Xueao Zhang

High responsivity phototransistors based on few-layer ReS2 for weak signal detection

Two-dimensional transition metal dichalcogenides are emerging with tremendous potential in many optoelectronic applications due to their strong light-matter interactions. To fully explore their potential in photoconductive detectors, high responsivity and weak signal detection are required. Here, we present high responsivity phototransistors based on few-layer rhenium disulfide (ReS2). Depending on the back gate voltage, source drain bias and incident optical light intensity, the maximum attainable photoresponsivity can reach as high as 88,600 A W-1, which is a record value compared to other two-dimensional materials with similar device structures and two orders of magnitude higher than that of monolayer MoS2. Such high photoresponsivity is attributed to the increased light absorption as well as the gain enhancement due to the existence of trap states in the few-layer ReS2 flakes. It further enables the detection of weak signals, as successfully demonstrated with weak light sources including a lighter and limited fluorescent lighting. Our studies underscore ReS2 as a promising material for future sensitive optoelectronic applications.

Broadband Photovoltaic Detectors Based on an Atomically Thin Heterostructure

van der Waals junctions of two-dimensional materials with an atomically sharp interface open up unprecedented opportunities to design and study functional heterostructures. Semiconducting transition metal dichalcogenides have shown tremendous potential for future applications due to their unique electronic properties and strong light-matter interaction. However, many important optoelectronic applications, such as broadband photodetection, are severely hindered by their limited spectral range and reduced light absorption. Here, we present a p-g-n heterostructure formed by sandwiching graphene with a gapless band structure and wide absorption spectrum in an atomically thin p-n junction to overcome these major limitations. We have successfully demonstrated a MoS2-graphene-WSe2 heterostructure for broadband photodetection in the visible to short-wavelength infrared range at room temperature that exhibits competitive device performance, including a specific detectivity of up to 10(11) Jones in the near-infrared region. Our results pave the way toward the implementation of atomically thin heterostructures for broadband and sensitive optoelectronic applications.

High-performance and compact binary blazed grating coupler based on an asymmetric subgrating structure and vertical coupling

A high-performance and compact fiber-to-waveguide binary blazed subwavelength grating coupler was designed based on silicon-on-insulator. By the appropriate choice of waveguide/grating parameters, including thicknesses, periods, height, and fill factor, to optimize the mode matching, a relatively high coupling efficiency was obtained for the fiber and waveguide interface. Moreover, perfectly vertical fiber coupling is achieved by using an asymmetric subgrating structure in which a period consists of two subgratings with identical etching height and different widths. Coupling efficiency as high as 69% at a wavelength of 1.52 μm and 65% at a wavelength of 1.55 μm is calculated. Simultaneously, the 1 dB wavelength bandwidth is around 80 nm. The coupling efficiency can reach up to 80% or so if Bragg reflector layers are added. Finally, the device layout is simple, feasible, one-step etched, and compatible with standard complementary metal-oxide semiconductor technology processing.

The Raman redshift of graphene impacted by gold nanoparticles

The influence of gold nanoparticles (GNPs) on graphene was studied by Raman spectroscopy. It was found that the contact of GNPs could induce the whole Raman spectrum of graphene to redshift. And the shift of the 2D peak is more obvious than that of the G peak. A model of local strain was brought forward to explain the shift of Raman spectrum, which comes from the charges transfer between the GNPs and graphene. The observation of the Raman shifts helps us to gain more physical insights into the graphene-related systems.

The nonlinear optical properties of coupling and decoupling graphene layers

Third-order optical nonlinearities of graphene from monolayer to multilayers were investigated in the femtosecond regime, and the contribution of interlayer coupling to the nonlinearities was studied. The nonlinear refractive index γ of the order of 10−9 cm2/W and the nonlinear absorption coefficient β of 10−6 cm/W were obtained. By systematically investigating the nonlinear optical properties with the number of layers and comparing the coupling graphene with the decoupling superimposed graphene, we found that the coupling of interlayers has large effect upon the nonlinear refraction. These results provide an effective approach for developing graphene-based nonlinear photonic devices.

High Efficient Subwavelength Binary Blazed Grating Beam Splitter via Vertical Coupling

We propose a novel broadband beam splitter (BS) with a single-layer and compact grating vertical coupling structure, which is based on the form birefringence of subwavelength binary blazed grating and effective-medium theory. Rigorous coupled-wave analysis is used to optimize the design of this beam splitter. The simulation and analysis show that the BS for transverse electric (TE) light are designed to split the incident light beam into two beams of equal power (nearly 50% split), which travel in opposite directions in the waveguide. The coupling length is about 9 . The coupling efficiency for the right and the left branches of waveguide are 47% and 52%, respectively. The power difference of two output ports is less than 15% over a 75-nm wavelength bandwidth range.

Current induced doping in graphene-based transistor with asymmetrical contact barriers

The metal/graphene contacts play a very important role in the performance of graphene-based devices. We report here a unique observation of current-induced doping in graphene transistors. The charge carrier type and the concentration in graphene can be manipulated by the current flowing through the graphene device, arising from the asymmetrical metal/graphene barriers between the source and drain electrodes and the accompanied current crowding effect.

Investigation of Radiation Collection by InSb Infrared Focal-Plane Arrays with Micro-optic Structures

Three designs of micro-optic structures have been analyzed by two-dimensional simulation. Compared with traditional spherical microlenses, the micro-optic structures have the same ability to collect radiation and do not have the disadvantages of traditional microlenses. In our analysis the micro-optic structures are simple grooved notches above the space between two adjacent mesas. We also investigate the characteristics of InSb focal-plane arrays with both spherical microlenses and micro-optic structures under oblique incident radiation. Empirical formulas were derived to describe the response and crosstalk as a function of incident radiation angle. Our results show that the micro-optic structures can be effectively used in radiation collection for InSb infrared focal-plane arrays.

High electrical conductivity of individual epitaxially grown MoO2 nanorods

Molybdenum dioxides (MoO2) have potential applications in batteries owing to their good electrical conductivity. Here, we report the electrical properties of high-quality MoO2 nanorods grown using chemical vapor deposition which are partially wrapped in MoS2 on c-sapphire [α-Al2O3(0001)] substrates and subsequently transferred onto Si wafers for device fabrication. The as-fabricated devices with the individual MoO2 nanorods showed a high electrical conductivity of 6.04 × 103 S/cm and a low contact resistance of 33 Ω, thus demonstrating a superior electrical performance than in any other previous reports on MoO2-based devices. The MoS2 wrapping around the rods had a negligible effect on the conductivity. The electrical conductivity of the MoO2 nanorods was observed to decline in air when a high voltage was applied; this could be mitigated by packaging the nanorods using SiO2 or holding them under high vacuum. Our results provide the foundation for understanding the properties and potential applications of ...

Experimental study of plasmon in a grating coupled graphene device with a resonant cavity

Plasmon was probed from graphene which was grown by chemical vapor deposition using terahertz time-domain spectroscopy at room temperature. Graphene was laid on a resonant cavity, and metal grating was then deposited on top of them. For the THz light polarized along the grid fingers, the optical conductivity of graphene changed from Drude response into strongly Lorentz behavior with a peak formed in the THz-region. These experimental results are highly consistent with the theoretical prediction of a single layer graphene. It confirms that the graphene plasmon frequency can be tuned by the length of grating. Moreover, the extinction in the transmission of single-layer graphene can also be increased beyond 60%.