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Journal of the Physical Society of Japan

On “the Anomalous Effect” of the Magnetic Circular Polarization of the F Center Luminescence

Norio Akiyama, Katsuyasu Kawano, Yuzo Mori, Hiroshi Ohkura

Published Jun 15, 1984• 1 citation
Publisher

Abstract

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.

DOI
10.1143/jpsj.53.1927

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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.
The magnetic nature of the relaxed excited state (RES) of F centers and spin-flipping processes in the optical pumping cycle have been clarified by studying the magnetic circular polarization (MCP) of emission when the pumping light is modulated between right and left circular polarization at the frequency of (ω/2π). Paramagnetic component Δ p (ω) of MCP is found to be proportional to the spin polarization P * (ω) in the RES with a proportionality constant F p . From the analysis of the dependence of Δ p (ω) at particular frequencies on the pumping photon energy E p h , F p is estimated. Thus, the E p h -dependence of the net spin-mixing parameter ε t is determined. The values of F p and ε t are found to be smaller than those determined previously. The source of discrepancy is discussed. It is suggested that the spin-mixing in the non-radiative transition process is not neglected.
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.
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.
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.
The magnetic property of the relaxed excited state (RES) of the F center has been manifested by the experimental and theoretical study of the magnetic circular polarization (ΔMCP) of the F center emission. The para- and diamagnetic components (Δp and Δd) of ΔMCP as well as the g-shift of the RES have been experimentally determined. From the comparison of these values with those computed values on the vibronic theory, the spin-orbit interaction constant, the orbital g-factor, and the spin polarization of the RES are determined. The tuning photon energy dependence of Δp is observed. The spin-memory-loss during radiationless decay is found to be significant. The temperature dependence of Δd is mentioned.
The photoluminescence of the negatively charged nitrogen-vacancy (${\mathrm{NV}}^{\ensuremath{-}}$) center displays anomalous saturation behavior at high excitation pulse energies. Where the luminescence is expected to approach a maximum value asymptotically, we have found that it instead drops by as much as a factor of two. In this report, we present evidence that these effects are caused by optical spin depolarization of the ${\mathrm{NV}}^{\ensuremath{-}}$ center. We show that the presence of an external magnetic field results in a marked decrease in the anomalous character of the ${\mathrm{NV}}^{\ensuremath{-}}$ centers' luminescence and demonstrate that low-energy pulses applied after a strong depolarizing pulse can repolarize the spin of the center. We also offer a model and derive important parameters describing dynamics in the NV center and its interaction with light, which quantitatively explain all the observations.