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Physical Review

Optically Induced Magnetization in Ruby

Jan P. van der Ziel, N. Bloembergen

Published May 17, 1965• 57 citations
Publisher

Abstract

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 $^{4}A_{2}$ ground state to the levels of the $\overline{E}(^{2}E)$ excited state. The change in ${M}_{z}$ is linearly proportional to $H$ except near the anticrossing points of the $^{4}A_{2}$ 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 $^{4}A_{2}$ 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 ${M}_{z}$ 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.

DOI
10.1103/physrev.138.a1287

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Optically Induced Magnetization in Ruby

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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.

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Optically Induced Precessing Magnetization in Ruby near the Level Anticrossing Points in 4A2

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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.

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Doubly resonant Raman electron paramagnetic transitions in ruby (Al${}_{2}$O${}_{3}$:Cr${}^{3+}$) have been investigated using dye laser excitation tuned across the range of the Zeeman components of its well-known ${R}_{1}$ emission line. With magnetic field B normal to c, the optic axis of ruby, it is seen that the Raman electron paramagnetic resonance (EPR) lines exhibit significant intensity enhancements due to the simultaneous occurrence of ``in resonance'' and ``out resonance,'' as visualized in the Kramers-Heisenberg formalism of inelastic light scattering. The specific Raman EPR features observed in the present study, together with their resonances, however, differ markedly from those observed in our previous investigation with $\mathbf{B}\ensuremath{\parallel}\mathbf{c}$. These differences can be traced to level mixing effects within the Zeeman multiplet of the ${}^{4}{A}_{2}$ (ground) state on the one hand, and the negligible value of ${g}_{\ensuremath{\perp}}$ for the $\mathrm{E\ifmmode \bar{}\else \={}\fi{}}$ (excited) state of the ${R}_{1}$ emission line on the other. Furthermore, we note that with B\ensuremath{\perp}c and under resonant conditions, the experimentally observed \ensuremath{\Delta}$m$ $=$ \ifmmode\pm\else\textpm\fi{}1 pair of Stokes and anti-Stokes Raman lines are not connected by time-reversal symmetry.

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Optical detection and phonon bottleneck of the direct decay within the E¯(E2) state of ruby

J. I. Dijkhuis, K. Huibregtse, H. W. de Wijn

Direct relaxation within the optically excited $\overline{E}(^{2}E)$ 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 $\overline{E}$ 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 ${\mathrm{Cr}}^{3+}$ ions, explains the lattice-bath equilibration. Finally, the prospect is discussed of using the $\overline{E}$ 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 ${\mathrm{cm}}^{\ensuremath{-}1}$.

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Magnetically induced suppression and enhancement of optical excitation of ruby at anticrossing points

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Sharp resonances in optical excitation of ruby by laser pulses are observed as Zeeman sublevels anticross in an external magnetic field. For the ${R}_{1}$ optical line of ruby $(694.3\phantom{\rule{0.3em}{0ex}}\mathrm{nm})$ excitation drops when the external magnetic field strength approaches $4.14\phantom{\rule{0.3em}{0ex}}\mathrm{kG}$ while for the ${R}_{2}$ line $(692.9\phantom{\rule{0.3em}{0ex}}\mathrm{nm})$ it increases in $4.14$ and $2.07\phantom{\rule{0.3em}{0ex}}\mathrm{kG}$ fields. A simple theoretical description of the observed effects on the basis of level mixing explains both situations.

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Magnetic-field dependence of photon-echo decays in ruby

Joseph Ganem, Y. P. Wang, Richard S. Meltzer +1 more

Measurements of photon-echo decays in dilute ruby (${\mathrm{Cr}}_{2}$${\mathrm{O}}_{3}$ concentration of 0.018 at. %) in magnetic fields up to 29.5 kG are reported. Echoes from the $^{4}$${\mathit{A}}_{2}$(-3/2)${\mathrm{\ensuremath{\leftrightarrows}}}^{2}$E,E\ifmmode\bar\else\textasciimacron\fi{}(-1/2) and $^{4}$${\mathit{A}}_{2}$(-1/2)${\mathrm{\ensuremath{\leftrightarrows}}}^{2}$E,E\ifmmode\bar\else\textasciimacron\fi{}(-1/2) transitions have dephasing times (${\mathit{T}}_{2}$) that are directly proportional to the ratio of the magnetic field (B) to the temperature (T), for the range 2.5B/T20 kG/K. A transition in the decay from exponential to nonexponential behavior for B/T>10 kG/K is observed. Decay times for the $^{4}$${\mathit{A}}_{2}$(-1/2)${\mathrm{\ensuremath{\leftrightarrows}}}^{2}$E,E(-1/2) echo are roughly a factor of 3 longer than for the $^{4}$${\mathit{A}}_{2}$(-3/2)${\mathrm{\ensuremath{\leftrightarrows}}}^{2}$E,E\ifmmode\bar\else\textasciimacron\fi{}(-1/2) echo. We attribute the lengthening of ${\mathit{T}}_{2}$ with increasing B/T to the suppression of Cr-Cr spin flip-flops as the population of the upper $^{4}$${\mathit{A}}_{2}$ levels decreases.

Physical review. B, Condensed matter•Apr 1, 1991•10 citations
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 QQQ-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^{4}A_{2}4A2​ ground state to the levels of the E‾(2E)\overline{E}(^{2}E)E(2E) excited state. The change in Mz{M}_{z}Mz​ is linearly proportional to HHH except near the anticrossing points of the 4A2^{4}A_{2}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^{4}A_{2}4A2​ wave functions. When HHH has a small component perpendicular to the zzz 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{M}_{z}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.
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.
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.
Doubly resonant Raman electron paramagnetic transitions in ruby (Al2{}_{2}2​O3{}_{3}3​:Cr3+{}^{3+}3+) have been investigated using dye laser excitation tuned across the range of the Zeeman components of its well-known R1{R}_{1}R1​ emission line. With magnetic field B normal to c, the optic axis of ruby, it is seen that the Raman electron paramagnetic resonance (EPR) lines exhibit significant intensity enhancements due to the simultaneous occurrence of ``in resonance'' and ``out resonance,'' as visualized in the Kramers-Heisenberg formalism of inelastic light scattering. The specific Raman EPR features observed in the present study, together with their resonances, however, differ markedly from those observed in our previous investigation with $\mathbf{B}\ensuremath{\parallel}\mathbf{c}$. These differences can be traced to level mixing effects within the Zeeman multiplet of the 4A2{}^{4}{A}_{2}4A2​ (ground) state on the one hand, and the negligible value of ${g}_{\ensuremath{\perp}}$ for the $\mathrm{E\ifmmode \bar{}\else \={}\fi{}}$ (excited) state of the R1{R}_{1}R1​ emission line on the other. Furthermore, we note that with B\ensuremath{\perp}c and under resonant conditions, the experimentally observed \ensuremath{\Delta}mmm === \ifmmode\pm\else\textpm\fi{}1 pair of Stokes and anti-Stokes Raman lines are not connected by time-reversal symmetry.
Direct relaxation within the optically excited E‾(2E)\overline{E}(^{2}E)E(2E) level of 700-ppm ruby is investigated in magnetic fields up to 65 kG making an angle to the ccc 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∗{N}^{*}N∗ is observed of the equilibrium population of the upper Zeeman level of E‾\overline{E}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+{\mathrm{Cr}}^{3+}Cr3+ ions, explains the lattice-bath equilibration. Finally, the prospect is discussed of using the E‾\overline{E}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 ${\mathrm{cm}}^{\ensuremath{-}1}$.
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.
Measurements of photon-echo decays in dilute ruby (Cr2{\mathrm{Cr}}_{2}Cr2​O3{\mathrm{O}}_{3}O3​ concentration of 0.018 at. %) in magnetic fields up to 29.5 kG are reported. Echoes from the 4^{4}4A2{\mathit{A}}_{2}A2​(-3/2)${\mathrm{\ensuremath{\leftrightarrows}}}^{2}$E,E\ifmmode\bar\else\textasciimacron\fi{}(-1/2) and 4^{4}4A2{\mathit{A}}_{2}A2​(-1/2)${\mathrm{\ensuremath{\leftrightarrows}}}^{2}$E,E\ifmmode\bar\else\textasciimacron\fi{}(-1/2) transitions have dephasing times (T2{\mathit{T}}_{2}T2​) that are directly proportional to the ratio of the magnetic field (B) to the temperature (T), for the range 2.5B/T20 kG/K. A transition in the decay from exponential to nonexponential behavior for B/T>10 kG/K is observed. Decay times for the 4^{4}4A2{\mathit{A}}_{2}A2​(-1/2)${\mathrm{\ensuremath{\leftrightarrows}}}^{2}$E,E(-1/2) echo are roughly a factor of 3 longer than for the 4^{4}4A2{\mathit{A}}_{2}A2​(-3/2)${\mathrm{\ensuremath{\leftrightarrows}}}^{2}$E,E\ifmmode\bar\else\textasciimacron\fi{}(-1/2) echo. We attribute the lengthening of T2{\mathit{T}}_{2}T2​ with increasing B/T to the suppression of Cr-Cr spin flip-flops as the population of the upper 4^{4}4A2{\mathit{A}}_{2}A2​ levels decreases.