Search

The beaming effect and interference patterns on the exit surface of multiple nano metallic slits with micrometer pitch were observed by scanning near-field optical microscopy. The near-field light intensity for multiple slits had a longer propagation tail and a smaller diverging angle as compared to that in a single slit. From finite-difference time-domain calculations, we verify that these fringe patterns come from the interference effect of longitudinal electric field, which is propagating along the surface. The beaming light in the slit openings is also attributed to the addition of longitudinal fields.

Optical near-field in subwavelength air-dielectric structure is studied by a collection-mode near-field optical microscopy. Concentrated optical near-field and large polarization anisotropy at the dielectric regions are measured. From the simulation of finite-difference time-domain method, we conclude the edge diffraction dominating the optical near-field distribution.

Phase separations in conjugated polymer (MEH-PPV) and non-conjugated polymer (PMMA) blended films are studied by photoluminescent (PL) near-field scanning optical microscope and spectrometer. The morphologies of blended films changed from stripe patterns to isolated islands as PMMA concentrations increased. The PL spectra in isolated micron dots are the same as MEH-PPV. Nevertheless, we found new PL spectra, peaked at 500 nm wavelength, in the PMMA matrix. Since, PMMA does not emit light, the new spectra indicate some MEH-PPV diluted in the PMMA rich regions. The excitons of MEH-PPV in the PMMA matrix are localized and emit light as PPV oligomers.

Subwavelength focusing in mesoscale structures was measured in the near field. Specifically, we found that plane waves form focused beams in higher topographic regions when they propagate through mesoscale transparent air-dielectric structures. By finite-difference time-domain simulations we verified that light diffracted off topographic edges and its convergence at higher topographic regions are the mechanisms for focus. This subwavelength focusing effect provides a simple way for mass production of mesoscale periodic structures. We made subwavelength gratings and hole arrays to demonstrate their feasibility.

This research investigates the propagation behavior of the optical near field between gold-coated tapered fiber tips and magnetic thin films. From three-dimensional finite-difference time-domain calculations, we verified that p-polarized incident light generated a strong longitudinal electric field in the fiber tip. This longitudinal electric field can propagate in the nanogap with the aid of surface-plasmon waves. The surface-plasmon wave and the transverse magneto-optical Kerr effect were used in near-field microscopy to measure the magnetic images of submicron Ni–Fe dot arrays. Changes of magnetic domains under different external magnetic fields were experimentally verified by near-field magneto-optical microscopy.

Optical contrast on the surface of mesoscale air–dielectric structures was evaluated with collection-mode near-field scanning optical microscopy. When air holes (300 nm) were smaller than the incident wavelength (400 or 600 nm), the optical field was concentrated in the dielectric region. When air holes (1000 nm) were larger than the incident wavelength (400 or 600 nm), there was topographic edge enhancement. After confirmation with finite-difference time-domain simulation, we conclude that light diffracted off topographic edges is a major contributor to contrast in collection-mode near-field optical images.

The propagation and the distribution of the optical near field in nanometallic slits are measured by a near-field scanning optical microscope. The optical near field for the p-polarized wave is confined to the middle of the slit. In contrast, the near field for the s-polarized wave is located at the edges. A simulation by the finite-difference time-domain method verifies that the near-field distribution for the s-polarized wave is due to the propagation of the surface plasmon wave (SPW) at the air-metal surface. The existence of the SPW also accounts for the extraordinary transmittance of s-polarized light, which is one order of magnitude larger than that of p-polarized light.