Photoconductive Substrate Research Articles

Selective Induction of Molecular Assembly to Tissue-Level Anisotropy on Peptide-Based Optoelectronic Cardiac Biointerfaces.

The conduction efficiency of ions in excitable tissues and of charged species in organic conjugated materials both benefit from having ordered domains and anisotropic pathways. In this study, a photocurrent-generating cardiac biointerface is presented, particularly for investigating the sensitivity of cardiomyocytes to geometrically comply to biomacromolecular cues differentially assembled on a conductive nanogrooved substrate. Through a polymeric surface-templated approach, photoconductive substrates with symmetric peptide-quaterthiophene (4T)-peptide units assembled as 1D nanostructures on nanoimprinted polyalkylthiophene (P3HT) surface are developed. The 4T-based peptides studied here can form 1D nanostructures on prepatterned polyalkylthiophene substrates, as directed by hydrogen bonding, aromatic interactions between 4T and P3HT, and physical confinement on the nanogrooves. It is observed that smaller 4T-peptide units that can achieve a higher degree of assembly order within the polymeric templates serve as a more efficient driver of cardiac cytoskeletal anisotropy than merely presenting aligned -RGD bioadhesive epitopes on a nanotopographic surface. These results unravel some insights on how cardiomyocytes perceive submicrometer dimensionality, local molecular order, and characteristics of surface cues in their immediate environment. Overall, the work offers a cardiac patterning platform that presents the possibility of a gene modification-free cardiac photostimulation approach while controlling the conduction directionality of the biotic and abiotic components.

A Diamond Terahertz Large Aperture Photoconductive Antenna Biased by a Longitudinal Field

The novel design of a terahertz large aperture photoconductive antenna (LAPCA) is reported. It features a longitudinal orientation of the bias electric field within the photoconductive substrate, and has the advantage of a small interelectrode gap, resulting in a higher field for the same applied voltage. The proposed LAPCA configuration has been tested with a nitrogen-doped (∼10 ppm) synthetic monocrystalline diamond, which is a promising material for high-intensity and high-power terahertz sources. Two antennas with different high-voltage electrode realizations were assembled, pumped by a 400 nm femtosecond laser, and tested for THz emitter function. The experimental data are found to be in good correlation with the numerical simulation results. The performance of antennas with the conventional transverse E-field configuration and the novel longitudinal configuration is compared and discussed.

Optoelectronic Trajectory Reconfiguration and Directed Self-Assembly of Self-Propelling Electrically Powered Active Particles.

Self-propelling active particles are an exciting and interdisciplinary emerging area of research with projected biomedical and environmental applications. Due to their autonomous motion, control over these active particles that are free to travel along individual trajectories, is challenging. This work uses optically patterned electrodes on a photoconductive substrate using a digital micromirror device (DMD) to dynamically control the region of movement of self-propelling particles (i.e., metallo-dielectric Janus particles (JPs)). This extends previous studies where only a passive micromotor is optoelectronically manipulated with a translocating optical pattern that illuminates the particle. In contrast, the current system uses the optically patterned electrode merely to define the region within which the JPs moved autonomously. Interestingly, the JPs avoid crossing the optical region's edge, which enables constraint of the area of motion and to dynamically shape the JP trajectory. Using the DMD system to simultaneously manipulate several JPs enables to self-assemble the JPs into stable active structures (JPs ring) with precise control over the number of participating JPs and passive particles. Since the optoelectronic system is amenable to closed-loop operation using real-time image analysis, it enables exploitation of these active particles as active microrobots that can be operated in a programmable and parallelized manner.

Improving the efficiency of an optical-to-terahertz converter using sapphire fibers

Objectives. The study aims to improve the efficiency of a large-area photoconductive terahertz (THz) emitter based on an optical-to-terahertz converter (OTC) having a radiating area of 0.3 × 0.3 mm2 for generating high-power THz radiation by using an array of close-packed profiled sapphire fibers having a diameter in the range of 100–300 μm as focusing optics.Methods. As a photoconductive substrate, we used a semi-infinite LT-GaAs layer (low-temperature grown GaAs; GaAs layer grown by molecular beam epitaxy at a low growth temperature). Additional Si3N4 and Al2O3 layers are intended for reducing leakage currents in the OTC and reducing the reflection of the laser pump pulse from the air/semiconductor interface (Fresnel losses), respectively, at a gap width of 10 μm. For forming the antenna electrodes and feed strips, the Ti/Au metal system was used. The simulation was carried out by the finite element method in the COMSOL Multiphysics environment.Results. The use of a profiled sapphire fiber whose diameter has been optimized with respect to the gap parameters to significantly increase the concentration of charge carriers in the immediate vicinity of the electrodes of an OTC is demonstrated. The integrated efficiency of a large-area photoconductive THz emitter was determined taking into account the microstrip topology of the array with a characteristic size of feed strips proportional to the gap width in the OTC and with the upper (masking) metal layer. The maximum localization of the electromagnetic field in close proximity to the edges of electrodes at the “fiber–semiconductor” interface is achieved with a profiled sapphire fiber diameter of 220 μm.Conclusions. By optimizing the diameter of the sapphire fiber, the possibility of improving the localization of incident electromagnetic waves in close proximity to the edges of the OTC electrodes by ~40 times compared to the case without fiber, as well as increasing the overall efficiency of a large-area emitter by up to ~7–10 times, was demonstrated.

Facile activation of a GaAs substrate with electron beam irradiation for THz photoconductive antenna

One of the most important technological challenges that photoconductive devices in terahertz systems encounter is the viability of cost-effective and large-scale device production. We introduce an economical and mass-producible approach to fabricating a substrate material for photoconductive devices. By using an electron beam irradiator, the material properties of GaAs were engineered in a controllable manner, achieving comparable performance to that of a well-known photoconductive substrate, LT-GaAs. THz emission of the irradiated substrates was tested and found to be superior to a commercial device in terms of high-power THz signal emission and ability to withstand a high bias voltage.

Near-field mapping of high permittivity dielectric microwave resonator modes via optically induced conductance.

In this paper, we demonstrate a straightforward, low-cost, and high resolution optical-based method to measure the three-dimensional relative electric field magnitude in microwave circuits without the need to monitor reflected laser beams or the requirement of photoconductive substrates for the device under test. The technique utilizes optically induced conductance, where a focused laser beam excites electron-hole-pairs (EHPs) in a semiconductor thin film placed in the near-field of a microwave circuit. The generated EHPs create localized loss in the resonator and modulate the transmitted microwave signal, proportional to the local microwave electric field. As a proof of principle, several different modes of a high permittivity (ɛ ∼ 80) cylindrical dielectric resonator are mapped.

Silicon based photo-controlled deformable mirror

The fabrication and characterization of a Silicon based photo-controlled deformable mirror (PCDM) for spatial light modulation and wavefront control is reported. The device is a membrane mirror based on a 1-inch monolithic non-pixelated Silicon photoconductive substrate with a transparent ITO electrode on one side coupled with a 5 µm thick nitrocellulose membrane. The device is driven applying an AC voltage and it is characterized by measuring the generated wavefront using a Shack-Hartmann wavefront sensor. Upon uniform illumination with a NIR LED, the deformation increases with the voltage applied, its frequency and the light intensity. The maximum deformation achieved is approximately 2 µm PtV (wavefront). The repeatability of the induced deformation is also evaluated together with its time stability, proving that the device is reliable in the working conditions. The response time is evaluated and it is in the range of 10 ms. The device is suitable for an efficient adaptable focusing element with an important focus control.

Design of highly directive lens-less photoconductive dipole antenna array with frequency selective surface for terahertz imaging applications

In this paper, a highly directive photoconductive dipole array antenna is presented for imaging system operating at terahertz (THz) frequencies. The array antenna is formed on a single photoconductive substrate to make it compact and suitable for THz imaging applications. The presented photoconductive dipole array antenna improves the gain as well as directivity and therefore, is useful to enhance the imaging capabilities to address the considerations such as limited depth-of-field (DoF) that is the distance over which an object is considered in focus and size-weight-and-power (SWaP) of THz source for imaging applications. These are important considerations for applications like stand-off imaging and surveillance of moving targets where the high angular resolution as well as extended DoF is the key for successful detection of hidden explosives and illicit drugs. In addition to this, a technique of using frequency selective surface with photoconductive array antenna is also presented to further enhance the directivity of the radiating structure to yield high image resolution. Moreover, by using this technique, the radiation energy has been confined to the desired frequency band rather than spreading over a wide spectrum range.

Optical Gating of Graphene on Photoconductive Fe:LiNbO3.

We demonstrate experimentally nonvolatile, all-optical control of graphene's charge transport properties by virtue of an Fe:LiNbO3 photoconductive substrate. The substrate can register and sustain photoinduced charge distributions which modify locally the electrostatic environment of the graphene monolayer and allow spatial control of graphene resistivity. We present light-induced changes of graphene sheet resistivity as high as ∼370 Ω/sq (∼2.6-fold increase) under spatially nonuniform light illumination. The light-induced modifications in the sheet resistivity are stable at room temperature but can be reversed by uniform illumination or thermal annealing (100 °C for 4 h), thus restoring graphene's electrical properties to their initial, preillumination values. The process can be subsequently repeated by further spatially nonuniform illumination.

Optically controlled electrophoresis with a photoconductive substrate

A photoconductive substrate is used to perform electrophoresis. Light-induced micro-particle flow manipulation is demonstrated without using a fabricated flow channel. The path along which the particles were moved was formed by an illuminated light pattern on the substrate. Because the substrate conductivity and electric field distribution can be modified by light illumination, the forces acting on the particles can be controlled. This technique has potential applications as a high functionality analytical device.

Optimization of the liquid crystal light valve for signal beam amplification

A liquid crystal light valve is an optically controlled spatial light modulator made as a cell with liquid crystal on a photoconductive substrate. Dynamic holograms recorded in such a hybrid device may give rise to the amplification of one of the recording beams at the expense of the other. In the present work, liquid crystal light valves with different thicknesses of the liquid crystal layer are studied at various conditions of the dynamic grating recording aiming to maximize the signal beam amplification in the infrared. A seventeen-fold gain is achieved in the improved cell for optimal recording conditions.

Zinc selenide-based large aperture photo-controlled deformable mirror.

Realization of large aperture deformable mirrors with a large density of actuators is important in many applications, and photo-controlled deformable mirrors (PCDMs) represent an innovation. Herein we show that PCDMs are scalable realizing a 2-inch aperture device based on a polycrystalline zinc selenide (ZnSe) as the photoconductive substrate and a thin polymeric reflective membrane. ZnSe is electrically characterized and analyzed through a model that we previously introduced. The PCDM is then optically tested, demonstrating its capabilities in adaptive optics.

3-D Non-UV Digital Printing of Hydrogel Microstructures by Optically Controlled Digital Electropolymerization

A technique using digital masks without ultraviolet light to rapidly print 3-D biopolymer structures with complex microarchitectures in a microfluidic chip has been demonstrated. In this approach, a customized system is used to project light images on a photoconductive substrate in order to create localized virtual electrodes when an alternating electric field is applied across the fluidic medium in an optically controlled digital electropolymerization chip. Upon these virtual electrodes, the localized electric fields are generated, which could activate the polymerization of acrylate-based molecules, such as poly(ethylene glycol) diacrylate (PEGDA), to form microstructures with the same shapes as the projected light images. We have demonstrated that the 3-D PEGDA microstructures with the customized shapes could be fabricated rapidly through a layer-by-layer process by applying a series of digital masks (projected light images). With our current projection and microscopy system, the fabrication of microhydrogel structures with a lateral resolution of 3 $\mu \text{m}$ and an adjustable thickness ranging from tens of nanometers to hundreds of micrometers has been demonstrated. In summary, this novel technique provides an efficient process for the rapid printing of the 3-D biopolymer-based microstructures, and could enable many future applications in a mechanoanalysis of cancer cells, tissue engineering, and drug screening. [2015-0110]

Conductivity and magnetoresistance of La0.7Ce0.3MnO3−δ thin films under photoexcitation

La0.7Ce0.3MnO3 thin films of different thicknesses, degrees of CeO2-phase segregation and oxygen deficiency, grown on SrTiO3 single crystal substrates, were comparatively investigated with respect to both their spectral and temperature-dependent photoconductivity (PC) and their magnetoresistance (MR) behaviour under photoexcitation. While as-grown films were insensitive to optical excitation, oxygen reduction appeared to be an effective way to decrease the film resistance, but the film thickness was found to play a minor role. However, from the evaluation of the spectral behaviour of the PC and the comparison of the MR of the LCeMO/substrate-samples with a bare substrate under illumination we find that the photoconductivity data reflects not only contributions from (i) photogenerated charge carriers in the film and (ii) carriers injected from the photoconductive substrate (as concluded from earlier works), but also (iii) a decisive parallel photoconduction in the SrTiO3 substrate. Furthermore—also by analyzing the MR characteristics—the unexpected occurence of a strong electroresistive effect in the sample with the highest degree of CeO2 segregation and oxygen deficiency could be attributed to the electroresistance of the SrTiO3 substrate as well. The results suggest a critical reconsideration and possibly a reinterpretation of several previous photoconductivity and electroresistance investigations of manganite thin films on SrTiO3.

Light-Addressed Electrodeposition of Polysaccharide Chitosan Membranes on a Photoconductive Substrate

This paper describes a light-addressed electrolytic system used to perform an electrodeposition of polysaccharide chitosan membranes on a photoconductive substrate using a digital micromirror device (DMD). In this system, a patterned light illumination is projected onto a photoconductive substrate serving as a photo-cathode to electrolytically produce hydroxide ions, which leads to an increased pH gradient. The high pH generated at the cathode can cause a local gelation of chitosan through sol-gel transition. By controlling the illumination pattern on the DMD, a light-addressed electrodeposition of chitosan membranes with different shapes and sizes, as well as multiplexed micropatterning, was performed. The effect of the illumination time of the light pattern and the applied current density on the dimensional resolution of chitosan membrane formation was examined experimentally. The spatial resolution in our proposed method was demonstrated about 40 mm by patterning with a variety of lines with different widths and different spaces between the lines. We anticipate that photo-electro-chemically-induced deposition of chitosan-based hydrogels may offer interesting opportunities for the integration of biological systems into miniaturized devices.

Light-Addressable Electrodeposition of Magnetically-Guided Cells Encapsulated in Alginate Hydrogels for Three-Dimensional Cell Patterning

This paper describes a light-addressable electrolytic system used to perform an electrodeposition of magnetically-guided cells encapsulated in alginate hydrogels using a digital micromirror device (DMD) for three-dimensional cell patterning. In this system, the magnetically-labeled cells were first manipulated into a specific arrangement by changing the orientation of the magnetic field, and then a patterned light illumination was projected onto a photoconductive substrate serving as a photo-anode to cause gelation of calcium alginate through sol-gel transition. By controlling the illumination pattern on the DMD, we first successfully produced cell-encapsulated multilayer alginate hydrogels with different shapes and sizes in each layer via performing multiplexed micropatterning. By combining the magnetically-labeled cells, light-addressable electrodeposition, and orientation of the magnetic fields, we have successfully demonstrated to fabricate two layers of the cell-encapsulated alginate hydrogels, where cells in each layer can be manipulated into cross-directional arrangements that mimic natural tissue. Our proposed method provides a programmable method for the spatiotemporally controllable assembly of cell populations into three-dimensional cell patterning and could have a wide range of biological applications in tissue engineering, toxicology, and drug discovery.

34.1: Invited Paper: Slowing Light in Liquid Crystals

AbstractSteep dispersion is obtained by exploiting the large optically induced refractive index change in liquid crystal media. This effect is observed either in devices made by the association of liquid crystals with a photoconductive substrate or by employing the photo‐isomerization of azo‐dye molecules hosted in a chiral liquid crystal structure. When interacting in these media, a pulse of light that is relatively long becomes delayed, hence, slow light phenomena are obtained with steep dispersion, a feature that can be exploited for improving the performances of optical sensors.

The Mn2+/Mn3+ state of La0.7Ce0.3MnO3 by oxygen reduction and photodoping

Films of cerium-doped LaMnO3, which has been intensively discussed as an electron-doped counterpart to hole-doped mixed-valence lanthanum manganites during the past decade, were analyzed by x-ray photoemission spectroscopy with respect to their manganese valence under photoexcitation. The comparative analysis of the Mn 3s exchange splitting of La0.7Ce0.3MnO3 (LCeMO) films in the dark and under illumination clearly shows that both oxygen reduction and illumination are able to decrease the Mn valence towards a mixed 2+/3+ state, independently of the film thickness and the degree of CeO2 segregation. Charge-injection from the photoconductive SrTiO3 substrate into the Mn eg band with carrier lifetimes in the range of tens of seconds and intrinsic generation of electron–hole pairs within the films are discussed as two possible sources of the Mn valence shift and the subsequent electron doping.

21308 光伝導性基板を用いた光制御可能な電気泳動法の開発

This paper introduces optically controllable electrophoresis with a photoconductive substrate. Objective of this study is to control micro particles in fluid with laser irradiation. Micro Total Analysis Systems (μTAS) and Lab-on-a-chip have been developed in a medical and biological applications, and they have been intensively studied. The systems are expected to have more quick procedures and higher functionality. Photoconductive substrate is a kind of semiconductor, and its conductance is increased by optical irradiation. Electrophoresis is the motion of dispersed micro particles relative to a fluid under the influence of a spatially uniform electric field. By combining property of photoconductive substrate and electrophoresis, we can achieve micro particles operation without flow channel. We believe that this technique contributes to development of the high functionality analysis devices.

Light-addressed electrodeposition of enzyme-entrapped chitosan membranes for multiplexed enzyme-based bioassays using a digital micromirror device.

This paper describes a light-addressed electrolytic system used to perform an electrodeposition of enzyme-entrapped chitosan membranes for multiplexed enzyme-based bioassays using a digital micromirror device (DMD). In this system, a patterned light illumination is projected onto a photoconductive substrate serving as a photo-cathode to electrolytically produce hydroxide ions, which leads to an increased pH gradient. The high pH generated at the cathode can cause a local gelation of chitosan through sol-gel transition. By controlling the illumination pattern on the DMD, a light-addressed electrodeposition of chitosan membranes with different shapes and sizes, as well as multiplexed micropatterning, was performed. The effect of the illumination time of the light pattern on the dimensional resolution of chitosan membrane formation was examined experimentally. Moreover, multiplexed enzyme-based bioassay of enzyme-entrapped chitosan membranes was also successfully demonstrated through the electrodeposition of the chitosan membranes with various shapes/sizes and entrapping different enzymes. As a model experiment, glucose and ethanol were simultaneously detected in a single detection chamber without cross-talk using shape-coded chitosan membranes entrapped with glucose oxidase (GOX), peroxidase (POD), and Amplex Red (AmR) or alcohol oxidase (AOX), POD, and AmR by using same fluorescence indicator (AmR).