Far- and near-field characterization of small radiation sources

Abstract

We present experimental studies on small radiation sources such as subwavelength apertures, photonic crystal lasers, and microdisk lasers in their far- and near-field regimes. For the far-field characterization of highly divergent emissions, we develop a solid angle scanner which is capable of scanning a detector and necessary optics over the surface of a sphere centered at the sources. Polarization-resolved two-dimensional angular distributions are measured with this instrument. For the near-field characterization of subwavelength structures, we develop a near-field scanning optical microscope which scans a fiber probe over the source surface with shear-force distance regulation using dual tuning forks. Spectrally-resolved near-field images are measured with this instrument. In the studies on the subwavelength apertures, the far-field emissions are decomposed into low-order vector spherical waves. Based on the analyses, a general multipole model for NSOM probes is proposed. Moreover, we find that surface waves are excited at the aperture and propagate along a polarization direction. The surface waves menifest themselves as sidelobes and polarization-dependent fringes in near- and far-field regimes of the aperture on a corrugated plane. In the studies on the photonic crystal lasers, various lasing modes excited in the hexagonal cavities defined by two-dimensional photonic crystals are identified and classified into one- and two-dimensional modes. The one(two)-dimensional modes lase when the lattice constant is small(large). The polarization state of one-dimensional modes are found to be transverse-electric, while two-dimensional modes have various polarization states. We find that the lasing modes decay in a few peiods in the photonic crystal regions. In the studies on the microdisk lasers, the azimuthal mode number of a whispering-gallery mode is experimentally determined by counting the number of bright regions in the far-field emission pattern. The polari...