Shiqiao Qin

Dielectric loaded graphene plasmon waveguide

Dielectric loaded graphene plasmon waveguide (DLGPW) is proposed and investigated. An analytical model based on effective-index method is presented and verified by the finite element method simulations. The mode effective index, propagation loss, cutoff wavelength of higher order modes and single-mode operation region were derived at mid-infrared spectral region. By changing Fermi energy level, the propagation properties of fundamental mode could be tuned flexibly. The structure of the DLGPW is simple and easy for fabrication. It provided a new freedom to manipulate the graphene surface plasmons, which may led to new applications in actively tunable integrated optical devices.

Monolayer-graphene-based perfect absorption structures in the near infrared.

Subwavelength perfect optical absorption structures based on monolayer-graphene are analyzed and demonstrated experimentally. The perfect absorption mechanism is a result of critical coupling relating to a guided mode resonance of a low index two-dimensional periodic structure. Peak absorption over 99% at wavelength of 1526.5 nm with full-width at half maximum (FWHM) about 18 nm is demonstrated from a fabricated structure with period of 1230 nm, and the measured results agree well with the simulation results. In addition, the influence of geometrical parameters of the structure and the angular response for oblique incidence are analyzed in detail in the simulation. The demonstrated absorption structure in the presented work has great potential in the design of advanced photo-detectors and modulators.

Region-confined restoration method for motion-blurred star image of the star sensor under dynamic conditions.

Under dynamic conditions, the centroiding accuracy of the motion-blurred star image decreases and the number of identified stars reduces, which leads to the degradation of the attitude accuracy of the star sensor. To improve the attitude accuracy, a region-confined restoration method, which concentrates on the noise removal and signal to noise ratio (SNR) improvement of the motion-blurred star images, is proposed for the star sensor under dynamic conditions. A multi-seed-region growing technique with the kinematic recursive model for star image motion is given to find the star image regions and to remove the noise. Subsequently, a restoration strategy is employed in the extracted regions, taking the time consumption and SNR improvement into consideration simultaneously. Simulation results indicate that the region-confined restoration method is effective in removing noise and improving the centroiding accuracy. The identification rate and the average number of identified stars in the experiments verify the advantages of the region-confined restoration method.

Compound parabolic concentrator applied as receiving antenna in scattering optical communication

The two-dimensional (2D) compound parabolic concentrators (CPC) characteristics are analyzed. It is shown that CPCs height is taller and its light collecting ability is stronger with the CPCs field of view decreasing when the bottom radius is unchanged. According to the ZEMAX analysis, CPC is good at collecting optical signal, and the antenna combining CPC with hemispherical lens can gather more optical signal than a single CPC or CPCs combined in series. The light propagation of scattering optical communication based on multiple scattering is simulated by Monte Carlo method, and the results show that using CPC as receiving antenna can strengthen communication systems signal collecting ability and increase its communication distance.

A new integrated Gaussian-Markov process model for precision shipboard transfer alignment

In shipboard transfer alignment (TA), Kalman filter used to estimate the misalignment angle requires accurate ship dynamic flexure model. Traditionally, the ship dynamic flexure is modelled as a second-order Gaussian-Markov process according to experience. However, this model has not been validated in real applications, when the current model is differ with this used in Kalman filter design will result in a large measurement error. To solve this problem, an integrated Gaussian-Markov process model is proposed in this contribution which is based on the hydroelastic analysis and statistic from previous measured data. Specifically, theoretical analysis shows ship dynamic flexure is the response of elastic ship hull to sea wave loads, while the sea wave spectrum is of double-peaked in frequency filed for the swell and wind sea waves occurring simultaneously, and therefore, the dynamic flexure is also with double-peaked spectrum distribution. Furthermore, the frequency analysis based on our previous measured ship dynamic flexure data also demonstrates the power spectrum density (PSD) of actual dynamic flexure angle is of double-peaked distribution, which can be modelled more accurately by combined using two independent second-order Gaussian-Markov process models. Experimental results show that Kalman filter utilizing the proposed integrated Gaussian-Markov process model provides more accurate measurement of the misalignment angle compared with using traditional second-order Gaussian-Markov process model in shipboard TA.

Centroid error compensation method for a star tracker under complex dynamic conditions

The traditional approach of a star tracker for reducing the dynamic error concentrates on a single frame of star images. Through correlating adjacent star images together with their angular relations sensed by a gyroscope unit (GU), the attitude-correlated frames (ACF) approach expands the view from one single frame to frame sequences. However, the star centroid is shifted from the star true position at the center time of the exposure period under complex dynamic conditions, which is called the complex motion induced error (CMIE) in this paper. The CMIE has a large effect on the performance of the ACF approach. This paper presents a method to compensate the CMIE through reconstructing the star trajectory with the angular velocity of the star tracker sensed by a GU, which achieves effective compensation of the CMIE crossing the boresight direction. Since the observation sensitivity to the CMIE along the boresight direction is low, the attitudes from two different fields of view (FOVs) are combined to improve its compensation accuracy. Then the ACF approach is applied to the obtained results where the CMIE has already been compensated completely. Simulations under shipboard dynamic conditions and experiments under oscillating conditions indicate that the proposed method is effective in improving the performance of the ACF approach and reducing the dynamic error of a star tracker under complex dynamic conditions.

Adaptive iteration method for star centroid extraction under highly dynamic conditions

Star centroiding accuracy decreases significantly when star sensor works under highly dynamic conditions or star images are corrupted by severe noise, reducing the output attitude precision. Herein, an adaptive iteration method is proposed to solve this problem. Firstly, initial star centroids are predicted by traditional method, and then based on initial reported star centroids and angular velocities of the star sensor, adaptive centroiding windows are generated to cover the star area and then an iterative method optimizing the location of centroiding window is used to obtain the final star spot extraction results. Simulation results shows that, compared with traditional star image restoration method and Iteratively Weighted Center of Gravity method, AWI algorithm maintains higher extraction accuracy when rotation velocities or noise level increases.

Optical property of nanocomposite of mesoporous silica thin films incorporated with gold nanoparticles

Amino-functionalized mesoporous silica thin films (MTFs) are produced using surface active agent F127, and then gold nanoparticles are introduced into the pore channels to prepare the Au/SiO2 nanocomposite. After assembling the gold, the amino-functionalized MTF undergoes some shrinkage but remains a periodic structure as demonstrated by X-ray diffraction (XRD) patterns. The nanocomposite shows an acute characteristic diffraction peak assigned to (111) plane of the face-centered-cubic structure of gold, indicating that gold nanoparticles crystallize well and grow in a preferred orientation in the pore channels. The surface plasma resonance (SPR) absorption peak near 570 nm undergoes a red-shift accompanied by a strengthening of intensity when HAuCl4 is used to react with the amino groups on the internal pore surfaces for 4, 6, and 8 h. The simulative results are consistent with the experimental ones shows that the absorption property of the Au/SiO2 nanocomposite is influenced by the dipping time, which affects the size and volume fraction of embedded gold nanoparticles.

Electrically tunable graphene polarization beam splitting utilizing Brewster effect

We theoretically and numerically demonstrate that electrically tunable polarization beam splitting (PBS) can be realized by the structure of graphene ribbons supported on a dielectric film. The mechanism comes from the fact that the TE mode (like S wave polarization) is highly reflected by the plasmonic resonances of graphene ribbons, and the TM mode (like P wave polarization) is largely transmitted utilizing the Brewster effect. The results of full-wave numerical simulations reveal that a transmission splitting ratio of about 20 dB and a reflection splitting ratio of around 28 dB can be achieved with the Brewster angle incident. The proposed polarization beam splitting can be electrically tuned, which facilitates many practical applications.

Optical Nonlinearity of Mesoporous Silica Thin Films Embedded with Gold Nanoparticles

The mesoporous silica thin films (MSTFs) synthesized by evaporation-induced self-assembly were loaded with gold nanoparticles (GNPs). The TEM and XRD characterizations showed that the template of MSTF possessed a well-ordered mesostructure, and GNPs can be well crystallized in the thin film by neutralization and hydrogen-reduction reactions. The z-scan technique was used for studying the nonlinear optical properties of the GNPs/MSTF nanocomposite. The results showed that this kind of material was possessed of a relatively feeble nonlinear absorption but an obvious nonlinear refraction. The nonlinear refractive index was estimated to be 7.9 × 10−11 cm2/W and the theoretical fitting well matched with the measured data.