Engineering Faculty

We have proposed a new ultracompact optical demultiplexer based on metalinsulator-metal plasmonic waveguides aperturecoupled to the ring resonators. Our proposed device has high performance, small footprint, and high potential for integration and development to more channels.

Although the conductance and dielectric function of graphene can be tuned by applying external voltage, the tunability is less than 3%. Hybridizing graphene with other two-dimensional transition metal dichalcogenides (TMDs) can improve the adjustability and tunability of the optical properties of graphene-based structures at near-infrared frequencies. In this paper, we theoretically compute the dielectric function of graphene-MoTe 2graphene and graphene-MoTe 2 -graphene heterostructures utilizing the quantum electrostatic heterostructure (QEH) model, which is an ab-initio method. Utilizing the QEH results, we propose a hyper crystal (HC) absorber at near-infrared frequencies. Hence, we use the transfer matrix method to investigate our proposed absorber analytically. Moreover, we simulate the graphene-TMD-graphene (G-TMD-G) absorbers by the numerical finite difference time domain method. The results of the numerical solution are consistent with those of the analytical method. Due to the dependency of the Fermi level of graphene on the direct bandgap of the TMDs, the dielectric function of the G-TMD-G heterostructure can be tuned and enhanced further by changing the number of TMD layers. Finally, we demonstrate that the full absorption of the heterostructures can be achieved at different frequencies for transverse magnetic polarization. Since the thicknesses of the layers in the HC are lower than the wavelength of the light, no diffracted bands are ubiquitous, and the absorption can be observed for a wide range of incidence angles and bandwidths at near-infrared frequencies. Because of utilizing graphene-based HCs, in addition to the feasibility of design compared to the complex metasurfaces, the absorption bandwidth is significant for a wide range of incidence angles. This kind of HC absorber can be used in the design of sensitive optical devices, such as tunable filters, detectors, and photovoltaic applications.

In this study, we benefit from the nonlinear optical tunability of the graphene-transition metal dichalcogenide (G-TMDC) heterostructure and the strong confinement of the electromagnetic fields of surface plasmon polaritons (SPPs) on graphene in order to propose a highly tunable nonlinear optical Bragg reflector. Recently, two-dimensional (2D) TMDCs are the subject of intense researches because of their nonlinear optical properties at near infrared wavelengths which are very intriguing for various optical applications. We choose two kinds of 2D-TMDCs, MoSe 2 and WSe 2 , with the strongest second order optical nonlinearity at near infrared range to properly design the periodic variation of the propagating SPP waves on the graphene layer. We utilize theoretical method of quantum electrostatic heterostructure to compute the dielectric function of graphene-TMDCs. Different nonlinearities of two TMDCs lead to noticeable tuning of the full width at half maximum (FWHM) and the central Bragg wavelength of the reflector which let design various optical devices. We design an add/drop filter, a nonlinear switch, and an AND/OR optical logic gate based on our proposed Bragg reflector. Our finite difference time domain numerical and transfer matrix analytical results reveal that by increment of the optical intensity up to 6 MW/cm 2 which is below the pulse damage threshold of graphene, due to the second order nonlinearity, a 10-nm red-shift in central Bragg wavelength is observed and the 20-nm FWHM at linear regime decreases to 1.5 nm. The SPP intensities of 0.8 MW/cm 2 and 1.53 MW/cm 2 fulfill the requirements for AND and OR logical operations with 57 and 66.51 dB extinction ratios, respectively.

In this study, a refractive index sensor based on nonlinear graphene-transition metal dichalcogenides (TMDs) Bragg reflector (BR) is proposed and analyzed. The Bragg wavelength and the reflection width can be engineered by the number of TMD layers and the intensity of the input, this case is dissimilar to the specified resonance wavelength of the surface plasmon resonances (SPRs) with constant resonance wavelength. The low loss nature and high nonlinearity of TMDs accompanying by graphene brilliant properties let the design of more flexible, narrower resonance, and lower loss sensors. We theoretically compute the dielectric function of the structure utilizing quantum electrostatic heterostructure method and numerically simulate our proposed sensor by three dimensional finite difference time domain. The simulation results reveal that the effective refractive index of the sensor directly controls the Bragg wavelength of the proposed sensor. We tune the Bragg wavelength and the bandwidth of the reflection of the BR sensor by input intensity. In our proposed sensor, the full width at half maximum (FWHM) of the BR reflection spectra decreases from 12 nm in the linear regime to 1.5 nm at the nonlinear regime, which let have 1099 (nm/RIU) sensitivity with input intensity of 6.18 (MW/cm2). In the nonlinear regime, a variation of 0.01 in the refractive index, cause 20 nm redshift in BR wavelength and with respect to 1.5-nm FWHM, the reflection spectra are completely recognizable. Due to its sensitivity, this kind of heterostructure sensor can be utilized in the design of sensitive optical biosensors.

Semiconducting transition‐metal dichalcogenides (TMDCs) provide a fascinating discovery platform for strong light–matter interaction effects in the visible spectrum at ambient conditions. While most of the works have focused on hybridizing excitons with resonant photonic modes of external mirrors, cavities, or nanostructures, intriguingly, TMDC flakes of subwavelength thickness can themselves act as nanocavities. Herein, the optical response of such freestanding planar waveguides of WSe2 by means of cathodoluminescence spectroscopy is determined. Strong exciton–photon interaction effects that foster long‐range propagating exciton–polaritons and enable direct imaging of the energy transfer dynamics originating from cavity‐like Fabry–Pérot resonances are revealed. Furthermore, confinement effects due to discontinuities in the flakes are demonstrated as an efficient means to tailor mode energies, spin–momentum couplings, and the exciton–photon coupling strength, as well as to promote p...

Van der Waals materials such as thin films of transition-metal dichalcogenides (TMDCs) manifest strongly bound exciton states in the visible spectrum at ambient conditions that provide an ideal platform for exciton-photon couplings. Utilizing semiconducting TMDCs in the form of multilayer structure combined with metals can increase significantly the light-matter interaction. In this way, the interaction between excitons and surface-plasmon polaritons emerge as a platform for transferring the electromagnetic energy at confined modal volumes. Here, we theoretically investigate how moving electrons can be used as a probe of hybrid exciton-plasmon polaritons of gold-WSe2 multilayers within the context of electron energy-loss spectroscopy (EELS) and cathodoluminescence (CL) spectroscopy. Interestingly, and in contrast to WSe2 slab waveguides where quasi-propagating photonic modes interact with only exciton A, in gold-WSe2 multilayer, exciton A and exciton B can both strongly interact wit...

Semiconducting transition-metal dichalcogenides (TMDCs) provide a fascinating discovery platform for strong light-matter interaction effects in the visible spectrum at ambient conditions. While most of the work has focused on hybridizing excitons with resonant photonic modes of external mirrors, cavities, or nanostructures, intriguingly, TMDC flakes of subwavelength thickness can themselves act as nanocavities. Here, we determine the optical response of such freestanding planar waveguides of WSe2, by means of cathodoluminescence spectroscopy. We reveal strong exciton-photon interaction effects that foster long-range propagating exciton-polaritons and enable direct imaging of the energy transfer dynamics originating from cavity-like Fabry-Pérot resonances. Furthermore, confinement effects due to discontinuities in the flakes are demonstrated as an efficient means to tailor the excitonphoton coupling strength, along the edges of natural flakes. Our combined experimental and theoretica...

The study of spin-orbit coupling (SOC) of light is crucial to explore the light-matter interactions in subwavelength nanostructures with broken symmetries. In noncentrosymmetric photonic crystals, the SOC results in the splitting of the otherwise degenerate energy bands. Herein, we explore the SOC in a noncentrosymmetric plasmonic crystal, both theoretically and experimentally. Cathodoluminescence (CL) spectroscopy combined with the numerically calculated photonic band structure reveals an energy band splitting that is ascribed to the broken symmetries in the noncentrosymmetric plasmonic crystal. By shifting the impact position of the electron beam throughout a unit cell of the plasmonic crystal, we show that the emergence of the energy band splitting strongly depends on the excitation position of the surface plasmon (SP) waves on the crystal. Moreover, we exploit angle-resolved CL and dark-field polarimetry to demonstrate polarization-dependent scattering of SP waves interacting with the plasmonic crystal. The scattering direction of a given polarization is determined by the transverse spin angular momentum inherently carried by the SP wave, which is in turn locked to the direction of SP propagation. Our study gives insight into the design of novel plasmonic devices with polarization-dependent directionality of the Bloch plasmons. We expect spin-orbit plasmonics will find much more scientific interests and potential applications with the continuous development of nanofabrication methodologies and uncovering new aspects of spin-orbit interactions.

The study of spin–orbit coupling (SOC) of light is crucial to explore the light–matter interactions in sub-wavelength structures. By designing a plasmonic lattice with chiral configuration that provides parallel angular momentum and spin components, one can trigger the strength of the SOC phenomena in photonic or plasmonic crystals. Herein, we explore the SOC in a plasmonic crystal, both theoretically and experimentally. Cathodoluminescence (CL) spectroscopy combined with the numerically calculated photonic band structure reveals an energy band splitting that is ascribed to the peculiar spin–orbit interaction of light in the proposed plasmonic crystal. Moreover, we exploit angle-resolved CL and dark-field polarimetry to demonstrate circular-polarization-dependent scattering of surface plasmon waves interacting with the plasmonic crystal. This further confirms that the scattering direction of a given polarization is determined by the transverse spin angular momentum inherently carrie...

Transition-metal dichalcogenides with their exciton-dominated optical behavior emerge as promising materials for realizing strong light-matter interactions in the visible range and at ambient conditions. When these materials are combined with metals, the energy confining ability of plasmon polaritons in metals below the diffraction limit, allows for further enhancing and tailoring the light-matter interaction, due to the formation of plexcitons in hybrid metal-TMDC structures at the interface. Herein, we demonstrate that the coupling between quasi-propagating plasmons in plasmonic crystals and excitons in WSe2, provides a multi-oscillator playground for tailoring the band structure of plasmonic crystal structures and results in emerging flat bands. The cathodoluminescence spectroscopy and angle-resolved measurements combined with the numerically calculated photonic band structure confirm a strong exciton-plasmon coupling, leading to significant changes in the band diagram of the hybrid lattice and the ability to tailor the band diagram via strong coupling. The hybrid plexcitonic crystal structures investigated here sustain optical waves with remarkably low group velocities. These results could be used for designing tunable slow-light structures based on the strong-coupling effect and pave the way toward plexcitonic topological photonic structures.

der Waals materials such as thin films of transition-metal dichalcogenides (TMDCs) manifest strongly bound exciton states in the visible spectrum at ambient conditions that provide an ideal platform for exciton-photon couplings. Utilizing semiconducting TMDCs in the form of multilayer structure combined with metals can increase significantly the light-matter interaction. In this way, the interaction between excitons and surface-plasmon polaritons emerge as a platform for transferring the electromagnetic energy at confined modal volumes. Here, we theoretically investigate how moving electrons can be used as a probe of hybrid exciton-plasmon polaritons of gold-WSe2 multilayers within the context of electron energy-loss spectroscopy (EELS) and cathodoluminescence (CL) spectroscopy. Interestingly, and in contrast to WSe2 slab waveguides where quasi-propagating photonic modes interact with only exciton A, in gold-WSe2 multilayer, exciton A and exciton B can both strongly interact with surface plasmon polaritons. Hence, we observe CL emission suppression at excitonic or plasmonic peaks, which reveals the energy transfer between excitons and plasmons in the form of nonradiating guided waves. Our work provides a systematic study for deeper understanding of the effect of the configuration and the thickness of layers on the photonic and plasmonic modes and hence on the strength of the coupling between excitons and surface-plasmon polaritons. Our findings pay the way for designing efficient photodetectors, sensitive sensors, and light emitting devices based on metal/semiconducting hybrid materials. , and 𝛿 𝛼𝑧 is the Kronecker-delta function, 𝑘 0 = 𝜔 𝑐 ⁄ . 𝐴 ̃𝑛1 𝛼 and 𝐴 ̃𝑛2 𝛼 defines the unknown coefficients of the plane waves propagating inside each layer in the upward and downward directions, respectively, that should be obtained using the boundary conditions. The index n spans the domain [1, ⋯ , 𝑁], where N is the number of the layers. Using the solution for the vector potentials, the electric field components are calculated as and for the magnetic field we obtain Using the Poynting's theorem, the radiated power to the far field zone along the z-axis is recast as 𝑅𝑒 ∫ 𝑑𝑥 ∫ 𝑑𝑦(𝐸 𝑥 𝐻 𝑦 * -𝐸 𝑦 𝐻 𝑥 * ) = ∫ 𝑑𝑘 𝑥 ∫ 𝑑𝑘 𝑥 𝑝 ̃𝑧(𝜔; 𝑘 𝑥 , 𝑘 𝑦 ).

Semiconducting transition-metal dichalcogenides (TMDCs) provide a fascinating discovery platform for strong light-matter interaction effects in the visible spectrum at ambient conditions. While most of the work has focused on hybridizing excitons with resonant photonic modes of external mirrors, cavities, or nanostructures, intriguingly, TMDC flakes of subwavelength thickness can themselves act as nanocavities. Here, we determine the optical response of such freestanding planar waveguides of WSe2, by means of cathodoluminescence spectroscopy. We reveal strong exciton-photon interaction effects that foster long-range propagating exciton-polaritons and enable direct imaging of the energy transfer dynamics originating from cavity-like Fabry-Pérot resonances. Furthermore, confinement effects due to discontinuities in the flakes are demonstrated as an efficient means to tailor mode energies, spin-momentum couplings, and the exciton-photon coupling strength, as well as to promote photon-mediated exciton-exciton interactions. Our combined experimental and theoretical results provide a deeper understanding of exciton-photon self-hybridization in semiconducting TMDCs and may pave the way to optoelectronic nanocircuits exploiting exciton-photon interaction beyond the routinely employed two-oscillator coupling effects. The phenomenon of spontaneous collective coherence in condensed matter quantum systems, as most notably manifested by Bose-Einstein condensation, continues to be a subject of

The study of spin-orbit coupling (SOC) of light is crucial to explore the light-matter interactions in subwavelength structures. By designing a plasmonic lattice with chiral configuration that provides parallel angular momentum and spin components, one can trigger the strength of the SOC phenomena in photonic or plasmonic crystals. Herein, we explore the SOC in a plasmonic crystal, both theoretically and experimentally. Cathodoluminescence (CL) spectroscopy combined with the numerically calculated photonic band structure reveals an energy band splitting that is ascribed to the peculiar spin-orbit interaction of light in the proposed plasmonic crystal. Moreover, we exploit angle-resolved CL and dark-field polarimetry to demonstrate circular-polarization-dependent scattering of surface plasmon waves interacting with the plasmonic crystal. This further confirms that the scattering direction of a given polarization is determined by the transverse spin angular momentum inherently carried by the SP wave, which is in turn locked to the direction of SP propagation. We further propose an interaction Hamiltonian based on axion electrodynamics that underpins the degeneracy breaking of the surface plasmons due to the spin-orbit interaction of light. Our study gives insight into the design of novel plasmonic devices with polarization-dependent

Topological plasmonic provides a new insight for the manipulation of light. Analogous to exotic nature of topological edge states in topological photonics, topological plasmonic combines concepts from topology and plasmonics. By utilizing topological protection, plasmons can be made to propagate without significant scattering or decay, even in the presence of defects or disorder. Herein, we present a study on the design, characterization, and manipulation of topological plasmonic chains of discs based on the Su-Schrieffer-Heeger model made into a ring resonator. The investigation focuses on exploring the unique properties of these resonators and their potential to support topologically protected edge modes, within the continuum of rotationally symmetric optical modes of a ring. To observe the topological edge modes in rotationally symmetric chains, we employ a symmetry-breaking excitation technique based on electron beams. It analyzes the influence of parameters such as dimerization and loop numbers on the presence of topological modes. Additionally, we explore the manipulation of electron impact positions to control the direction of propagation and selectively excite specific bulk or edge modes. The findings contribute to a deeper understanding of topological effects by numerically investigating their design principles and exploring techniques to manipulate topological edge modes. The insights gained from this study have implications for the development of nanoscale plasmonic systems with customized functionalities, potentially impacting areas such as nanophotonics and quantum information processing.

Topological plasmonics offers new ways to manipulate light by combining concepts from topology and plasmonics, similar to topological edge states in photonics. However, designing such topological states remains challenging due to the complexity of the high-dimensional design space. We present a novel method that uses supervised, physics-informed deep learning and surrogate modeling to design topological devices for specific wavelengths. By embedding physical constraints in the neural network's training, our model efficiently explores the design space, significantly reducing simulation time. Additionally, we use non-planar wavefront excitations via electron beams to probe topologically protected plasmonic modes, making the design and training process nonlinear. Using this approach, we design a topological device with unidirectional edge modes in a ring resonator at specific operational frequencies. Our method reduces computational cost and time while maintaining high accuracy, highlighting the potential of combining machine learning and advanced techniques for photonic device innovation.

Long-lived coherent quasiparticles are a promising foundation for novel quantum technologies, where maintaining quantum coherence is crucial. Decoherence, driven by finite emitter lifetimes, remains a central challenge in quantum computing. Here, we control the dynamics of spatially separated quantum emitters via preserving their phase information by introducing a topological waveguide as a robust chiral reservoir. Incoherent quantum emitters randomly positioned near a perturbed honeycomb plasmonic interface and couple to the mutual topological interface mode. Using the S3 Stokes parameter, we trace far-field polarization patterns that reflect emitter coherence and spin-momentum locking. We show that even weakly coupled emitters exhibit coherent excitation and imprint phase on the emission. Time-domain dynamics reveal signatures of superradiance and subradiance that correlate with spatial interference in S3. These spatialtemporal features confirm that the observed polarization patterns arise from coherent quantum many-body dynamics, not classical interference. This challenges the conventional dichotomy between incoherent and coherent regimes, revealing that topological chiral photonic environments mediate long-range quantum correlations beyond standard waveguide QED.