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Journal of the Optical Society of America

Ruby as a Macroscopic Fluorescing and Laser Material*†

Archie I. Mahan, C. V. Bitterli, S. M. Cannon, D. G. Grant

Published Jan 1, 1969• 8 citations
Publisher

Abstract

We consider ruby as a linear, uniaxial, maxwellian medium, which, in its absorbing state, has axially dependent material constants. It is made active by pumping with electric fields, in the region of shorter-wavelength absorption bands, which, by means of an energy transfer, induce source-type electric fields having constant amplitudes in the R1 and R2 regions, and make possible absorption, spontaneous emission, and stimulated emission. Experiments indicate that the absorption, fluorescence, and laser action are all maximum for electric fields perpendicular to the optic axis, and minimum when the electric field is parallel to the optic axis; the relative irradiances and shapes of the R1 and R2 bands in all cases depend upon the orientation of the plane of polarization of the emerging radiant flux relative to the optic axis. Attempts are made to compute the spectral distributions in the R1 and R2 bands for the ordinary and extraordinary waves, neglecting all boundary-value problems. Agreement was found to be possible only in the region below the threshold of laser action.

DOI
10.1364/josa.59.000049

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Stimulated and Fluorescent Optical Emission in Ruby From 4, 2 to 300°K : Zero-field Splitting and Mode Structure

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Optically Induced Magnetization in Ruby

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Journal of the Optical Society of America•Dec 1, 1965•2 citations

Relation between Absorption and Emission in the Region of the R Lines of Ruby

Donald F. Nelson, Michael D. Sturge

We have measured the radiative efficiency (ratio of $R$-line photons to the total number of photons emitted) and absorption cross sections for the $R$ lines of dilute ruby, as a function of polarization and temperature between 20 and 373\ifmmode^\circ\else\textdegree\fi{}K. We have also measured the fluorescent lifetime from 20 to 373\ifmmode^\circ\else\textdegree\fi{}K in optically thin crystals, and from 20 to 273\ifmmode^\circ\else\textdegree\fi{}K in optically thick crystals. The difference between the reciprocal lifetimes in thin and thick crystals is directly comparable (via the Einstein relations) with the integrated absorption in the $R$ lines. This comparison shows that for the $R$ lines at 20 and 77\ifmmode^\circ\else\textdegree\fi{}K detailed balance holds to well within the experimental error of 5%, a precision never to our knowledge previously approached in solids. An upper limit of 0.002 ${\mathrm{cm}}^{\ensuremath{-}1}$ was placed on any Stokes shift of the ${R}_{1}$ line, confirming that it is a no-phonon line. Comparison of $\ensuremath{\sigma}$ and $\ensuremath{\alpha}$ spectra confirms that the $R$ lines and their vibronic satellites are predominately electric-dipole in character. The radiative efficiency of the $R$ lines is strongly polarization dependent, a result which differs from that of previous workers. Its temperature dependence agrees well with that calculated from the observed vibronic spectrum at 77\ifmmode^\circ\else\textdegree\fi{}K. The integrated absorption in the $R$ lines increases by about 20% between 20 and 373\ifmmode^\circ\else\textdegree\fi{}K. The absorption in the vibronic satellites on the high-frequency side of the $R$ lines appears to be weaker than the corresponding emission on the low-frequency side. The temperature variation of the lifetime agrees well with that calculated from the absorption and radiative efficiency, but about 10% of the decay at 77\ifmmode^\circ\else\textdegree\fi{}K and 30% at 373\ifmmode^\circ\else\textdegree\fi{}K is either nonradiative or by emission at wavelengths outside the range of observation. The implications of our results for laser work are discussed.

Physical Review•Feb 15, 1965•257 citations
We consider ruby as a linear, uniaxial, maxwellian medium, which, in its absorbing state, has axially dependent material constants. It is made active by pumping with electric fields, in the region of shorter-wavelength absorption bands, which, by means of an energy transfer, induce source-type electric fields having constant amplitudes in the R1 and R2 regions, and make possible absorption, spontaneous emission, and stimulated emission. Experiments indicate that the absorption, fluorescence, and laser action are all maximum for electric fields perpendicular to the optic axis, and minimum when the electric field is parallel to the optic axis; the relative irradiances and shapes of the R1 and R2 bands in all cases depend upon the orientation of the plane of polarization of the emerging radiant flux relative to the optic axis. Attempts are made to compute the spectral distributions in the R1 and R2 bands for the ordinary and extraordinary waves, neglecting all boundary-value problems. Agreement was found to be possible only in the region below the threshold of laser action.
Several experiments on optically excited ruby are described. First, absorption transitions initiated from the metastable states,t\min{2}\max{3}2Eandt\min{2}\max{3}2T1, are investigated in the photon energy range from 5500 to 45 000 cm-1. The absorption spectrum of these excited states is determined in both\pi-and σ-polarizations and assignments for the observed absorption bands are achieved. Obtained results are compared with other experiments reported in the literature. Next, emission properties of pink ruby are studied in the region near theRlines. The emission spectrum is determined point by point by means of intense flash excitation. The observed weak bands are ascribed to theR'and phonon assistedRtransitions. Laser actions in these bands are discussed and experimental results on phonon terminated laser amplification are presented. An experiment using a giant pulse as an excitation source reveals that the transition time fromt\min{2}\max{3}2Etot\min{2}\max{3}2T1is shorter than a fraction of 1μs at room temperature.
The spectral character of the stimulated emission from ruby has been investigated; we found that the zero-field splitting (0.38 cm−1) of the A42 ground level and the cavity optical length and its reflectivity determine the spectral character of this emission. At 300°K the system is induced into oscillation in a number of cavitymodes separated by Δv̄ = (2lε12)−1, where l is the length of the resonator and ε12 is the index of refraction for the ordinary ray. At T≅80°K the Ē(2E) → ±32(A2) and the Ē(2E) → ±12(4A2) transitions were identified in the stimulated emission spectrum. Each can oscillate in an axial cavity mode (or modes) plus an off-axis mode at the center of the transition. At 4.2°K the system oscillates in off-axis modes, unless the length of the cavity l is such that it has axial resonances coincident with the transition frequencies. An analysis of the temperature dependence of the oscillation threshold for the separate transitions is presented and is compared with the observed behavior.
The spectral character of the stimulated emission from ruby has been investigated; we found that the zero-field splitting (0.38 cm−1) of the 4A2 ground level and the cavity optical length and its reflectivity determine the spectral character of this emission. At 300°K the system is induced into oscillation in a number of cavity modes separated by Δv = (2le12)−1, where l is the length of the resonator and e12 is the index of refraction for the ordinary ray. At T≅80°K the Ē(2E) → ±32(A2) and the Ē(2E) → ±12(4A2) transitions were identified in the stimulated emission spectrum. Each can oscillate in an axial cavity mode (or modes) plus an off-axis mode at the center of the transition. At 4.2°K the system oscillates in off-axis modes, unless the length of the cavity l is such that it has axial resonances coincident with the transition frequencies. An analysis of the temperature dependence of the oscillation threshold for the separate transitions is presented and is compared with the observed behavior.
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.
In this paper, four different ways are described in which homogeneous, isotropic, nonferromagnetic, Maxwellian media at rest may exhibit absorption and stimulated emission. These four ways may be identified by a positive or negative conductivity, a sink or source-type electric field, a complex dielectric constant, and a complex permeability. The forms of the electromagnetic waves, the refractive index, and the positive or negative extinction coefficient are determined for each process for both absorbing and active media. Spontaneous emission is associated with source-type electric and magnetic fields which generate conduction or displacement currents in the medium with improper phases to contribute to the radiant flux in the incident electromagnetic wave. Stimulated emission is characterized by exponentially increasing incident electric and magnetic fields and an excess of the time average of the outflow of radiant flux over the surfaces of an arbitrary volume. Stimulated emission appears when the time average of the rate at which the source-type electric and magnetic fields do work in carrying certain conduction or displacement currents in directions opposite to the incident electric and magnetic fields is greater than the time average of the rate at which the incident electric and magnetic fields dissipate energy by doing work in carrying other conduction or displacement currents in directions parallel to the incident electric and magnetic fields.
We have measured the radiative efficiency (ratio of RRR-line photons to the total number of photons emitted) and absorption cross sections for the RRR lines of dilute ruby, as a function of polarization and temperature between 20 and 373\ifmmode^\circ\else\textdegree\fi{}K. We have also measured the fluorescent lifetime from 20 to 373\ifmmode^\circ\else\textdegree\fi{}K in optically thin crystals, and from 20 to 273\ifmmode^\circ\else\textdegree\fi{}K in optically thick crystals. The difference between the reciprocal lifetimes in thin and thick crystals is directly comparable (via the Einstein relations) with the integrated absorption in the RRR lines. This comparison shows that for the RRR lines at 20 and 77\ifmmode^\circ\else\textdegree\fi{}K detailed balance holds to well within the experimental error of 5%, a precision never to our knowledge previously approached in solids. An upper limit of 0.002 ${\mathrm{cm}}^{\ensuremath{-}1}$ was placed on any Stokes shift of the R1{R}_{1}R1​ line, confirming that it is a no-phonon line. Comparison of $\ensuremath{\sigma}$ and $\ensuremath{\alpha}$ spectra confirms that the RRR lines and their vibronic satellites are predominately electric-dipole in character. The radiative efficiency of the RRR lines is strongly polarization dependent, a result which differs from that of previous workers. Its temperature dependence agrees well with that calculated from the observed vibronic spectrum at 77\ifmmode^\circ\else\textdegree\fi{}K. The integrated absorption in the RRR lines increases by about 20% between 20 and 373\ifmmode^\circ\else\textdegree\fi{}K. The absorption in the vibronic satellites on the high-frequency side of the RRR lines appears to be weaker than the corresponding emission on the low-frequency side. The temperature variation of the lifetime agrees well with that calculated from the absorption and radiative efficiency, but about 10% of the decay at 77\ifmmode^\circ\else\textdegree\fi{}K and 30% at 373\ifmmode^\circ\else\textdegree\fi{}K is either nonradiative or by emission at wavelengths outside the range of observation. The implications of our results for laser work are discussed.