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Polishing is a main process in achieving high-quality surfaces of optical components. However, effective assessing technique for rough stage in the polishing process is sparse so that hinders the study on the mechanism and optimization of polishing. In this study, we propose an inspection method for polishing quality based on quasi Brewster angle technology (qBAT). An equivalent medium layer theory and its model are developed to accurately describe the optical reflection characteristics of a rough surface. Hence both rough- and fine-polishing qualities can be evaluated by the qBAT approach. To demonstrate the newly proposed method, laser crystal, gadolinium gallium garnets, is studied by computational simulation and experiments. A high consistence is achieved between theoretical simulation and experimental results, which proves the robustness of the method to the rough surfaces. To our knowledge, it is the first time that the qBAT is developed to evaluate the polishing quality covering rough- and fine-polishing stages. This capability shows great potential in fundamental research and process optimization of polishing brittle-hard materials.

Low-symmetry 2D materials are reviving recently owing to their azimuth-angle-dependent physical properties, which can be applied in polarization-sensitive optical and photoelectric devices. In this work, layered V2P4S13 with a novel low-symmetry porous structure was introduced as a promising highly anisotropic 2D material. We systematically investigated the structural, phonon-vibrational, and optical properties of 2D V2P4S13 by transmission electron microscopy, angle-resolved polarized Raman spectroscopy, angle-resolved reflection, and absorption spectroscopy. We discovered that 2D V2P4S13 shows strong crystallographic anisotropy, highly porous structure, and unique bi-vanadium atom coupling in the 2D plane. Polarized Raman spectroscopy indicated the strong phonon vibration anisotropy of 2D V2P4S13. Additionally, angle-resolved difference reflection and absorption spectra demonstrated optical anisotropy with a high anisotropic absorption ratio of 1.55 at 468 nm. Remarkably, we discovered unusual linear dichroism conversion of V2P4S13 from 468 nm to 600 nm. Such unique structural features of 2D V2P4S13, in combination with high optical anisotropy, make it an ideal platform for novel polarization-sensitive devices.

We report the first experimental study of the piezoelectricity of 2D SnSe crystals. SnSe demonstrates a maximum piezoelectric output voltage of ∼760 mV, which is the highest voltage generated by a single piece of 2D material reported so far. The power density reaches 28 mW m−2 with a mechanical-to-electrical energy conversion efficiency of 6.5%. The piezoelectricity of SnSe is highly orientation dependent, and the piezoelectric signal along the armchair orientation is ∼5 times larger than that along the zigzag orientation. A SnSe piezoelectric nanogenerator is able to monitor human body motion and vital health signs, such as the heart rate, because of its unprecedented piezoelectricity. We demonstrate a solely 2D-material self-powered sensor unit (SPSU) by integrating a SnSe nanogenerator and a MoS2 multi-functional sensor on one chip. This all-in-one system powered by heartbeats and periodical bending demonstrates superb pH, illumination intensity, and humidity sensing performance. The sensing mechanism of the SPSU is investigated. The discovery of piezoelectricity in SnSe provides a way for achieving novel self-powered atomic-scale electromechanical systems which could stimulate further fundamental research and potential applications.

Low-symmetry two-dimensional (2D) semiconductors have attracted great attention because of their rich in-plane anisotropic optical, electrical, and thermoelectric properties and potential applications in multifunctional nanoelectronic and optoelectronic devices. However, anisotropic 2D semiconductors with high performance are still very limited. Here, we report the systematic study of in-plane anisotropic properties in few-layered b-As that is a narrow-gap semiconductor, based on the experimental and theoretical investigations. According to experimental results, we have come up with a simple method for identifying the orientation of b-As crystals. Meanwhile, we show that the maximum mobility of electrons and holes was measured in the in-plane armchair (AC) direction. The measured maximum electron mobility ratio is about 2.68, and the hole mobility ratio is about 1.79.

In-plane optical anisotropy has been detected from monolayer MoS2 grown on a-plane (1120) sapphire substrate in the ultraviolet–visible wavelength range. Based on the measured optical anisotropy, ...

Inefficient sample preparation methods hinder the performance of high-throughput single-molecule force spectroscopy (H-SMFS) for viscous damping among reactants and unstable linkage. Here, we demonstrated a sample preparation method for H-SMFS systems to achieve a higher ratio of effective target molecules per sample cell by gas-phase silanization and reactant hydrophobization. Digital holographic centrifugal force microscopy (DH-CFM) was used to verify its performance. The experimental result indicated that the DNA stretching success ratio was improved from 0.89% to 13.5%. This enhanced efficiency preparation method has potential application for force-based DNA stretching experiments and other modifying procedures.

Mechanical strain induced changes in the electronic properties of two-dimensional (2D) materials is of great interest for both fundamental studies and practical applications. The anisotropic 2D materials may further exhibit different electronic changes when the strain is applied along different crystalline axes. The resulting anisotropic piezoresistive phenomenon not only reveals distinct lattice-electron interaction along different principle axes in low-dimensional materials but also can accurately sense/recognize multidimensional strain signals for the development of strain sensors, electronic skin, human-machine interfaces, etc. In this work, we systematically studied the piezoresistive effect of an anisotropic 2D material of rhenium disulfide (ReS2), which has large anisotropic ratio. The measurement of ReS2 piezoresistance was experimentally performed on the devices fabricated on a flexible substrate with electrical channels made along the two principle axes, which were identified noninvasively by the reflectance difference microscopy developed in our lab. The result indicated that ReS2 had completely opposite (positive and negative) piezoresistance along two principle axes, which differed from any previously reported anisotropic piezoresistive effect in other 2D materials. We attributed the opposite anisotropic piezoresistive effect of ReS2 to the strain-induced broadening and narrowing of the bandgap along two principle axes, respectively, which was demonstrated by both reflectance difference spectroscopy and theoretical calculations.

Low-symmetry two-dimensional (2D) materials with unique in-plane anisotropy can promote both fundamental science and practical applications in optics, optoelectronics, electronics, and polarization detection. As a member of 2D materials, doping/alloying material systems have gained great attention owing to the tunable bandgap and special properties. However, the in-plane anisotropic optical and electrical properties of these 2D alloy materials have rarely been reported. In this work, low-symmetry 2D Ge(1−x)SnxSe2 (x = 0–1.0) alloy flakes have been synthesized by chemical vapor deposition (CVD) with a bandgap varying from 1.55 eV (GeSe2, x = 0) to 1.90 eV (SnSe2, x = 1.0). Angle-resolved polarized Raman spectroscopy (ARPRS) is used to confirm the in-plane vibrational anisotropy, and azimuth-dependent reflectance difference microscopy (ADRDM) is applied to visualize the in-plane optical anisotropy. Polarization-dependent transmission spectroscopy (PDTS) is carried out to reflect the in-plane absorptional anisotropy and linear dichroism, and birefringence characteristics are also found in the subsequent studies. All of the results indicate the unique in-plane optical anisotropy and birefringence characteristics of the 2D Ge(1−x)SnxSe2 alloy flakes, providing new opportunities for polarization-controlled devices, optical wave plates, and polarizers.

Birefringence and dichroism are very important properties in optical anisotropy. Understanding the intrinsic birefringence and dichroism of a material can provide great help to utilize its optical anisotropy. But the direct experimental investigation of birefringence in nanoscale materials is rarely reported. As typical anisotropic transition metals trichalcogenides (TMTCs) materials with quasi-1D structure, TiS3 and ZrS3 have attracted extensive attention due to their special crystal structure and optical anisotropy characteristics. Here, the optical anisotropy properties such as birefringence and dichroism of two kinds of quasi-1D TMTCs, TiS3 and ZrS3 , are theoretically and experimentally studied. In experimental results, the anisotropic refraction and anisotropic reflection of TiS3 and ZrS3 are studied by polarization-resolved optical microscopy and azimuth-dependent reflectance difference microscopy, respectively. In addition, the birefringence and dichroism of ZrS3 nanoribbon in experiment are directly measured by spectrometric ellipsometry measurements, and a reasonable result is obtained. This work provides the basic optical anisotropy information of TiS3 and ZrS3 . It lays a foundation for the further study of the optical anisotropy of these two materials and provides a feasible method for the study of birefringence and dichroism of other nanomaterials in the future.

The yield of a large-area ultra-thin display panel depends on the realization of designed thickness of multilayer films of all pixels. Measuring the thicknesses of multilayer films of a single pixel is crucial to the accurate manufacture. However, the thinnest layer is reaching the sub-20nm level, and different layers feature remarkable divergence in thickness with similar optical constants. This turns to a key obstruction to the thickness characterization by optical spectroscopy. Based on the tiny differences in absorptivity, a fast method for measuring the film thickness in a single pixel is proposed which combines the layer number reducing model and micro-area differential reflectance spectroscopy. The lower layers can be considered as semi-infinite in the corresponding spectral range whose thickness is infinite in the fitting algorithm. Hence, the thickness of the upper layer is fitted in a simplified layer structure. For demonstration, a multilayer silicon microstructure in a single pixel, p-Si/a-Si/n-Si (10nm/950nm/50nm) on complex substrate, is measured. The light spot diameter is about 60 microns with measuring-time in 2 seconds. The measurement deviation is 3% compared by a commercial ellipsometer. To conclude, the proposed method realizes the layer number reduction for fitting multilayer thickness with large thickness difference and similar optical constants, which provides a powerful approach for multilayer microstructure characterizations.

Many proteins and peptides have been identified to effectively and specifically bind on certain surfaces such as silica, polystyrene and titanium dioxide. It is of great interest, in many areas such as enzyme immobilization, surface functionalization and nanotechnology, to understand how these proteins/peptides bind to solid surfaces. Here we use single-molecule force spectroscopy (SMFS) based on atomic force microscopy to directly measure the adhesion force between a silica-binding peptide SB7 and glass surface at single molecule level. SMFS results show that the adhesion force of a single SB7 detaching from the glass surface distributes in two populations at ~220 pN and 610 pN, which is higher than the unfolding forces of most mechanically stable proteins and the unbinding forces of most stable protein-protein interactions. Molecular dynamics simulation reveals that the electrostatic interactions between positively charged arginine residues and the silica surface dominates the binding of SB7 on silica. Our study provides experimental evidence and molecular mechanism at the single-molecule level for the SB7-based immobilization of proteins on silica-based surface, which is able to withstand high mechanical forces, making it an ideal fusion tag for silica surface immobilization or peptide-base adhesive materials.

Transparent layers are critical for enhancing optical contrast of graphene on a substrate. However, once the substrate is fully covered by large-area graphene, there are no accurate transparent layer and reference for optical contrast calculations. The thickness uncertainty of the transparent layer reduces the analytical accuracy of graphene. Thus, in this Letter, we propose a reference-aided differential reflection (DR) method with a dual-light path. The accurate thickness of the transparent layer is obtained by improving the DR spectrum sensitivity using a designable reference. Hence, the analytical accuracy of graphene thickness is guaranteed. To demonstrate this concept, a centimeter-scale chemical-vapor-deposition-synthesized graphene was measured on a S i O 2 / S i substrate. The thickness of underlying S i O 2 was first identified with the 1 nm resolution by the DR spectrum. Then, the thickness distribution of graphene was directly deduced from a DR map with submonolayer resolution at a preferred wavelength. The results were also confirmed by ellipsometry and atomic force microscopy. As a result, this new method provides an extra degree of freedom for the DR method to accurately measure the thickness of large-area two-dimensional materials.

Optical tweezers is one of the commonly used technologies to research on protein force spectroscopy. However, whether optical tweezers system has the capability of force spectroscopy measurement at the molecular scale is vital to single molecule experiments. In this paper, we test the capability of our home-built dual-trap optical tweezers system by stretching polyprotein (NuG2)8 which is made of eight identical tandem repeats of NuG2. With the constant velocity stretching and relaxation mode, we achieve a lot of experimental data and get the contour length increment of (NuG2)8 rapidly from the unfolding processes after fitting these data. The result is consistent with existing reports, which demonstrates optical tweezers system has the force spectroscopy test ability and (NuG2)8 can be used as a new standard sample to evaluate the test performance of optical tweezers. Using polyprotein (NuG2)8 as standard sample has two advantages: stretching polyprotein can help improve the efficiency of data statistics and a large number of experiments can reduce the randomness of the system when testing.

Lu2O3 has attracted widespread attention in high-power lasers, scintillators, and semiconductors due to its high thermal conductivity, high stability, and large band gap. Due to the limitations of the growth process, high-precision characterization of polished quality and optical properties for large-size Lu2O3 single crystal is extremely scarce, which hinders the design and optimization of optics based on Lu2O3. In this study, we design a systematic machining scheme for large-size Lu2O3 single crystal thick chip to obtain polished crystal with virtually no surface and subsurface damage. The surface/subsurface quality was evaluated utilizing optical profilometer, scanning electron microscope, and transmission electron microscope. Variable-angle spectroscopic ellipsometer was employed to measure the optical constants over wide spectral region (210-1690 nm), removing the lack of optical constants of Lu2O3 single crystal in the ultraviolet region. The band gap is analyzed based on the optical constants results. The consistency of the measured data with the relevant literature verifies the reliability and accuracy of the measurement method along with the data. The surface/subsurface damage inspection method provides an effective approach for ultra-precision machining mechanism research and process optimization of Lu2O3 and other hard and brittle materials. The band gap and the optical constants in wide spectral region provide critical fundamental data for the design and performance optimization of related optics.

Low-symmetry layered two-dimensional (2D) materials with strong in-plane optical anisotropy can potentially be applied for polarization photodetection. This is especially true for those 2D materials with a direct band gap, which can efficiently absorb light with specific axial polarization. However, discovering such new anisotropic 2D materials with a direct band structure is still extremely challenging. Here, we fabricate a photodetector using a pseudo-one-dimensional (pseudo-1D) Nb(1–x)TixS3 alloy device and demonstrate that it is highly sensitive to the polarized light because of the strong in-plane optical anisotropy and direct band gap of the alloy by combining angle-resolved polarization Raman spectroscopy, azimuth-dependent reflectance difference microscopy, polarization-dependent absorption spectroscopy, and hybrid functional theory calculations. As a consequence, the polarization photodetector of the Nb(1–x)TixS3 alloy shows a large photocurrent anisotropic ratio and a high photoresponse. The cho...

The air instability of black phosphorus (BP) severely hinders the development of its electronic and optoelectronic applications. Although a lot of effort has been made to passivate it against degradation in ambient conditions, approaches to further manipulate the properties of passivated BP are still very limited. Herein, we report a simple and low-cost chemical method that can achieve BP passivation and property tailoring simultaneously. The method is conducted by immersing a BP sample in the solution containing both 2,2,6,6-tetramethylpiperidinyl-N-oxyl (TEMPO) and triphenylcarbenium tetrafluorobor in a mixture of water and acetone (v/v = 1:1). After the treatment, the BP sample is functionalized with TEMPO, which not only efficiently passivates BP but also p-dopes BP to a degenerated density level of 1013 cm–2. The performance of the BP field effect transistor is improved after functionalization with a high Ion/Ioff ratio of 106 and carrier mobility of 881.5 cm2/(V·s). The functionalization-induced dop...

The discontinuity of medium at the boundary produces optically anisotropic response which makes reflectance difference microscopy (RDM) a potential method for nanometre-thickness microstructure measurements. Here, we present the methodology of RDM for the edge measurement of ultrathin microstructure. The RD signal of microstructure's boundary is mathematically deduced according to boundary condition and polarization optics theory. A normal-incidence RDM setup was built simply with one linear polarizer, one liquid crystal variable retarder and one 5 × objective. Then, the performance of the developed setup was identified using homogenous reflection mirror and high quality linear polarizer. For demonstration, microstructures array with 100 nm step height was measured. The results show that the RD signal is sensitive to the edge and its sign reflects the change direction of the edge. Furthermore, a height sensitivity of better than 10 nm and a spatial resolution of ∼3 μm offer this technique a good candidate for characterizing ultrathin microstructures.

Optical anisotropy is one of the most fundamental physical characteristics of emerging low-symmetry two-dimensional (2D) materials. It provides abundant structural information and is crucial for creating diverse nanoscale devices. Here, we have proposed an azimuth-resolved microscopic approach to directly resolve the normalized optical difference along two orthogonal directions at normal incidence. The differential principle ensures that the approach is only sensitive to anisotropic samples and immune to isotropic materials. We studied the optical anisotropy of bare and encapsulated black phosphorus (BP) and unveiled the interference effect on optical anisotropy, which is critical for practical applications in optical and optoelectronic devices. A multi-phase model based on the scattering matrix method was developed to account for the interference effect and then the crystallographic directions were unambiguously determined. Our result also suggests that the optical anisotropy is a probe to measure the thickness with monolayer resolution. Furthermore, the optical anisotropy of rhenium disulfide (ReS2), another class of anisotropic 2D materials, with a 1T distorted crystal structure, was investigated, which demonstrates that our approach is suitable for other anisotropic 2D materials. This technique is ideal for optical anisotropy characterization and will inspire future efforts in BP and related anisotropic 2D nanomaterials for engineering new conceptual nanodevices.

The ability to detect light in photodetectors is central to practical optoelectronic applications, which has been demonstrated in inorganic semiconductor devices. However, so far, the study of polarization-sensitive organic photodetectors, which have unique applications in flexible and wearable electronics, has not received much attention. Herein, the construction of polarization-sensitive photodetectors based on the single crystals of a superior optoelectronic organic semiconductor, 2,6-diphenyl anthracene (DPA), is demonstrated. The systematic characterization of two-dimensionally grown DPA crystals with various techniques definitely show their strong anisotropy in molecular vibration, optical reflectance and optical absorption. In terms of polarization sensitivity, DPA-crystal based photodetectors exhibit a linear dichroic ratio up to ≈1.9. Theoretical calculations confirm that intrinsic linear dichroism, originated from the anisotropic in-plane crystal structure, is responsible for the polarization sensitivity of DPA crystals. This work opens up a new door for exploiting organic semiconductors for developing highly compact polarization photodetectors and providing new functionalities in novel flexible optical and optoelectronic applications.

Due to their low-symmetry lattice characteristics and intrinsic in-plane anisotropy, 2D pentagonal materials, a new class of 2D materials composed entirely of pentagonal atomic rings, are attracting increasing research attention. However, the existence of these 2D materials has not been proven experimentally until the recent discovery of PdSe2 . Herein, penta-PdPSe, a new 2D pentagonal material with a novel low-symmetry puckered pentagonal structure, is introduced to the 2D family. Interestingly, a peculiar polyanion of [SePPSe]4- is discovered in this material, which is the biggest polyanion in 2D materials yet discovered. Strong intrinsic in-plane anisotropic behavior endows penta-PdPSe with highly anisotropic optical, electronic, and optoelectronic properties. Impressively, few-layer penta-PdPSe-based phototransistor not only achieves excellent electronic performances, a moderate electron mobility of 21.37 cm2 V-1 s-1 and a high on/off ratio of up to 108 , but it also has a high photoresponsivity of ≈5.07 × 103 A W-1 at 635 nm, which is ascribed to the photogating effect. More importantly, penta-PdPSe also exhibits a large anisotropic conductance (σmax /σmax = 3.85) and responsivity (Rmax /Rmin = 6.17 at 808 nm), superior to most 2D anisotropic materials. These findings make penta-PdPSe an ideal material for the design of next-generation anisotropic devices.

We discuss the processes involved in the formation of a few monolayer thick pentacene films on the SiO2 surface. The evolution of the optical properties has been monitored in situ by optical spectroscopy and the interface morphology was investigated ex situ by atomic force microscopy. The sub monolayer shows the identical characteristic optical response with the intensity proportional to the coverage. From the second monolayer growth on, the optical properties are significantly changed. This experimental evidence indicates the formation of the first monolayer layer comprising the so-called “orthorhombic phase” crystallites with near-vertically oriented pentacene molecules, and the multilayer above comprises the so-called “thin-film phase” crystallites. Most importantly, the combination of optical spectroscopy and atomic force microscopy (AFM) investigations reveals that the first monolayer crystallize into the orthorhombic phase and the structure is persevered even after being buried by the multilayer. These observations suggest that the thickness-driven phase transformation from the orthorhombic phase to the thin-film phase starts from the second monolayer on rather than the first monolayer. This result provides a new insight into the structural evolutions and growth of pentacene thin films on SiO2.

van der Waals (vdW) magnetic insulators are of significance in both fundamental research and technological application, but most two-dimensional (2D) vdW magnetic systems are unstable and of high lattice symmetry. Stable 2D vdW magnetic insulators with anisotropic structure are needed to modulate the properties and unlock potential applications. Here we present a stable vdW antiferromagnetic material, CrOCl, with low-symmetry orthorhombic structure, and investigate systematically its magnetism, phase transition behavior, and optical anisotropy. Spin-phonon coupling effects are uncovered by the abnormal frequency shifts of Raman-active modes, suggesting the formation of a magnetic superstructure. The sizable abnormal change of interplanar spacing indicates the presence of a structural transition at around 27 K. Further in-plane vibrational, reflectional, and absorptional anisotropic properties are explored both experimentally and theoretically, revealing a highly polarization sensitive characteristic and linear dichroism in 2D CrOCl. Meanwhile, the particularly high polarization dependency of the second-harmonic generation and the nonlinear susceptibility of ∼2.24 × 10-11 m/V make it suitable in the field of polarization-dependent nonlinear optics. The findings on the intricate properties of 2D CrOCl lay foundations for future applications of low-symmetry vdW magnets in spin-dependent electronic and optoelectronic devices.

We study computationally the design of a high extinction ratio nano polarizer based on black phosphorus (BP). A scattering-matrix calculation method is applied to compute the overall polarization extinction ratio along two orthogonal directions. The results reveal that, with a resonance cavity of SiO2, both BP/ SiO 2 /Si and h-BN/BP/ SiO 2 /Si configurations can build a linear polarizer with extinction ratio higher than 16 dB at a polarized wavelength in the range of 400 nm&ndash, 900 nm. The polarization wavelength is tunable by adjusting the thickness of the BP layer while the thicknesses of the isotrocpic layers are in charge of extinction ratios. The additional top layer of h-BN was used to prevent BP degradation from oxidation and strengthens the practical applications of BP polarizer. The study shows that the BP/ SiO 2 /Si structure, with a silicon compatible and easy-to-realize method, is a valuable solution when designing polarization functional module in integrated photonics and optical communications circuits.

2D ternary materials exhibit great promise in the field of polarization-sensitive photodetectors due to the low-symmetry crystal structure. However, the realization of ternary material growth is still a huge challenge because of the complex reaction process. Here, for the first time, 2D ternary In2 SnS4 flakes are obtained via synergistic additive of salt and molecular sieve-assisted chemical vapor deposition. Raman vibration mode of In2 SnS4 flakes exhibits polarization-dependent properties. The polarization-resolved absorption spectroscopy and azimuth-dependent reflectance difference microscopy further confirm its anisotropy of in-plane optical absorption and reflection. Besides, the In2 SnS4 flake based device on mica shows ultrafast rising and decay rates of ≈20 and 20 µs. Impressively, In2 SnS4 flake based phototransistor demonstrates giant gate-tunable polarization-sensitive photoresponse: the dichroic ratio can be adjusted in the range of 1.13-1.70 with gate voltage varying from -35-35 V. This work provides an effective means for modulating the polarization-sensitive photoresponse, which may significantly promote the research progress of polarization-sensitive photodetectors.

Graphene and WO3 co-modified TiO2 nanotube array (GR-WO3/TNA) photoelectrodes were prepared through in-situ anodization and electrodeposition process. The morphology and structure were characterized by scanning electron microscopy (SEM), transmission electron microscope (TEM), X-ray diffraction (XRD), Raman spectroscopy and X-ray photoelectron spectroscopy (XPS). Optical and photoelectrochemical (PECH) properties were investigated by UV–Vis diffuse reflectance spectroscopy (UV–Vis DRS), photovoltage and photocurrent density. The photodecomposition performance was investigated through photocatalytic (PC) degradation of dimethyl phthalate (DMP) under Xenon light illumination. It was found that WO3 and GR were successfully electrodeposition onto the surface of TNA and all photoelectrodes showed highly ordered and vertically aligned nanotube array with anatase and rutile TiO2 mixcrystal structure. The decorated WO3 and GR on TNA photoelectrode reduced the recombination of photogenerated hole-electron pairs and prolonged the lifetime of photogenerated holes, thereby the visible light absorption and the PECH response of TNA photoelectrode were improved obviously, which would facilitate the formation of reactive species and improve the PC performance for DMP degradation. The kinetic constant of GR-WO3/TNA photoelectrode for PC degradation of DMP was 1.41 times that of TNA photoelectrode.

Optical waveguides on three material platforms of silicon-on-insulator (SOI), amorphous silicon (a-Si:H) and silicon nitride are analyzed for realization of an optical compact link compatible with CMOS technology. The three material properties are presented. The optical components on the three potential material platforms are put together for a comparison, in which, straight, bent waveguides, and multimode-interference beam splitters for an optical link are designed, fabricated, and measured. The comparison results of simulation and measurement show that a-Si:H has a similar performances with SOI material but lower cost and good feasibility of photonic integration on silicon.

An analysis is developed for integrated acousto-optical (AO) switches with multi-modulated-arms in a parallel configuration. The analysis shows that an integrated AO switch with multi-arms can improve contrast ratio largely. The optimum SAW phase of closing arms is presented to achieve highest contrast ratio for the integrated AO switches with multi-modulated-arms. A trade-off of fabrication tolerance and higher on/off contrast ratio is discussed, in which, larger number of modulated arms shows a higher on/off contrast ratio, but a stricter fabrication. The research shows that an AO switch with 8-modulated-arms is an optimum structure as an on/off switch with a higher contrast ratio of −75 dB and less number of arms.

High-precision axial displacement tracking of trapped beads is an indispensable feature of optical tweezers in advanced single-molecule studies. Here, we demonstrate an alternative method that enables axial calibration and tracking to be carried out on the same sample to avoid unnecessary errors. This method works by applying a dynamic force balance on a bead trapped between a piezoelectrically driven glass slide and an optical trap; in this configuration, the bead can be stopped precisely in different positions and imaged by a camera. A simple gradient algorithm is used to process the images into calibration data. After optimization of the calibration method and samples, our method exhibited better than 5 nm experimental axial resolution, with a measurement range of +/-500 nm around the objective focus at video speed. Moreover, for the first time, the deviation of the focusing plane in dual-trap optical tweezers was measured. We confirmed the axial deviation between two optical traps in our setup to be ~10 nm, corresponding to a force spectroscopy gage error of ~1 pN. This approach offers a favorable solution for in-use setup updating, as it can be seamlessly integrated into any optical tweezers system without requiring new hardware updates.

Developing angular trapping methods, which will enable optical tweezers to rotate a micronized bead, is of great importance for the studies of biomacromolecules during a wide range of torque-generation processes. Here we report a novel controlled angular trapping method based on composite Janus particles. We used a chemically synthesized Janus particle, which consists of two hemispheres made of polystyrene (PS) and poly(methyl methacrylate) (PMMA) respectively, as a model system to demonstrate this method. Through computational and experimental studies, we demonstrated the feasibility to control the rotation of a Janus particle in a linearly polarized laser trap. Our results showed that the Janus particle aligned its two hemisphere's interface parallel to the laser propagation direction as well as the laser polarization direction. In our experiments, the rotational state of the particle can be easily and directly visualized by using a CMOS camera, and does not require complex optical detection system. The rotation of the Janus particle in the laser trap can be fully controlled in real time by controlling the laser polarization direction. Our newly developed angular trapping technique has the great advantage of easy implementation and real time controllability. Considering the easy chemical synthesis of Janus particles and implementation of the angular trapping, this novel method has the potential of becoming a general angular trapping method. We anticipate that this new method will significantly broaden the availability of angular trapping in the biophysics community, and expand the scope of the research that can be enabled by the angular trapping approach., Comment: 16 pages, 5 figures

An atomically thin MoSe2 layer has been synthesized on mica using molecular beam epitaxy (MBE). The polymorphous of the MoSe2 layer depends on the coverage and the growth temperature. At low coverages and low growth temperature, 1T-MoSe2 forms in addition to a comparable quantity of 2H-MoSe2. The metastable 1T-MoSe2 transfers gradually to the stable 2H-MoSe2 before the completion of the first monolayer. The current result sheds some light on the complexity of the nucleation and growth of transition metal dichalcogenide (TMDC) monolayers and implies a possible route for a phase selective synthesis using MBE.

Structural symmetry is a simple way to quantify the anisotropic properties of materials toward unique device applications including anisotropic transportation and polarization-sensitive photodetection. The enhancement of anisotropy can be achieved by artificial symmetry-reduction design. A core-shell SbI3 /Sb2 O3 nanowire, a heterostructure bonded by van der Waals forces, is introduced as an example of enhancing the performance of polarization-sensitive photodetectors via symmetry reduction. The structural, vibrational, and optical anisotropies of such core-shell nanostructures are systematically investigated. It is found that the anisotropic absorbance of a core-shell nanowire is obviously higher than that of two single compounds from both theoretical and experimental investigations. Anisotropic photocurrents of the polarization-sensitive photodetectors based on these core-shell SbI3 /Sb2 O3 van der Waals nanowires are measured ranging from ultraviolet (UV) to visible light (360-532 nm). Compared with other van der Waals 1D materials, low anisotropy ratio (Imax /Imin ) is measured based on SbI3 but a device based on this core-shell nanowire possesses a relatively high anisotropy ratio of ≈3.14 under 450 nm polarized light. This work shows that the low-symmetrical core-shell van der Waals heterostructure has large potential to be applied in wide range polarization-sensitive photodetectors.

The precisely controlled growth of transition metal dichalcogenide (TMDC) monolayers requires sensitive and nondestructive techniques to monitor the morphology and coverage in situ and in real time. In the current work, differential reflectance spectroscopy (DRS) was applied to monitor the molecular beam epitaxy (MBE) growth of atomically thin MoSe2 layers on mica. The optical evolution exhibits an oscillation with monolayer periodicity, revealing a two-dimensional (2D) layer-by-layer growth of the MoSe2 thin films. The observed sensitivity of DRS to the step density is associated to the modified electronic structures at the edges of TMDC monolayers. As DRS works in any transparent ambient, we speculate that it could be of great use for realizing precisely controlled growth of TMDC monolayers using not only MBE but also chemical vapor deposition (CVD).

In-plane anisotropy of layered materials adds another dimension to their applications, opening up avenues in diverse angle-resolved devices. However, to fulfill a strong inherent in-plane anisotropy in layered materials still poses a significant challenge, as it often requires a low-symmetry nature of layered materials. Here, we report the fabrication of a member of layered semiconducting AIIIBVI compounds, TlSe, that possesses a low-symmetry tetragonal structure and investigate its anisotropic light–matter interactions. We first identify the in-plane Raman intensity anisotropy of thin-layer TlSe, offering unambiguous evidence that the anisotropy is sensitive to crystalline orientation. Further in-situ azimuth-dependent reflectance difference microscopy enables the direct evaluation of in-plane optical anisotropy of layered TlSe, and we demonstrate that the TlSe shows a linear dichroism under polarized absorption spectra arising from an in-plane anisotropic optical property. As a direct result of the line...

We combined reflection difference microscopy, electron transport measurements, and atomic force microscopy to characterize the mechanical and electrical anisotropy of few-layer black phosphorus. We were able to identify the lattice orientations of the two-dimensional material and construct suspended structures aligned with specific crystal axes. The approach allowed us to probe the anisotropic mechanical and electrical properties along each lattice axis in separate measurements. We measured the Young's modulus of few-layer black phosphorus to be 58.6 ± 11.7 and 27.2 ± 4.1 GPa in zigzag and armchair directions. The breaking stress scaled almost linearly with the Young's modulus and was measured to be 4.79 ± 1.43 and 2.31 ± 0.71 GPa in the two directions. We have also observed highly anisotropic transport behavior in black phosphorus and derived the conductance anisotropy to be 63.7%. The test results agreed well with theoretical predictions. Our work provided very valuable experimental data and suggested an effective characterization means for future studies on black phosphorus and anisotropic two-dimensional nanomaterials in general.

In this paper, a ring frequency shifter using surface acoustic waves (SAW) is developed on a silicon-on-insulator (SOI) technology, in which, a design is introduced to make SOI waveguide piezoelectric by putting ZnO on SOI layer. A simulation shows that the refractive index change of SOI waveguide with an overlayer of ZnO is about 10−3 by SAW of 100 mW. A finite-difference time domain (FDTD) is used to model the new concept of a ring frequency shifter using SAW of 100 mW. The results show that the resonance wavelength of a ring resonator with a radius of 50 μm shifts 22.7% of a free spectral range (FSR) due to the refractive index change of waveguide by SAW without change in transmission on resonances and finesse, a tiny change in bandwidth, Q-factor and FSR of a ring resonator.

In order to increase the number of channels that could be multiplexed or demultiplexed in the dense wavelength division multiplexed (DWDM) system based on the resonators on silicon-on-insulator (SOI) technology, the Vernier effect in the series-coupled racetrack resonators is presented to extend the free spectral range (FSR) of the DWDM systems. A method is developed based on a matrix approach to simulate Vernier devices. A three-dimensional full vectorial finite difference (FVFD) model, specifically suited for high index contrast and smaller size waveguides, for example, a waveguide in SOI technology, is developed to obtain the properties of a waveguide. Finally, the Vernier effect in the two series-coupled racetrack resonators is experimentally verified with an improved FSR and interstitial resonance suppression.

The liquid crystal variable retarder (LCVR), as a controllable phase modulator, works in a setting voltage or modulated mode and has been applied in the field of microscopic polarimetry. However, the modulation period of an LCVR is normally limited to dozens to hundreds of milliseconds, which is not suitable for a rapid measurement. Based on this feature, in this work, one rapid measuring strategy was reported. Only two frames were needed for a normalized-intensity-difference microscopic anisotropy measurement. The working principle and instrumentation were presented. For demonstration, a flake of graphene was measured by this method and compared by the reported way. An approximately 30× speed improvement was realized with the clear signal measurement. This proposed method will help a fast in situ characterization of ultrathin films and 2D materials.The liquid crystal variable retarder (LCVR), as a controllable phase modulator, works in a setting voltage or modulated mode and has been applied in the field of microscopic polarimetry. However, the modulation period of an LCVR is normally limited to dozens to hundreds of milliseconds, which is not suitable for a rapid measurement. Based on this feature, in this work, one rapid measuring strategy was reported. Only two frames were needed for a normalized-intensity-difference microscopic anisotropy measurement. The working principle and instrumentation were presented. For demonstration, a flake of graphene was measured by this method and compared by the reported way. An approximately 30× speed improvement was realized with the clear signal measurement. This proposed method will help a fast in situ characterization of ultrathin films and 2D materials.

Pentacene adsorbed on the $\mathrm{Cu}(110)\ensuremath{-}\mathrm{c}(6\ifmmode\times\else\texttimes\fi{}2)\mathrm{O}$ surface has been investigated using scanning tunneling microscopy and spectroscopy and density functional theory calculations. The molecules lie almost flat on the surface and are anchored via the end carbon ring to a Cu atom of the topmost surface layer, leading to a break of the mirror symmetry of pentacene parallel to the short molecular axis. As a result the molecular electronic structure of the frontier orbitals exhibits a local and gentle modification around the anchoring site but is essentially unaffected in the remaining part of the molecule. The enhancement of the conductance peak related to the second highest occupied molecular orbital (HOMO-2) at the anchoring site, however, is suggestive of a resonance with a surface state or with the Cu $d$-band.

A novel process to control light through the coupling modulation by surface acoustic wave (SAW) is presented in an optical micro resonator. An optical waveguide modulator of a racetrack resonator on silicon-on-insulator (SOI) technology is took as an example to explore the mechanism. A finite-difference time-domain (FDTD) is developed to simulate the acousto-optical (AO) modulator using the mechanism. An analytical method is presented to verify our proposal. The results show that the process can work well as an optical modulator by SAW.

Germanium diselenide (GeSe2) has recently emerged as a new member of in-plane anisotropic 2D materials, notable for its wide bandgap of 2.74 eV, excellent air stability, and high performance in polarization-sensitive photodetection. However, the interlayer interaction in GeSe2 has never been reported, which usually plays an important role in layer-number-dependent physical properties. Here, the interlayer coupling in GeSe2 is systematically investigated from theory to experiment. Unexpectedly, all of density functional theory (DFT) calculations about layer-dependent band structures, cleavage energy, binding energy, translation energy, and interlayer differential charge density demonstrate the much weaker interlayer interaction in GeSe2 when compared with black phosphorus (BP). Furthermore, both thickness-dependent and temperature-dependent Raman spectra of GeSe2 flakes, which exhibit no detectable changes of Raman peaks with the increase in thickness and a small first-order temperature coefficient of -0.0095 cm-1 K-1, respectively, experimentally confirm the weakly coupled layers in GeSe2. The results establish GeSe2 as an unusual member of in-plane anisotropic 2D materials with weak interlayer interaction.

We report on both the theoretical and experimental design of a black phosphorus (BP)-based reflective linear polarizer on Si/SiO2 substrate in visible range using the Fabry–Perot cavities method. Thanks to the optical anisotropy of BP, polarization wavelength regulation and a high extinction ratio are achievable via optimizing the thickness of BP. Using azimuth-dependent reflectance difference microscopy, we directly measured a huge optical anisotropy of 1.58, corresponding to an extinction ratio of ∼9 dB, from a 96 nm BP on a silicon substrate capped by 260 nm thermally oxidized silicon at a wavelength of 690 nm for the first time, to the best of our knowledge. Our results not only provide a new route to designing nanoscale polarizers based on anisotropic two-dimensional (2D) materials, promoting the application of 2D materials in integrated optoelectronics and system-on-chip, but also suggest a modulation technique for optical anisotropy by integrating the BP film with cavity structures.

Anisotropic 2D materials exhibit unique optical, electrical, and thermoelectric properties that open up possibilities for diverse angle-dependent devices. However, the explored anisotropic 2D materials are very limited and the methods to identify the crystal orientations and to study the in-plane anisotropy are in the initial stage. Here azimuth-dependent reflectance difference microscopy (ADRDM), angle-resolved Raman spectra, and electrical transport measurements are used to systematically characterize the influence of the anisotropic structure on in-plane optical and electrical anisotropy of 2D GeAs, a novel group IV–V semiconductor. It is proved that ADRDM offers a way to quickly identify the crystal orientations and also to directly characterize the in-plane optical anisotropy of layered GeAs. The anisotropic electrical transport behavior of few-layer GeAs field-effect transistors is further measured and the anisotropic ratio of the mobility is as high as 4.6, which is higher than the other 2D anisotropic materials such as black phosphorus. The dependence of the Raman intensity anisotropy on the sample thickness, excitation wavelength, and polarization configuration is investigated both experimentally and theoretically. These data will be useful for designing new high-performance devices and the results suggest a general methodology for characterizing the in-plane anisotropy of low-symmetry 2D materials.