Engineering a strong and stable ultraviolet chiroptical effect in a large-area chiral plasmonic shell.

Ultraviolet chiral metamaterials (UCM) are highly desired for their strong interaction with the intrinsic resonance of molecules and ability in manipulating the polarization state of high energy photons, but rarely reported to date due to their small feature size and complex geometry. Herein, we design and fabricate a kind of novel ultraviolet chiral plasmonic shell (UCPS) by combing the stepwise Al deposition and colloid-sphere assembled techniques. The cancellation effect originated from the disorder lattices of micro-domains in the colloid monolayer has been successfully overcome by optimizing the deposition parameters, and a strong CD signal of larger than 1 deg in the UV region is demonstrated both in simulation and experiment. This strong ultraviolet chiroptical resonances mainly come from the surface chiral lattice resonance mode, the whispering gallery mode and also the interaction between neighbor shells, and can be effectively tuned by changing structural parameters, for example, the sphere diameter, or even slightly increasing the deposition temperature in experiment. To improve the stability, the fabricated UCPSs are protected by N2 in the deposition chamber and then passivated by UV-ozone immediately after each deposition step. The formed UCPS show an excellent stability when exposing in the atmospheric environment. The computer-aided geometrical model, electromagnetic modes, and the tunable chiroptical resonance modes have been systematically investigated.

Dual-task convolutional neural network based on the combination of the U-Net and a diffraction propagation model for phase hologram design with suppressed speckle noise.

In this paper, a dual-task convolutional neural network based on the combination of the U-Net and a diffraction propagation model is proposed for the design of phase holograms to suppress speckle noise of the reconstructed images. By introducing a Fresnel transmission layer, based on angular spectrum diffraction theory, as the diffraction propagation model and incorporating it into U-Net as the output layer, the proposed neural network model can describe the actual physical process of holographic imaging, and the distributions of both the light amplitude and phase can be generated. Afterwards, by respectively using the Pearson correlation coefficient (PCC) as the loss function to modulate the distribution of the amplitude, and a proposed target-weighted standard deviation (TWSD) as the loss function to limit the randomness and arbitrariness of the reconstructed phase distribution, the dual tasks of the amplitude reconstruction and phase smoothing are jointly solved, and thus the phase hologram that can produce high quality image without speckle is obtained. Both simulations and optical experiments are carried out to confirm the feasibility and effectiveness of the proposed method. Furthermore, the depth of field (DOF) of the image using the proposed method is much larger than that of using the traditional Gerchberg-Saxton (GS) algorithm due to the smoothness of the reconstructed phase distribution, which is also verified in the experiments. This study provides a new phase hologram design approach and shows the potential of neural networks in the field of the holographic imaging and more.

Optical differentiation based on weak measurements.

Optical differentiation shows much potential to be applied in computation due to its strong parallelizability. Currently, each optical differential method can only obtain partial differential information. Here, we propose a general approach to obtain complete differentiation. Compared to previous methods, we can separately obtain the differentiation of amplitude and phase, reserve the negative value of the differentiation, and acquire the differentiation in arbitrary directions of the two-dimensional field. We measure the differentiation of the Gaussian beam to demonstrate this method. A practical experiment of identifying the move direction of the motion-blurred image is also presented to verify the practicability of our method. Our method can further be applied to intelligence algorithms, image identification, and optical analog computing.

U-Net convolutional neural network-based modification method for precise fabrication of three-dimensional microstructures using laser direct writing lithography.

In this paper, a modification method based on a U-Net convolutional neural network is proposed for the precise fabrication of three-dimensional microstructures using laser direct writing lithography (LDWL). In order to build the correspondence between the exposure intensity distribution data imported to the laser direct writing system and the surface profile data of the actual fabricated microstructure, these two kinds of data are used as training tensors of the U-Net convolutional neural network, which is proved to be capable of generating their accurate mapping relations. By employing such mapping relations to modify the initial designed exposure intensity data of the parabolic and saddle concave micro-lens with an aperture of 24µm×24µm, it is demonstrated that their fabrication precision, characterized by the mean squared error (MSE) and the peak signal-to-noise ratio (PSNR) between the fabricated and the designed microstructure, can be improved significantly. Specifically, the MSE of the parabolic and saddle concave micro-lens decreased from 100 to 17 and 151 to 50, respectively, and the PSNR increased from 22dB to 29dB and 20dB to 25dB, respectively. Furthermore, the effect of laser beam shaping using these two kinds of micro-lens has also been improved considerably. This study provides a new solution for the fabrication of high-precision three-dimensional microstructures by LDWL.

Measurement of hysteresis loop based on weak measurement.

In this Letter, we propose a technique for hysteresis loop measurement based on weak measurement. By using the photonic spin Hall effect (PSHE) as a probe and combining the quantum weak measurement, the technique's noise can be suppressed greatly. A theoretical model to describe the numerical relation between the amplified shift and Kerr rotation angle is established. Through detecting the amplified shift of the PSHE based on weak measurement, we experimentally measure the hysteresis loops of Ni-Fe alloy film, iron-phthalocyanine (FePc) monolayer film, and Co/FePc double-layer film. The results show that the precision can reach about $ \sim {10^{ - 6}} \;{\rm rad} $∼10-6rad under ordinary experimental conditions, which may have an important application prospect in magneto-optic parameters measurement.

Active perfect absorber based on planar anisotropic chiral metamaterials.

Active chiral plasmonics have attracted a considerable amount of research interest for their power to switch the handedness of chiral metamaterials and the potential applications in highly integrated polarization sensitive devices, stereo display fields, and so on. In this work, we propose a kind of active chiral metamaterial absorber (ACMA) composed by planar anisotropic chiral metamaterials (PACMs) and a metal layer. Our in-depth theoretical analysis indicates that the circular conversion dichroism (CCD) from PACMs plays a crucial role to achieve the active chiroptical effect. The CCD effect can enable a differentiated microcavity-interference effect between the left and right circular incident lights and results in a chiroptical effect related to the equivalent optical length between the PACMs and the metal layer. In simulations, a high-performance ACMA, which are composed by the 'Z'-shaped PACMs, is designed, and the maximum reflection CDR from ACMA can reach 0.882. Meanwhile, the minimum reflection CDR can reach to 0, resulting a very large adjustable range of from 0 to 0.882. The maximum modulation sensitivity, which is defined as Mn=∂CDR/∂n and Md=∂CDR/∂d, can reach to about 1368.252 for d=100um and 0.06157 nm-1 for n=4.5,respectively. In addition to the active chiroptical effect, the designed ACMA also shows excellent performance as a sensor, such as when it is being used as a highly-sensitive temperature sensor. In that case, the minimum detected precision can reach approximately 3.067 * 10-8 °C, if VO2 is used to fill the FP cavity.

Unveiling the spin Hall effect of light in Imbert-Fedorov shift at the Brewster angle with weak measurements.

The Imbert-Fedorov (IF) shift is defined as the transverse shift of barycenter of the entire beam when a circular or elliptically polarized incident beam is reflected. In this work, we examine the IF shift of Gaussian beam at the Brewster angle. Interestingly, the spin Hall effect of light takes place in the IF shift at the same time. Furthermore, this interesting phenomenon is experimentally observed using weak measurements. These findings may have useful applications in spin optics.

Binary wavefront optimization using a simulated annealing algorithm.

We propose an idea using a simulated annealing algorithm for amplitude modulation to focus light through disordered media. Using 4096 independently controlled segments of an incident wavefront, the intensity of the target signal is enhanced 73 times over the original intensity of the same output channel. The simulated annealing algorithm and existing amplitude control algorithms for focusing through scattering media are compared experimentally. It is found that the simulated annealing algorithm achieves the highest enhancement when the number of iterations required for optimization is the same.

Precision improvement of surface plasmon resonance sensors based on weak-value amplification.

In this paper, a method to improve the precision of prism-coupler-based surface plasmon resonance (SPR) sensors using weak-value amplification (WVA) is presented. A general theory model to describe the post-selected SPR sensing system is established and the polarization-dependent amplitude and phase evolutions induced by the SPR are regarded as the weak interactions in the WVA formalism. As a consequence of WVA, the detected signal can be enhanced with respect to the technical noise which is dependent on the post-selected light intensity. The precision of our method reaches 2.9 × 10-7RIU, which is improved nearly one order of magnitude compared to the conventional sensing technique.

Optoplasmonic probe to realize scanning near-field Raman microscopy.

Tip-enhanced Raman spectroscopy (TERS) is a powerful scanning probe technique for Raman detections in nanotechnology to date. However, limited by the physical principles of a nanosize tapered metal (or metal-coated) probe used in a TERS device, only far-field without near-field Raman signal can be collected by the TERS with the metal probe. This makes conventional TERS lower in efficiency and cannot be a real near-field Raman microscopy. In this paper, we propose a simple and realizable optoplasmonic probe model, which is composed of a dielectric microsphere and a metal nanobowtie, to realize an ideal scanning near-field Raman microscopy (SNRM). Using finite-difference time-domain (FDTD) method, calculation results of electric field distributions of the proposed probe demonstrate that the probe provides three outstanding characteristics, including strong enhancement of local electric field, nanoscale distributions of the produced electric filed, and collection enhancement of emitted energy with wide wavelength range in near field. These characteristics of the probe resolve the detecting restrictions of metal probes and provide a real near-field scanning method. Therefore, a potentially novel SNRM can be expected to extend Raman application range greatly.

Estimation of optical rotation of chiral molecules with weak measurements.

From its beginning, the measurement of optical rotation (OR) is crucial for the analysis of chiral molecules in many fields. However, the precision measurement of a weak OR signal with conventional setup remains elusive. In this Letter, we experimentally propose a precision method to determine the OR of glucose and fructose based on weak measurements. By using the spin Hall effect of light (SHEL) as a probe, a nonlinear weak measurements model is established that is applicable beyond the Aharonov-Albert-Vaidman (AAV) limit. Due to the high sensitivity of weak measurements amplification with respect to the OR, a tremendous variation of the amplified beam displacement of the SHEL is observed, while the concentration of glucose and fructose is slightly changed.

Pure magnetic resonances controlled by the relative azimuth angle between meta-atoms.

A plasmonic molecule showing strong magnetic resonance modes and flexible tunability is proposed. The molecule is composed of two elements, a crescent shaped metallic disk and a smaller one embedded in the cavity of the larger one. The cavity and gap formed by these two elements enable the molecule to support magnetic resonances in the visible and near infrared spectral region, while electric resonances are much weaker to be detected. We show that by changing the relative orientation angle of these two meta-atoms, the resonance wavelength can be changed from the visible to near infrared without modification of the size of the molecule. Anti-crossings and crossings of resonance energy levels, which stem from the coupling effect, are analyzed. When resonating magnetically, the local electric field enhancement at the crescent tip can reach up to hundreds of times with high spatial confinement, which renders the molecule promising applications in many fields.

Diffraction-dependent spin splitting in spin Hall effect of light on reflection.

We report on a diffraction-dependent spin splitting of the paraxial Gaussian light beams on reflection theoretically and experimentally. In the case of horizontal incident polarization, the spin splitting is proportional to the diffraction length of light beams near the Brewster angle. However, the spin splitting is nearly independent with the diffraction length for the vertical incident polarization. By means of the angular spectrum theory, we find that the diffraction-dependent spin splitting is attributed to the first order expansion term of the reflection coefficients with respect to the transverse wave-vector which is closely related to the diffraction length.

Incident-polarization-sensitive and large in-plane-photonic-spin-splitting at the Brewster angle.

In this Letter, we report a phenomenon of large in-plane-photonic-spin-splitting (IPPSS) in the case of a linear polarized Gaussian light beam reflected from an air-glass interface at the Brewster angle. The IPPSS-induced displacement reaches ∼12.4 µm, which is quite larger than the previously reported value. Particularly, the IPPSS is extremely sensitive (∼70 µm/deg) to the incident polarization. We also find that the direction of the spin accumulation can be switched by adjusting the incident polarization slightly. These findings may have useful applications in spin manipulation and precise polarization metrology.

Scaling phenomenon of graphene surface plasmon modes in grating-spacer-graphene hybrid systems.

We investigate the excitations of graphene surface plasmon waves in grating-spacer-graphene hybrid systems. It is demonstrated that the resonant absorption rate is scaling invariant as the geometric parameters of the hybrid system are scaled, and this phenomenon is nearly unaffected by the dispersions of the optical parameters of graphene and the grating material. We present an analytical model to calculate the absorption rate and elucidate that the scaling invariant phenomenon originates from the scalabilities of the graphene surface plasmon modes. This study could benefit the development of graphene plasmonic devices at infrared and terahertz frequencies.

Carbon and Silicon Impurity Defects in GaN: Simulating Single-Photon Emitters by First Principles.

Defect single-photon emitters (SPE) in gallium nitride (GaN) have garnered great attentions in recent years due to the advantages they offer, including the ability to operate at room temperature, narrow emission linewidths, and high brightness. Nevertheless, the precise nature of the single-photon emission mechanism remains uncertain due to the multitude of potential defects that can form in GaN. In this work, our systematical investigation with the ab initio calculation indicates that carbon and silicon, as common dopants in gallium nitride, can interact with intrinsic defects in GaN and form new high-speed defect single-photon sources. Our findings identify a ternary defect NGaVNCN that possesses a short lifetime of less than 1 ns and a small zero-photon line (ZPL) of 864 nm. In other words, this defect can serve as a high-speed single photon source in the short wavelength window for fiber communication. In sharp contrast, the Si-supported defect NGaVNSiN has a higher unoccupied defect energy level which enters the conduction band and is therefore unsuitable for single photon emission. A systematic investigation has been conducted into the potential defects, thermal stability, and single-photon emission properties. The relaxation calculation and self-consistent calculations employed the Perdew-Burke-Ernzerhof exchange-correlation functional and Heyd-Scuseria-Ernzerhof exchange-correlation functional, respectively. These findings indicate the potential for high-performance single-photon sources through carbon or silicon doping of GaN.

Folic acid functionalized gadolinium-doped carbon dots as fluorescence / magnetic resonance imaging contrast agent for targeted imaging of liver cancer.

Gadolinium-doped carbon dots (Gd-CDs), as a new class of nanomaterial, has a wide application prospect in targeted imaging and monitoring diagnosis and treatment of liver cancer because of their good fluorescence (FL)-magnetic resonance (MR) imaging properties. First, Gd-CDs were synthesized by hydrothermal method with gadodiamide as gadolinium source, citric acid as carbon source and silane coupling agent (KH-792) as coupling agent with FL quantum yield (QY) of 48.2%. Then, folic acid (FA), which is highly expressed in liver cancer, was used as a targeting component to modify Gd-CDs to obtain targeted imaging agent (Gd-CDs-FA). The results showed that Gd-CDs and Gd-CDs-FA have low cytotoxicity and good biocompatibility, and the targeting and selectivity of Gd-CDs-FA to HepG2 cells could be observed under confocal laser scanning microscope (CLSM). The T1 longitudinal relaxation rates (r1) of Gd-CDs and Gd-CDs-FA are 15.92 mM-1s-1 and 13.56 mM-1s-1, respectively. They showed good MR imaging ability in vitro and in vivo, and MR imaging in nude mice further proved the targeting imaging performance of Gd-CDs-FA. Therefore, Gd-CDs-FA with higher QY showed good FL-MR targeting imaging ability of liver cancer, which broke through the limitations of single molecular imaging probe in sensitivity and soft tissue resolution. This study provides a new idea for the application of Gd-CDs in FL and MR targeting imaging of liver cancer.

Design and fabrication of three-dimensional chiral nanostructures based on stepwise glancing angle deposition technology.

The chiral structures have displayed some inevitable and fascinating properties in many research fields, such as chemistry, biology, mathematics, and physics. In this Article, we report the use of stepwise glancing angle deposition technology to produce the 3D chiral nanostructures. Through the optimization of deposition parameters (such as the orientation angle of poly styrene spheres (PSs) array, the deposition angle, thickness, and number), a great number of chiral structures have been achieved, and their size depends on the diameter of PS spheres. These chiral structures all can be simulated and predesigned through the use of a 3D geometrical model, which greatly improves the efficiency of this method. In addition, the circular dichroism spectrum shows that these chiral structures own an obvious Cotton effect, indicating their potential application as 3D chiral metamaterials.

Surface-plasmon-polaritons-assisted nanolithography with dual-wavelength illumination for high exposure depth.

We propose a new direct writing nanolithography approach using a plasmonic focusing device and a nano silver mirror with dual-wavelength illumination for high exposure depth. Arrays of pyramid aperture are used to focus the incident light beams into 80 nm light spots. The pyramid combined with a thin silver film coated on the substrate constructs a surface plasmon polaritons (SPP) coupling cavity, which amplifies the intensity of the light field in it by SPP effect and resonance. The transmission depth of the standing wave formed by forward and reflected light could reach hundreds of nanometers. Two lasers with different wavelengths are used as illumination sources to homogenize the light field through complementation between the two standing waves. Simulation results show by using 355 nm and 441 nm wavelengths, a space of 44 nm at the bottom of the photoresist could be obtained after exposure and development. The feature size of resist patterns could be further scaled down, depending on the optimization of parameters of photoresist exposure and development, illumination wavelengths, etc.

Preparation, Properties, and Application of Graphene-Based Materials in Tissue Engineering Scaffolds.

Tissue engineering has a great application prospect as an effective treatment for tissue and organ injury, functional reduction, or loss. Bioactive tissues are reconstructed and damaged organs are repaired by the three elements, including cells, scaffold materials, and growth factors. Graphene-based composites can be used as reinforcing auxiliary materials for tissue scaffold preparation because of their large specific surface area, and good mechanical support. Tissue engineering scaffolds with graphene-based composites have been widely studied. Part of research have focused on the application of graphene-based composites in single tissue engineering. The basic principles of graphene materials used in tissue engineering are summarized in some research. Some studies emphasized the key problems and solutions urgently needed to be solved in the development of tissue engineering and discussed their application prospect. Some related studies mainly focused on the conductivity of graphene and discussed the application of electroactive scaffolds in tissue engineering. In this review, the composite materials for preparing tissue engineering scaffolds are briefly described, which emphasizes the preparation methods, biological properties, and practical applications of graphene-based composite scaffolds. The synthetic techniques, with stressing solvent casting, electrospinning, and three-dimensional printing, are introduced in detail. The mechanical, cell-oriented, and biocompatible properties of graphene-based composite scaffolds in tissue engineering are analyzed and summarized. Their applications in bone tissue engineering, nerve tissue engineering, cardiovascular tissue engineering, and other tissue engineering are summarized systematically. In addition, this work also looks forward to the difficulties and challenges in the future research, providing some references for the follow-up research of graphene-based composites in tissue engineering scaffolds. Impact statement Regeneration and repair of tissue and organ injury has become a new research hotspot in recent years. Tissue engineering scaffolds prepared with graphene-based materials have good biocompatibility, excellent mechanical properties, and strong cell orientation, which can fully induce the proliferation and differentiation of seed cells. This review briefly describes the basic materials for the preparation of tissue engineering scaffolds, and focuses on the preparation, performance, and application of graphene-based materials in tissue engineering, providing sufficient understanding of graphene applied in regenerative medicine.