Photoconductive optically driven deformable membrane under high-frequency bias: fabrication, characterization, and modeling

The fabrication and characterization of an optically addressable deformable mirror for a spatial light modulator are described. Device operation utilizes an electrostatically driven pixelated aluminized polymeric membrane mirror supported above an optically controlled photoconductive GaAs substrate. A 5 mum thick grid of patterned photoresist supports the 2 mum thick aluminized Mylar membrane. A conductive ZnO layer is placed on the backside of the GaAs wafer. Similar devices were also fabricated with InP. A standard Michelson interferometer is used to measure mirror deformation data as a function of illumination, applied voltage, and frequency. The device operates as an impedance distribution between two cascaded impedances of deformable membrane substrate, substrate, and electrode. An analysis of device's operation under several bias conditions, which relates membrane deformation to operating parameters, is presented.     

Photoconductive optically driven deformable membrane for spatial light modulator applications utilizing GaAs substrates

The fabrication and characterization of an optically addressable deformable mirror for a spatial light modulator is described. Device operation utilizes an electrostatically driven pixellated aluminized polymeric membrane mirror supported above an optically controlled photoconductive GaAs substrate. A 5 microm thick grid of patterned photoresist supports the 2 microm thick aluminized Mylar membrane. A conductive ZnO layer is placed on the back side of the GaAs wafer. A standard Michelson interferometer is used to measure mirror deformation data as a function of illumination, applied voltage, and frequency. A simplified analysis of device operation is also presented.     

Backside calibration chronopotentiometry: using current to perform ion measurements by zeroing the transmembrane ion flux

A recent new direction in ion-selective electrode (ISE) research utilizes a stir effect to indicate the disappearance of an ion concentration gradient across a thin ion-selective membrane. This zeroing experiment allows one to evaluate the equilibrium relationship between front and backside solutions contacting the membrane by varying the backside solution composition. This method is attractive since the absolute potential during the measurement is not required, thus avoiding standard recalibrations from the sample solution and a careful control of the reference electrode potential. We report here on a new concept to alleviate the need to continuously vary the composition of the backside solution. Instead, transmembrane ion fluxes are counterbalanced at an imposed critical current. A theoretical model illustrates the relationship between the magnitude of this critical current and the concentration of analyte and countertransporting ions and is found to correspond well with experimental results. The approach is demonstrated with lead(II)-selective membranes and protons as dominating interference ions, and the concentration of Pb(2+) was successfully measured in tap water samples. The principle was further evaluated with calcium-selective membranes and magnesium as counterdiffusing species, with good results. Advantages and limitations arising from the kinetic nature of the perturbation technique are discussed.     

Fringe visibility improvement using an asynchronous image-subtracting optically addressed spatial light modulator

We demonstrate the application of an asynchronous image-subtraction optically addressed spatial light modulator to particle image velocimetry fringe processing. The device comprises an amorphous silicon p-i-n-i-p photosensor and a ferroelelectric liquid-crystal light-modulating layer. The images to be subtracted are encoded on two separate wavelengths. The operation of the device is described, and characterization shows a frame rate of 100 Hz, a resolution of 3 line pairs/mm, and a write-light sensitivity of ≈1 mW/cm(2) at a wavelength of 514 nm. The device is read by the use of light with a 633-nm wavelength whereas the subtraction light is at a wavelength of 670 nm. Using this device to subtract a nonuniform pedestal from the optically computed power spectral density function (the Young's fringe pattern), we find we can improve the signal-to-clutter ratio of peaks in the image-transmittance autocorrelation function of particle image velocimetry transparencies. The device also permits processing of very low-visibility fringe patterns, generated from doubly exposed images, in which one image has half the transmittance of the other. These could not be processed with a nonsubtracting, binary, liquid-crystal optically addressed spatial light modulator.     

Diffraction efficiency analysis of a parallel-aligned nematic-liquid-crystal spatial light modulator

An optically addressed parallel-aligned nematic-liquid-crystal spatial light modulator is developed for applications in optical information processing and interferometry. Its performance, including diffraction efficiency, is measured, and a theoretical analysis of diffraction efficiency is performed. A comparison between the experimental results and the theoretical analysis shows good agreement. The diffraction efficiency of near-sinusoidal gratings written on the device is of the order of 30%, which is close to theoretical maximum.     

A Solar‐Blind UV Detector Based on Graphene‐Microcrystalline Diamond Heterojunctions

An ultraviolet detector is demonstrated through a whole‐wafer, thin diamond film transfer process to realize the heterojunction between graphene and microcrystalline diamond (MCD). Conventional direct transfer processes fail to deposit graphene onto the top surface of the MCD film. However, it is found that the 2 µm thick MCD diamond film can be easily peeled off from the growth silicon substrate to expose its smooth backside for the graphene transfer process for high‐quality graphene/MCD heterojunctions. A vertical graphene/MCD/metal structure is constructed as the photodiode device using graphene as the transparent top electrode for solar‐blind ultraviolet sensing with high responsivity and gain factor. As such, this material system and device architecture could serve as the platform for next‐generation optoelectronic systems.     

Self-biasing effect in colour sensitive photodiodes based on double p-i-n a-SiC:H heterojunctions

A large area colour imager optically addressed is presented. The colour imager consists of a thin wide band gap p-i-n a-SiC:H filtering element deposited on the top of a thick large area a-SiC:H(-p)/a-Si:H(-i)/SiC:H(-n) image sensor, which reveals an intrinsic colour filter. In order to tune the external applied voltage for full colour discrimination, the photocurrent generated by a modulated red light is measured under different optical and electrical bias. Results show that the device, under appropriated readout voltages, behaves itself as an imager and a filter giving information not only on the position where the optical image is absorbed but also on its wavelength and intensity. Identification of the red, green and blue components of the spectrum and simultaneous image recognition were achieved at readout voltages that are able to cancel the self-bias effect due to the different light penetration depth. These voltages shift from positive to negative values as the wavelength of the impinging photons across the back absorber increases. A numerical simulation supports the colour filter analysis.     

Simple, low-cost technique for photolithographic self-aligned top metal contacts to nanowires and nanotubes

We propose a new simple, low-cost method for providing all-round metal contacts to one-dimensional structures such as carbon nanotubes and nanowires on a transparent substrate. The nanostructures are first positioned in place to bridge a electrode gap by dielectrophoresis. The electrode structure is then used as a self-aligned mask during the subsequent photolithography through illumination from the substrate backside. This is followed by metallization and lift-off. Our measurements on multi-walled carbon nanotubes thus contacted show reasonable yield and good electrical contacts for the process carried out on a glass slide as the substrate.     

Electroluminescence from silicon nanoparticles fabricated from the gas phase

Electroluminescence from as-prepared silicon nanoparticles, fabricated by gas phase synthesis, is demonstrated. The particles are embedded between an n-doped GaAs substrate and a semitransparent indium tin oxide top electrode. The total electroluminescence intensity of the Si nanoparticles is more than a factor of three higher than the corresponding signal from the epitaxial III-V semiconductor. This, together with the low threshold voltage for electroluminescence, shows the good optical properties of these untreated particles and the efficient electrical injection into the device. Impact ionization by electrons emitted from the top electrode is identified as the origin of the electrically driven light emission.     

Optimization of multichannel parallel joint transform correlator for accelerated pattern recognition

The multibeam parallel joint transform correlator for optical pattern recognition, which was recently proposed by the authors [Appl. Opt. 37, 5408 (1998)], can increase parallelism without accumulating zero-order background level at the first Fourier transform plane. To evaluate the throughput capability, an experimental trial was made, achieving a 67-ms recognition rate per face per channel, which is limited by the response of the optically addressed liquid-crystal spatial light modulator. A general design theory is developed for dense packing of the optical channels for a given spatial light modulator resolution, considering the bandwidth requirement of the target image. Then the condition for submillisecond throughput with state-of-the-art device technology is discussed.     

Resolution and response-time dependence of ferroelectric liquid-crystal optically addressed spatial light modulators on grating profiles

We introduce a model to analyze the time response of an optically addressed spatial light modulator (OASLM) when a two-dimensional image is written on it. Comparison with experimental results is performed by use of gratings as input images. For a given spatial frequency, we show that the response time of the device depends on the grating profile. The effect of the time-constant mechanism is demonstrated theoretically and experimentally for a sinusoidal-wave and a square-wave intensity profile. An alternative explanation for the resolution limitation of the OASLM that is related to the time-constant mechanism and a new method for measuring the resolution of the device are proposed.     

Oscillator Circuit for a Vibrating Capacitor Driven by an RF Electric Field

The vibrating capacitor to be discussed consists of a metalized membrane, clamped at its edges, between two electrodes. The membrane is driven by an RF electric field (309 kHz), which is amplitude modulated at the natural frequency (6 kHz) of the membrane. This RF field is caused by a voltage applied to one of the electrodes, which, together with the membrane, forms the driving capacitor. The other electrode, and the membrane, constitute the vibrating capacitor. A special RF oscillator has been designed, of which an essential part is the driving capacitor. This capacitor causes the RF voltage to be amplitude modulated at the natural frequency of the membrane. The oscillation conditions of this electromechanical system are derived from the equation of motion of the membrane and the electrical properties of the driving circuit. An oscillator circuit with optimum performance is designed with the aid of a computer. Finally, the computed results are verified by measurements.     

Small hysteresis nanocarbon-based integrated circuits on flexible and transparent plastic substrate

We report small hysteresis integrated circuits by introducing monolayer graphene for the electrodes and a single-walled carbon nanotube network for the channel. Small hysteresis of the device originates from a defect-free graphene surface, where hysteresis was modulated by oxidation. This uniquely combined nanocarbon material device with transparent and flexible properties shows remarkable device performance; subthreshold voltage of 220 mV decade(-1), operation voltage of less than 5 V, on/off ratio of approximately 10(4), mobility of 81 cm(2) V(-1) s(-1), transparency of 83.8% including substrate, no significant transconductance changes in 1000 times of bending test, and only 36% resistance decrease at a tensile strain of 50%. Furthermore, because of the nearly Ohmic contact nature between the graphene and carbon nanotubes, this device demonstrated a contact resistance 100 times lower and a mobility 20 times higher, when compared to an Au electrode.     

Polar‐Electrode‐Bridged Electroluminescent Displays: 2D Sensors Remotely Communicating Optically

A novel geometry for electroluminescent devices, which does not require transparent electrodes for electrical input, is demonstrated, theoretically analyzed, and experimentally characterized. Instead of emitting light through a conventional electrode, light emission occurs through a polar liquid or solid and input electrical electrodes are coplanar, rather than stacked in a sandwich configuration. This new device concept is scalable and easily deployed for a range of modular alternating‐current‐powered electroluminescent light sources and light‐emitting sensing devices. The polar‐electrode‐bridged electroluminescent displays can be used as remotely readable, spatially responsive sensors that emit light in response to the accumulation and distribution of materials on the device surface. Using this device structure, various types of alternating current devices are demonstrated. These include an umbrella that automatically lights up when it rains, a display that emits light from regions touched by human fingers (or painted upon using a mixture of oil and water), and a sensor that lights up differently in different areas to indicate the presence of water and its freezing. This study extends the dual‐stack, coplanar‐electrode device geometry to provide displays that emit light from a figure drawn on an electroluminescent panel using a graphite pencil.     

Spatial light modulators for high-brightness projection displays

We fabricated polymer-dispersed liquid-crystal light valves (PDLCLV's) consisting of a 30-mum-thick hydrogenated amorphous-silicon film and a 10-mum-thick polymer-dispersed liquid-crystal (PDLC) film composed of nematic liquid-crystal (LC) microdroplets surrounded by polymer. The device can modulate high-power reading light, because the PDLC becomes transparent or opalescent independent of the polarization state of the reading light when either sufficient or no writing light is incident on the PDLCLV. This device has a limiting resolution of 50 lp/mm (lp indicates line pairs), a reading light efficiency of 60%, a ratio of intensity of light incident on the PDLC layer to intensity of light radiated from the layer, and an extinction ratio of 130:1. The optically addressed video projection system with three PDLCLV's, LC panels of 1048 x 480 pixels as input image sources, a 1-kW Xe lamp, and a schlieren optical system projected television (TV) pictures of 600 and 450 TV lines in the horizontal and the vertical directions on a screen with a diagonal length of 100 in. The total output flux of this system was 1500 lm.     

Characterization of an Optimized Nonlinear Joint Transform Correlator

Abstract

The performance of a nonlinear joint transform correlator which employs a GEC-Marconi optically addressed spatial light modulator in the Fourier plane is investigated in terms of the device operating conditions and other system parameters. We discuss the optimum performance characteristics of the correlator and the sensitivity to changes in the rotation, scale and separation of the objects in the input plane.     

Dynamic holographic interconnects that use ferroelectric liquid-crystal spatial light modulators

Dynamic interconnect holograms are designed by the use of a simulated annealing algorithm and written to a 128 × 128 pixel ferroelectric spatial light modulator that is used in a binary-phase mode. Dynamic holograms are used to implement a 2 × 2 crossbar with single-mode fiber inputs and outputs, which function with as high as 27 dB of isolation between output ports. The principle is extended to two-dimensional interconnection holograms, and arbitrary fan-out to as high as 64 points is demonstrated with good performance.

Images of interconnection holograms are transferred from the spatial light modulator to an optically addressed spatial light modulator that is used in a binary-phase mode. The addition of a fixed array generator computer-generated hologram permits replication of the hologram image, thus creating a larger hologram with a high space-bandwidth product on the optically addressed spatial light modulator.

Results of a preliminary experiment are presented.

     

Amorphous indium tin oxide electrodes for piezoelectric and light-emitting device deposited by vacuum roll to roll process

The amorphous indium tin oxides (ITOs) and the transparent conducting oxides (TCOs) electrode on the flexible polymer substrate, polyvinylidene difluoride (PVDF), and their application to the suggested combinational device system were studied. The lower range ordered crystalline structure of the ITOs enable the reduction of micro-cracks or defect grains under flexible conditions. With the piezoelectric function of substrate polymer, which is added to the display function with OLED (Organic light emitting diodes), the pattern design with the separated electrodes for two devices for emitting light and vibration was considered. From these results, the concept of a combinational flexible electronic device system with a functional polymer substrate could be suggested.     

Node-based reconfigurable volume interconnections. 1. Principles and optical design

We present a general-purpose three-dimensional interconnection network that models various parallel operations between two data planes. This volume interconnection system exhibits reconfigurable capabilities because of parallel and externally weighted interconnection modules, called nodes. We propose a generic optical implementation based on the cascading of two planar hologram arrays, coupled with a bistable optically addressed spatial light modulator. The role of this component is discussed in terms of energy regeneration and spatial cross-talk limitation. As an example, a binary matrix-matrix multiplier is implemented that uses a ferroelectric liquid-crystal light valve.     

Carrier Multiplication in PbS Quantum Dots Anchored on a Au Tip using Conductive Atomic Force Microscopy

Carrier multiplication (CM) is the amplification of the excited carrier density by two times or more when the incident photon energy is larger than twice the bandgap of semiconductors. A practical approach to demonstrate the CM involves the direct measurement of photocurrent in the device. Specifically, photocurrent measurement in quantum dots (QDs) is typically limited by high contact resistance and long carrier-transfer length, which yields a low CM conversion efficiency and high CM threshold energy. Here, the local photocurrent is measured to evaluate the CM quantum efficiency from a QD-attached Au tip of a conductive atomic force microscope (CAFM) system. The photocurrent is efficiently measured between the PbS QDs anchored on a Au tip and a graphene layer on a SiO₂/Si substrate as a counter electrode, yielding an extremely short channel length that reduces the contact resistance. The quantum efficiency extracted from the local photocurrent data with an incident photon energy exhibits a step-like behavior. More importantly, the CM threshold energy is as low as twice the bandgap, which is the lowest threshold energy of optically observed QDs to date. This enables the CAFM-based photocurrent technique to directly evaluate the CM conversion efficiency in low-dimensional materials.