Jianfei Jiang

Graphene-Based Floating-Gate Nonvolatile Optical Switch

We proposed a nonvolatile optical waveguide switch by utilizing a floating-gate (FG) configuration whose FG layer is a single-layer graphene. The switching signal can be removed after the optical switching is accomplished. The propagation state of light then can be retained by charges trapped in the graphene layer until the next erasing signal. Depending on waveforms of driving signals, the device can work as either a phase shifter or an intensity switch. In the phase shifter mode, a 646-μm-long device can achieve a phase shift of π. Corresponding energy consumptions to program/erase, the π phase shift is 82.8 and 118.2 pJ, respectively. In the intensity switch mode, a 328-μm-long device is able to attenuate the light by 20 dB. Energies consumed by the programming and the erasing operations are 35.7 and 45.4 pJ, respectively.

Atomic picture of elastic deformation in a metallic glass

The tensile behavior of a Ni60Nb40 metallic glass (MG) has been studied by using ab initio density functional theory (DFT) calculation with a large cell containing 1024 atoms (614 Ni and 410 Nb). We provide insight into how a super elastic limit can be achieved in a MG. Spatially inhomogeneous responses of single atoms and also major polyhedra are found to change greatly with increasing external stress when the strain is over 2%, causing the intrinsically viscoelastic behavior. We uncover the origin of the observed super elastic strain limit under tension (including linear and viscoelastic strains) in small-sized MG samples, mainly caused by inhomogeneous distribution of excess volumes in the form of newly formed subatomic cavities.

Silicon Add-Drop Filter Based on Multimode Bragg Sidewall Gratings and Adiabatic Couplers

A silicon photonic add-drop filter is demonstrated with sidewall Bragg gratings in a multimode strip waveguide. The operating principle is based on the contradirectional coupling between the TE0 mode and different higher-order modes inside the multimode waveguide, which depends on the symmetry property of the periodic refractive-index perturbations. In order to relax the fabrication tolerance, a pair of adiabatic couplers is used to add/drop the optical signal at the output/input ports of the multimode waveguide. Experimental results show that the functionality of add/drop filtering is successfully achieved with a low insertion loss of 0.6 dB. We also show that by sweeping the multimode waveguide width from 700 to 1400 nm, the bandwidth can be flexibly tuned from 16 to 2 nm.

Size effect on atomic structure in low-dimensional Cu-Zr amorphous systems

The size effect on atomic structure of a Cu64Zr36 amorphous system, including zero-dimensional small-size amorphous particles (SSAPs) and two-dimensional small-size amorphous films (SSAFs) together with bulk sample was investigated by molecular dynamics simulations. We revealed that sample size strongly affects local atomic structure in both Cu64Zr36 SSAPs and SSAFs, which are composed of core and shell (surface) components. Compared with core component, the shell component of SSAPs has lower average coordination number and average bond length, higher degree of ordering, and lower packing density due to the segregation of Cu atoms on the shell of Cu64Zr36 SSAPs. These atomic structure differences in SSAPs with various sizes result in different glass transition temperatures, in which the glass transition temperature for the shell component is found to be 577 K, which is much lower than 910 K for the core component. We further extended the size effect on the structure and glasses transition temperature to Cu64Zr36 SSAFs, and revealed that the Tg decreases when SSAFs becomes thinner due to the following factors: different dynamic motion (mean square displacement), different density of core and surface and Cu segregation on the surface of SSAFs. The obtained results here are different from the results for the size effect on atomic structure of nanometer-sized crystalline metallic alloys.

Broadband tunable filter based on the loop of multimode Bragg grating

A broadband tunable silicon filter has been demonstrated on silicon-on-insulator platform. The device is based on the loop of multimode anti-symmetric waveguide Bragg grating. A wide bandwidth tunability about 1.455 THz (0.117-1.572 THz) is achieved. The device, functions like a ring, can realize the bandwidth tunable of the drop port and the through port. And, its feature has simultaneous wavelength tuning and no free space ranges limitation. A high out-of-band contrast of 30 dB is achieved with a bandwidth of 1.572 THz (Δλ = 13 nm). The out-of-band contrast is 18 dB at the minimum bandwidth 0.117 THz (Δλ = 1.0 nm).

Plasmonic reflection color filters with metallic random nanostructures

We develop reflective color filters with randomly distributed nanodisks and nanoholes fabricated with hydrogen silsesquioxane and Ag films on silicon substrate. They exhibit high resolution, angle-independence and easily up-scalable fabrication, which are the most important factors for color filters for industrial applications. We uncover the underlying mechanism after systematically analyzing the localized surface plasmon polariton coupling in the electric-field distribution. The agreement of the experimental results with those from the simulation indicates that tunable colors across the visible spectrum can be obtained by simply varying the diameter of the nanodisks, promoting their applications.

Structural evolution and atomic dynamics in Ni–Nb metallic glasses: A molecular dynamics study

The composition and temperature dependence of static and dynamic structures in NixNb1-x (x = 50-70 at. %) were systematically studied using molecular dynamics with a new-released semi-empirical embedded atom method potential by Mendelev. The calculated pair correlation functions and the structure factor match well with the experimental data, demonstrating the reliability of the potential within relatively wide composition and temperature ranges. The local atomic structures were then characterized by bond angle distributions and Voronoi tessellation methods, demonstrating that the icosahedral ⟨0,0,12,0⟩ is only a small fraction in the liquid state but increases significantly during cooling and becomes dominant at 300 K. The most abundant clusters are identified as ⟨0,0,12,0⟩ and distorted icosahedron ⟨0,2,8,2⟩. The large fraction of these two clusters hints that the relatively good glass forming ability is near the eutectic point. Unlike Cu-Zr alloys, both the self-diffusion coefficient and shear viscosity are insensitive to compositions upon cooling in Ni-Nb alloys. The breakdown of the Stokes-Einstein relation happens at around 1.6Tg (Tg: glass transition temperature). In the amorphous state, the solid and liquid-like atoms can be distinguished based on the Debye-Waller factor ⟨u2⟩. The insensitivity of the dynamic properties of Ni-Nb alloys to compositions may result from the relatively simple solidification process in the phase diagram, in which only one eutectic point exists in the studied composition range.

Silicon Add-Drop Filter Based on Multimode Grating Assisted Couplers

We demonstrate the design, fabrication, and characterization of a two-mode silicon add-drop filter, which is based on multimode grating-assisted contra-directional couplers. The mode conversions between two asymmetry strip waveguides are realized by using spatially periodic corrugations in one side wall of a multimode waveguide at the phase-match condition. Experimental results show that the fundamental mode of a single-mode waveguide is successfully coupled to the first two modes of the multimode waveguide in the opposite direction. The corresponding mode conversion bandwidths and extinction ratios are also measured. The demonstrated add-drop filter may find their applications in a hybrid multiplexing system by combing wavelength-division multiplexing technology and mode-division multiplexing technology for on-chip optical interconnect.

Design of ultrafast laser-driven microactuator based on photoacoustic mechanism.

In this work, an ultrafast laser-driven microactuator based on the photoacoustic mechanism was proposed with large amplitude and high response frequency. The microactuator was fabricated by LIGA technology. The displacement of the microactuator could be up to 11 μm at resonance state when the repeat frequency was around 14 kHz using a nanosecond pulse laser. Theoretical model was set up and the calculated results agree reasonably well with the experimental data. The microactuator based on the photoacoustic mechanism provides a more efficient actuation method.

Dynamic properties of a metal photo-thermal micro-actuator

This work presents the design, modeling, simulation, and characterization of a metal bent-beam photo-thermal micro-actuator. The mechanism of actuation is based on the thermal expansion of the micro-actuator which is irradiated by a laser, achieving noncontact control of the power supply. Models for micro-actuators were established and finite-element simulations were carried out to investigate the effects of various parameters on actuation properties. It is found that the thermal expansion coefficient, thermal conductivity, and the geometry size largely affected actuation behavior whereas heat capacity, density, and Youngs modulus did not. Experiments demonstrated the dynamic properties of a Ni micro-actuator fabricated via LIGA technology with 1100/30/100 μm (long/wide/thick) arms. The tip displacement of the micro-actuator could achieve up to 42 μm driven by a laser beam (1064 nm wavelength, 1.2 W power, and a driving frequency of 1 HZ). It is found that the tip displacement decreases with increasing laser driving frequency. For 8 Hz driving frequency, 17 μm (peak-valley value) can be still reached, which is large enough for the application as micro-electro-mechanical systems. Metal photo-thermal micro actuators have advantages such as large displacement, simple structure, and large temperature tolerance, and therefore they will be promising in the fields of micro/nanotechnology.