Xiaochun Dong

Intergrain current flow in a randomly oriented polycrystalline SmFeAsO0.85 oxypnictide

We report a direct current transport study of the local intergrain connections in a polycrystalline SmFeAsO0.85 (Sm1111) bulk, for which we earlier estimated significant intergranular critical current density Jc. Our combined low temperature laser scanning microscopy and scanning electron microscopy observations revealed only few grain-to-grain transport current paths, most of which switched off when a magnetic field was applied. These regions typically occur where current crosses Fe–As, which is a normal-metal wetting-phase that surrounds Sm1111 grains, producing a dense array of superconducting-normal-superconducting contacts. Our study points out the need to reduce the amount of grain boundary-wetting Fe–As phase, as well as the crack density within pnictide grains, as these defects produce a multiply connected current-blocking network.

Re-entrant spin glass behavior in Mn-rich YMnO3

We use magnetism and specific heat measurements to investigate the hexagonal Mn-rich YMnO3. It is found that upon cooling from a high temperature, the compound first orders antiferromagnetically at TN∼72K and then undergoes a re-entrant spin glass (RSG) transition at TSG∼42K. This RSG behavior results from the competition between the ferromagnetic interaction and the antiferromagnetic interaction, which is related to the intrinsic geometric magnetic frustration in this system.

A high refractive index metamaterial at visible frequencies formed by stacked cut-wire plasmonic structures

A type of metamaterial composing of stacked cut-wire plasmonic structures has been designed, fabricated, and characterized. The excitation of electric resonance caused by the surface plasmons effect dominates the electric field states, so that the effective refractive index can behave as a dramatic increase at visible frequencies, which is intimately associated with the resonance strength and can be modulated by reasonably changing the structure geometries. The phenomenon has been demonstrated with the measured transmittances being in agreement with simulation results. And the effective parameters judged by Kramers–Kronig relations have been uniquely retrieved from the simulated transmission and reflection data.

Localized surface plasmon nanolithography with ultrahigh resolution

A localized surface plasmon nanolithography (LSPN) technique is proposed and demonstrated to produce patterns with a sub-20nm line width. High transmission efficiency is realized by adjusting the period of grating. The well-regulated grating structures in metallic mask are employed to excite surface plasmon polaritons (SPPs) on the illuminated side. The SPP waves propagate toward the tip along the taper surfaces which cause most of energy accumulation at the tip and give rise to high local field enhancements in a near-field region around the tip. The amplitude of local electric field intensity is quite large and the line width can be confined within sub-20nm, at the same time, the contrast and spatial resolution are greatly enhanced, which can facilitate nanolithography efficiently with simple ultraviolet light sources.

Profile control technology for high-performance microlens array

A profile formation and control approach has been developed for manufacturing micro-optical elements with continuous profile and deep relief depth. Based on Dills exposure model, an effective expression for determining the exposure dose function is established by using a supposition of equivalent exposure threshold inside a resist layer. An analytical simplified formula is further deduced by taking absorbance as constant B, and the approximate condition is discussed. For evaluating the simplified formula, the profile error was calculated and analyzed by simulation. With the exposure dose function, the binary mask for manipulating the light distribution by means of a moving-mask lithographic method can be designed. Experimental results are given and show the comparative performance to the required profile and relief depth. A series of refractive microlens arrays with aspherical profiles, a wide range of numerical apertures (0.005 to 0.6), and high fill factors were accomplished in the lab and may be applied to many systems

Nanolithography method by using localized surface plasmon mask generated with polydimethylsiloxane soft mold on thin metal film

We propose a photolithographic method to fabricate nanostructures by employing a localized surface plasmon (LSP) mask generated by a soft mold on a thin metal film. The soft mold can be formed by transparent materials, such as polydimethylsiloxane, contacting firmly to the metal film. The pattern edges of the mold, serving as the fine tapers, can be used to excite LSPs and accumulate a large amount of localized energy from the incident light field, providing a modulated optical field in the resist with nanometer feature size. Nanolithographic results with a minimum feature size of 30 nm are demonstrated.

Geometrical characterization issues of plasmonic nanostructures with depth-tuned grooves for beam shaping

Design of an enhanced surface plasmon polaritons (SPPs) based nanostructure for the purpose of beam shaping is discussed. An indentation with depth-tuned grooves is presented to realize the beam shaping and extraordinary transmission. The nanostructure is directly fabricated using focused ion beam (FIB) milling on an Ag thin film coated on quartz with a thickness of 200 nm. A large measurement error is found during geometrical characterization of the nanostructures by use of an atomic force microscope (AFM) working in tapping mode. Apex wearing and 34 deg full cone angle of the probe generate the measurement errors during the characterization of nanostructures with a feature size of 200 nm and below. To solve this problem, an FIB trimmed AFM probe is employed in the geometrical characterization. The results show that the error is improved greatly using the trimmed probe. The desired excitation of the SPPs is derived using an optical fiber coupled CCD spectrometer after the modified geometrical characterization. The designed structure can be used as an optical probe for future inspection and detection use. (c) 2006 Society of Photo-Optical Instrumentation Engineers.

General conformal transformation method based on Schwarz-Christoffel approach

A general conformal transformation method (CTM) is proposed to construct the conformal mapping between two irregular geometries. In order to find the material parameters corresponding to the conformal transformation between two irregular geometries, two polygons are utilized to approximate the two irregular geometries, and an intermediate geometry is used to connect the mapping relations between the two polygons. Based on these manipulations, the approximate material parameters for TE and TM waves are finally obtained by calculating the Schwarz-Christoffel (SC) mappings. To demonstrate the validity of the method, a phase modulator and a plane focal surface Luneburg lens are designed and simulated by the finite element method. The results show that the conformal transformation can be expanded to the cases that the transformed objects are with irregular geometries.

Experimental study of a multiwavelength photon sieve designed by random-area-divided approach

In this paper, a design method for a multiwavelength photon sieve is described based on a random-area-divided approach, where the whole aperture of a multiwavelength imaging photon sieve is divided into multiple discrete spaces corresponding to the number of the selected working wavelengths. The micropinhole distribution in each discrete space can be calculated for the defined wavelength with one fixed focal length in terms of the normal design for photon sieve. A three-wavelength photon sieve was designed and fabricated in the lab, and its imaging properties are analyzed in the experimental optical system with satisfactory results.

Surface pattern and large low-field magnetoresistance in La0.5Ca0.5MnO3 films

Formation of an ordered surface structure in La0.5Ca0.5MnO3 films due to the mismatch of the thermal expansion coefficient between the film and the substrate has been investigated. The surface pattern consists of grain chains located on regular orthogonal cracks. The cracks serve as weak-link grain boundaries, and unusually enhanced low-field magnetoresistance (−14.4% in 400 Oe at 90 K) has been observed, which may be explained by spin-polarized tunneling across the grain boundaries.