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Physical Review

Spectroscopy and Optical Maser Action in Sr F2 : Sm2+

P. P. Sorokin, M. J. Stevenson, J. R. Lankard, George D. Pettit

Published Jul 15, 1962• 66 citations
Publisher

Abstract

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.

DOI
10.1103/physrev.127.503

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Physical Review Letters•Oct 30, 1967•16 citations
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 (\ensuremathΔ\ensuremathν<0.6\ensuremath{\Delta}\ensuremath{\nu}<0.6\ensuremathΔ\ensuremathν<0.6 cm\ensuremath−1{\mathrm{cm}}^{\ensuremath{-}1}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 \ensuremathτ\ensuremath{\tau}\ensuremathτ is approximately 1.5\ifmmode\times\else\texttimes\fi{}10\ensuremath−6{10}^{\ensuremath{-}6}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 \ensuremathτ\ensuremath{\tau}\ensuremathτ 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−5d4f\ensuremath{-}5d4f\ensuremath−5d band lies \ensuremath{\sim}600 cm\ensuremath−1{\mathrm{cm}}^{\ensuremath{-}1}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−5d4f\ensuremath{-}5d4f\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.
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 ${\mathrm{cm}}^{\ensuremath{-}2}$. 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.
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+.
One-color two-photon excitation spectra of the 7^{7}7F0{\mathit{F}}_{0}F0​${\ensuremath{\rightarrow}}^{5}$D0{\mathit{D}}_{0}D0​ and 7^{7}7F0{\mathit{F}}_{0}F0​${\ensuremath{\rightarrow}}^{5}$D2{\mathit{D}}_{2}D2​ transitions of Sm2+{\mathrm{Sm}}^{2+}Sm2+ in SrF2{\mathrm{SrF}}_{2}SrF2​ have been measured at low temperature and their dependences upon photon polarization analyzed. Linewidths on the order of 0.2, 0.6, and 1.0 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ are obtained for the 7^{7}7F0{\mathit{F}}_{0}F0​${\ensuremath{\rightarrow}}^{5}$D0{\mathit{D}}_{0}D0​(A1g{\mathit{A}}_{1\mathit{g}}A1g​), 7^{7}7F0{\mathit{F}}_{0}F0​${\ensuremath{\rightarrow}}^{5}$D2{\mathit{D}}_{2}D2​(Eg{\mathit{E}}_{\mathit{g}}Eg​), and 7^{7}7F0{\mathit{F}}_{0}F0​${\ensuremath{\rightarrow}}^{5}$D2{\mathit{D}}_{2}D2​(T2g{\mathit{T}}_{2\mathit{g}}T2g​) Stark components observed at 15 K, respectively. The 7^{7}7F0{\mathit{F}}_{0}F0​${\ensuremath{\rightarrow}}^{5}$D0{\mathit{D}}_{0}D0​(A1g{\mathit{A}}_{1\mathit{g}}A1g​) transition is observed to be isotropic within experimental error, while the Eg{\mathit{E}}_{\mathit{g}}Eg​ and T2g{\mathit{T}}_{2\mathit{g}}T2g​ components of the 7^{7}7F0{\mathit{F}}_{0}F0​${\ensuremath{\rightarrow}}^{5}$D2{\mathit{D}}_{2}D2​ transition exhibit the anisotropic patterns expected in Oh{\mathrm{O}}_{\mathit{h}}Oh​ symmetry. Calculations of two-photon transition intensities are conducted within the framework of the phenomenological model previously used for Sm2+{\mathrm{Sm}}^{2+}Sm2+ in MClF (M=Ba,Sr) and Eu3+{\mathrm{Eu}}^{3+}Eu3+ in LuPO4{\mathrm{LuPO}}_{4}LuPO4​, where the rare-earth ion occupies sites of C4v{\mathit{C}}_{4\mathit{v}}C4v​ and D2d{\mathit{D}}_{2\mathit{d}}D2d​ symmetries, respectively. The present results are compared and contrasted with those obtained in these previous cases.
We have found direct evidence of upward, phonon annihilation, processes in the decay patterns of two metastable states $^{5}D_{0}(14542 {\mathrm{cm}}^{\ensuremath{-}1})$ and $^{5}D_{1}(15878 {\mathrm{cm}}^{\ensuremath{-}1})$ of BaClF: Sm2+{\mathrm{Sm}}^{2+}Sm2+. Under pulse excitation, the decay curve of the 5D1^{5}D_{1}5D1​ fluorescence includes a long tail, with a time constant equal to the lifetime of 5D0^{5}D_{0}5D0​, which appears at \ensuremath{\sim}280\ifmmode^\circ\else\textdegree\fi{}K and grows in intensity with temperature; finally at \ensuremath{\sim}400\ifmmode^\circ\else\textdegree\fi{}K both states decay with the same time constant. This finding is consistent with the thermal dependence of the ratio of the intensities of the 5D1^{5}D_{1}5D1​ to the 5D0^{5}D_{0}5D0​ flourescence.