Zichen Zhang

Flexible quantum dot light emitting diodes based on ZnO nanoparticles

Flexible quantum dot light emitting diodes (QLEDs) have attracted extensive attention owing to the advantages of foldability and their broad application in flexible display devices. In this work, we report high performance, mechanically flexible QLEDs based on ZnO nanoparticles used as an electron transfer layer (ETL). The QLEDs have been fabricated on poly(ethylene-terephthalate) (PET) substrates utilizing a unique structure consisting of bilayered hole transport films and ZnO nanoparticles acting as an ETL to improve the device performance owing to its appropriate energy band position and high charge mobility. The QLEDs exhibited high performance, such as a lowered turn on voltage of 1.6 V and improved current and power efficiencies of 5.20 cd A−1 and 1.80 lm W−1, respectively. They presented good flexibility with a critical bending radius of 4.5 mm, suggesting the broad application potential of flexible QLEDs.

Diffraction based phase compensation method for phase-only liquid crystal on silicon devices in operation

A method to measure the optical response across the surface of a phase-only liquid crystal on silicon device using binary phase gratings is described together with a procedure to compensate its spatial optical phase variation. As a result, the residual power between zero and the minima of the first diffraction order for a binary grating can be reduced by more than 10 dB, from -15.98 dB to -26.29 dB. This phase compensation method is also shown to be useful in nonbinary cases. A reduction in the worst crosstalk by 5.32 dB can be achieved when quantized blazed gratings are used.

Surface plasmon-enhanced quantum dot light-emitting diodes by incorporating gold nanoparticles.

Surface plasmon-enhanced electroluminescence (EL) has been demonstrated by incorporating gold (Au) nanoparticles (NPs) in quantum dot light-emitting diode (QLED). Time-resolved photoluminescence (TRPL) spectroscopy reveals that the EL enhancement is ascribed to the near-field enhancement through an effective coupling between excitons of the quantum dot emitters and localized surface plasmons around Au NPs. It is found that the size of Au NPs and the distance between the Au NPs and the emissive layer have significant effects on the performance of QLED. The enhancement can be maximized as the SP resonance wavelength of Au NPs matches well with the PL emission wavelength of the QD film and the distance between Au NPs and the emissive layer maintains 15 nm. The photoluminance (PL) and EL intensity can be enhanced by 4.4 and 1.7 folds with the incorporation of Au NPs. The maximum current efficiency of 4.56 cd/A can be achieved for the resulting QLEDs by incorprating Au NPs with an enhancement factor of 2.0. In addition, the enhancement ratio of 2.2 can be achieved for the lifetime of resulting QLED.

Origin of Transient Crosstalk and Its Reduction in Phase-only LCOS Wavelength Selective Switches

The transient crosstalk in a phase-only liquid crystal on silicon (LCOS) based wavelength selective switch using a Fourier transform setup was investigated. Its origin was identified using an in situ test procedure and found to be related to the transient phase patterns displayed by the LCOS device during the switching. Two different methods were proposed to reduce the transient crosstalk without the need to modify the optics or electronics in use. Experimental results show both methods are able to reduce the worst-case transient crosstalk by at least 5 dB.

High Quality Assembly of Phase-Only Liquid Crystal on Silicon (LCOS) Devices

Liquid crystal on silicon (LCOS) for phase-only holography is ideally made to better optical tolerance than that for conventional amplitude modulating applications. Die-level assembly is suited to custom devices and pre-production prototypes because of its flexibility and efficiency in conserving the silicon backplane. Combined with automated assembly, it will allow high reproducibility and fast turnaround time, paving the way for pre-production testing and customer sampling before mass production. Pre-assembly optical testing is the key element in the process. By taking into account the flatness of both the backplane and the front glass plate, we have assembled high quality LCOS devices. We have reached our aim of less than one quarter wavelength phase distortion across the active area.

CNT-FED Architecture Based on a Gate Electrode of Diabolo Mode

Summary form only given. Recently, the carbon nanotube field emission display has drawn more and more concern. With its excellent image display quality and low fabrication expense, the printable CNT-FED will take the place of cathode ray tube as the major consumer-priced and mass-market display. A normal CNT-FED device is composed of the CNT cathode, the gate electrode, the anode and dielectric insulator layers. The gate electrode in CNT-FED is used to modulate and address the electron beam. Because some electrons may be collected by the gate electrode in a triode, the brightness of the image is decreased. This paper proposes a novel gate structure fabricated on a glass plate with diabolo funnels. Chemical corrosion was utilized to produce diabolo funnels in the glass dielectric layer and HF with the 40% concentration with sulfuric acid is used. A metal layer was screen-printed on the lower surface of the glass-mesh plate as an electron extraction gate electrode. MgO film was vaporized on the surface of the diabolo funnels. The electrons from CNT are transported in a diabolo mode via secondary electron emission over the wall of the diabolo funnel placed above the sub emitters. As a result, many electrons including the secondary electrons and backscatters are generated and the brightness of the diabolo mode gate structure CNT-FED could increase obviously. In the paper, we give the results of numerical simulation of the secondary electron emission process with Monte Carlo method. In this paper, we also study the influence of a MgO layer on the surface of the diabolo funnel. Comparisons are made between a bare glass diabolo funnel and a MgO layer coated on the surface of the funnel. As the simulation results and experiment results shown, the triode structure in which a MgO layer is vaporized on the surface of the funnel can get higher brightness

Reduction of crosstalk in a colourless multicasting LCOS-based wavelength selective switch by the application of wavefront encoding

To optimise the design of a wavelength selective switch based on a phase-only liquid crystal on silicon spatial light modulator and mitigate crosstalk, we propose using a technique referred to as wavefront encoding that involves purposefully building a wavefront error into the optical system. Experimental results taken at 674nm are presented that show wavefront encoding based on defocus can reduce the worst case crosstalk by >10dB compared to a standard Fourier transform set-up. In the case of the WSS we propose using wavefront encoding based on astigmatism.

Application of the fractional Fourier transform to the design of LCOS based optical interconnects and fiber switches

It is shown that reflective liquid crystal on silicon (LCOS) spatial light modulator (SLM) based interconnects or fiber switches that use defocus to reduce crosstalk can be evaluated and optimized using a fractional Fourier transform if certain optical symmetry conditions are met. Theoretically the maximum allowable linear hologram phase error compared to a Fourier switch is increased by a factor of six before the target crosstalk for telecom applications of -40 dB is exceeded. A Gerchberg-Saxton algorithm incorporating a fractional Fourier transform modified for use with a reflective LCOS SLM is used to optimize multi-casting holograms in a prototype telecom switch. Experiments are in close agreement to predicted performance.

Use of wavefront encoding in optical interconnects and fiber switches for cross talk mitigation

A technique of cross talk mitigation developed for liquid crystal on silicon spatial light modulator based optical interconnects and fiber switches is demonstrated. By purposefully introducing an appropriate aberration into the system, it is possible to reduce the worst-case cross talk by over 10 dB compared to conventional Fourier-transform-based designs. Tests at a wavelength of 674 nm validate this approach, and show that there is no noticeable reduction in diffraction efficiency. A 27% spot increase in beam diameter is observed, which is predicted to reduce at longer datacom and telecom wavelengths.

High-Performance Photo-Modulated Thin-Film Transistor Based on Quantum dots/Reduced Graphene Oxide Fragment-Decorated ZnO Nanowires

In this paper, a photo-modulated transistor based on the thin-film transistor structure was fabricated on the flexible substrate by spin-coating and magnetron sputtering. A novel hybrid material that composed of CdSe quantum dots and reduced graphene oxide (RGO) fragment-decorated ZnO nanowires was synthesized to overcome the narrow optical sensitive waveband and enhance the photo-responsivity. Due to the enrichment of the interface and heterostructure by RGO fragments being utilized, the photo-responsivity of the transistor was improved to 2000 A W−1 and the photo-sensitive wavelength was extended from ultraviolet to visible. In addition, a positive back-gate voltage was employed to reduce the Schottky barrier width of RGO fragments and ZnO nanowires. As a result, the amount of carriers was increased by 10 folds via the modulation of back-gate voltage. With these inherent properties, such as integrated circuit capability and wide optical sensitive waveband, the transistor will manifest great potential in the future applications in photodetectors.