arXiv subjects

Zexiang Shen

Publications and source records attributed to Zexiang Shen.

18 recordsLinked to original sources

arXiv2026

Phase singularity enabled polarization switchable analog spatial differentiation in an atomic MoS$_2$ planar Fabry-P\'erot cavity

Reconfigurable analog optical computing requires rapid and efficient switching between core mathematical operations, such as first- and second-order spatial differentiation. Here, we demonstrate a monolayer MoS$_2$ integrated a planar Fabry-P\'erot (F-P) cavity that performs polarization switchable analog spatial differentiation under oblique incidence. By exploiting polarization dependent phase singularities, the device satisfies distinct optical transfer functions for different-order differentiation at the same operating condition. As a result, first-order and second-order derivatives of input images are experimentally realized by simply switching the incident polarization. These results establish the planar cavity as a compact reconfigurable spatial differentiator, whose computational order is controlled solely by light polarization. This approach provides a fast, convenient, and integration friendly strategy for tunable optical computing, and enables polarization switchable edge detection for image processing, with potential applications in real-time object recognition, feature extraction, and optical data compression.

physics.optics↗

arXiv2025

Guiding polaritonic energy and momentum through two-dimensional Bravais lattices

The strong exciton absorption in monolayer transition metal dichalcogenides provides a promising platform for studying polaritons with tunable dispersions, which are crucial for controlling polaritonic energy and momentum, but remain underexplored. In this work, monolayer MoS$_2$ is coupled with a Fabry-P\'erot microcavity to form polaritons. Five types of Bravais lattices with sub-wavelength periods, based on polymethyl methacrylate (PMMA) nanopillars, are intentionally designed. The energy overlap between the periodic PMMA scattering wave and the polariton establishes a coupling channel that controls the directional flow of polaritonic energy, as demonstrated through angle-resolved reflectance measurements. Back-space image measurements further demonstrate that the dispersion in reciprocal space can be directly and manually tuned, allowing for control over their number and their positions. The coupling between the polariton and PMMA scattering wave is further demonstrated by analyzing the reflectance using the two-port two-mode model. The symmetries of 2D Bravais lattices allow the angle between energy and momentum flow to vary widely, from 90{\deg}, 60{\deg}, 45{\deg}, and 30{\deg} to arbitrary values. By adjusting the lattice vector lengths, the position of the dispersion branch in a specific direction can be fine-tuned, enabling full-range control over polariton dispersion. This work presents the first theoretical and experimental demonstrations of guiding the direction of polaritonic energy and momentum through Bravais lattice design.

physics.optics↗

arXiv2025

Fabry-Perot resonance modes in a MoS$_2$-based vertical stacking cavity for strong light-matter coupling and topological phase singularity

Rich dielectric properties in atomic transition metal dichalcogenides (TMDs) enhance light-matter interactions and contribute to a variety of optical phenomena. The direct transfer of TMDs onto photonic crystals facilitates optical field confinement and modifies photon dispersion through the generation of polaritons. However, light-matter interaction is severely limited by this stacking method. This limitation can be significantly improved by constructing a vertical stacking cavity with alternating layers of dielectric material and monolayer MoS$_2$. This multilayer structure is proven to be a compact, versatile, and customizable platform for controlling Fabry-Perot cavity resonance mode. Angle-resolved reflectance further aids in studying resonance mode dispersion. Moreover, the strong light-matter interaction results in multiple perfect absorptions, with the monolayer MoS$_2$ significantly contributing to the absorption in this system, as schematically revealed by the electric field distribution. The multiple perfect absorptions produce an unusual amounts of phase singularities with topological pairs, whose generation, evolution, and annihilation can be controlled by adjusting cavity parameters. Our findings provide a flexible and consistent framework for optimizing light-matter interactions and supporting further studies on wavefront shaping, optical vortices, and topological variants.

physics.optics↗

arXiv2023

Retrieving positions of closely packed sub-wavelength nanoparticles from their diffraction patterns

Distinguishing two objects or point sources located closer than the Rayleigh distance is impossible in conventional microscopy. Understandably, the task becomes increasingly harder with a growing number of particles placed in close proximity. It has been recently demonstrated that subwavelength nanoparticles in closely packed clusters can be counted by AI-enabled analysis of the diffraction patterns of coherent light scattered by the cluster. Here we show that deep learning analysis can determine the actual position of the nanoparticle in the cluster of subwavelength particles from a sing-shot diffraction pattern even if they are separated by distances below the Rayleigh resolution limit of a conventional microscope.

physics.optics↗

arXiv2016

Large-area and high-quality 2D transition metal telluride

Atomically thin transitional metal ditellurides like WTe2 and MoTe2 have triggered tremendous research interests because of their intrinsic nontrivial band structure. They are also predicted to be 2D topological insulators and type-II Weyl semimetals. However, most of the studies on ditelluride atomic layers so far rely on the low-yield and time-consuming mechanical exfoliation method. Direct synthesis of large-scale monolayer ditellurides has not yet been achieved. Here, using the chemical vapor deposition (CVD) method, we demonstrate controlled synthesis of high-quality and atom-thin tellurides with lateral size over 300 μm. We found that the as-grown WTe2 maintains two different stacking sequences in the bilayer, where the atomic structure of the stacking boundary is revealed by scanning transmission electron microscope (STEM). The low-temperature transport measurements revealed a novel semimetal-to-insulator transition in WTe2 layers and an enhanced superconductivity in few-layer MoTe2. This work paves the way to the synthesis of atom-thin tellurides and also quantum spin Hall devices.

cond-mat.mtrl-sci↗

arXiv2011

Hydrogen bond relaxation dynamics and the associated vibronic and volumetric anomalies of H2O upon frozen

A combination of the extended Ice Rule of Pauling, Raman spectroscopy, and molecular dynamics calculations has enabled us to clarify the bonding origin of the anomalous volume expansion, Raman phonon relaxation, and the stiffness and fragility of H2O upon frozen. We found that the initially shorter-and-stronger intramolecular "H+/p-O2-" bond follows the regular rule of cooling-contraction while the initially longer-and-weaker intermolecular "O2- : H+/p" nonbond turns to be even longer yet stiffer in the "O2- : H+/p-O2-" hydrogen-bond of H2O upon frozen, as a consequence of the polarization and Coulomb repulsion between the unevenly-bounded bonding and nonbonding electron pairs. The elongation of the nonbond and the polarization of the nonbonding lone pair are responsible, respectively, for the volume expansion and the stiffness and the fragility of ice. Findings should form important impact to the understanding of the physical anomalies of H2O under other stimuli such as pressure and confinement.

cond-mat.other↗

arXiv2011

Surface energy engineering of graphene

Contact angle goniometry is conducted for epitaxial graphene on SiC. Although only a single layer of epitaxial graphene exists on SiC, the contact angle drastically changes from 69° on SiC substrates to 92° with graphene. It is found that there is no thickness dependence of the contact angle from the measurements of single, bi, and multi layer graphene and highly ordered pyrolytic graphite (HOPG). After graphene is treated with oxygen plasma, the level of damage is investigated by Raman spectroscopy and correlation between the level of disorder and wettability is reported. By using low power oxygen plasma treatment, the wettability of graphene is improved without additional damage, which can solve the adhesion issues involved in the fabrication of graphene devices.

cond-mat.mtrl-sci↗

arXiv2010

Electronic Structure and Structural Evolutions of Hydrogenated Graphene Probed by Raman Spectroscopy

The electronic structure and structural evolution of hydrogenated graphene are investigated by Raman spectroscopy with multiple excitations. The excitation energy dependent saturation effect on the ratio of integrated intensities of D and G modes (ID/IG) is revealed and further developed as a quick method for estimation of inter-defect distance and defect density in hydrogenated graphene. At low hydrogen coverage, the chemisorbed H atoms behave like defects in sp2 C=C matrix; while for a high hydrogen coverage, the sp3 C-H bonds become coalescent clusters, resulting in confinement effect on the sp2 C domains. Electronic structure changes caused by varying hydrogen coverage are evidenced by excitation energy dependent red shift of D and 2D bands. Our results provide a useful guide for developing applications of hydrogenated graphene, as well as using Raman spectroscopy as quick metrology of the defect density in further exploring other kinds of graphene derivatives.

cond-mat.mtrl-sci↗

arXiv2010

Large Scale Synthesis of Bi-layer Graphene in Strongly Coupled Stacking Order

Large scale synthesis of single layer graphene (SLG) by chemical vapor deposition (CVD) has received a lot of attention recently. However, CVD synthesis of AB stacked bi-layer graphene (BLG) is still a challenging work. Here we report synthesis of BLG homogeneously in large area by thermal CVD. The 2D Raman band of CVD BLG splits into four components, suggesting splitting of electronic bands due to strong interlayer coupling. The splitting of electronic bands in CVD BLG is further evidenced by the study of near infrared (NIR) absorption and carrier dynamics probed by transient absorption spectroscopy. Ultraviolet photoelectron spectroscopy invesigation also indiates CVD BLG possesses different electronic structures from those of CVD SLG. The growth mechanism of BLG is found to be related to catalystic activity of copper (Cu)surface, which is determined by purity of Cu foils employed in CVD process. Our work showsthat strongly coupled or even AB stacked BLG can be grown on Cu foils in large scale, which isof particular importance for device applications based on their split electronic bands

cond-mat.mtrl-sci↗

arXiv2010

Monolayer Graphene as Saturable Absorber in Mode-locked Laser

We demonstrate that the intrinsic properties of monolayer graphene allow it to act as a more effective saturable absorber for mode-locking fiber lasers compared to multilayer graphene. The absorption of monolayer graphene can be saturated at lower excitation intensity compared to multilayer graphene, graphene with wrinkle-like defects, and functionalized graphene. Monolayer graphene has a remarkable large modulation depth of 95.3%, whereas the modulation depth of multilayer graphene is greatly reduced due to nonsaturable absorption and scattering loss. Picoseconds ultrafast laser pulse (1.23 ps) can be generated using monolayer graphene as saturable absorber. Due to the ultrafast relaxation time, larger modulation depth and lower scattering loss of monolayer graphene, it performs better than multilayer graphene in terms of pulse shaping ability, pulse stability and output energy.

physics.optics↗

arXiv2010

Ultrafast carrier dynamics in pristine and FeCl3-intercalated bilayer graphene

Ultrafast carrier dynamics of pristine bilayer graphene (BLG) and bilayer graphene intercalated with FeCl3 (FeCl3-G), were studied using time-resolved transient differential reflection (delta R/R). Compared to BLG, the FeCl3-G data showed an opposite sign of delta R/R, a slower rise time, and a single (instead of double) exponential relaxation. We attribute these differences in dynamics to the down-shifting of the Fermi level in FeCl3-G, as well as the formation of numerous horizontal bands arising from the d-orbitals of Fe. Our work shows that intercalation can dramatically change the electronic structure of graphene, and its associated carrier dynamics.

cond-mat.mes-hall↗

arXiv2010

FeCl3 based Few-Layer Graphene Intercalation Compounds: Single Linear Dispersion Electronic Band Structure and Strong Charge Transfer Doping

Graphene has attracted great attentions since its first discovery in 2004. Various approaches have been proposed to control its physical and electronic properties. Here, we report that graphene based intercalation compounds is an efficient method to modify the electronic properties of few layer graphene (FLG). FeCl3 intercalated FLG were successfully prepared by two-zone vapor transport method. This is the first report on full intercalation for graphene samples. The features of the Raman G peak of such few-layer graphene intercalation compounds (FLGIC) are in good agreement with their full intercalation structures. The FLGIC presents single Lorentzian 2D peak, similar to that of single layer graphene, indicating the loss of electronic coupling between adjacent graphene layers. First principle calculations further reveal that the band structure of FLGIC is similar to single layer graphene but with strong doping effect due to the charge transfer from graphene to FeCl3. The successful fabrication of FLGIC opens a new way to modify properties of FLG for fundamental studies and future applications.

cond-mat.mtrl-sci↗

arXiv2010

Stacking Dependent Optical Conductivity of Bilayer Graphene

The optical conductivities of graphene layers are strongly dependent on their stacking orders. Our first-principle calculations show that while the optical conductivities of single layer graphene (SLG) and bilayer graphene (BLG) with Bernal stacking are almost frequency independent in the visible region, the optical conductivity of twisted bilayer graphene (TBG) is frequency dependent, giving rise to additional absorption features due to the band folding effect. Experimentally, we obtain from contrast spectra the optical conductivity profiles of BLG with different stacking geometries. Some TBG samples show additional features in their conductivity spectra in full agreement with our calculation results, while a few samples give universal conductivity values similar to that of SLG. We propose those variations of optical conductivity spectra of TBG samples originate from the difference between the commensurate and incommensurate stackings. Our results reveal that the optical conductivity measurements of graphene layers indeed provide an efficient way to select graphene films with desirable electronic and optical properties, which would great help the future application of those large scale misoriented graphene films in photonic devices.

cond-mat.mtrl-sci↗

arXiv2009

G band Raman double resonance in twisted bilayer graphene: an evidence of band splitting and folding

The stacking faults (deviates from Bernal) will break the translational symmetry of multilayer graphenes and modify their electronic and optical behaviors to the extent depending on the interlayer coupling strength. This paper addresses the stacking-induced band splitting and folding effect on the electronic band structure of twisted bilayer graphene. Based on the first-principles density functional theory study, we predict that the band folding effect of graphene layers may enable the G band Raman double resonance in the visible excitation range. Such prediction is confirmed experimentally with our Raman observation that the resonant energies of the resonant G mode are strongly dependent on the stacking geometry of graphene layers.

cond-mat.mtrl-sci↗

arXiv2009

Comment on "Raman spectra of misoriented bilayer graphene"

In a recent paper [Phys. Rev. B 78, 113407 (2008)], Poncharal et al. studied the Raman spectra of misoriented bilayer graphene. They found that the blueshift of 2D band of misoriented graphene relative to that of single layer graphene shows a strong dependence on the excitation laser energy. The blueshift increases with decreasing excitation energy. This finding contradicts our explanation of reduction of Fermi velocity of folded/misoriented graphene [Ni et al. Phys. Rev. B 77, 235403 (2008)]. In this comment, we present more experimental results from our group as well as from others to show that the blueshift is indeed only weakly dependent on excitation energy. We therefore suggest that our explanation of 2D blushift of folded graphene due to reduction of Fermi velocity is still valid.

cond-mat.mtrl-sci↗

arXiv2008

Visibility study of graphene multilayer structures

The visibility of graphene sheets on different types of substrates has been investigated both theoretically and experimentally. Although single layer graphene is observable on various types of dielectrics under an optical microscope, it is invisible when it is placed directly on most of the semiconductor and metallic substrates. We show that coating of a resist layer with optimum thickness is an effective way to enhance the contrast of graphene on various types of substrates and makes single layer graphene visible on most semiconductor and metallic substrates. Experiments have been performed to verify the results on quartz and NiFe-coated Si substrates. The results obtained will be useful for fabricating graphene-based devices on various types of substrates for electronics, spintronics and optoelectronics applications.

cond-mat.mtrl-sci↗

arXiv2008

Reduction of Fermi velocity in folded graphene observed by resonance Raman spectroscopy

The 1+1 layer folded graphene sheets that deviate from AB stacking are successfully fabricated and their electronic structures are investigated by Raman spectroscopy. Significant blue shift of the 2D band of folded graphene compared to that of single layer graphene (SLG) is observed. This is attributed to SLG-like electronic structure of folded graphene but with slowing down of Fermi velocity (as much as ~5.6%). Different amount of blue shift of 2D band is observed for different folded graphenes, which may correspond to the different twist angle and/or separation between the two layers, resulting in different Fermi velocity of folded graphenes. Electronic structure of 1+1 folded graphene samples with different stacking order (twist and separation between the two layers, and in-plane shift of the two layers) can be investigated by Raman spectroscopy.

cond-mat.mtrl-sci↗

arXiv2008

Interference enhancement of Raman signal of graphene

Raman spectroscopic studies of graphene have attracted much interest. The G-band Raman intensity of a single layer graphene on Si substrate with 300 nm SiO2 capping layer is surprisingly strong and is comparable to that of bulk graphite. To explain this Raman intensity anomaly, we show that in addition to the interference due to multiple reflection of the incident laser, the multiple reflection of the Raman signal inside the graphene layer must be also accounted for. Further studies of the role of SiO2 layer in the enhancement Raman signal of graphene are carried out and an enhancement factor of ~30 is achievable, which is very significant for the Raman studies. Finally, we discuss the potential application of this enhancement effect on other ultra-thin films and nanoflakes and a general selection criterion of capping layer and substrate is given.

cond-mat.mtrl-sci↗