Optical spin polarisation

For an understanding of ESR in crystals, a detailed discussion of the molecular fundamentals is necessary. We deal with this primarily in Sections 7.2 and 7.3. There, the spin quantisation in triplet states, magnetic dipole-dipole couphng, zero-field splitting, Zeeman spHtting and fine structure are explained. These fundamentals apply both to isolated molecules and to excitons (Sects. 7.4 and 7.5). In the two later Sects. 7.6 and 7.7 of this chapter, the so called optical spin polarisation in excited triplet states and dynamic nuclear spin polarisation will be treated. 

Fig. 7.25 Triplet exciton ESR lines from naphthalene crystals at room temperature and y = 35 GHz. a) Bol y, non-doped crystal the different intensities result from a weak optical spin polarisation, b) Naphthalene -i- 0.5% quinoxaline, with Bolly + 10.5° the low-field transition takes place by stimulated emission, but the high-field transition exhibits absorption of microwaves. From [24].

Figure 7.28 lists all the rate constants in zero field and in an applied magnetic field which must be experimentally determined in order to have a complete understanding of the dynamics of the triplet state and therefore a quantitative understanding of optical spin polarisation the individual population rates Sj (u = x,y,z)... 

The differences in the population and depopulation rate constants and the phosphorescence probabilities of the three components of the triplet states form the basis of all the methods for Optical Detection of Magnetic Resonance in triplet states of jr-electron systems. These methods were developed after the discovery of optical spin polarisation and extended to inorganic solids. The essential physical difference from the optical double resonance in atoms developed by Alfred Kastler is to be found in the selection mechanism in optical double resonance, the polarisation of the resonant UV light, i.e. the symmetry of an applied field, is responsible for the selection. In optical spin polarisation, the selection is due to the spin-orbit coupling, and thus to an internal field. 

Optical electron spin polarisation (OEP) is the term used to describe a non-Boltzmann distribution of the populations of the three zero-field or Zeeman components of an optically-excited triplet state. This non-thermal equilibrium can be a stationary or a non-stationary state. The optical excitation, that is e.g. the UV excitation, must be neither narrow-band nor polarised, and at low temperatures, OEP is the normal case for most triplet states in organic tt-electron systems. The OEP is... 

A well-known and important phenomenon in the area of nuclear-spin resonance (NMR) in gases, liquids, or solid samples is dynamic nuclear-spin polarisation (DNP) (see e.g. [M6]). This term refers to deviations of the nuclear magnetisation from its thermal-equilibrium value, thus a deviation from the Boltzmann distribution of the populations of the nuclear Zeeman terms, which is produced by optical pumping (Kastler [31]), by the Overhauser effect [32], or by the effet solide or solid-state effect [33]. In all these cases, the primary effect is a disturbance of the Boltzmann distribution in the electronic-spin system. In the Overhauser effect and the effet solide, this disturbance is caused for example by saturation of an ESR transition. Owing to the hyperfine coupling, a nuclear polarisation then results from coupled nuclear-electronic spin relaxation processes, whereby the polarisation of the electronic spins is transferred to the nuclear spins. 

Fig. 7.31 The steady-state value of the optical nuclear-spin polarisation p = pi n a fluorene-dghio crystal doped with acridine, measured at room temperature, as a function of the orientation of the polarising magnetic field whose value is Bp = S mT. Pi is the equilibrium polarisation, and T5 is the nuclear-spin temperature. From [35].

In the method known as ADMR, the microwave transitions between the individual triplet sublevels are detected by means of a change in the optical absorption of the singlet ground state. This is based on a redistribution of the population between the three different triplet substates (zero-field states without, or Zeeman states with an external magnetic field) with different decay constants ( spin-polarisation ) if for example the spin population is pumped by microwave irradiation from a shortlived sublevel to a long-Hved one, then the overall triplet population is increased, while the population of the singlet ground state decreases. 

The well-characterised spin-qubit system of NV defects can be used for the QIT the quantum processor implementation can be realized by the coherent spin manipulation. The first step for coherent spin manipulation is to prepare a pure state of the internal spin structure. For the [N-V]" centre, spin state initialisation can be easily achieved by optical pumping with a polarised laser beam tuned above the absorption band. Decay from the 3E level via optical emission dominates but conserves spin. On the other hand, decay via the metastable singlet level 1A is slower since it does not conserve spin. Competition between those two processes at room temperature leads to spin polarization of the 3A ground level, making populated mainly the ms = 0 substate [40]. Once... 

Moreover, the manipulation of trapped radioactive ions with laser light opens up possibilities to study questions of the Standard Model. By optical pumping within the hyperfine levels of the ground state, the nuclear spins of radioactive nuclides can be polarised with high efficiency. The detection of the asymmetry of beta decay, for example, will allow one to explore deviations from the vector/axial-... 

Figure 9.22a shows an image of the patterned j X retarder made by the technology explained in this Chapter, viewed in between two crossed polaris-ers. The retarder is spin-coated on top of a colour filter. If the orientation of the optic axis of the A A retarder is parallel to the transmissive axis of one of the polarisers, the effective retardation is zero. An effective retarda-... 

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