Photoconductive Substrate Research Articles

Transmissive liquid crystal light-valve for near-infrared applications

An optical valve is realized by associating a nematic liquid crystal layer with a Cr-doped gallium arsenide as a photoconductive substrate. The light-valve is shown to efficiently operate in transmission at 1.06 μm optical wavelength. The optical phase shift and refractive index change are measured as a function of the incident light intensity and of the voltage applied. Additionally, the light-valve is shown to act as a self-defocusing medium. Combining transmissive properties and nonlinear features, applications for dynamic holography in the near-infrared region of the spectrum can be envisaged.

Enhancement of Terahertz Pulse Emission by Optical Nanoantenna

Bridging the gap between ultrashort pulsed optical waves and terahertz (THz) waves, the THz photoconductive antenna (PCA) is a major constituent for the emission or detection of THz waves by diverse optical and electrical methods. However, THz PCA still lacks employment of advanced breakthrough technologies for high-power THz emission. Here, we report the enhancement of THz emission power by incorporating optical nanoantennas with a THz photoconductive antenna. The confinement and concentration of an optical pump beam on a photoconductive substrate can be efficiently achieved with optical nanoantennas over a high-index photoconductive substrate. Both numerical and experimental results clearly demonstrate the enhancement of THz wave emission due to high photocarrier generation at the plasmon resonance of nanoantennas. This work opens up many opportunities for diverse integrated photonic elements on a single PCA at THz and optical frequencies.

1P1-L01 光伝導性基板を用いた光制御可能な電気泳動法の開発(光応用-オプトロニクスの未来-)

We develop optically controllable electrophoresis with a photoconductive substrate. We demonstrate simultaneous manipulation of microparticles without any flow channels.

Optical switching of terahertz radiation from meta-atom-loaded photoconductive antennas

Optical switching of the spectrum and polarization of terahertz radiation from split-ring resonator-loaded photoconductive antennas has been demonstrated. The switching is based on the sensitivity of the resonance of a split-ring resonator on a photoconductive substrate to a change in the capacitance induced by optical pulse irradiation. The spectral and polarization characteristics of the split-ring resonator-loaded photoconductive antennas are discussed in terms of the coupling between the electric dipole induced by the pump laser and the eigenmodes of the split-ring resonators.

Light-addressable electrodeposition of cell-encapsulated alginate hydrogels for a cellular microarray using a digital micromirror device

This paper describes a light-addressable electrolytic system used to perform an electrodeposition of calcium alginate hydrogels using a digital micromirror device (DMD). In this system, a patterned light illumination is projected onto a photoconductive substrate serving as a photo-anode to electrolytically produce protons, which can lead to a decreased pH gradient. The low pH generated at the anode can locally release calcium ions from insoluble calcium carbonate (CaCO(3)) to cause gelation of calcium alginate through sol-gel transition. By controlling the illumination pattern on the DMD, a light-addressable electrodeposition of calcium alginate hydrogels with different shapes and sizes, as well as multiplexed micropatterning was performed. The effects of the concentration of the alginate and CaCO(3) solutions on the dimensional resolution of alginate hydrogel formation were experimentally examined. A 3 × 3 array of cell-encapsulated alginate hydrogels was also successfully demonstrated through light-addressable electrodeposition. Our proposed method provides a programmable method for the spatiotemporally controllable assembly of cell populations into cellular microarrays and could have a wide range of biological applications in cell-based biosensing, toxicology, and drug discovery.

Optimization of thin‐film configuration for light‐addressable stimulation electrode

AbstractLight addressing is an emerging technique to optically address a virtual electrode on a photoconductive substrate. A thinner photoconductive layer of a light‐addressable planar electrode can improve the spatial resolution of the light‐addressed electrode. Voltage application to the electrode, however, causes a strong electric field across the thin photoconductive layer with a significant avalanche effect, which induces an undesired increase of dark current. In order to overcome this problem, we investigated how photoconductive‐layer thickness and passivation‐layer conductivity affect voltage‐application‐induced bright and dark charge densities. Suppression of the dark charge density with a thick photoconductive layer and a low‐conductive passivation layer is found to be a key factor for optimization of the light‐addressable electrode. With this design strategy, we developed a novel light‐addressable electrode using titanium dioxide as a photoconductor. To suppress the avalanche effect, the thickness of the titanium‐dioxide layer was designed to be 1.5 μm. The fabricated electrode turned out to have sufficient photoelectric properties: the bright charge density reached 70 μC/cm2 and the bright‐to‐dark charge density ratio was greater than 10, which allows stimulation to cultured dissociated neurons. © 2010 Wiley Periodicals, Inc. Electron Comm Jpn, 94(1): 61–68, 2011; Published online in Wiley Online Library (wileyonlinelibrary.com). DOI 10.1002/ecj.10241

Continuous photocontrolled deformable membrane mirror

We present a deformable mirror that is composed by a metalized membrane with a monolithic nonpixelated photoconductive Bi12SiO20 substrate. The assembly constitutes a continuous photocontrolled deformable mirror and is driven through suitable light intensity distributions illuminating the photoconductive element. The continuous deformation of the reflective surface provides an all-optical and dynamical control of the incoming wave front, without spatial segmentation and with a deformation as large as a few microns.

光アドレス型刺激電極の薄膜構成最適化

Light addressing is an emerging technique to optically address a virtual electrode on a photoconductive substrate. A thinner photoconductive layer of a light-addressable planar electrode can improve the spatial resolution of the light-addressed electrode. Voltage application to the electrode, however, causes a strong electric field across the thin photoconductive layer with a significant avalanche effect, which induces an undesired increase of dark current. In order to overcome this problem, we investigated how photoconductive-layer thickness and passivation-layer conductivity affect voltage-application-induced bright and dark charge densities. Suppression of the dark charge density with a thick photoconductive layer and a low-conductive passivation layer is found to be a key factor for optimization of the light-addressable electrode. With this design strategy, we developed a novel light-addressable electrode using titanium dioxide as a photoconductor. To suppress the avalanche effect, the thickness of the titanium-dioxide layer was designed to be 1.5 μm. The fabricated electrode turned out to have sufficient photoelectric properties: the bright charge density reached 70 μC/cm2 and the bright-to-dark charge density ratio was greater than 10, which allows stimulation to cultured dissociated neurons. © 2010 Wiley Periodicals, Inc. Electron Comm Jpn, 94(1): 61–68, 2011; Published online in Wiley Online Library (wileyonlinelibrary.com). DOI 10.1002/ecj.10241

2121 任意基板への微小要素の集積化のためのマニピュレーションシステム(要旨講演,一般セッション:マイクロナノメカトロニクス)

We developed, for the first time, the manipulation system based on optoelectronic tweezers (OET) which manipulates multiple nano/micro components on an arbitrary substrate with concurrent observation of the component motion. It uses CdS as a photoconductive film. Since CdS transmits red light and absorbs blue light, red light is utilized for the observation and blue light is utilized for the manipulation. The photoconductive electrode substrate which generates the nonuniform electric field for the component manipulation using dielectrophoresis consists of CdS on comb ITO electrode. Using the substrate, it was successfully demonstrated to manipulate a silica particle. Light projected CdS/ITO area had the higher electric field and repelled silica particle. By appling 10 V at 100 kHz, manipulation force of 0.6 pN was induced and manipulated silica particle speed was about 7 μm/s.

Optically addressed microelectromechanical systems driven with high-frequency modulated light

We propose and analyze a new mode of operation for an optically addressed deformable mirror device. The device consists of an array of metallized membrane mirrors supported above an optically addressed photoconductive substrate. A conductive transparent electrode is deposited on the backside of the substrate. A variable polarity voltage is applied between the membrane and the back electrode of the device accompanied with high-frequency modulated light. The membrane is deformed when a modulated light illuminates the backside of the device. This occurs due to impedance and bias redistribution between the two cascaded impedances. This operating mechanism of a microelectromechanical systems device is suitable for detecting moving targets.

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.

Optically driven microelectromechanical-system deformable mirror under high-frequency AC bias

A new, optically addressed deformable mirror device is demonstrated. The device consists of a pixellated metalized polymeric membrane mirror supported above an optically addressed photoconductive substrate. A conductive transparent ZnO layer is deposited on the back side of the substrate. A very high-frequency AC bias is applied between the membrane and the back electrode of the device. The membrane is deformed when the back of the device is illuminated because of impedance and bias redistribution between two cascaded impedances. We fabricated, demonstrated, and modeled the operation of this device.

Synthesis and electrooptical characterization of polysiloxanes containing indolyl groups acting as photoconductive substrates for photorefractive materials

Abstract Synthesis and electrooptical analysis of polysiloxane and poly-N-vinyl derivatives containing indolyl groups are reported. The indole group and some of its methyl derivatives have been taken into account in order to evaluate their behaviour, with respect to that of the widely employed carbazole group, when used as photoconductive centres attached to a polymer chain. The obtained data show that photoconductivity and traps formation mechanism can be inferred as functions of the physico-chemical parameters (electric dipole moment and ionization potential) of the different groups and polymers involved. To this end such parameters have been carefully computed for a series of pyrrole, indole and carbazole derivatives. The 2,3-dimethylindole derivative appears to be particularly promising due to its electrooptical behaviour in the red absorption region where measurements have been accomplished and are shown to be consistent with the theoretical predictions.

Two-photon photoconductive terahertz generation in ZnSe

We report on the generation of free-space terahertz (THz) radiation using polycrystalline ZnSe as the photoconductive (PC) substrate. It is found that the ZnSe emitter can be operated at peak fields up to 125 kV/cm without dielectric breakdown, and can be photoexcited (through two-photon absorption) at pump energy fluences up to 28 mJ/cm2 without saturating. The relationship between the THz field strength and the excitation conditions of the PC gap are interpreted through a photocarrier transport model.

Influence of substrate-lens design in terahertz time-domain spectroscopy

We describe measurements of the angular radiation patterns from lens-coupled terahertz antennas fabricated on photoconductive substrates. These measurements were performed with a novel terahertz (THz) time-domain spectrometer in which the femtosecond optical pulses used to gate the emitter and receiver antennas were delivered by optical fiber. We used this system to perform a comparison between the two substrate-lens designs commonly used in THz time-domain spectrometers. We measured both E-plane and H-plane emission patterns for a 90° bow-tie antenna. By comparing these experimental results with simulations based on Fresnel–Kirchoff diffraction, we find that the choice of substrate-lens design is important in determining not only the directivity of the emitted beam but also the spectral bandwidth. These results emphasize the significance of this crucial component in the design of broadband THz spectrometers.

Optically controlled photoconductive N-bit switched microwave signal attenuator

An optical architecture for N-bit digital control of microwave signals is introduced that uses the photoconductive effect in microwave waveguides for variable rf attenuation control. A 2-bit optically controlled microwave attenuator based on a transmission line fabricated on a silicon photoconductive substrate is experimentally demonstrated at 990 MHz. This attenuator provided 0, 5.8, 11.2, and 15.6 dB of independent optically switched attenuation levels.

Effect of AlGaAs window and buffer layers on the response characteristics of GaAs photoconductive detectors

Five comparative GaAs-based photoconductive detectors were fabricated to investigate the effects of AlGaAs window and buffer layers on device performance. Steady-state and transient photoconductive responses were measured on structures with and without an AlGaAs window layer over the photoconductive layer, and with and without an AlGaAs buffer layer between the photoconductive layer and substrate. The structure with both an AlGaAs window and AlGaAs buffer layer shows the lowest dark current (<1 nA) and the highest responsivity (∼ 0.2 A/W). However, the structure on a bare semi-insulating GaAs substrate shows the fastest pulse response of ∼ 800 ps full width at half maximum, indicating that interfacial rather than bulk properties dominate the photoconductive characteristics.

Gallium arsenide (GaAs)-based photoconductive switches for pulse generation and sampling applications in the nanosecond regime

The design is described of a set of optoelectronic switches having an interdigitated electrode structure and implemented with high-resistivity GaAs photoconductive substrates. A theoretical analysis is developed for determining the pulsed light ON-state resistance (peak conductance), OFF-state (dark) resistance, and the associated capacitances for the various designed gap geometries. Data are provided on the processing steps used in successfully fabricating a working set of switches based on the theoretical design. A test apparatus is used to make measurements of the pulsed light conductance of these devices having nominal gap spacings of 5, 10, 20 and 40 mu m.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>