Jiayu Zhang

Enhanced light transmission through cascaded metal films perforated with periodic hole arrays

We report experimental results on enhanced light transmission through two periodically perforated metal films separated by a layer of dielectric. A perforated metal film (single metallic structure) exhibits extraordinary optical transmission, and when two such perforated metal films are spaced by a dielectric layer (cascaded metallic structure), the transmission is further increased. The maximum transmission of the cascaded metallic structure, which depends on the distance between the two metal films, can be more than 400% greater than that of a corresponding single metallic structure. It is proposed that the coupling of surface plasmon polaritons between the two metal films is involved in the process.

Experimental imaging properties of immersion microscale spherical lenses

Using the immersion lensing technique, the resolution of a conventional spherical lens can be improved by a factor of 1/n over its value in air (n, the refractive index of the immersion medium). Depending on the relative position between an object and a lens, either a real or a virtual image is formed. Here we report a new physical phenomenon experimentally observed in the microscale lens imaging. We find that when a microscale spherical lens is semi-immersed in a medium, the resolution of the lens is improved as it can intercept more fine details of the object. However, the microscale lens has two image channels for the fine and coarse details and two images corresponding to the two components can be formed simultaneously. Our findings will advance the understanding of the super-resolution imaging mechanisms in microscale lenses.

Bicolor Mn-doped CuInS2/ZnS core/shell nanocrystals for white light-emitting diode with high color rendering index

We synthesized bicolor Mn-doped CuInS2 (CIS)/ZnS core/shell nanocrystals (NCs), in which Mn2+ ions and the CIS core were separated with a ZnS layer, and both Mn2+ ions and CIS cores could emit simultaneously. Transmission electron microscopy and powder X-ray diffraction measurements indicated the epitaxial growth of ZnS shell on the CuInS2 core, and electron paramagnetic resonance spectrum indicated that Mn2+ ions were on the lattice points of ZnS shell. By integrating these bicolor NCs with commercial InGaN-based blue-emitting diodes, tricolor white light-emitting diodes with color rendering index of 83 were obtained.

Effect of shell thickness on two-photon absorption and refraction of colloidal CdSe/CdS core/shell nanocrystals

Colloidal CdSe nanocrystals (NCs) are successively overcoated with CdS monolayers (MLs), and the effective two-photon absorption (TPA) coefficient and refractive index of this series of CdS/CdSe core/shell NCs are measured by a Z-scan technique at 800 nm wavelength. The TPA and nonlinear refraction are enhanced dramatically with the CdS shell growth towards 3 MLs, but decreased with the further shell growth. The effects of surface states, local field, and intrinsic piezoelectric polarization are discussed to explain the optical nonlinearity of the colloidal core/shell NCs.

Experimental far-field imaging properties of a ~5-μm diameter spherical lens

Microscale lenses are mostly used as near-sighted lenses. The far-field imaging properties of a microscale spherical lens, where the lens is spatially separated from the object, are experimentally studied. Our experimental results show that, for a blu-ray disc (an object) whose spacing is 300 nm, the lens can magnify the stripe patterns of the disc when the lens is spatially separated from the object. In the experimentally tested range (0-14 μm), all the magnified images are virtual images. When the distance is increased from 0 to 14 μm the magnification decreases from 1.47× to 1.20× and the field of view increases from 3.8 to 12.2 μm. The image magnification cannot be described by standard geometrical optics.

Voltage-dependent electroluminescence from colloidal CdSe∕ZnS quantum dots

Electroluminescence (EL) was obtained in the hybrid film of colloidal CdSe∕ZnS quantum dots (QDs) and poly(9-vinylcarbazole) (PVK), and the voltage and temperature dependences of the EL were measured. The quantum-confined Stark effect of colloidal QDs is clearly observed, and the QDs’ EL intensity rises with temperature. Electrons and holes are proposed to be separately transported in QDs and PVK, respectively.

Role of shape in middle-infrared transmission enhancement through periodically perforated metal films.

We report experimental results on the role of aperture shape in middle-infrared enhanced transmission through a metal film perforated with a periodic aperture array. The transmission spectrum is highly dependent on the aperture shape. When the aperture shape changes from circular to cross-dipole or the cross-dipole arm length increases, the transmission spectrum exhibits a large redshift. The enhanced transmission is proposed to be an interplay of surface plasmon polaritions at the metal surface and localized surface plasmons (LSPs) inside the apertures. The shift of the transmission bands is proposed to be due to the redshift of the LSP resonance spectrum of the individual apertures. The role of shape in enhanced transmission in the cascaded structure is also studied.

Enhanced transmission through metal films perforated with circular and cross-dipole apertures

We reported here the experimental results of the metal film perforated with circular and cross-dipole apertures. The transmission spectrum of the sample with a complex unit cell is substantially different from that of the corresponding sample with a simple unit cell. The localized surface plasmon polaritons are suggested to be involved into the enhanced transmission.

Diffraction inhibition in two-dimensional photonic crystals

We show the existence of flat equal-frequency contour across the entire first Brillouin zone in two-dimensional photonic crystals. Under this condition, light diffraction can be inhibited. Moreover, the beam can be highly localized between two neighboring rows of the defect-free photonic crystal. Such a finding may have novel applications in light beam manipulations and on-chip integrated photonic devices.

Highly efficient directional emission using a coupled multi-channel structure to a photonic crystal waveguide with surface modification

We propose a system for achieving highly efficient directional emission using a coupled multi-channel structure to a photonic crystal waveguide with a modified surface region. A highly directional beam with about 50% transmission efficiency at large distances can be obtained. The simulation results show that the surface modes in the modified region are easy to couple to the multi-channel structure, which can generate many light beams in the channels. The interference of the light beams exiting from the channels results in a highly efficient directional emission. Additionally, the size effect of the coupled multi-channel structure on the directional emission is also considered. When the row number of the coupled multi-channel structure is larger than a certain value, a steady state of highly efficient directional emission can be obtained.