Youwen Liu

Electrical and optical properties of Nd3+-doped Na0.5Bi0.5TiO3 ferroelectric single crystal

Sodium bismuth titanate Na0.5Bi0.5TiO3 (NBT) single crystal doped with Nd3+ was grown by a top-seeded solution growth method. Powder x-ray diffraction revealed a pure perovskite structure with the rhombohedral phase. We found that the dielectric and ferroelectric properties were enhanced by the Nd3+ dopant. After poling along the [1?1?1] direction, transmittance was enhanced dramatically. The Sellmeier dispersion equation and energy band gaps were obtained. The absorption band around 808?nm has high full-width at half-maximum and large absorption cross-section, which is suitable for AlGaAs diode-laser pumping. A strong emission transition band of Nd3+ at around 1066?nm was observed; a long radiation lifetime 324??s shows a low quenching effect. These results indicate that Nd3+-doped NBT crystal could be applied in photonic or integrated optoelectronic devices as a multi-functional crystal.

Composition dependence of dispersion and bandgap properties in PZN-xPT single crystals

Refractive indices and optical transmittance spectra of (1-x)Pb(Zn1/3Nb2/3)O3-xPbTiO3 (PZN-xPT) single crystals poled along [001] direction have been investigated. Cauchy dispersion equations of the refractive indices were obtained by least square fitting, which can be used to calculate the refractive indices in the low absorption wavelength range. After poled along [001] direction, the transmittance of PZN–12%PT single crystal is more than 65% from 0.5 to 5.8 μm, which is much higher than that of PZN–5%PT and PZN–8%PT single crystals. Optical energy bandgap was obtained from absorption coefficient spectra. Our results show that as the PT content increases, the refractive indices of PZN-xPT single crystals increase, while the optical bandgap decreases.

Composition and orientation dependence of high electric-field-induced strain in Pb(In1/2Nb1/2)O3–Pb(Mg1/3Nb2/3)O3–PbTiO3 single crystals

Relaxor based xPb(In1/2Nb1/2)O3–(1-x-y)Pb(Mg1/3Nb2/3)O3–yPbTiO3 single crystals [PIMNTx/(1-x-y)/y] have broader temperature usage range and comparable piezoelectric properties to (1-x)Pb(Mg1/3Nb2/3)O3–xPbTiO3 crystals. In this work, electric-field-induced strain behavior of PIMNT crystals with different orientation and composition was investigated. High strain with low hysteresis was observed in rhombohedral PIMNT25/44/31 crystal oriented along 〈001〉, rather than 〈011〉 and 〈111〉. Ultrahigh strain level up to 0.75% with linear curve can be achieved under 5u2009kV/mm electric field. The electric field induces phase transformation of PIMNT crystals is much higher than that of PIMNT crystals. These properties make PIMNT crystals promising candidate for high performance solid-state actuators.

Optical transmission and dispersion of 0.25Pb(In1/2Nb1/2)O3- (0.75 − x) Pb(Mg1/3Nb2/3)O3-xPbTiO3 single crystals

Optical transmission spectra and refractive index dispersions of ternary relaxor-based ferroelectric 0.25Pb(In1/2Nb1/2)O3–(0.75u2009−u2009x)Pb(Mg1/3Nb2/3)O3–xPbTiO3 [PIMNT0.25/(0.75u2009−u2009x)/x] single crystals were studied after poled along [001] direction. Sellmeier dispersion equations of the refractive index were obtained by least square fitting, which can be used to calculate the refractive indices in visible and near infrared spectra. The optical transmission of tetragonal PIMNT0.25/0.37/0.38 single crystal is about 70% from 0.5 to 5.8u2009μm, which is much higher than that of rhombohedral PIMNT0.25/0.45/0.30 and morphotropic phase boundary PIMNT0.25/0.41/0.34 single crystals. Optical interband direct and indirect transitions were investigated by absorption spectra. Our results show that as the PbTiO3 content increases the refractive indices of PIMNT single crystals increase, while the optical band gap decreases.

Tunable Multichannel Plasmonic Filter Based on a Single Graphene Sheet on a Fibonacci Quasiperiodic Structure

We propose a tunable multichannel plasmonic mid-infrared filter of a single graphene sheet depositing on a Fibonacci quasiperiodic structure. The transmission spectra are numerically analyzed by a finite-difference time-domain (FDTD) method, and the results show that the filtering properties of the proposed structure can be tuned by the chemical potentials of graphene, the width and depth of air grooves, and the refractive index of the substrate. The simulation results are discussed analytically by deducing the dispersion relation of surface plasmon polariton propagating on four layers structure of air/graphene/air/dielectric. The proposed structure may find potential applications in tunable filters, sensors, and integrated photonic circuits.

Reversal construction of polarization-controlled focusing field with multiple focal spots

Abstract. We propose an approach to control the polarization and intensity of the focusing field with multiple focal spots in a high-numerical-aperture imaging system. This focused field is explained as the reversal radiation of the dipole array with presupposed oscillating direction in the focal volume. By engineering the structure of the electric dipole array and reversing the radiation from this dipole array, the local manipulation of intensity and polarization in the focal region is achieved, and the required field at the pupil plane is derived. A focusing field with six vectorial focal spots is created as an example to verify this method. This multifocal field will meet potential application in polarization-dependent optical microscope and optical trapping for multiple particles.

Non-invasive depth-resolved imaging through scattering layers via speckle correlations and parallax

Various approaches for imaging through scattering layers have been proposed, but very few of them provide depth-resolved images. Here, we propose and experimentally demonstrate an approach for non-invasive depth-resolved imaging of objects hidden behind a scattering medium based on speckle correlations and parallax. The relative intensities of the objects autocorrelations and the positions of the cross-correlations, both change from different apertures. The reconstruction of objects is achieved by subtraction of the autocorrelations and the conventional Fienup-type iterative phase-retrieval algorithm, and the objects relative position is retrieved from the relative position of the cross-correlation to the central; moreover, the depth information of objects is decoded by the displacement of the cross-correlations through different apertures. This multi-aperture technique allows sharp imaging of objects with full actual depth information at once.

Tunable plasmonic filter based on graphene-layered waveguide

We propose a tunable band-stop plasmonic filter based on monolayer graphene with different thickness of structure, and the corresponding transmission characteristic is numerically investigated by using finite-difference time-domain (FDTD) method. The results show that the proposed filter can achieve a broad stopband that can be tuned by various physical parameters such as the chemical potential of graphene, the thickness of packing layers and so on. Our studies may be important for designing tunable optical filter, the fabrication of nano-integrated plasmonic circuits and the refractive index sensitive sensors.

Speckle reduction in optical coherence tomography by adaptive total variation method

An adaptive total variation method based on the combination of speckle statistics and total variation restoration is proposed and developed for reducing speckle noise in optical coherence tomography (OCT) images. The statistical distribution of the speckle noise in OCT image is investigated and measured. With the measured parameters such as the mean value and variance of the speckle noise, the OCT image is restored by the adaptive total variation restoration method. The adaptive total variation restoration algorithm was applied to the OCT images of a volunteer’s hand skin, which showed effective speckle noise reduction and image quality improvement. For image quality comparison, the commonly used median filtering method was also applied to the same images to reduce the speckle noise. The measured results demonstrate the superior performance of the adaptive total variation restoration method in terms of image signal-to-noise ratio, equivalent number of looks, contrast-to-noise ratio, and mean square error.

Controlled focus shaping with generalized cylindrical vector beam

An optical system is proposed for the controlled focus shaping of generalized cylindrical vector beams by a high-numerical-aperture lens. A segmented electro-optical filter with four concentric belts is introduced and works with a double-λ/2-plate. By controlling the rotational angle of the plate and the voltage applied to the filter with proper azimuth angle, a three-dimensional engineered focusing field, such as an electrically controlled axial-shifted focus, extended depth of focus, and a diffraction-limited optical tube, can be achieved. The main advantage of this focusing system is that the focused field can be adjusted and formed by the applied voltage and the rotational angle of the half-wave plates. It can provide a new focus shaping technique with free adjustability and flexibility.