Hongda Chen

Optically tuned terahertz modulator based on annealed multilayer MoS2.

Controlling the propagation properties of terahertz waves is very important in terahertz technologies applied in high-speed communication. Therefore a new-type optically tuned terahertz modulator based on multilayer-MoS2 and silicon is experimentally demonstrated. The terahertz transmission could be significantly modulated by changing the power of the pumping laser. With an annealing treatment as a p-doping method, MoS2 on silicon demonstrates a triple enhancement of terahertz modulation depth compared with the bare silicon. This MoS2-based device even exhibited much higher modulation efficiency than the graphene-based device. We also analyzed the mechanism of the modulation enhancement originated from annealed MoS2, and found that it is different from that of graphene-based device. The unique optical modulating properties of the device exhibit tremendous promise for applications in terahertz switch.

Effect of shape, height, and interparticle spacing of Au nanoparticles on the sensing performance of Au nanoparticle array

The effect of shape, height, and interparticle spacing of Au nanoparticles (NPs) on the sensing performance of Au NP array is systematically investigated. Lengthening the major axis of elliptical NPs with the minor axis kept constant will cause the redshift of the local surface plasmon (LSP) resonance mode, enhance the sensitivity, and widen the resonance peaks. Larger height corresponds to smaller LSP resonance wavelength and narrower resonance peak. With each NP size unchanged, larger interparticle spacing corresponds to larger resonance wavelength and smaller full-width at half-maximum (FWHM). Moreover, duty cycle is important for sensitivity, which is largest when the duty cycle is 0.4.

Enhanced sensitivity of gold elliptic nanohole array biosensor with the surface plasmon polaritons coupling

Surface plasmon resonance (SPR) spectroscopy of the metal nanostructure (MN) is widely used in chemical and biological sensing. The biosensor characteristics of the MN are affected by the size, shapes, substrate, and so on. Especially, some studies about the mode of the MN show that the surface plasmon polaritons (SPPs) coupled mode excitation has a significant impact on the sensitivity of the MN biosensor. To reach the aim of obtaining the high sensitivity MN biosensor, the coupled mode of the gold elliptic nanohole array (GENA) was simulated and analyzed. It shows that the coupled mode was influenced by the refractive indexes of medium and substrate. The coupled mode resonance peak shows a higher sensitivity than that of other SPPs mode peaks. The GENA biosensor chips were fabricated and integrated with polydimethylsiloxane (PDMS) microfluidics. The sensitivity of the GENA is characterized by transmission spectra of GENA in deionized water and aqueous NaCl solution. The coupled mode peak sensitivity of the biosensor reaches 549 nm/RIU. An antigen-antibody interaction experiment was also adopted to verify the sensitivity of the surface binding reaction. The sensitivity of detected concentration of the AFP (α-fetoprotein) in our experiment has been reached 25 ng/ml closed to the clinic concentration. The GENA biosensor chip has potential application in label-free chemical and biomedical fields, especially, cancer biomarker testing.

Laser direct writing of SiO2/TiO2 sol-gel films to fabricate stripe optical waveguides

SiO2-TiO2 planar optical waveguides are fabricated on silicon wafer substrate by dip-coating technique with the Sol-Gel solutions, based on which the stripe optical waveguides are patterned by laser direct writing of the Sol-Gel films using an Ytterbium fiber laser and followed by chemical etching. The effects of the laser processing parameters on the microstructure of the core layer films are investigated. The relative chemical etching rates of the non-irradiated area in Sol-Gel films that are haeted at different temperature are characterized. The optical fields and propagation losses of the optical waveguides at the wavelength of 1550 nm are characterized by multi-channel fiber/waveguides coupling system. The experimental results demonstrate that the composition, the post heat treatment temperature and laser power density have a big effect on the widths of the stripe optical waveguides, and the minimum widths about 25 μm can be fabricated with the suitable parameters. The core layer of the planar optical waveguides as received by Sol-Gel method is loose in structure, and a shrinkage concave groove forms in the laser irradiated area. The microstructure and forming mechanisms of the stripe waveguides by laser direct writing Sol-Gel films are discussed. The minimum propagation loss of the fabricated stripe waveguides is about 1.77dB/cm at 1550nm. Better results are expected by improving the film composition and laser processing parameters further.

Surface characteristics of SiO2-TiO2 strip fabricated by laser direct writing

SiO2-TiO2 sol-gel films are deposited on SiO2/Si by dip-coating technique. The SiO2-TiO2 strips are fabricated by laser direct writing using all ytterbium fiber laser and followed by chemical etching. Surface structures, morphologies and roughness of the films and strips are characterized. The experimental results demonstrate that the SiO2-TiO2 sol-gel film is loose in Structure and a shrinkage concave groove forms if the film is irradiated by laser beam. The surface roughness of both non-irradiated and laser irradiated areas increases with the chemical etching time. But the roughness of laser irradiated area increases more than that of non-irradiated area under the same etching time. After being etched for 28 s, the surface roughness value of the laser irradiated area increases from 0.3 nm to 3.1 nm.

Fabrication of SiO2-TiO2 strip waveguides by laser direct writing

SiO2-TiO2 strip optical waveguides have been fabricated by laser densification of SiO2-TiO2 sol-gel films using a Ytterbium fiber lasers and following chemical etching process. Effects of laser processing parameters and dried temperature of SiO2-TiO2 sol-gel films on the dimensions of strip optical waveguides were systematically studied. And the mechanism on laser densification of SiO2-TiO2 films was discussed. The experimental results demonstrate that the width and thickness of strip waveguides increase with laser power density until SiO2-TiO2 films are damaged by laser irradiation. The available range of laser power density for laser densification increase through enhancing dried temperature of SiO2-TiO2 films, which mainly thanks to the increasing stress capacity in the films. The width of strip optical waveguides can be markedly decreased from 110 to 25 μm in the case of the same thickness by enlarging available range of laser power density. The mechanism on laser densification of SiO2-TiO2 films is mainly based on shrinkage of nanoscale pore within SiO2-TiO2 films duo to the energy transferred from silicon substrate.

Optical Controlled Terahertz Modulator Based on Tungsten Disulfide Nanosheet

The terahertz (THz) modulator, which will be applied in next-generation wireless communication, is a key device in a THz communication system. Current THz modulators based on traditional semiconductors and metamaterials have limited modulation depth or modulation range. Therefore, a THz modulator based on annealed tungsten disulfide (WS2, p-type) and high-resistivity silicon (n-type) is demonstrated. Pumped by a laser, the modulator presents a laser power-dependent modulation effect. Ranging from 0.25 to 2 THz, the modulation depth reaches 99% when the pumping laser is 2.59 W/cm2. The modulator works because the p-n heterojunction can separate and limit carriers to change the conductivity of the device, which results in a modulation of the THz wave. The wide band gap of WS2 can promote the separation and limitation of carriers to obtain a larger modulation depth, which provides a new direction for choosing new materials and new structures to fabricate a better THz modulator.

Lowering the threshold current of photonic crystal vertical-cavity surface-emitting lasers

In recent years, photonic crystal vertical-cavity surface-emitting lasers (PhC-VCSELs) have attracted significant attention. Characteristics such as single-mode operation in a large cavity area (which results in high single-mode output power),1, 2 highspeed modulation,3 and a small circular divergence angle of the output beam profile4 make them suitable for optical communications and sensing applications. In addition, they are easy to design and fabricate,5 and polarization-stable devices with single fundamental-mode operation have been fabricated.6 However, the abnormally high PhC-VCSEL threshold current has puzzled researchers. Reducing the threshold current is necessary to reduce power consumption and thermal losses, and improve the PhC-VCSEL devices’ stability and reliability. We used theory and experiment to optimize the relation between the oxide aperture and light emission aperture of the PhCVCSELs so that we obtained both high single-mode output and low threshold current (see Figure 1). We used the full 3D (FDTD) method to analyze the effect of the oxide aperture diameter on the optical characteristics. FDTD takes not just the oxide aperture diameter into account but the whole structure. We used cavity mode loss analysis to investigate the single-mode operation of the laser and find the minimum oxide aperture required to maintain the single-mode performance of the device with a low threshold current. The simulation results shown in Figure 2(a and b) show that the oxide aperture diameters in an appropriate range determine the threshold current and also, for those devices with a 7 m light emission aperture diameter, affect the higher-order modes. The oxide aperture provides additional transverse confinement to the transverse mode and reduces higher modes more Figure 1. Schematic 3D view of the photonic crystal vertical-cavity surface-emitting laser (PhC-VCSEL) containing 22.5 pairs of p-type top distributed Bragg reflectors (DBRs) and 34.5 pairs of n-type bottom DBRs. Both consist of aluminum gallium arsenide (Al0:9Ga0:1As and Al0:12Ga0:88As, except for a 30nm Al0:98Ga0:02As oxidation layer inserted in the base of the DBRs.)