Narrow Results

We explain the mechanism of defect screening in GaInN/GaN quantum wells, which are used as active layers in white and blue light emitting diodes (LEDs). Despite the fact that these devices have now been commercially available for some time, the reason for the high luminescence efficiency had not been really understood. The high defect densities in these devices commonly would not allow the use as an optical emitter. We present the mechanism turning an actually poor-quality material into a powerful optical emitter.

Scanning near-field optical microscope (SNOM) was employed to investigate the room temperature photoluminescence (PL) of single ZnO nanowires with different radii excited by 325 nm laser. Two-dimensional distribution of their PL intensity is provided for the analysis of intensity decay from emission source. It is found that the PL intensity at both ends of each ZnO nanowire (end emission) was much stronger than that at the sides of the wire (side emission). Further investigation indicates that the quality of end emission depends on the diameters of the wires. Some of the ZnO nanowires with special diameters emit stronger light, and the shape of the light is close to Gauss beam. In addition, the Gauss shape light can diffuse longer distance than what the side emission does, typically in the range of a few micrometers. It is a sign of the fact that special guided modes of the PL light are formed in the nanowires. The calculation results predicate that the special guiding mode strongly relies on the diameters of the ZnO nanowires. The good directional property and high intensity of the end emission have many potential applications, including optical switch and microanalysis. It has been shown that SNOM can provide direct evidence of light emission properties from single nanowires, and hence provide the clue of increasing light efficiency and the improvement of light-propagating mode.

The 5,10,15,20-tetrakis[3,4-bis(2-ethylhexyloxy)phenyl]-21H,23H-porphinato zinc(II) (ZnEHO) is highly stable and exhibits a colorful absorption spectrum in the visible range. Exposure of a chloroform solution of ZnEHO to amines is shown to induce changes in the characteristic optical spectrum owing to charge transfer between the amine and the delocalized π-electron system within the highly conjugated molecule. Solid state Langmuir Blodgett (LB) films containing only ZnEHO are compared to films containing a mixture of ZnEHO and calix[8]arene. The transparent calix[8]arene does not change the optical response but aids the diffusion of the amine gas into the LB films. Atomic force microscopy (AFM) and scanning near-field optical microscopy (SNOM) images demonstrate the topological and compositional differences between the samples. The response of the LB films of ZnEHO and calix[8]arene to a variety of different amines demonstrates that this is a good material system for use as an amine sensor.

Optical imaging with nanoscale resolution, beyond that possible with conventional diffraction-limited microscopy, may be achieved by scanning a nano-antenna in close proximity to a sample surface. This review will first aim to provide an overview of the basic principles of the technique of scanning near-field optical microscopy, before moving on to consider its most widely implemented form, in which the sample is illuminated through a small aperture held less than 10nm to the sample surface, for optical imaging with a resolution of about 50 nm. The exciting new possibilities for high-resolution optical imaging and spectroscopy promised by “apertureless” near-field optical microscopy are then considered. Such techniques may involve local scattering of light from a sample surface by a tip, local enhancement of an optical signal by a metal tip, or the use of a fluorescent molecule or nanoparticle attached to a tip as a local optical probe of a surface. These new optical nanoprobes offer the promise of optical microscopy with true nanometer spatial resolution.

We study numerically the constant-height image in transmission mode of photon scanning tunneling microscopy (PSTM). The local field inside the probe tip is calculated, which scans over a dielectric rectangular structure. The substrate is illuminated by a s-polarized incident light with oblique angle. The influence by the dielectric functions of the tip and the lateral size of the structure on the optical signal are analyzed. We find out that the transmitted intensity is correlated with the shape of the sample and the assumption of passive probe may be a reasonable approximation when the optical constant of the tip is close to that of air.