Spin polarization in near-field optical microscopy

Abstract

The possibility of spin polarization and coherence detection is discussed theoretically as a strong coupling limit of a probe-tip-sample system in near-field optical microscopy and spectroscopy. A novel detection scheme is proposed, and the characteristic behavior is numerically analyzed on the basis of the field propagator method and density operator method for a nanometric probe-tip and a quasi-two-level sample system. It is shown that the spin polarization and coherence of the sample give different peak intensities and peak shifts of the near-field signal as a function of the probe-tipsample distance, depending on the linear polarization of the probe beam. The results suggest that one may use the characteristics in order to sense a sample state.Recent advances in near-field optical microscopy and spectroscopy have expanded the potential for using local plasmon spectroscopy [1-3] and single molecule spectroscopy [4] in addition to high-resolution imaging of a variety of samples with lateral resolution on a nanometer scale and perpendicular resolution on a sub-nanometer scale [5][6][7]. Examples of nanometric structure manipulation include fundamental experiments on ultra-high-density storage devices. Moreover, atomic guidance by an evanescent field in micron-tosubmicron hollow fibers has been demonstrated in research on nano/atom photonics [8]. Thus, both superresolution beyond the diffraction limit and size-dependent field confinement have been confirmed as inherent characteristics of nearfield optical microscopy and spectroscopy.Prior to these advances, atomic behavior near the substrate associated with the energy shift of resonance, the line shape, the strength, and the width of resonance has been investigated in detail theoretically and experimentally by optical pump-probe spectroscopy [9][10][11][12][13][14]. For example, the spin polarization and magnetic resonance of alkali-metal atomic vapors (cesium and rubidium) have been measured by selective reflection optical pumping [11]. An evanescent field used as a probe beam showed different signal behavior from a propagating field in the presence of sodium atoms [12].The optical spin orientation on III-V semiconductor materials such as GaAs has likewise been extensively studied to determine the details of their band structure, the types of optical transition and relaxation processes, and so on [15]. These materials have also been investigated from the viewpoint of spin-polarized electron tunneling [16,17], while the spin polarization of magnetic materials has been systematically studied by scanning tunneling microscopy [18,19].When one tries to apply the near-field technique to the above problems, benefiting from the resolution improvement, it becomes important to consider the efficiency tradeoff between light illumination and detection power. In addition, we have to bear in mind that physical quantities behave differently depending on the probe size and probe-sample distance, as emphasized in single molecule near-field spectroscopy relating to the quenching ...

Quantum theoretical approach to a near‐field optical system

“…Such experimental results exemplify the need for quantum theoretical treatment of optical near‐field problems. Several authors, in fact, have semi‐classically analysed near‐field spectroscopic problems (Girard et al ., 1995; Cho et al ., 1996; Kobayashi, 1998), formulating electronic structures in terms of quantum mechanics and optical near fields in terms of classical electromagnetics.…”

<title>Quantum theory and virtual photon model of near-field optics</title>

“…The third term in Eq. (15) represents the interactions between photons and excitons in a multipolar QED Hamiltonian,39 where /1 (f') is the dipole operator of matter and D (j) is the displacement field operator that is given by the conjugate momentum operator II ( of the vector potential operator A (iv) expanded in terms of plane wave bases as…”