Abstract:Washboard belt-like zinc selenide (ZnSe) nanostructures are successfully prepared by a simple chemical vapor deposition (CVD) technology without catalyst. The phase compositions, morphologies and optical properties of the nanostructures are investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM) and photoluminescence (PL) spectroscop, respectively. A vapor-liquid mechanism is proposed for the formation of ZnSe belt-like structures. Strong PL from the ZnSe nanostructure can be tuned from 462 nm to 440 nm with temperature varying from 1000 °C to 1200 °C, and it is demonstrated that the washboard belt-like ZnSe nanostructures have potential applications in optical and sensory nanotechnology. This method is expected to be applied to the synthesis of other II–VI groups or other group’s semiconducting materials.

Abstract:The absorption enhancements of silicon layer in silicon solar cells with three kinds of Ag nanoparticles including sphere, cylinder and cuboid are studied by the finite difference time domain (FDTD) method, respectively. The results show that the light absorption of silicon is significantly improved due to the localized surface plasmon (LSP) resonance. The relations of the absorption enhancement with the parameters of nanoparticles are thoroughly analyzed. The optimal absorption enhancement can be achieved by adjusting the relevant parameters. Among the three types of Ag nanoparticles, i.e., sphere, cylinder and cuboid, the silicon with the cubical Ag nanopaticles shows the most efficient absorption enhancement at optimal conditions, its maximum absorption enhancement factor is 1.35, and that with the spherical Ag nanopaticles gets the lowest absorption enhancement. The work is useful for the further theoretical study and design for the plasmonic thin-film solar cell.

Abstract:We report a fiber Bragg grating (FBG) vibration transducer based on an equal strength cantilever with enhanced sensitivity design. The sensitivity of the transducer is improved by a buffer layer of polyimide which increases the effective distance between the FBG and the neutral axis of the cantilever. The thickness of the polyimide layer is further optimized by finite element analysis (FEA). Vibration test results demonstrate that the sensitivity is enhanced by about 3.34 times than the conventional design, from the original 10.2 pm/g to 44.3 pm/g, which is consistent with the FEA. It is also shown experimentally that the sensitivity enhancement does not degrade the fundamental vibration characteristics of the cantilever, especially the resonant frequency.

Abstract:A novel composite ultraviolet (UV)/blue photodetector is proposed in this paper. Lateral ring-shaped PN junction is used to separate photogenerated carriers and inject the non-equilibrium excess carriers to the bulk, changing the bulk potential and shifting the threshold voltage of the metal-oxide-semiconductor field-effect transistor (MOSFET) as well as the drain current. Numerical simulation is carried out, and the simulation results show that the composite photodetector has the enhanced responsivity for UV/blue spectrum. It exhibits very high sensitivity to weak and especially ultra-weak light. A responsivity of 7000 A/W is obtained when the photodetector is illuminated under incident optical power of 0.01 μW. As a result, this proposed combined photodetector has great potential for UV/blue and ultra-weak light applications.

Abstract:Recirculating frequency shifter (RFS) can generate stable multi-carrier, but the carrier-to-noise-ratio (CNR) is limited because noise is accumulated round by round in the recirculation loop structure. A modified RFS with a delay interferometer (DI) is proposed in this paper. The DI can suppress the accumulated noise, and enhance the CNR of multi-carrier. The principle of the generation of high quality multi-carrier based on an RFS with DI is analyzed theoretically. 50 stable carriers with spacing of 10.7 GHz are successfully generated. The flatness of 50 stable carriers is 3 dB, and the CNR is 43 dB which is increased by 3 dB compared with the structure without DI.

Abstract:A high temperature sensor based on the multi-parameter temperature dependent characteristic of photoluminescence (PL) of quantum dot (QD) thin film is demonstrated by depositing the CdSe/ZnS core/shell QDs on the SiO₂ glass substrates. The variations of the intensity, the peak wavelength and the full width at half maximum (FWHM) of PL spectra with temperature are studied experimentally and theoretically. The results indicate that the peak wavelength of the PL spectra changes linearly with temperature, while the PL intensity and FWHM vary exponentially for the temperature range from 30 °C to 180 °C. Using the obtained temperature dependent optical parameters, the resolution of the designed sensor can reach 0.1 nm/°C.

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Abstract:A novel polarizer with a silver nanoribbon added into a traditional waveguide directional coupler is designed to realize the polarized output of TE mode. A high extinction ratio can be obtained because of the selectivity of surface plasmon polaritons (SPPs) on polarization. The effects of the polarizer parameters on coupling efficiency and extinction ratio are discussed. Simulation results indicate that the coupling efficiency for TE mode can reach about 95%, but only 3% for TM mode, with the extinction ratio of TE mode about 15 dB when the light wavelength is 1550 nm. The polarizer may have potential applications in photonic integrated circuits and quantum information technology.

Abstract:A novel switchable dual-wavelength erbium-doped fiber (EDF) laser is demonstrated. The wavelength selection element consists of two fiber Bragg gratings (FBGs), a polarization controller (PC) and a 3 dB coupler forming a Sagnac loop inserted with two FBGs. We study the effect of coupling ratio on filtering performance in this paper. By adjusting PC, we can change the wavelength-dependent loss, and then using nonlinear polarization rotation effect to suppress the mode competition caused by the homogeneous broadening of EDF, we obtain single- and double-wavelength laser outputs. At room temperature, under 200 mW pump power, dual-wavelength laser is achieved, and the center wavelengths are 1545.34 nm and 1548.20 nm, respectively. The peak power values are ?13.36 dBm and ?14.58 dBm, and side mode suppression ratios (SMSRs) are 41.10 dB and 39.88 dB, respectively. Within two hours, the maximum fluctuation of peak power is less than 0.7 dB, which shows that the demonstrated fiber laser is stable. Moreover, by adjusting PC, singel-wavelength laser output is obtained, the peak power is ?6.27 dBm or ?5.45 dBm, and SMSR is 40.03 dB or 39.96 dB at 1545.34 nm or 1548.20 nm, respectively.

Abstract:In this paper, we present the design of a new photonic crystal fiber (PCF) gas sensor for evanescent-field sensing in terahertz (THz) wave band. This sensor can be used to identify the gas, and its size is very large, so that it is beneficial to fill it with the test substance. Based on simulation, we demonstrate that the gas sensor using PCFs with four noncircular large holes in the cladding has high sensitivity and low loss, the confinement loss is less than 0.007 dB/m, and the bending loss is very small. The new PCF gas sensor can detect kinds of gases, for example, if test gas is water vapor, it has obvious absorption peaks in THz band, and the sensitivities of gas sensor are 64% and 73% at 1.097 THz and 0.752 THz, respectively. Due to the ultra-low loss and high sensitivity of the model, the novel steering-wheel structured fiber is very suitable for evanescent-field sensing and the detection of chemical and biological products.

Abstract:An efficient LED projector is designed for the LED flat lamp without light guide plate (LGP), in order to get high-efficiency lighting capablility and low cost of LED flat lamp. By employing refraction and total reflection, uniform intensity distribution of the emergent light and high-efficiency lighting capablility are achieved. The simulation results show that the output efficiency and intensity distribution on LED flat lamp panel depend on the distance between projector and output panel, the rotation angle of the projector and the gap between projector and LED. It performs well in both lighting efficiency and uniformity, while a distance of 5 mm, a rotation angle of 2° and a gap of 1.2 mm are used with the panel size of 240 mm×360 mm in the LED flat lamp without LGP. And 96.6% of optical uniformity is achieved.

Abstract:A novel tunable optoelectronic oscillator (OEO) based on a photonic radio frequency (RF) phase shifter is proposed and analyzed, which consists of a dual-drive Mach-Zehnder modulator (DMZM), a 90° hybrid coupler and a tunable microwave amplifier (TMA). This tunable OEO has a simple configuration, and the frequency of the oscillating signal can be tuned by adjusting the amplification factor of the tunable amplifier. The simulation results show that the maximum frequency shift from the center oscillation frequency is 1.48 MHz when the amplification factor of the TMA is set at 10.

Abstract:Cu(In,Ga)Se₂ (CIGS) thin films are prepared by a single-stage process and a three-stage process at low temperature in the co-evaporation equipment. The quite different morphologies of CIGS thin films deposited by two methods are characterized by scanning electron microscopy (SEM). The orientation of CIGS thin films is identified by X-ray diffraction (XRD) and Raman spectrum, respectively. Through analyzing the film-forming mechanisms of two preparation processes, we consider the cause of such differences is that the films deposited by three-stage process at low temperature evolve from Cu-poor to Cu-rich ones and then back to Cu-poor ones. The three-stage process at low temperature results in the CIGS thin films with the (220)/(204) preferred orientation, and the ordered vacancy compound (OVC) layer is formed on the surface of the film. This study has great significance to large-scale industrial production.

Abstract:The Lorentz model and modified Debye model (MDM) parameters for Si_0.6Ge_0.4 are presented. A nonlinear optimization algorithm is developed. The obtained parameters are used to determine the complex relative permittivity of Si_0.6Ge_0.4, and compared with the experimental data for validation. Finally the obtained parameters are used to analyze the properties of symmetric surface plasmon polariton (SPP) mode propagation in a dielectric-metal-dielectric (DMD) material constructed with silver (Ag) and Si_0.6Ge_0.4 for further verifying the extracted parameters.

Abstract:We present a theoretical description of surface plasmon polariton (SPP) on a graphene surface excitated by four-wave mixing using two laser beams. The phase-matched incident angles of the two beams that meet the SPP excitation conditions are calculated, and the discussion of the generated SPP power dependence on the incident angles is presented. We demonstrate that there is an enhancement in the peak power at the optimum angles at 633 nm and 921 nm, which is about 30 and 850 times larger than the corresponding peak power on a gold surface, respectively.

Abstract:The Er³⁺/Ce³⁺ co-doped tellurite-based glasses (TeO₂-Bi₂O₃-TiO₂) modified with various WO₃ contents are prepared using conventional melt-quenching technique. The X-ray diffraction (XRD) patterns and Raman spectra of glass samples are measured to investigate the structures. The absorption spectra, the up-conversion emission spectra, the 1.53 μm band fluorescence spectra and the lifetime of Er³⁺:⁴I_13/2 level are measured, and the amplification quality factors of Er³⁺ are calculated to evaluate the effect of WO₃ contents on the 1.53 μm band spectroscopic properties. With the introduction of WO₃, it is found that the prepared tellurite-based glasses maintain the amorphous structure, while the 1.53 μm band fluorescence intensity of Er³⁺ is improved evidently, and the fluorescence full width at half maximum (FWHM) is broadened accordingly. In addition, the prepared tellurite-based glass samples have larger bandwidth quality factor than silicate and germanate glasses. The results indicate that the prepared Er³⁺/Ce³⁺ co-doped tellurite-based glass with a certain amount of WO₃ is an excellent gain medium applied for the 1.53 μm band Er³⁺-doped fiber amplifier (EDFA).

Abstract:After taking the higher-order dispersion and three kinds of saturable nonlinearities into account, we investigate the characteristics of modulation instability (MI) in real units in the positive refractive region of metamaterials (MMs). The results show that the gain spectra of MI consist of two spectral regions, one of which is close to and the other is far from the zero point. In particular, the spectral region far from the zero point also has high cut-off frequency but narrow spectral width just as those revealed in the negative refractive region. Moreover, the gain spectra can change with the normalized angular frequency, the normalized optical power and the form of the saturable nonlinearity. Concretely, the spectral width increases with increase of the normalized angular frequency. But both of the spectral width and the peak gain increase and then decrease with increase of the normalized optical power. In other words, the MI characteristics and MI related applications can be controlled by adjusting the structure of the MMs, the form of the saturable nonlinearity and the normalized optical power.

Abstract:An effective hierarchical reliable belief propagation (HRBP) decoding algorithm is proposed according to the structural characteristics of systematically constructed Gallager low-density parity-check (SCG-LDPC) codes. The novel decoding algorithm combines the layered iteration with the reliability judgment, and can greatly reduce the number of the variable nodes involved in the subsequent iteration process and accelerate the convergence rate. The result of simulation for SCG-LDPC(3969,3720) code shows that the novel HRBP decoding algorithm can greatly reduce the computing amount at the condition of ensuring the performance compared with the traditional belief propagation (BP) algorithm. The bit error rate (BER) of the HRBP algorithm is considerable at the threshold value of 15, but in the subsequent iteration process, the number of the variable nodes for the HRBP algorithm can be reduced by about 70% at the high signal-to-noise ratio (SNR) compared with the BP algorithm. When the threshold value is further increased, the HRBP algorithm will gradually degenerate into the layered-BP algorithm, but at the BER of 10^?7 and the maximal iteration number of 30, the net coding gain (NCG) of the HRBP algorithm is 0.2 dB more than that of the BP algorithm, and the average iteration times can be reduced by about 40% at the high SNR. Therefore, the novel HRBP decoding algorithm is more suitable for optical communication systems.

Abstract:An electric encoded/optical transmission system of code division multiple access (CDMA) is proposed. It encodes the user signal in electric domain, and transfers the different code slice signals via the different wavelengths of light. This electric domain encoder/decoder is compared with current traditional encoder/decoder. Four-user modulation/demodulation optical CDMA (OCDMA) system with rate of 2.5 Gbit/s is simulated, which is based on the optical orthogonal code (OCC) designed in our laboratory. The results show that the structure of electric encoding/optical transmission can encode/decode signal correctly, and can achieve the chip rate equal to the user data rate. It can overcome the rate limitation of electronic bottleneck, and bring some potential applications in the electro-optical OCDMA system.

Abstract:The analytical expressions for the average intensity, root mean square (RMS) beam width and angular spread of Gaussian Schell-model (GSM) beams propagating under slant atmospheric turbulence are derived, and they are used to study the influence of different propagation paths on the propagation of laser beams in atmospheric turbulence. It is shown that under the same condition, the influence of atmospheric turbulence along a downlink path on the GSM beam propagation is the smallest among the three paths. Therefore, the downlink propagation is more beneficial to the beam propagation through atmospheric turbulence compared with the uplink propagation and horizontal propagation.

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