arXiv subjects

Mengtao Sun

Publications and source records attributed to Mengtao Sun.

17 recordsLinked to original sources

arXiv2026

Tunable Mid-Infrared Chiral Selective Absorber Based on Asymmetric V-Shaped Metasurfaces Driven by Chiral quasi-Bound State in the Continum

Efficient discrimination of circularly polarized light (CPL) is of great significance in photonics. In this study, we propose a nanophotonic device based on asymmetric V-shaped metasurfaces that exhibits strong discrimination between left- and right-circularly polarized (LCP and RCP) light at the resonant wavelengths of the absorption spectra. The chiral-selective response originates from a quasi-bound state in the continuum (qBIC) mode enabled by controlled symmetry breaking in the V-shaped structure, which exhibits high absorption under LCP illumination while the resonance is strongly suppressed under RCP incidence, leading to a large absorption contrast between the two circular polarizations. This significant absorption difference enables highly efficient chiral discrimination. Furthermore, the resonant wavelength can be broadly tuned from 5200 to 6200 nm by scaling the structural dimensions without compromising the high absorption contrast between LCP and RCP. This work not only enables high-performance chiral detection and separation, but also offers valuable guidance for reconfigurable chiral nanodevices, with promising applications in areas such as enantiomer sensing, chiral imaging, and CPL spectroscopy.

physics.optics↗

arXiv2026

Broadband Asymmetric Transmission with Wide Spectral Tunability based on Substrate-Embedded Silicon Nanoring Arrays

In this work, we theoretically propose a broadband asymmetric transmission (AT) device based on periodic Si nanoring arrays embedded in a SiO2 substrate. Results indicate that the device achieves a remarkable broadband AT effect in the near-infrared region (1750-2400 nm), with forward transmissivity exceeding 0.8 (maximum of 0.98), backward transmissivity less than 0.15 (minimum of 0.015) and an isolation ratio (IR) reaching a maximum of 17.8 dB at 2280 nm. Furthermore, the transmissivity spectrum exhibits excellent scalability and tunability through uniform scaling of the structure, allowing the operational band to be tailored across a wide spectral range, from 890 to 3300 nm. This Si-based nanostructure offers a robust and flexible platform for applications in optical isolation, multi-channel sensing, and integrated photonic circuits.

physics.optics↗

arXiv2026

Broadband asymmetric transmission with tunable bilayer silicon nanoarrays: from visible to near-infrared

A kind of asymmetric transmission (AT) device based on bilayer silicon arrays (BSA) nanostructure is theoretically explored, which achieves high forward transmissivity and suppressed backward transmissivity for broadband by simply adjusting the parameters of the structure. The structure consists of two silicon cylinder arrays, one on the SiO2 substrate and the other embedded in the substrate. Particularly, three AT devices with different configurations are designed, which exhibit broadband AT with high isolation ratios in the wavelength ranges of 685-807 nm, 866-1029 nm, and 1285-1536 nm, respectively. A comprehensive analysis of the BSA structure's performance across different array periods highlights its potential for broadband optical applications, such as optical isolation and multi-channel optical sensors.

physics.optics↗

arXiv2025

Dual-Mode Asymmetric Transmission based on Asymmetric and Orthogonal Gratings: Polarization-Dependent and -Independent Modes

A dual-mode asymmetric transmission (AT) nanodevice based on the asymmetric and orthogonal grating-film-grating (AO-GFG) structure is proposed and systematically investigated theoretically. The device supports two distinct localized surface plasmon resonance (LSPR) modes for forward transmission, corresponding to the polarization-dependent (M1) and the polarization-independent (M2) resonances, respectively. This results in the fact that when x-polarized light is incident, only M2 exists; when y-polarized light is incident, both M1 and M2 exist. Besides, both modes yield the maximum isolation ratio of more than 10 dB. The electric field distributions further indicate that M2 exhibits strong confinement and efficient tunneling through the metallic film, while M1 shows weaker but more polarization-sensitive hybridization. The coexistence and tunability of these two modes constitute the physical basis of dual-mode AT, highlighting the AO-GFG structure as a promising platform for high-isolation and polarization-tunable plasmonic devices in the visible and near-infrared regions.

physics.optics↗

arXiv2025

Extend the waveband of high isolation ratio and narrow band asymmetric transmission based on grating-film-grating nanostructure

A serial asymmetric transmission (AT) nanodevices based on the grating-film-grating (G-F-G) structure are proposed and studied. By showing the results of three different nanodevices as examples, it is proved that this kind of G-F-G nanostructure can achieve high isolation ratio AT at arbitrary wavelength within a certain range by designing the parameters of the structure. These three nanodevices can achieve high forward transmittivities of 0.69, 0.71, and 0.87 at the wavelengths of 714, 810, and 905 nm, respectively, and the isolation ratio between forward and backward transmittivities are all more than 10 dB. In addition, the effects of the incidence angle and lateral displacement of upper gratings on device performance were also investigated, revealing that optimal AT is achieved under normal incidence and zero displacement. The proposed G-F-G structure provides a solution for passive and easy to manufacture optical isolators with potential applications in optical communication and sensing systems.

physics.optics↗

arXiv2023

Mechanisms of chiral plasmonics -- scattering, absorption and photoluminescence

Chirality is a concept that one object is not superimposable on its mirror image by translation and rotation. In particular, chiral plasmonics have been widely investigated due to their excellent optical chiral properties, and have led to numerous applications such as optical polarizing element etc. In this study, we develop a model based on the concept of the interaction between harmonic oscillators to investigate and explain the optical chiral mechanisms of strongly coupled metal nanoparticles (MNPs). The chirality of the scattering, absorption, and photoluminescence spectra are carefully discussed in detail. The results show that the chirality of the system originates not only from the orientations of the MNPs, but also from the different eigen parameters between them. Specifically, the derived three factors contribute to the chirality: the symmetry, the coupling strength, and the coherent superposition of the emitted electric field. This work provides a deeper understanding on the chiral plasmonics and may guide relevant applications in theory.

physics.optics↗

arXiv2022

Unified Treatment for Scattering, Absorption, and Photoluminescence of coupled Metallic Nanoparticles with Vertical Polarized Excitation

Optical properties of coupled metallic particles (MNPs) have been widely reported due to their unique characteristics such as peak shift/splitting of the coupling spectra and electromagnetic enhancement at sub-wavelength scale, etc. In a previous work, we have investigated the coupling spectra of two coupled MNPs with parallel polarized excitation. In this study, we investigate the vertical polarization case in detail. Different from the parallel one, the vertical one has its unique properties: (a) three coupling coefficients; (b) positive coupling terms in the coupling equations; (c) blue-shifts of the peaks with the increasing coupling strength for identical MNPs spectra, including scattering, absorption, and photoluminescence. Comparison with published experimental results shows the validity of this model. This work provides a deeper understanding on the optical properties of coupled MNPs and is beneficial to relevant applications.

physics.optics↗

arXiv2022

Absorption and photoluminescence properties of coupled plasmon-exciton (plexciton) systems

Plexciton is the formation of new hybridized energy states originated from the coupling between plasmon and exciton. To reveal the optical properties of both exciton and plexciton, we develop a classic oscillator model to describe the behavior of them. Particularly, the coupling case, i.e., plexciton, is investigated theoretically in detail. In strong coupling, the electromagnetically induced transparency is achieved for the absorption spectra; the splitting behaviors of the modes are carefully analyzed, and the splitting largely depends on the effective number of the electrons and the resonance coupling; the photoluminescence spectra show that the spectral shapes remain almost unchanged for weak coupling and change a lot for strong coupling; the emission intensity of the exciton is strongly enhanced by the plasmon and can reach to the order of $10^{10}$ for a general case. We also show the comparisons between our model and the published experiments to validate its validity. This work may be useful for understanding the mechanism of the plexciton and for the development of new applications.

physics.optics↗

arXiv2021

Unified Treatment for Scattering and Photoluminescence Properties of Strongly Coupled Metallic Nanoparticle Chains based on a Coupling Classic Harmonic Oscillator Model

We present a multimer coupling classic harmonic oscillator model to reveal the scattering and photoluminescence (PL) properties of metallic nanoparticle chains. Taking particle number from 1 to 6 as examples, we compare the calculated spectra with the experimental ones from other researchers' work, and they agree well with each other. Furthermore, scattering and PL properties are analyzed carefully varying with particle number $n$, coupling strength $g$ and effective free electron number $N$. Results indicates larger red-shift and smaller full width at half maximum (FWHM) of the scattering spectra with larger $n$ or/and larger $g$. Meanwhile, the splitting of PL modes increases as $g$ increases, and the amplitudes are dependent on the excitation wavelength. This classic model is simple and shows a unified treatment for understanding the scattering and PL properties of multimer coupled systems.

physics.optics↗

arXiv2021

Understanding photoluminescence of coupled metallic nanostructures based on a coupling classic harmonic oscillator model

Photoluminescence (PL) phenomenon from metallic nanostructures has been explained and understood by several point of views. One of them is based on the classic harmonic oscillator model, which describes PL of single mode. In this study, we continue to expand this classic model to a coupling case, which involves two oscillators that interact with each other together with the excitation electric field. The new generated modes due to the coupling are carefully analyzed, including their behaviors varying with the coupling coefficients in different cases. Furthermore, for practical purpose, PL spectra and white light scattering spectra of two individual metallic nanostuctures are calculated as examples employing the model to verify its validity. This work would give a deeper understanding on coupling PL phenomena and is helpful to relative applications.

physics.optics↗

arXiv2020

Plasmon-Enhanced Interface Charge Transfer Exciton of 2D In-Plane Lateral and van Der Waals MoS$_2$/WS$_2$ Heterostructures

The multi-scale computational method of combining first-principles calculation and finite element electromagnetic simulations is used to study plasmon-enhanced interlayer charge transfer (ICT) exciton of 2D lateral and van der Waals MoS2/WS2 heterostructure with 2H phase. The weak ICT excitons are observed in the 2H lateral and van der Waals MoS$_2$/WS$_2$ heterostructures. Theoretical results reveal the physical principle of plexciton resulting from strong coupling between plasmonic and ICT exciton. The weak CT exciton can be strongly enhanced by metal plasmon, can provide a new way to observe the weak CT exciton. Our results can promote deeper understanding of the plexciton resulting from strong coupling interaction between plasmon and exciton of lateral and van der Waals heterostructures.

physics.optics↗

arXiv2019

Visualizations of electric and magnetic interactions in ECD and ROA

The chiral source and its mechanism in the molecular system are of great significance in many fields. In this work, we proposed visualized methods to investigate physical mechanism of chiral molecule, where the electric and magnetic interactions are visualized with the transitional electric dipole moment, the transitional magnetic dipole moment and the transitional electric quadrupole moment, and their tensor product. The relationship between molecular ROA response and molecular structure was analyzed in an intuitive way. The relationship between chromophore chirality and molecular vibration mode are revealed via interaction between the transition electric dipole moment and the transition magnetic dipole moment. The molecular chirality is derived from the anisotropy of the molecular transition electric dipole moment and the transition magnetic dipole moment. The anisotropic dipole moment localized molecular chromophore is the source of the vibration mode in which the ROA responds to the reverse.

physics.chem-ph↗

arXiv2016

Ultrafast Dynamics of Plasmon-Exciton Interaction of Ag Nanowire-Graphene Hybrids for Surface Catalytic Reactions

Using the ultrafast pump-probe transient absorption spectroscopy, the femtosecond-resolved plasmon-exciton interaction of graphene-Ag nanowire hybrids is experimentally investigated, in the VIS-NIR region. The plasmonic lifetime of Ag nanowire is about 150 femtosecond (fs). For a single layer of graphene, the fast dynamic process at 275 fs is due to the excitation of graphene excitons, and the slow process at 1.4 picosecond (ps) is due to the plasmonic hot electron interaction with phonons of graphene. For the graphene-Ag nanowire hybrids, the time scale of the plasmon-induced hot electron transferring to graphene is 534 fs, and the metal plasmon enhanced graphene plasmon is about 3.2 ps in the VIS region. The graphene-Ag nanowire hybrids can be used for plasmon-driven chemical reactions. This graphene-mediated surface-enhanced Raman scattering substrate significantly increases the probability and efficiency of surface catalytic reactions co-driven by graphene-Ag nanowire hybridization, in comparison with reactions individually driven by monolayer graphene or single Ag nanowire. This implies that the graphene-Ag nanowire hybrids can not only lead to a significant accumulation of high-density hot electrons, but also significantly increase the plasmon-to-electron conversion efficiency, due to strong plasmon-exciton coupling.

cond-mat.mes-hall↗

arXiv2015

Ultralong Lifetime Plasmons on Picosecond Time Scale Enabled by Hybrid Plasmon-Phonon Polaritons

Graphene plasmonics is of great interest for compact optical devices working in broad frequency domains with ultrahigh speed and very low energy consumption. However, graphene plasmons damp out quickly on most substrates mainly due to scattering loss from substrate surface phonons and impurities. Here we discover a new hybridized plasmon-phonon polariton mode in graphene/h-BN van der Waals heterostructures, which enables ultralong hybrid plasmon lifetime up to 1.6 picosecond, the longest plasmon lifetime ever demonstrated. Such remarkably long lifetime arises from the coupling of long-lifetime h-BN transverse optical phonon with graphene plasmons, which uniquely exists in monolayer heterostructures. Our findings and understanding of this unexploited hybrid mode offer a novel approach to tune the plasmon behaviours in the frequency, time and space domains. This can potentially introduce a new paradigm to generate highly-confined plasmons with ultra-long lifetime for various applications, such as deep-subwavelength metamaterials, ultra-low-loss waveguides, and ultrafast optical switches.

cond-mat.mtrl-sci↗

arXiv2015

Formation of Enhanced Uniform Chiral Fields in Symmetric Dimer Nanostructures

Chiral fields with large optical chirality are very important in chiral molecules analysis, sensing and other measurements. Plasmonic nanostructures have been proposed to realize such super chiral fields for enhancing weak chiral signals. However, most of them cannot provide uniform chiral near-fields close to the structures, which makes these nanostructures not so efficient for applications. Plasmonic helical nanostructures and blocked squares have been proved to provide uniform chiral near-fields, but structure fabrication is a challenge. In this paper, we show that very simple plasmonic dimer structures can provide uniform chiral fields in the gaps with large enhancement of both near electric fields and chiral fields under linearly polarized light illumination with polarization off the dimer axis at dipole resonance. An analytical dipole model is utilized to explain this behavior theoretically. 30 times of volume averaged chiral field enhancement is gotten in the whole gap. Chiral fields with opposite handedness can be obtained simply by changing the polarization to the other side of the dimer axis. It is especially useful in Raman optical activity measurement and chiral sensing of small quantity of chiral molecule.

physics.optics↗

arXiv2014

Effects of Electric Field Gradient on Sub-nanometer Spatial Resolution of Tip-enhanced Raman Spectroscopy

Tip-enhanced Raman spectroscopy (TERS) with sub-nanometer spatial resolution has been recently demonstrated experimentally. However, the physical mechanism underlying is still under discussion. Here, we theoretically investigate the electric field gradient of a coupled tip-substrate system. Our calculations suggest that the ultra-high spatial resolution of TERS can be partially attributed to the electric field gradient effect owning to its tighter spatial confinement and sensitivity to the infrared (IR)-active of molecules.

cond-mat.mes-hall↗

arXiv2011

Activated Vibrational Modes and Fermi Resonance in Tip-Enhanced Raman Spectroscopy

Using p-aminothiophenol (PATP) molecules on a gold substrate as prototypical examples and high vacuum tip-enhanced Raman spectroscopy (HV-TERS), we show that the vibrational spectra of those molecules are distinctly different from those in typical surface-enhanced Raman spectroscopy. Detailed first-principles calculations help to assign the Raman peaks in the TERS measurements as Raman active and infrared (IR) active vibrational modes of dimercaptoazobenzene (DMAB), thus providing strong spectroscopic evidence for the conversion of PATP dimerization to DMAB. The activation of the IR active modes is due to enhanced electromagnetic field gradient effects within the gap region of the highly asymmetric tip-surface geometry. Our TERS measurements also realize splitting of certain vibrational modes due to Fermi resonance between a fundamental mode and the overtone of a different mode or a combinational mode. These findings help to broaden the versatility of TERS as a promising technique for ultrasensitive molecular spectroscopy.

physics.atm-clus↗