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Journal of the Physical Society of Japan

Magnetic Circular Polarization of F Center Emission and the Spin Mixing Processes in Optical Cycle

Hiroshi Ohkura, Kiyokazu Iwahana, Masaji Tanaka, Katsushi Tara, Koichi Imanaka

Published Dec 1, 1980• 22 citations
Publisher

Abstract

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.

DOI
10.1143/jpsj.49.2296

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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 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.
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.
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.
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 experimentally study the Rb-Xe spin-exchange optical pumping process. The dependence of the spin-exchange rate on the intensity of the pumping light is measured at different temperatures under low magnetic fields. We demonstrate that the spin-exchange rate will decrease as the Rb spin polarization increases, which agrees well with the theoretical prediction. In our measurement, three spin exchange and spin relaxation mechanisms, namely, the spin exchange rates between Rb and Xe atoms caused by van der Waals molecule (${\mathrm{\ensuremath{\Gamma}}}_{\mathrm{vdW}}$) and binary collision (${\mathrm{\ensuremath{\Gamma}}}_{\mathrm{bin}}$) processes, and the spin relaxation rate due to the wall collision of Xe atoms (${\mathrm{\ensuremath{\Gamma}}}_{\mathrm{w}}$), have comparable magnitudes. These rates are extracted separately from the measured data with different Rb spin polarization at various temperatures. Our work provides a comprehensive confirmation of the physical picture of the spin-exchange optical pumping process.
Initializing a set of qubits to a given quantum state is a basic prerequisite for the physical implementation of quantum-information protocols. Here, we discuss the polarization of the electronic and nuclear spin in a single nitrogen vacancy center in diamond. Our initialization scheme uses a sequence of laser, microwave and radio-frequency pulses, and we optimize the pumping parameters of the laser pulse. A rate equation model is formulated that explains the effect of the laser pulse on the spin system. We have experimentally determined the population of the relevant spin states as a function of the duration of the laser pulse by measuring Rabi oscillations and Ramsey-type free-induction decays. The experimental data have been analyzed to determine the pumping rates of the rate equation model.