Multiple necks around biaxially loaded holes in sheets

Experiments are described in which holes in sheets of 3 and 4 mm thick half-hard commercially pure aluminium are equibiaxially expanded by hydraulic bulging using an undersheet of 1 mm thick mild steel. Discrete radial necks are formed within the annulus (not at the bore), the number of which depends on the initial hole diameter/sheet thickness ratio. Prediction of the amount of deformation at the onset of neck formation requires the deformation history of elements to be followed. The number of necks depends approximately linearly on the ratio of the initial diameter of the hole to the sheet thickness. Further loading produces radial cracks within some (but not all) of the thinning necks.     

Surface second-harmonic generation from vertical GaP nanopillars

We report on the experimental observation and analysis of second-harmonic generation (SHG) from vertical GaP nanopillars. Periodic arrays of GaP nanopillars with varying diameters ranging from 100 to 250 nm were fabricated on (100) undoped GaP substrate by nanosphere lithography and dry etching. We observed a strong dependence of the SHG intensity on pillar diameter. Analysis of surface and bulk contributions to SHG from the pillars including the calculations of the electric field profiles and coupling efficiencies is in very good agreement with the experimental data. Complementary measurements of surface optical phonons by Raman spectroscopy are also in agreement with the calculated field intensities at the surface. Finally, polarization of the measured light is used to distinguish between the bulk and surface SHG from GaP nanopillars.     

New Optical Fiber Coherence Tomography— Compact and Stabile System for Medical Application

A new idea of the optical fiber coherence tomography system designed for medical application is presented in this paper. Application as a source the femtosecond 400 mW Ti:Al₂O₃ pulse laser operates at wavelength 798 nm with spectral pulse bandwidth about 145 nm provides the longitudinal resolution about 2 mum. The system can be used for scanning the interior of a human body owing to application of a special optical head based on single-mode fiberscope. The structure comprises a set of two fiber-optic interferometers. The first is a Michelson type which enables the doubled pulses generation with spatial separation directly correlated with a deep scanning in the investigated sample. The second one is a partially delayed Fizeau interferometer formed on the end of optical fiber providing a high temperature and polarization system stability. The discussed setup consists also of a new signal processing structure based on synchronous detection scheme and application of the lock-in amplifier allowing investigation of the reflection coefficient of investigated sample via a suitable numerical processing of the first two harmonic amplitudes of the detected signals. The experimental results of OCT reconstruction of a tree leaf in area 150 ×  1750 mum giving the possibility of recognizing various kind of tissues are also presented.     

Wave propagation through a photonic crystal in a negative phase refractive-index region

The wave propagation through a photonic crystal with a triangular lattice of air holes realized in the InP-InGaAsP heterostructure are studied theoretically for the transverse magnetic modes. The photonic crystal possesses a negative refractive index, and the self-focus of the beam is successfully observed. The weak side beams are observed due to high-order Bloch waves in the photonic crystal. The coupling efficiency for the outgoing waves to a waveguide is also studied.     

Models and measurements for the transmission of submicron-widthwaveguide bends defined in two-dimensional photonic crystals

One of the essential building-blocks of miniature photonic crystal (PC)-based photonic integrated circuits (PICs) is the sharp bend. Our group has focused on the 2-D photonic crystal based on a triangular lattice of holes perforating a standard heterostructure. The latter, GaAlAs-based or InP-based, is vertically a monomode waveguide. We consider essentially one or two 60° bends defined by one to five missing rows, spanning both cases of monomode and multimode channel waveguides. From intensive modeling and various experimental measurements (both on GaAs and InP), we point out the origin of the present level of bend insertion losses and discuss the merits of the many roads open for improved design