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Physical Review Letters

Microwave-Modulated Fluorescence in Ca F2 : Eu2+

Lloyd L. Chase

Published Sep 23, 1968• 11 citations
Publisher

Abstract

Microwave absorption in the spin levels of the excited state of an ionic system can modulate the circularly polarized fluorescence from the state at right angles to an applied, static magnetic field. This modulation is detected in the broad, vibronic, sideband fluorescence of the ${\mathrm{Eu}}^{2+}$ ion in Ca${\mathrm{F}}_{2}$ for a modulation frequency of 9.5 GHz.

DOI
10.1103/physrevlett.21.888

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Physical Review A•Jan 1, 1996•21 citations
Microwave absorption in the spin levels of the excited state of an ionic system can modulate the circularly polarized fluorescence from the state at right angles to an applied, static magnetic field. This modulation is detected in the broad, vibronic, sideband fluorescence of the Eu2+{\mathrm{Eu}}^{2+}Eu2+ ion in CaF2{\mathrm{F}}_{2}F2​ for a modulation frequency of 9.5 GHz.
The paramagnetic circular dichroism of the $4f\ensuremath{-}5d$ absorption bands of CaF2{\mathrm{F}}_{2}F2​:Tm2+{\mathrm{Tm}}^{2+}Tm2+ was investigated at 2.0\ifmmode^\circ\else\textdegree\fi{}K and 9.0kG. Using group theory it is shown that the bands can be separated into E52{E}_{\frac{5}{2}}E25​​ and G32{G}_{\frac{3}{2}}G23​​ character. Experimentally it was found that in this case the bands are primarily G32{G}_{\frac{3}{2}}G23​​ in character. Optical pumping experiments were performed where changes as large as \ifmmode\pm\else\textpm\fi{}20% were easily observed in the electron-paramagnetic-resonance absorption signal at XXX-band frequencies and 1.5\ifmmode^\circ\else\textdegree\fi{}K by changing the polarization of the pump light from right to left circular. A method for optically monitoring the difference in the population of the two Zeeman components of the ground state is also reported on.
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.
A treatment of resonance fluorescence is given in which the properties of the incident and scattered light are expressed in terms of coherency matrices. Calculations are made for two successive scattering processes corresponding to the experiments described in an accompanying paper. For the first process the scattering atoms are subjected to a magnetic field consisting of parallel static and oscillating components. The fluorescent light is frequency-modulated and may also be polarization-modulated. This light excites fluorescence in a second cell in which the atoms are subjected to a static magnetic field. The equations show under what conditions the second fluorescence is intensity-modulated and describe the resonances in the amplitude of modulation which occur when the modulation frequency matches a Zeeman interval of the excited atoms.
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.
Spectral hole burning on the $4{f}^{7}(^{8}S_{\frac{7}{2}})\ensuremath{\rightarrow}4{f}^{6}5d$ transition of Eu2+{\mathrm{Eu}}^{2+}Eu2+ is observed to occur by two mechanisms in CaF2{\mathrm{F}}_{2}F2​:Eu2+{\mathrm{Eu}}^{2+}Eu2+. At zero magnetic field, persistent spectral hole burning occurs by two-step photoionization where the hole burning survives cycling to room temperature. The observed hole spectrum is compared with a calculation which considers the small Eu2+{\mathrm{Eu}}^{2+}Eu2+ ground-state splitting and both the ground- and excited-state hyperfine interactions. In a magnetic field above 1 T, transient spectral hole burning occurs by population redistribution among the ground-state hyperfine levels. Long-lived (minutes) holes are observed for a half integral spin system. The resulting hole spectrum consists of holes, antiholes, and a nuclear spin-flip sideband and can be explained from the hyperfine interactions in the ground and excited states. The dominant hole decay occurs by phonon-induced transitions among hyperfine levels of the ${M}_{S}=\ensuremath{-}\frac{7}{2}$ and ${M}_{S}=\ensuremath{-}\frac{5}{2}$ electron-spin sublevels. Central hole linewidths of 200 MHz are observed at zero magnetic field. Hole linewidths at high magnetic fields are as narrow as 40 MHz but these are inhomogenously broadened by superhyperfine interactions with the FFF nuclear spins. Estimates of hole-burning quantum efficiency are also obtained.
Circular dichroism has been detected in the polarization of the fluorescence resulting from the decay of excited calcium ions. Synchrotron radiation was used to both ionize and excite calcium atoms from their ground state, and the photoejected electron was detected in coincidence with the polarized fluorescent photon. By introducing a degree of left, then right circularly polarized light into the incident photon beam the circular dichroism could be measured. The analysis of the results was carried out using recent developments of the statistical tensor method, and it was possible to determine the sign of the phase difference between the outgoing waves.