A proper excitation wavelength and strong hot-spot effect are of great importance for the application of surface- enhanced Raman spectroscopy (SERS) in the field of biochemistry. As a popular SERS substrate, noble metal nanoparticles dimer is analyzed systematically in this paper. The hot-spot effect and the resonance peak position with varying geometric parameters of metal nanoparticles are investigated. Moreover, the excitation wavelength of bimetallic structure is discussed as well. The conclusions can provide reference for the selection of metal material at different excitation wavelengths.

Energy transfer process and direct charge trapping are two main emission mechanisms in phosphorescent organic light-emitting diodes (PhOLEDs) with host-guest system. Here, to investigate the relationship between luminescence mechanism and performances in host-guest system thoroughly, we demonstrated a green PhOLED using 1,3-bis(carbazole-9-yl) benzene (mCP) as the host and the phosphorescent dye fac-tris(2-phenylpyridine) iridium (fac-Ir(ppy)3) as the guest, where the dopant concentration was variable. It had achieved maximum current efficiency and external quantum efficiency of 84.71 cd/A and 24.5% at 5wt% dopant concentration, respectively. The intrinsic emission mechanisms in the system have been discussed in detail. Moreover, by analyzing the constant current electroluminescence (EL) and the transient EL, the main cause of relationship between dopant concentration and luminescence mechanism has been revealed and the origin of the high performance is also unveiled.

Silica planar lightwave circuit (PLC) devices can offer great potential for quantum key distribution (QKD) with benefits of low-loss, low-cost, large-scale integration, miniaturization, stability and mass production. A complementary encoding-decoding system for QKD based on silica PLC technology was demonstrated, achieving two channels that have balanced outputs simultaneously. This system is consisted of two chips, and each chip is consisted of a variable optical splitter (VOS), an asymmetric Mach-Zehnder interferometer (AMZI) with a thermo-optic phase modulator (TOPM), a delay line (DL), and a directional coupler (DC). The measured delay times of pulse-pairs for two chips are 396 ps and 398 ps, respectively, and the system has shown relatively high interference visibility (IV) of 93.5% and 91.3% for two channels, respectively.

In this work, the light coupling efficiency of organic light-emitting diode (OLED) and polymer optical waveguide integrated device was improved by the grating coupler. To maximize light coupling efficiency, the grating coupler was optimized by finite-difference time-domain (FDTD) method. Based on the simulation results, the grating coupler was fabricated via laser interference lithography process and an OLED was integrated on the surface of it. Comparing the integrated devices without and with grating coupler, light coupling efficiency of the grating-based integrated device was improved by about 5%. The proposed integrated device has the potential application for low-cost and flexible monolithic optical sensors.

To discuss the relative slipping at the interface between a flexibly embedded fiber Bragg grating sensor and a substrate, the strain transfer was derived for an ideal case between the materials of an embedded fiber sensor. ANSYS software was used to establish a simulation model and to analyze the effects of the axial tensile force and the semi-embedded length and encapsulation substrate for the axial strain relative errors with and without relative slipping. The results of the numerical simulations show that the relative strain errors are smaller at the ends of the fibers and larger in the middle for the same tensile force, indicating that the strain transfer effect is location dependent and that the choice of a semi-embedded length of the fibers greater than 40 mm helps to reduce the relative errors. Meanwhile, five flexible sensors with different half-embedding lengths were experimentally encapsulated and subjected to axial tension-strain experiments, which showed the best strain transfer at a half-embedding length of 60 mm, and the experimental results were consistent with the numerical simulation results. The experimental results provide some theoretical and experimental basis for parameter optimization of flexible fiber grating sensors.

division multiplexing (DCO-OFDM) visible light communication (VLC) system. This improved method combines the modified Park’s synchronization method with the pseudo-noise (PN) sequence. The synchronization performance of the proposed method is analyzed in details and compared with that of three traditional methods in DCO-OFDM system. It is demonstrated that the improved method can obtain only one narrow and accurate timing point, which increases the synchronization performance of DCO-OFDM VLC system. Furthermore, the simulated results reveal that the new method has higher timing accuracy and resolution than other conventional methods for different signal noise ratios (SNRs) and fast Fourier transform (FFT) block sizes. To further investigate the performance of time metric, experiment is employed and the results show that the proposed method outperforms the other methods.

Design analysis of high core count (36-core, 37-core) un-coupled multicore fiber (MCF) using the heterogeneous core has been proposed, to achieve the high spatial efficiency and ultra-low crosstalk (~-81 dB/100 km) under the limited cladding diameter of 300 μm. To realize the ultra-low crosstalk level between the neighboring cores, the combination of two crosstalk reduction approaches, trench-assisted (TA) core and propagation direction interleaving (PDI) technique has been used. The forward and backward crosstalk characterizations have been studied in heterogeneous MCF with respect to fiber bending radius, wavelength, transmission distance, and core-pitch with both normal step-index and trench-assisted cores under single-mode propagation condition.

2,4,6-trinitrotoluene (TNT) has a strong explosive force and environmental toxicity, with the increasing threat of terrorist attacks worldwide, the high sensitivity detection of nitroaromatic explosives has become an urgent problem to be solved, and now the commonly used detection method is to use the optical principle, combined with large and expensive equipment to detect it. In order to detect the content of TNT in bad environment quickly and in real time, surface acoustic wave technology was proposed to detect different concentrations of TNT. In this paper, an ultra-sensitive TNT sensor was fabricated based on the surface acoustic wave technique. Specific detection of TNT was achieved by recognizing the shift of resonance frequency. Moreover, the whole process for the detection was done in 30 min, dedicating the rapid/real-time application of the sensor. This study focused on the transfer characteristics of resonance frequencies at different concentrations. The frequency of surface sound waves varies greatly at high concentration because the modified IDT (interdigital transducers) electrodes were utilized, which is easy to work under different concentrations of TNT. The proposed sensor has the advantages of real-time, simple and convenient detection, which provides a valuable method for the real-time detection of TNT.

A simple method to measure the length of optical fiber is proposed using microwave photonic technology. The length of fiber is measured according to the free spectral range (FSR) of the microwave photonic system. The measurement accuracy is only related to the accuracy of FSR measurement. The accuracy is further improved by introducing a reference fiber, and the method was verified in the experiment with 2—10 m single-mode fiber.

The existing image encryption schemes are not suitable for the secure transmission of large amounts of data in range-gated laser imaging under high noise background. Aiming at this problem, a range-gated laser imaging image compression and encryption method based on bidirectional diffusion is proposed. The image data collected from the range-gated laser imaging source is sparsely represented by the discrete wavelet transform. Arnold chaotic system is used to scramble the sparse matrix, and then the measurement matrix is constructed by the quantum cellular neural network (QCNN) to compress the image. In addition, the random sequence generated by QCNN hyperchaotic system is used to carry out "bidirectional diffusion" operation on the compression result, so as to realize the security encryption of image data. The comparative analysis of the security encryption performance of different compression ratios shows that the histogram sample standard of the encrypted image can reach about 10, and the information entropy value is more than 7.99, which indicates that the encryption scheme effectively hides the plaintext information of the original image. When the encrypted image is attacked by different degrees of noise, this method can still reconstruct the image through the effective decryption process. The experimental results show that this method realizes the secure compression and encryption of gated-laser imaging image data, and effectively ensures the security of data while reducing the amount of channel transmission data.

We propose a four-state reference-frame-independent quantum key distribution (RFI-QKD) protocol with the heralded pair-coherent source (HPCS). We investigate the performance of the proposed protocol and simulation results show that our protocol can achieve a high key generation rate in long-distance transmission, taking source flaws and statistical fluctuations into consideration. Although fewer states are used, this protocol not only has a higher key generation rate at the same transmission distance but also a longer transmission distance with the same secure communication compared with the original six-state RFI-QKD protocol using the weak coherent source (WCS).