Weitao Jiang

A superconducting nanowire single photon detector on lithium niobate.

Superconducting nanowire single photon detectors (SNSPDs) are a key enabling technology for optical quantum information science. In this paper we demonstrate a SNSPD fabricated on lithium niobate, an important material for high speed integrated photonic circuits. We report a system detection efficiency of 0.15% at a 1 kHz dark count rate with a maximum of ~1% close to the critical current at 1550 nm wavelength for a parallel wire SNSPD with front side illumination. There is clear scope for improving on this performance with further materials optimization. Detector integration with a lithium niobate optical waveguide is simulated, demonstrating the potential for high single photon detection efficiency in an integrated quantum optic circuit.

Investigation of ink transfer in a roller-reversal imprint process

Roller imprint is considered as one of the processes suitable for patterning on a large-area flexible substrate, which is critical for macroelectronics manufacturing. In contrast to other published roller imprint processes (such as hot-embossing roller imprint or ultraviolet roller imprint), the roller-reversal imprint (RRI) process investigated in this paper starts with pattern coating of an ink (mostly a liquefied electronics material, such as a semiconductor polymer) on a mould roller and ends with transferring the ink already patterned on the roller to the substrate, so it can obtain micropatterns with a precise profile yet leave no residual film on the substrate. One of the critical issues in obtaining patterned ink on the substrate with the required profile by the RRI process is to ensure a complete ink transfer from the microcavities on the mould roller onto the substrate. In this paper, a mathematical model is proposed to analyze the mechanics of the ink transfer, and a criterion for a complete ink transfer is derived. Furthermore, the effects of imprint force on the ink transfer are also demonstrated by an analysis of the elastic deformation of the substrate. The simulations and corresponding experiments show that the ink transfer in the RRI process is strongly dependent on the ratio of work of adhesion at the ink–mould and ink–substrate interfaces, and the critical ratio for a complete ink transfer is determined mainly by the profile of microcavities on the mould featured by the aspect ratio and the sidewall angle. The ink transfer model can be used to select proper materials (including the ink, surface energies of the mould roller and substrate) in the RRI process, and can also be regarded as a guideline for profile designing of the microcavities on the mould roller used in the RRI process.

Reversible Bending Behaviors of Photomechanical Soft Actuators Based on Graphene Nanocomposites.

Photomechanical nanocomposites embedded with light-absorbing nanoparticles show promising applications in photoresponsive actuations. Near infrared (nIR)-responsive nanocomposites based photomechanical soft actuators can offer lightweight functional and underexploited entry into soft robotics, active optics, drug delivery, etc. A novel graphene-based photomechanical soft actuators, constituted by Polydimethylsiloxane (PDMS)/graphene-nanoplatelets (GNPs) layer (PDMS/GNPs) and pristine PDMS layer, have been constructed. Due to the mismatch of coefficient of thermal expansion of two layers induced by dispersion of GNPs, controllable and reversible bendings response to nIR light irradiation are observed. Interestingly, two different bending behaviors are observed when the nIR light comes from different sides, i.e., a gradual single-step photomechanical bending towards PDMS/GNPs layer when irradiation from PDMS side, while a dual-step bending (finally bending to the PDMS/GNPs side but with an strong and fast backlash at the time of light is on/off) when irradiation from PDMS/GNPs side. The two distinctive photomechanical bending behaviors are investigated in terms of heat transfer and thermal expansion, which reveals that the distinctive bending behaviors can be attributed to the differences in temperature gradients along the thickness when irradiation from different sides. In addition, the versatile photomechanical bending properties will provide alternative way for drug-delivery, soft robotics and microswitches, etc.

Roller-reversal imprint process for preparation of large-area microstructures

The preparation of microstructures with certain patterns on a large-area substrate, especially on a flexible substrate, is a critical step in the development or production of flexible electronics (or macroelectronics). In this article, a novel roller-reversal imprint (RRI) process for the generation of large-area microstructures is proposed. In contrast to other published roller-imprint processes (such as hot embossing and ultraviolet roller-imprint), in which the material to be patterned is firstly film coated on the substrate and then the mold roller is pressed to the film, the RRI process starts with coating of the ink (mostly various liquefied electronics materials, such as a semiconductor polymer) on a patterned mold roller and then transferring the patterned ink to the substrate. The RRI process can be used to prepare micropatterns of various ink materials on a flexible substrate. By properly controlling the ink filling the microcavities on the mold roller and ink transfer to the substrate, the RRI ...

Strategy for a loading force induced overlay position shift in step imprint lithography

Abstract Imprint lithography has the potential to be a cost-effective successor to optical lithography, but there are a number of problems that need to be solved for this to happen. Imprint lithography is a mechanical process, and the overlay for imprint lithography depends not only on the alignment measurement accuracy but also on the loading force used for pattern transferring. A large force seriously deteriorates the overlay accuracy. How to eliminate the influence of loading force on the overlay accuracy is a practical challenge. This paper proposes a step imprint lithography tool for multilayer microstructure fabrication and emphasizes the importance of reporting the overlay process development. The loading process is investigated first. Relative movement and interaction force between the mould and the wafer are two main causes for an overlay position shift in the loading process. Though it can be compensated by the readjustment process, the deformation recoveries of the mechanical system in the demoulding process still bring about serious deterioration in pattern fidelity and overlay accuracy. An optimized overlay process is proposed to solve these problems. Some crucial issues in the optimized overlay process, like loading force optimization, the load pre-release process, and the demoulding process, are investigated by experiments to determine the best performing approaches using the developed step imprint lithography (SIL) tool. It was found that the proposed overlay process can eliminate the influence of loading force effectively.

Bio-inspired directional high-aspect-ratio nanopillars: fabrication and actuation

The Dynamic nature and responsive behaviour are the most attractive features of biological structures, and comprise the goals for next-generation smart materials. The nanostructured arrays provide unique topographic patterns that confer wetting, optical, and many other functions, but their actuation at the sub-micrometer scale is still a challenging goal. In this paper, we provide a simple route to fabricate ordered arrays of slender nanopillars with submicron diameters (400–500 nm) and high aspect ratios (20–40), with controllable slanted angles (60–90°). Experiments reveal that the fabricated slender nanopillars are flexible so that their orientation can be dynamically manipulated in response to an external electric field, while the stiffness can prevent ground or lateral collapse. The high aspect ratio nanopillars with orientation tunability can find applications in the development of smart materials, gecko-inspired reversible adhesion, etc.

Ultrasound-assisted recovery of free-standing high-aspect-ratio micropillars

High-aspect-ratio polymer micropillar arrays are widely employed in microfluidics and microdevices. The collapse of polymer micropillars, however, would induce device degradation, which calls for a recovery method for the collapsed micropillars to restore their performances. In this communication, we propose a cost-effective and robust approach for recovery of free-standing high-aspect-ratio micropillars by ultrasonic-assisted technology. The collapsed micropillar array immersed in a low-surface-energy solution will recover its upright state by ultrasonic stimulation and keep standing steadily due to the low-surface-energy treatment. The collapsed micropillars treated by the proposed approach are entirely able to spring back to their original upright position.

Enhanced photovoltaic performance of dye-sensitized solar cells with TiO2 micro/nano-structures as light scattering layer

Double layered TiO2 micro/nano-structured photo-anodes have been constructed for dye-sensitized solar cells (DSSCs). Mesoporous TiO2 nanoparticles serve as under layer to improve the dye absorption and TiO2 microflowers or nanorod aggregates serve as cover layer to provide prominent light scattering effect as well as electron pathways. The as-prepared architecture was characterized with field emission scanning electron microscopy. The TiO2 microflower scattering layer enhanced the photocurrent of DSSCs due to the increased light absorption by the strong light scattering effect, and reduced electron transfer resistance via the direct electron pathways. Enhanced light scattering effect was observed from TiO2 microflower based cell in the whole visible light range through the UV–Vis and IPCE measurements. Reduced electron transfer resistance was validated by electrochemical impedance test. The contrast experiments between the TiO2 microflowers and TiO2 nanorod aggregates further demonstrated that such open structured TiO2 microflowers are more effective to improve the performance of DSSCs. It is expected that the photo-anode design with TiO2 microflowers as scattering layer will be an alternative approach to improve the performance of DSSCs.

Nano-optical observation of cascade switching in a parallel superconducting nanowire single photon detector

The device physics of parallel-wire superconducting nanowire single photon detectors is based on a cascade process. Using nano-optical techniques and a parallel wire device with spatially separate pixels, we explicitly demonstrate the single- and multi-photon triggering regimes. We develop a model for describing efficiency of a detector operating in the arm-trigger regime. We investigate the timing response of the detector when illuminating a single pixel and two pixels. We see a change in the active area of the detector between the two regimes and find the two-pixel trigger regime to have a faster timing response than the one-pixel regime.

Selective-Filling Mold for Residual-Layer-Free Patterning of 3D Microstructures

We demonstrate a novel selective-filling mold containing physicochemically heterogeneous surfaces to achieve residual-layer-free patterning. Fabricated by femtolaser machining, the mold has hydrophilic surfaces in the concavity and hydrophobic surfaces on the protrusion. The experiments show that, after ink coating, the ink can wet and spread in concavity while dewets and splitted on protrusion, thus isolated ink filaments were finally formed in the concavity and separated by the dry protrusions. Without forming a residual layer, this technique offers an alternative method to fabricate isolated three-dimensional (3D) microstructures. It can be also used for large-area roll-to-roll fabrication of flexible electronics.