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Physical Review

Fluorescence and Optical Maser Effects in Ca F2 : Sm++

W. Kaiser, C. G. B. Garrett, Darwin L. Wood

Published Aug 1, 1961• 162 citations
Publisher

Abstract

Measurements are reported of absorption, emission, and activation spectra in Ca${\mathrm{F}}_{2}$: ${\mathrm{Sm}}^{++}$, and also of fluorescence lifetime. A revised level scheme is proposed. Observations of optical maser effects were made at liquid hydrogen and liquid helium temperatures over a wide range of pumping intensities. For illumination in the 6400-A band, the threshold intensity of illumination was about 20 w ${\mathrm{cm}}^{\ensuremath{-}2}$. Observations are reported of the dependence of the intensity of the maser beam on the pumping intensity. Photographic observations, made both with or without a Fabry-Perot etalon, show that on one particular sample five distinct frequencies were present in the maser signal, and that the number of modes excited was of the order of 1000.

DOI
10.1103/physrev.123.766

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Spectroscopy and Optical Maser Action in Sr F2 : Sm2+

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A crystal of Sr${\mathrm{F}}_{2}$: ${\mathrm{Sm}}^{2+}$ was operated as a pulsed optical maser at temperatures close to 4.2\ifmmode^\circ\else\textdegree\fi{}K. The 6969 \AA{} output corresponds to the wavelength of an exceedingly sharp line ($\ensuremath{\Delta}\ensuremath{\nu}<0.6$ ${\mathrm{cm}}^{\ensuremath{-}1}$) in which almost all of the low-temperature fluorescence is concentrated. In contrast to what prevails in Ca${\mathrm{F}}_{2}$: ${\mathrm{Sm}}^{+2}$ masers, the output is characterized by strong relaxation oscillations. This may be explained by the large difference in the fluorescent lifetimes in the two systems. While in Ca${\mathrm{F}}_{2}$: ${\mathrm{Sm}}^{2+}$ the value of $\ensuremath{\tau}$ is approximately 1.5\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}6}$ sec, in Sr${\mathrm{F}}_{2}$: ${\mathrm{Sm}}^{2+}$ it has a value of 15 msec. The reason for the large difference in $\ensuremath{\tau}$ values, in turn, depends upon the relative proximity of the long-wavelength edge of the red band to the position of the metastable $^{5}D_{0}$ level from which fluorescence occurs. Absorption measurements taken at helium temperatures on Sr${\mathrm{F}}_{2}$: ${\mathrm{Sm}}^{2+}$ show that the lowest vibrational level of the lowest lying $4f\ensuremath{-}5d$ band lies \ensuremath{\sim}600 ${\mathrm{cm}}^{\ensuremath{-}1}$ above the position of the $^{5}D_{0}$ level while in Ca${\mathrm{F}}_{2}$: ${\mathrm{Sm}}^{2+}$ there is near coincidence of the corresponding levels and consequently strong configurational mixing. Crystals of Sr${\mathrm{Cl}}_{2}$: ${\mathrm{Sm}}^{2+}$ were also studied in this regard and display an expected close similarity in spectral characteristics to Sr${\mathrm{F}}_{2}$: ${\mathrm{Sm}}^{2+}$. The vibrational structure on the $4f\ensuremath{-}5d$ bands is much more pronounced, however, than in Ca${\mathrm{F}}_{2}$: ${\mathrm{Sm}}^{2+}$ and Sr${\mathrm{F}}_{2}$: ${\mathrm{Sm}}^{2+}$, indicating weaker coupling to the lattice in the excited $5d$ state. A tentative interpretation for the splitting of the red band seen in the absorption spectra of ${\mathrm{Sm}}^{2+}$ ions is given.

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In the optical masers realized so far a Perot-Fabry device is used as a multimode cavity. It may be interesting to investigate the general properties of such a device when the emitting or the absorbing atoms are put inside the reflecting mirrors. Even in the case when this device works below the threshold of maser action it shows remarkable properties which are worthwhile studying experimentally. The following aspects are considered in the paper: In the case of external illumination, the distribution of light intensity inside a Perot-Fabry interferometer is calculated. It is shown that the local light intensity in the stationary waves inside can be much higher than the intensity of the incident light beam. The properties of light emitted by atoms inside the Perot-Fabry and emerging from it are investigated. Narrow fringes of very strong intensity can be obtained. If the emitting atoms are located in an atomic beam the central fringe has natural line width, the Doppler broadening being suppressed. The realization of a fluorescent medium of lamellar structure is discussed. This structure favors one special mode of emission fringes. Finally, the absorption of atoms inside the interferometer is studied. It is shown that this device is equivalent to a long absorption path in an ordinary light beam.

Applied Optics•Jan 1, 1962•123 citations

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Applied Optics•Jan 1, 1962•35 citations
Measurements are reported of absorption, emission, and activation spectra in CaF2{\mathrm{F}}_{2}F2​: Sm++{\mathrm{Sm}}^{++}Sm++, and also of fluorescence lifetime. A revised level scheme is proposed. Observations of optical maser effects were made at liquid hydrogen and liquid helium temperatures over a wide range of pumping intensities. For illumination in the 6400-A band, the threshold intensity of illumination was about 20 w cm\ensuremath−2{\mathrm{cm}}^{\ensuremath{-}2}cm\ensuremath−2. Observations are reported of the dependence of the intensity of the maser beam on the pumping intensity. Photographic observations, made both with or without a Fabry-Perot etalon, show that on one particular sample five distinct frequencies were present in the maser signal, and that the number of modes excited was of the order of 1000.
A crystal of SrF2{\mathrm{F}}_{2}F2​: Sm2+{\mathrm{Sm}}^{2+}Sm2+ was operated as a pulsed optical maser at temperatures close to 4.2\ifmmode^\circ\else\textdegree\fi{}K. The 6969 \AA{} output corresponds to the wavelength of an exceedingly sharp line ($\ensuremath{\Delta}\ensuremath{\nu}<0.6$ ${\mathrm{cm}}^{\ensuremath{-}1}$) in which almost all of the low-temperature fluorescence is concentrated. In contrast to what prevails in CaF2{\mathrm{F}}_{2}F2​: Sm+2{\mathrm{Sm}}^{+2}Sm+2 masers, the output is characterized by strong relaxation oscillations. This may be explained by the large difference in the fluorescent lifetimes in the two systems. While in CaF2{\mathrm{F}}_{2}F2​: Sm2+{\mathrm{Sm}}^{2+}Sm2+ the value of $\ensuremath{\tau}$ is approximately 1.5\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}6}$ sec, in SrF2{\mathrm{F}}_{2}F2​: Sm2+{\mathrm{Sm}}^{2+}Sm2+ it has a value of 15 msec. The reason for the large difference in $\ensuremath{\tau}$ values, in turn, depends upon the relative proximity of the long-wavelength edge of the red band to the position of the metastable 5D0^{5}D_{0}5D0​ level from which fluorescence occurs. Absorption measurements taken at helium temperatures on SrF2{\mathrm{F}}_{2}F2​: Sm2+{\mathrm{Sm}}^{2+}Sm2+ show that the lowest vibrational level of the lowest lying $4f\ensuremath{-}5d$ band lies \ensuremath{\sim}600 ${\mathrm{cm}}^{\ensuremath{-}1}$ above the position of the 5D0^{5}D_{0}5D0​ level while in CaF2{\mathrm{F}}_{2}F2​: Sm2+{\mathrm{Sm}}^{2+}Sm2+ there is near coincidence of the corresponding levels and consequently strong configurational mixing. Crystals of SrCl2{\mathrm{Cl}}_{2}Cl2​: Sm2+{\mathrm{Sm}}^{2+}Sm2+ were also studied in this regard and display an expected close similarity in spectral characteristics to SrF2{\mathrm{F}}_{2}F2​: Sm2+{\mathrm{Sm}}^{2+}Sm2+. The vibrational structure on the $4f\ensuremath{-}5d$ bands is much more pronounced, however, than in CaF2{\mathrm{F}}_{2}F2​: Sm2+{\mathrm{Sm}}^{2+}Sm2+ and SrF2{\mathrm{F}}_{2}F2​: Sm2+{\mathrm{Sm}}^{2+}Sm2+, indicating weaker coupling to the lattice in the excited 5d5d5d state. A tentative interpretation for the splitting of the red band seen in the absorption spectra of Sm2+{\mathrm{Sm}}^{2+}Sm2+ ions is given.
It was first shown by Axe and Sorokin that nonradiative decay of the terminal optical maser level (7F1^{7}F_{1}7F1​) CaF2{\mathrm{F}}_{2}F2​: Sm2+{\mathrm{Sm}}^{2+}Sm2+ is forbidden to first order even though many phonon states are available with the proper energy for the transition. At first sight this would indicate the 7F1^{7}F_{1}7F1​ level to be metastable. It is shown that a second-order two-phonon emission transition can alleviate this bottleneck, and the decay process rate is computed. This process, involving virtual transitions to a higher level (7F2^{7}F_{2}7F2​), is shown to be very effective and to lead to a nonradiative lifetime of \ensuremath{\sim}${10}^{\ensuremath{-}6}$ sec, in accord with the necessary conditions for the existence of optical maser action in CaF2{\mathrm{F}}_{2}F2​: Sm2+{\mathrm{Sm}}^{2+}Sm2+.
We have measured the pump-probe spectrum for CaF2:Sm2+ over a wide (360–1020 nm) spectral range. We observed gain throughout most of the 730-nm emission band, although there is some evidence of excited-state absorption (ESA) in this region. A much stronger ESA was observed near 500 nm. The temperature dependence of the ESA signal clearly indicates that the absorption is associated with divalent samarium. Because of the width and strength of this transition, we assign the ESA to a 4f55d → conduction band transition. We calculated the oscillator strength for this ESA band to be 0.10 and rationalized its magnitude based on an intensity-borrowing mechanism that involves the transition from the valence band to the conduction band.
In the optical masers realized so far a Perot-Fabry device is used as a multimode cavity. It may be interesting to investigate the general properties of such a device when the emitting or the absorbing atoms are put inside the reflecting mirrors. Even in the case when this device works below the threshold of maser action it shows remarkable properties which are worthwhile studying experimentally. The following aspects are considered in the paper: In the case of external illumination, the distribution of light intensity inside a Perot-Fabry interferometer is calculated. It is shown that the local light intensity in the stationary waves inside can be much higher than the intensity of the incident light beam. The properties of light emitted by atoms inside the Perot-Fabry and emerging from it are investigated. Narrow fringes of very strong intensity can be obtained. If the emitting atoms are located in an atomic beam the central fringe has natural line width, the Doppler broadening being suppressed. The realization of a fluorescent medium of lamellar structure is discussed. This structure favors one special mode of emission fringes. Finally, the absorption of atoms inside the interferometer is studied. It is shown that this device is equivalent to a long absorption path in an ordinary light beam.
In the optical masers realized so far a Perot-Fabry device is used as a multimode cavity. It may be interesting to investigate the general properties of such a device when the emitting or the absorbing atoms are put inside the reflecting mirrors. Even in the case when this device works below the threshold of maser action it shows remarkable properties which are worthwhile studying experimentally. The following aspects are considered in the paper: In the case of external illumination, the distribution of light intensity inside a Perot-Fabry interferometer is calculated. It is shown that the local light intensity in the stationary waves inside can be much higher than the intensity of the incident light beam. The properties of light emitted by atoms inside the Perot-Fabry and emerging from it are investigated. Narrow fringes of very strong intensity can be obtained. If the emitting atoms are located in an atomic beam the central fringe has natural line width, the Doppler broadening being suppressed. The realization of a fluorescent medium of lamellar structure is discussed. This structure favors one special mode of emission fringes. Finally, the absorption of atoms inside the interferometer is studied. It is shown that this device is equivalent to a long absorption path in an ordinary light beam.
In the optical masers realized so far a Perot-Fabry device is used as a multimode cavity. It may be interesting to investigate the general properties of such a device when the emitting or the absorbing atoms are put inside the reflecting mirrors. Even in the case when this device works below the threshold of maser action it shows remarkable properties which are worthwhile studying experimentally. The following aspects are considered in the paper: In the case of external illumination, the distribution of light intensity inside a Perot-Fabry interferometer is calculated. It is shown that the local light intensity in the stationary waves inside can be much higher than the intensity of the incident light beam. The properties of light emitted by atoms inside the Perot-Fabry and emerging from it are investigated. Narrow fringes of very strong intensity can be obtained. If the emitting atoms are located in an atomic beam the central fringe has natural line width, the Doppler broadening being suppressed. The realization of a fluorescent medium of lamellar structure is discussed. This structure favors one special mode of emission fringes. Finally, the absorption of atoms inside the interferometer is studied. It is shown that this device is equivalent to a long absorption path in an ordinary light beam.