \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}19}$ ${\mathrm{G}}^{\ensuremath{-}5}$${\mathrm{s}}^{\ensuremath{-}1}$ is close to the theoretical estimate based on static experiments in ruby. When increasing optical pumping, the phonon-bath conduction is demonstrated to become the limiting process due to resonant trapping of the phonons. Following pulsed excitation, a prolonging of the decay time by a factor up to 13 is attained at excited-state concentrations ${N}^{*}\ensuremath{\sim}2\ifmmode\times\else\texttimes\fi{}{10}^{17}/{\mathrm{cm}}^{3}$. In stationary experiments, a nonlinear increase with
N∗ is observed of the equilibrium population of the upper Zeeman level of
E to over a decade above the nonbottlenecked value, corresponding to bottlenecking factors of over \ensuremath{\sim}200. Under the assumption of a constant phonon lifetime of the order of the flight time through the illuminated region, rate equations appear to consistently account for the experiments. A Gaussian-type resonance line, fully homogenized supposedly by magnetic interactions with the surrounding Al nuclei and
Cr3+ ions, explains the lattice-bath equilibration. Finally, the prospect is discussed of using the
E direct transition in ruby as a tunable phonon generator and detector with bandwidth 0.002 ${\mathrm{cm}}^{\ensuremath{-}1}$ in the practical frequency range of 2-15 ${\mathrm{cm}}^{\ensuremath{-}1}$.
Alignment of Cr3+ in ruby has been accomplished by optically pumping a ruby sample with circularly polarized radiation, 6943 Å, from a high-power ruby laser. Alignment of Cr3+ is detected by the emf induced in a coil surrounding the ruby sample. Measurements of the percentage alignment and relaxation time of Cr3+ are obtained from oscillograms of the induced emf and of the circularly polarized radiation from the ruby laser. At room temperature the efficiency of alignment is several tens of percent and the relaxation time of Cr3+ is about 0.1 μsec.