GaAs nanowhisker arrays grown by magnetron sputter deposition

Arrays of cone-shaped GaAs nanowhiskers (NWs) with a surface number density of up to 10⁹ cm⁻², a height ranging from 300 to 10000 nm, and a diameter of about 200 nm at the base and from 200 to 10 nm and below at the top have been obtained by means of magnetron sputtering. The characteristic NW height is proportional to the effective thickness of a deposited material layer and inversely proportional to the transverse whisker size at the top. An analysis of the NW parameters confirms the validity of the Dubrovskii-Sibirev diffusion mechanism of NW formation.     

Growth of GaAs nanowhisker arrays by magnetron sputtering on Si(111) substrates

The growth of GaAs nanowhisker (NW) arrays on Si(111) substrates by magnetron sputtering is demonstrated. The characteristic NW length is proportional to the effective thickness of a deposited layer and inversely proportional to the transverse whisker size at the top. The results are explained in terms of the diffusion model of NW growth.     

Method for manufacturing zinc oxide nanowhisker arrays

Arrays of zinc oxide (ZnO) nanowhskers oriented perpendicularly to the surface of a substrate have been obtained by explosive laser evaporation of ZnO target on (0001)-oriented sapphire and unoriented silica, glass ceramic composite (Polycor), and glass substrates with magnetron-deposited ZnO sublayers layers. X-ray diffractograms of the nanowhisker arrays have been measured and features of their exciton luminescence have been studied.     

Interaction of TiW film with GaAs

The interactions of co-evaporated Ti_0.3W_0.7 films with GaAs have been studied after annealing at temperatures in the range of 650 to 900° C for 15 min employing Rutherford backscattering, transmission and scanning electron microscopy. X-ray diffraction and energy-dispersive X-ray analysis techniques. Reaction has been found to take place at 650° C as evidenced by the presence of an AsTi compound in the region near the interface of TiW film and GaAs. Gallium diffuses out to the surface at temperatures above 750° C and causes surface morphological degradation, which can be related to the instability of the TiW Schottky barrier height at higher temperatures 750° C as reported in the literature.     

Free-space optical interconnections with liquid-crystal microprism arrays

Liquid-crystal microprism arrays are shown to be useful for providing electrically controlled alignment of optical beams and fixed various free-space optical interconnections. They can deflect closely spaced micro-optical beams individually to any position with high transmittance (95%), high deflection angle (~10°), and low voltage (<2.8 V(rms)). Various fixed optical interconnections can be made simply by changes in the voltages applied to the microprism.     

Interaction of a crack with certain microcrack arrays

Interaction of a crack with microcracks (modelling “damage”) can significantly alter the stress concentration at the crack tip. Certain important effects of interaction—shielding effect (“toughening by microcracking”), amplification effect, influence of the orientations of microcracking and irregularities in its patterns, change of the character of interaction (shielding to amplification and vice versa) with change of the mode of loading, etc.—are demonstrated on several microcrack systems. It appears that these relatively simple systems exhibit the essential features of the crack-damage interactions. Consideration is based on the method of analysis of elastic solids with many cracks proposed recently [Kachanov, Int. J. Fracture 28, R11–R19 (1985); Kachanov, Int. J. Solids Structures (in press)] and briefly presented here.     

High frequency ultrasound imaging with optical arrays

Dynamically focused and steered high frequency ultrasound imaging systems require arrays with fine element spacing, wide bandwidths, and large apertures. However, these characteristics are difficult to achieve at frequencies greater than 30 MHz using conventional array construction methods. Optical schemes offer a solution. Focused laser beams incident on a suitable surface can generate and detect acoustic radiation. Precisely controlling the position and size of the beams defines points of transmission and detection, making it possible for pulse-echo image formation by synthetic aperture methods. An optical detection array was built, relying on a conventional piezoelectric transducer as an ultrasound source. The detection system, with near optimal resolution over a wide depth of field, demonstrates the potential for high frequency array implementation using optical techniques. A possible application is in pathology, where 2-D or 3-D fine resolution pulse-echo imaging can be performed in situ without the need for biopsies.     

GaAs/AIGaAs laser arrays with and without proton isolation

Abstract

Two types of GaAs/AIGaAs laser arrays, each consisting of five emitters and suitable for high power operations, have been fabricated and tested. They are easily fabricated, have high yield and deliver high power. The first array structure was fabricated using proton implantation to define the active lasing channels. Damage introduced by proton bombardment provides both electrical and optical confinement for the lasing channels. We have investigated the effect of heat treatment on the performance of these lasers and have found that the heat treated samples had a lower threshold current and higher quantum efficiency. For 400 μm long devices, with uncoated facets, we have measured a threshold current of 200 mA and a peak power of 231 mW/facet. The highest external quantum efficiency is 60%. We have also fabricated laser arrays with a simple, self‐aligned, index guided ridge waveguide structures. We have obtain a pulsed output power of up to 551 mW/facet with a quantum efficiency of 37.4%. The typical threshold current is 370 mA.     

Controllable optical trap arrays

A method of generating 3D optical trap arrays (OTAs) using a uniaxial crystal has been proposed and implemented. It is shown that the properties of obtained OTAs can be controlled by changing the parameters of the optical system and the state of radiation polarization past the crystal.     

Area-time trade-offs in arrays with optical pipelined buses

A general model for determining the computational efficiency of a particular class of electro-optical systems is described. The model is an abstraction of parallel systems that use digital electronic processors and optical pipelined buses for communication. Minimum requirements in terms of area (volume for three-dimensional structures) and time necessary in order to solve a problem are obtained. Different applications are investigated, and a matching area-time upper bound is given for the barrel-shift problem, simulated on an array with reconfigurable optical pipelined buses. The types of problems for which these lower bounds seem to be realistic are described.     

Compact optical imaging system for arrays of optical thyristors

A compact and modular optical system that employs gradient-refractive-index rod lenses to image arrays of Lambertian sources is characterized both experimentally and by ray-tracing simulations. A hybrid optical system that incorporates additional microlens arrays to reduce transmittance losses and aberrations is also modeled, and the two systems are compared.     

Polynomial-based optical true-time delay devices with microelectromechanical mirror arrays

We previously reported optical true-time delay devices, based on the White cell, to support phased-array radars. In particular, we demonstrated a quadratic device, in which the number of delays obtainable was proportional to the square of the number of times the light beam bounced in the cell. Here we consider the possibilities when a microelectromechanical (MEM) tip/tilt mirror array with multiple stable states is used. We present and compare designs for quadratic, quartic, and octic cells using MEM mirror arrays with two, three, and five micro-mirror tilt angles. An octic cell with a three-state MEM can produce 6,339 different delays in just 17 bounces.     

Tunable optical transmission through gold slit arrays with Z-shaped channels

The transmission of a normally incident wave through an array of subwavelength gold film with Z-shaped slits has been explored by using the finite-difference time-domain method. The results show that the transmission of a thinner metal film perforated with a Z-shaped slit array behaves nearly the same as that of a thicker metal film perforated with straight slit array with the same central slit length, which is useful for the miniaturization of the optical device. It is also presented that the transmission of a Z-shaped slit array sensitively depends on the slit geometrical parameters. By adjusting the width and length of each section of the Z-shaped slit, noticeable magnitude modification of the transmission, redshift, and blueshift of the resonance modes is found, which is useful for the design of frequency-selective and sensor optical devices.     

Efficiency enhancement InGaP/GaAs dual-junction solar cell with subwavelength antireflection nanorod arrays

The enhanced conversion efficiency of the InGaP/GaAs dual-junction solar cell was demonstrated utilizing broad-band and omnidirectional antireflection nanorod arrays. The nanorod arrays were fabricated by self-assembled Ni clusters, followed by inductively-coupled-plasma reactive ion etching. The conversion efficiency measured under one-sun air mass 1.5 global illuminations at room temperature was improved by 10.8%. The light absorption efficiencies of the top InGaP and bottom GaAs cells were also studied under the influence of nanorod arrays. The enhanced absorption efficiency was mostly contributed from the short wavelength absorption by top cell. Surface nanorod arrays served not only as broad-band omnidirectional antireflection layers but also scattering sources. The structure can be further optimized to obtain the maximum conversion efficiency.     

Broad-bandwidth radiation patterns of sparse two-dimensionalvernier arrays

The fabrication of a dense (one-half wavelength element spacing) two-dimensional (2D) transducer array suitable for medical ultrasound imaging is unrealistic using existing technology. Consequently, there is interest in developing sparse 2D transducer arrays. In this paper, we present the results of a study looking at the broad-bandwidth radiation patterns of 72 different sparse 2D vernier arrays. Suppression of grating lobes is achieved by choosing a different arrangement of transmit and receive elements using an analogy with a vernier scale. The broad-bandwidth radiation patterns were investigated by simulating volumetric sector scan of a point target. We summarize these results by deriving a set of design curves that predicts the minimum number of elements, element spacing, and apodization required for a desired beam width and maximum secondary lobe. The results show that a sparse vernier array can be designed with significantly lower average and peak secondary lobes compared with a sparse random array with the same number of elements and aperture size     

Single molecule tracking on supported membranes with arrays of optical nanoantennas

Coupling of the localized surface plasmons between two closely apposed gold nanoparticles (nanoantenna) can cause strong enhancements of fluorescence or Raman signal intensity from molecules in the plasmonic "hot-spot". Harnessing these properties for practical applications is challenging due to the need to fabricate gold particle arrays with well-defined nanometer spacing and a means of delivering functional molecules to the hot-spot. We report fabrication of billions of plasmon-coupled nanostructures on a single substrate by a combination of colloid lithography and plasma processing. Controlled spacing of the nanoantenna gaps is achieved by taking advantage of the fact that polystyrene particles melt together at their contact point during plasma processing. The resulting polymer thread shadows a gap of well-defined spacing between each pair of gold triangles in the final array. Confocal surface-enhanced Raman spectroscopy imaging confirms the array is functionally uniform. Furthermore, a fully intact supported membrane can be formed on the intervening substrate by vesicle fusion. Trajectories of freely diffusing individual proteins are traced as they sequentially pass through, and are enhanced by, multiple gaps. The nanoantenna array thus enables enhanced observation of a fluid membrane system without static entrapment of the molecules.     

Interaction of microwave radiation with an erosion plasma jet

The interaction of high-power pulsed microwave radiation with a plasma jet formed by a discharge in an ablative capillary is studied. A significant influence of microwave radiation on the plasma jet flow is found. Depending on the intensity of the initial perturbation of the jet, different scenarios of its evolution downstream are possible: attenuation or amplification accompanied with the development of turbulence up to the disruption of the flow if a certain threshold of the energy action is exceeded. A significant influence of the plasma jet and its state on the spatial position of the microwave energy release zone is found.     

Directional laser generation and detection of ultrasound with arrays of optical fibers

Some aspects of the generation and detection of ultrasound with laser beams are presented. The objective of the study is (1) to present some experimental results on the generation of ultrasound with an array of optical fibers, (2) to discuss the possibility of generating directional surface waves with acousto-optic scanning in a very narrow frequency band, thus increasing the signal-to-noise ratio, and (3) to discuss the feasibility of the directional detection of ultrasound by using an array of optical fibers as a receiver, also with the goal of increasing the signal-to-noise ratio.