3-dimensional optical vector field mapping using near-field scanning optical microscope probes

Abstract

In this thesis, I measured the direction of local electric field vectors in three-dimensional space with 100 nm resolution. A radial polarized light is generated by using a radial polarization converter and is focused by an objective lens. Gold nanoparticle functionalized probes are used as the local probes which scatter the focused field into the far-field region. The optical scattering properties of the gold nanoparticle functionalized probes are characterized by determining the polarizability tensors of them. I used rotational analyzer ellipsometry and Stokes parameters to reconstruct the local polarization states of the focused radial polarized light. Two distinct methods give consistent results with each other. I performed near-field measurements of transmission through nanometer-sized gaps at near infrared frequencies with varying the gap size from 1 nm to 10 nm. Field enhancement factors of the nanogaps were quantified by measuring transmission of the nanogaps using NSOM. The near-field measurements produce consistent result to the far-field measurement on quantifying the field enhancement factors of the nanogaps. In spite of the consistency, the near-field measurements have advantages of low background and accessibility to a single nanostructure. I demonstrated the mapping of optical magnetic field of the focused radial polarized light. Subwavelength apertures on metal films and on the apex of tapered fiber probes used as the local probes of the optical magnetic field. By scanning the apertures at the focal plane, I measured the optical magnetic field distribution of the focused radial polarized light. I compared the scattered signal from the apex of the conical probe and the collected signal through the aperture on the apex of the probe. From the measurements, I conclude that subwavelength apertures on the apex of the conical probe are sensitive to the horizontal components of the optical magnetic field. I fabricated near-field scanning optical microscopy (NSOM) probes with different geometrical factors which determine the selective sensitivity to the electric field and the magnetic field of light. The physical parameter dominating the preferential sensitivity is found to be the width of the metal rim surrounding aperture. I quantified the coupling ratio of the NSOM probes to the optical electric and magnetic field by measuring the scattering polarization of the probes. Using the characterized NSOM probes, I measured electromagnetic field distribution of vertical standing wave formed upon reflection at oblique incidence on a metal film. The vertical profiles of the collection signal from two different kinds of probes, the electric probes and the magnetic probes, appear to be out-of-phase.