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Physical Review Letters

Magnetic Circular Polarization of the T1 Luminescence in KI: Tl

Marco P. Fontana, Jeffrey A. Davis

Published Oct 27, 1969• 17 citations
Publisher

Abstract

The magnetic-field-induced circular polarization of the Tl characteristic emission in KI: Tl has been studied. The large size of the effect and its temperature dependence indicate that the emitting state sampled by our measurement is nearly degenerate. The data obtained support the theory on ${\mathrm{Tl}}^{+}$ luminescence in alkali halides recently proposed by Fukuda.

DOI
10.1103/physrevlett.23.974

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The magnetic-field-induced circular polarization of the Tl characteristic emission in KI: Tl has been studied. The large size of the effect and its temperature dependence indicate that the emitting state sampled by our measurement is nearly degenerate. The data obtained support the theory on Tl+{\mathrm{Tl}}^{+}Tl+ luminescence in alkali halides recently proposed by Fukuda.
When the pumping light is modulated between right and left circular polarization, the magnetic circular polarization of F center luminescence observed after passing through a linear polarizer has been called “the anomalous effect”. We measured this effect for the F center in KI as a function of the magnetic field H and the photon energy. These experimental data are explained by deriving a form which includes the parameter ( ε I ) for the unbalancing of circular polarization in pumping light and the spin-lattice relaxation rate of the relaxed excited state ( T 1 * ) -1 . The H -dependence of ( T 1 * ) -1 is presented.
We have solved the rate equations of the optical pumping cycle of the FFF center in the case of two spin-mixing parameters, ${\ensuremath{\epsilon}}_{+}$ and ${\ensuremath{\epsilon}}_{\ensuremath{-}}$. The solutions and the experimental results for the magnetic circular dichroism of the FFF-center emission are in complete agreement, and have allowed for the first time the measurement of the "anisotropy" of the electron-spin memory, i.e., $\frac{({\ensuremath{\epsilon}}_{\ensuremath{-}}\ensuremath{-}{\ensuremath{\epsilon}}_{+})}{({\ensuremath{\epsilon}}_{\ensuremath{-}}+{\ensuremath{\epsilon}}_{+})}$. This quantity will be very useful in order to clarify the role of the various processes responsible for the loss of the spin memory in the optical cycle. Moreover, from the rate equations we have been able to find the value of the polarization in the relaxed excited state, which is essential to calculate the spin-orbit splitting in emission.
Magnetic circular polarization (MCP) of F centar emission has been measured for KCl, KBr, KI, RbCl, and RbBr. The optical pumping cycle is proposed to include three spin mixing parameters in absoption, radiationless decay ( ε r1 ), and emission processes. From the analysis of experimental data using formulae derived from rate equations for the cycle, para- and dia-magnetic components ( Δ p and Δ d ) of MCP, and the spin mixing parameters are determined. Pumping photon energy dependence of Δ p are observed. It shows that ε r1 for KBr depends on photon energy and predominates the net spin mixing parameter ( ε t ). Temperature dependence of Δ d are shown to be relevant to the spin orbit interaction in the relaxed excited state of F center. The source of the discrepancy found between our determined values of ε t and those by other schools is clarified.
Helium atoms excited to an aligned (1s3d)3{)}^{3})3D state are produced by electron impact parallel to a magnetic field. The initial alignment of the atomic state evolves into an orientation with finite angular momentum along the magnetic field direction and thus gives rise to a circularly polarized component in the (1s3d)3{)}^{3})3D\ensuremath{\rightarrow}(1s2p)3{)}^{3})3P radiation propagating parallel to the magnetic field. Circular-polarization fractions of a few percent are observed, in general agreement with theory. The measured polarization fraction is found to be pressure dependent, indicating that secondary excitation and cascade processes must be included for a complete description.
We theoretically predict a strong influence of stimulated exciton-exciton scattering on semiconductor luminescence. The stimulated scattering causes circularly polarized instead of unpolarized emission at the biexciton emission line in a degenerate gas of partly spin polarized excitons. The biexciton polarization effect increases with increasing exciton densities and decreasing temperatures and approaches almost unity in the ultimate case of Bose-Einstein condensation. Time- and polarization-resolved luminescence measurements evidence the biexciton polarization effect both in ZnSe and GaAs quantum wells.
The alignment-orientation process is exactly treated for an alkali-like atom with purely electronic couplings. Analytic expressions for the circular polarization of the optical emission are given for specific types of anisotropic excitation in terms of the magnetic field and the spin-orbit parameter. No advanced formalism is used. It is shown how the hyperfine interaction with the electron orbital motion operates in a manner very similar to the purely electronic spin-orbit coupling, again leading to orbital orientation and to circularly polarized optical emission. Detailed computed results for alignment-orientation conversion for neutral helium are summarized, as are the results of experiments which directly exhibit circularly polarized emission due to the alignment-orientation effect in gases.