Abstract:A novel surface-enhanced Raman scattering (SERS)-active substrate based on Au nanoparticles (AuNPs)-coated silver nanowire (AgNW) is obtained by an effective and simple method. The results show that the hybrid structures prepared by this method are powerful SERS-active substrates for the detection of malachite green (MG) molecules with the limit of 1 nmol/L. The excellent enhancing ability mainly comes from two kinds of hot spots. One is from the gaps among the adjacent AuNPs, and the other is the presence of zone between AuNPs and AgNW. In particular, the AuNPs-coated AgNW can be viewed through the objective of the confocal Raman spectrometer due to the length of the AgNW reaches microns, which can improve the repeatability of detection. Moreover, it is of great significance in research of SERS mechanism and application.

Abstract:Chalcopyrite-type CuInSe₂ nanoparticles are successfully prepared by using In₂Se₃ nanoparticles as a precursor reacted with copper chloride (CuCl) solution via a phase transformation process in low temperature. The reaction time is a key parameter. After the reaction time increasing from 0.5 h to 8 h, In₂Se₃ and CuCl react with each other gradually via phase transformation into CuInSe₂ without any intermediate phase. The crystalline structure and morphology of the CuInSe₂ nanoparticles are characterized by X-ray diffraction (XRD) and field emission scanning electron microscopy (FESEM). The diameter of CuInSe₂ nanoparticles with good dispersibility ranges from 10 nm to 20 nm. The band gap of the CuInSe₂ nanoparticles is 1.04 eV calculated from the ultraviolet-visible (UV-VIS) spectrum.

Abstract:Multilayer dielectric thin film edge filter has serious polarization sensitivity under oblique incidence. The cutoff-bands of the s-polarization and p-polarization light in conventional edge filter will separate obviously under 45° oblique incidence, which limits its application. Based on the two chosen materials TiO₂ and SiO₂, a novel stack structure is proposed to design the non-polarization short-wave-pass thin film edge filter. By using the (4H 4L 4H) as the matching layers, the polarization separation at 3 dB transmittance for the thin film edge filter cutoff-band is less than 1 nm at the incident angle of 45°. In this way, the non-polarization short-wave-pass edge filter is easily designed and fabricated.

Abstract:The steady and dynamic properties are comparatively investigated for the n-doped and non-doped InGaN LEDs. The simulated results show that the n-doped LED exhibits the superior luminescence and modulation performance, which is mainly attributed to the higher carrier radiative rate of n-doped LED. The results can explain the reported experimental results perfectly.

Abstract:Silver nanoprisms (AgNPs) affect the performance of organic solar cells (OSCs) in different ways depending on their positions in the device. To investigate this issue, we incorporate AgNPs in different positions of OSCs and compare their performance. The power conversion efficiency (PCE) is improved by 23.60% to 3.98% when the AgNPs are incorporated in front of the active layer. On the other hand, when AgNPs are incorporated in the back of the active layer, the short-circuit current density (J _SC) is improved by 17.44% to 10.84 mA/cm². However, if AgNPs are incorporated in the active layer, both open-circuit voltage (V _OC) and J _SC are decreased. We discuss the position effect on the device performance, clarify the absorption shadow and exciton recombination caused by AgNPs, and finally indicate that the optimal position of plasmonic AgNPs is in front of the active layer.

Abstract:A dual-blue light-emitting diode (LED) with asymmetric AlGaN composition-graded barriers but without an AlGaN electron blocking layer (EBL) is analyzed numerically. Its spectral stability and efficiency droop are improved compared with those of the conventional InGaN/GaN quantum well (QW) dual-blue LEDs based on stacking structure of two In_0.18Ga_0.82N/GaN QWs and two In_0.12Ga_0.88N/GaN QWs on the same sapphire substrate. The improvement can be attributed to the markedly enhanced injection of holes into the dual-blue active regions and effective reduction of leakage current.

Abstract:Lithium lutetium fluoride (LiLuF₄) single crystals doped with different Dy³⁺ ion concentrations were grown by Bridgman method. The Judd-Ofelt (J-O) strength parameters (Ω ₂, Ω ₄, Ω ₆) of Dy³⁺ in LiLuF₄ crystal are calculated according to the measured absorption spectra and the J-O theory, by which the asymmetry of the Dy³⁺:LiLuF₄ single crystal and the possibility of attaining stimulated emission from ⁴F_9/2 level are analyzed. The capability of the Dy³⁺:LiLuF₄ crystal in generating white light by simultaneous blue and yellow emissions under excitation with ultraviolet light is produced. The effects of excitation wavelength and doping concentration on chromaticity coordinates and photoluminescence intensity are also investigated. Favorable CIE coordinates, x=0.319 3 and y=0.349 3, can be obtained for Dy³⁺ ion in 2.701% molar doping concentration under excitation of 350 nm.

Abstract:This paper provides the fabrication of Cd-free Cu(In,Ga)Se₂ (CIGS) solar cells on soda-lime glass substrates. A high quality ZnS buffer layer is grown by chemical bath deposition (CBD) process with ZnSO₄-NH₃-SC (NH₂)₂ aqueous solution system. The X-ray diffraction (XRD) result shows that the as-deposited ZnS film has cubic (111) and (220) diffraction peaks. Scanning electron microscope (SEM) images indicate that the ZnS film has a dense and compact surface with good crystalline quality. Transmission measurement shows that the optical transmittance is about 90% when the wavelength is beyond 500 nm. The bandgap (E _g ) value of the as-deposited ZnS film is estimated to be 3.54 eV. Finally, a competitive efficiency of 11.06% is demonstrated for the Cd-free CIGS solar cells with ZnS buffer layer after light soaking.

Abstract:InAs_0.6P_0.4 epilayers grown by low-pressure metal organic chemical vapor deposition (LP-MOCVD) on InP (100) substrate are investigated. The influence of growth temperature on crystalline quality of InAs_0.6P_0.4 epilayer is characterized by scanning electron microscopy (SEM), Hall measurements, photoluminescence (PL) spectra, and the Raman properties are analyzed by Raman scattering spectrum. The characterization results show that the crystalline quality and Raman property of InAs_0.6P_0.4 epilayers have close relation to the growth temperature. It indicates that 530 °C is the optimum growth temperature to get good quality and properties of InAs_0.6P_0.4 epilayers.

Abstract:In order to use white light emitting diode (LED) as a lighting source and communication part, a new modulation scheme called reverse dual header pulse interval modulation (RDH-PIM) is proposed for indoor visible light communications based on the analyses of the structures of on-off keying (OOK), pulse position modulation (PPM), digital pulse interval modulation (DPIM) and dual header pulse interval modulation (DH-PIM). After analyzing and comparing the symbol structure, bandwidth requirement and average transmission power of different modulation methods, the slot error rate is derived. Simulation results show that OOK has the minimum bandwidth, while RDH-PIM has the highest average transmission power and its bandwidth efficiency is obviously better than those of PPM and DPIM. Hence, RDH-PIM is superior in optical wireless communication systems.

Abstract:For the high peak-to-average power ratio (PAPR) of coherent optical orthogonal frequency division multiplexing (CO-OFDM) system, a novel joint technique which is the combination of iterative partial transmit sequence (IPTS) and clipping technique is proposed. Simulation results demonstrate that the PAPR and bit error rate (BER) performance of the proposed technique both outperform those of the single techniques. Under the same conditions, the threshold value and peak power of IPTS clipping joint technique are optimized by 3.44 dB and 0.86 dBm compared with those of IPTS techinque, respectively. At the BER of 10^?3, the optical signal to noise ratios (OSNRs) of the novel joint technique after 320 km and 400 km single-mode fiber (SMF) transmission are 0.68 dB and 1.18 dB smaller than those of clipping technique, respectively.

Abstract:For enhancing the coupling efficiency between the beam and the photodiode, a special optical taper is proposed for receiving optical signal. Based on the circular symmetric structure of special optical taper, the profile curve equations of it are deduced, including the trigonometric function type, parabolic type and exponential type. Moreover, the coupling efficiencies for special optical tapers with different profile curves are studied. The relationships of incident position, incident angle and coupling efficiency are analyzed. Finally, the comparison of coupling efficiency analytical results is also given.

Abstract:Based on aperture averaging effects on scintillation, a diversity reception system consisting of three apertures has been designed. The transmitted laser beam (λ=1.06 μm) propagates for 3 km, and is received through apertures with different sizes. Various numbers and configurations of apertures are studied to investigate the received laser beam’s spatial profile and quality. Our results show that the diversity reception system with three apertures can suppress atmospheric scintillation with relatively simple configuration and low cost.

Abstract:A construction scheme of variable-weight optical orthogonal codes (VW-OOCs) for asynchronous optical code division multiple access (OCDMA) system is proposed. According to the actual situation, the code family can be obtained by programming in Matlab with the given code weight and corresponding capacity. The formula of bit error rate (BER) is derived by taking account of the effects of shot noise, avalanche photodiode (APD) bulk, thermal noise and surface leakage currents. The OCDMA system with the VW-OOCs is designed and improved. The study shows that the VW-OOCs have excellent performance of BER. Despite of coming from the same code family or not, the codes with larger weight have lower BER compared with the other codes in the same conditions. By taking simulation, the conclusion is consistent with the analysis of BER in theory. And the ideal eye diagrams are obtained by the optical hard limiter.

Abstract:An efficient post-coding strategy is proposed in this letter to reduce the peak-to-average power ratio (PAPR) of orthogonal frequency division multiplexing (OFDM) signals for optical intensity modulated direct detection (IM/DD) systems. The post-coding scheme based on discrete cosine transform (DCT) is employed after the inverse fast Fourier transform (IFFT) in the transmitter to reduce the PAPR of OFDM signals. This method is different from the conventional pre-coding scheme which is employed before IFFT operation. Numerical simulations demonstrate that the new DCT post-coding strategy can significantly reduce the PAPR than the conventional pre-coding scheme. Meantime, the bit error rate (BER) performance of the proposed post-coding system can be improved compared with the conventional pre-coding scheme.

Abstract:A novel wavelet denoising (WD) assisted wavelength modulation technique is proposed for improving near-infrared detection performance on methane concentration based on tunable diode laser absorption spectroscopy (TDLAS). Due to the ability of multi-level analytical resolutions both in time- and frequency-domains, the noise contained in the differential signal is greatly suppressed. Sensor mechanical part, optical part and electrical part are integrated, and a portable detection device is finally developed. Theory and formulations of the WD-assisted wavelength modulation technique are presented, and experiments are carried out to prove the normal function on the extraction of the second harmonic (2f) signal from severely polluted differential signal by using the technique. By virtue of WD’s suppression on noises, the sensing characteristics on CH₄ concentration are improved, and the limit of detection (LOD) is decreased from 4×10^?6 (without WD processing) to 10^?6. The proposed technique can also be used for the measurement on the concentration of other gases with corresponding near-infrared distributed feedback lasers.

Abstract:Self-mixing interference (SMI) technique can be used for measuring vibration, displacement, velocity and absolute distance. In this paper, a simple demodulation algorithm for fast measuring frequency and amplitude of a simple harmonic vibration target is proposed based on the basic theoretical model of self-mixing interference effects. The simulative results show that the error between the vibration parameters which are demodulated by this algorithm and initial settings merely results from the sample rate. Further, the experimental system of self-mixing vibration measurement is built. The experimental results have a good agreement with simulation analyses. The maximum error of frequency demodulation is less than 1 Hz in our experiment.

Abstract:For fabric defect identification in the textile industry, a three-dimensional (3D) color phase shift profilometry (CPSP) method is proposed. The detecting system is mainly composed of one CCD camera and one digital-light-processing (DLP) projector. Before detection, the system should be calibrated to make sure the camera parameters. The CPSP color grating is projected to the measured fabric by DLP projector, and then it is collected by CCD camera to obtain the grating phase. The 3D measurement can be completed by the grating phase difference. In image acquisition, only invariable grating is projected to the object. In order to eliminate the interference from background light during the image acquisition, the brightness correction method is researched for improving the detection accuracy. The experimental results show that the false rate of detecting the fabric defects is 5.78%, the correct rates of detecting the fabric defects of hole and qualified fabric are both 100%, and the correct rates of detecting the fabric defect of scratch and fold are 98% and 96%, respectively. The experiment proves that the proposed method can accurately identify fabric defects.

Abstract:Aiming at these disadvantages like lack of details, poor contrast and blurry edges of infrared images reconstructed by traditional controllable microscanning super-resolution reconstruction (SRR), this paper proposes a novel algorithm, which samples multiple low-resolution images (LRIs) by uncontrollable microscanning, and then uses LRIs as chromosomes of genetic algorithm (GA). After several generations of evolution, optimal LRIs are got to reconstruct the high-resolution image (HRI). The experimental results show that the average gradient of the image reconstructed by the proposed algorithm is increased to 1.5 times of that of the traditional SRR algorithm, and the amounts of information, the contrast and the visual effect of the reconstructed image are improved.

Abstract:Based on the optical Tamm states (OTSs), a metal-distributed Bragg reflector (DBR) structure is presented. The existence of optical Tamm states is demonstrated by the dip of reflectivity spectrum in photonic band gap. The properties of the optical Tamm states are investigated by applying the transfer matrix method. The effects of different metal thicknesses and angles on the absorption are studied by numerical simulation. The wide-angle perfect absorption appears when the metal thickness is 39 nm.

Abstract: