Manipulating mode degeneracy for tunable spectral characteristics in multi-microcavity photonic molecules.

Optical microcavities are capable of confining light to a small volume, which could dramatically enhance the light-matter interactions and hence improve the performances of photonic devices. However, in the previous works on the emergent properties with photonic molecules composed of multiple plasmonic microcavities, the underlying physical mechanism is unresolved, thereby imposing an inevitable restriction on manipulating degenerate modes in microcavity with outstanding performance. Here, we demonstrate the mode-mode interaction mechanism in photonic molecules composed of degenerate-mode cavity and single-mode cavity through utilizing the coupled mode theory. Numerical and analytical results further elucidate that the introduction of direct coupling between the degenerate-mode cavity and single-mode cavity can lift the mode degeneracy and give rise to the mode splitting, which contributes to single Fano resonance and dual EIT-like effects in the double-cavity photonic molecule structure. Four times the optical delay time compared to typical double-cavity photonic molecule are achieved after removing the mode degeneracy. Besides, with the preserved mode degeneracy, ultra-wide filtering bandwidth and high peak transmission is obtained in multiple-cavity photonic molecules. Our results provide a broad range of applications for ultra-compact and multifunction photonic devices in highly integrated optical circuits.

Exciton polaritons in mixed-dimensional transition metal dichalcogenides heterostructures.

Transition metal dichalcogenides (TMDs) promise advanced optoelectronic applications thanks to their visible or near-infrared and layer-dependent bandgaps. Even more exciting phenomena happen via stacking the TMDs to form the vertical heterostructures, such as the exotic interlayer excitons in atomically rearranged bilayer TMDs, as the result of the tunable interlayer hopping of two monolayers. So far, those literature studies focus on either two-dimensional (2D) TMDs or the layered bulky three-dimensional (3D) TMDs. The mixed-dimensional TMDs remain a fundamental yet not fully appreciated curiosity. In this Letter, we have theoretically and numerically investigated the exciton polaritons in such a hybrid system composed by the nanostructured layered (3D) and monolayer (2D) TMDs. The strong coupling has been observed of the lattice mode in high index patterned 3D TMDs and exciton from the direct bandgaps of the 2D TMDs, with the tunable Rabi splitting by geometrically shaping the 3D TMDs. We believe that our mixed-dimensional system with the novel stacks of 2D/3D van der Waals heterostructures may allow for controlling the exciton transport for advanced quantum, polaritonic, and optoelectronic devices.

Ultra-compact high-sensitivity plasmonic sensor based on Fano resonance with symmetry breaking ring cavity.

Miniaturizing optical devices with desired functionality is a key prerequisite for nanoscale photonic circuits. Based on Fano resonance, an on-chip high-sensitivity sensor, composed of two waveguides coupling with a symmetry breaking ring resonator, is theoretically and numerically investigated. The established theoretical model agrees well with the finite-difference time-domain simulations, which reveals the physics of Fano resonance. Differing with the coupled cavities, the Fano resonance originates from the interference between symmetry-mode and asymmetry-mode in a single symmetry-broken cavity. The spectral responses and sensing performances of the plasmonic structure rely on the degree of asymmetry of cavity. In particular, the plasmonic sensor can detect the refractive index changes as small as 10-5, and the figure of merit (FOM) of symmetry-breaking cavity structure is 17 times larger than that of symmetrical cavity system. Additionally, the sensitivity to temperature of ethanol analyte achieves 0.701 nm/○C. Compared with the coupled cavities, the on-chip high-sensitivity sensor using a single cavity is more compact, which paves the way toward highly integrated photonic devices.

Broadband polarization resolving based on dielectric metalenses in the near-infrared.

We demonstrate a novel polarization-resolved device (PRD) with the ability to accurately resolve the polarization states via a simple measurement process. The PRD is composed of two elaborately designed metalenses, which are capable of focusing the two circularly polarized (CP) lights. Therefore, for an arbitrary polarized light (treated as a combination of the two CP lights), a discrepancy is exhibited on focusing efficiency, which inversely provides a way to calculate the ellipticity. With such a strategy, the generalized form for polarization resolving is derived, with which the ellipticity of the incident polarized light can be calculated (through just measuring the efficiencies of the two spots). This process is accomplished by utilizing the numerical simulations and theoretical analysis. Moreover, resolving the polarization states can be achieved within a wavelength range of 400nm, due to the broadband effect of the designed metalenses. With the merits of compact configuration, broadband and compatibility with the existing semiconductor technology, the designed PRD holds potential applications in characterizing the polarization states.

Confined surface plasmon of fundamental wave and second harmonic waves in graphene nanoribbon arrays.

The confined surface plasmon of fundamental wave and second harmonic wave (SHW) are investigated in graphene grating structure. The linear-optical absorption spectra with various fermi energy and carrier mobility are investigated with the finite difference time domain (FDTD) simulations and coupled mode theory (CMT). Based on the CMT, a theoretical model for the graphene grating is established to study the spectrum features of fundamental wave. The lifetimes of linear-optical resonant modes in theoretical model are investigated through the theoretical fitting of exact values in simulation, which are tunable with both the fermi energy and carrier mobility. We also have investigated the second-order nonlinearity of graphene grating by introducing the second-order nonlinear source. The proposed configuration and method are useful for research of the absorption, local field enhancement factor, lifetime of light, and nonlinear optical processes in highly integrated graphene photoelectric devices.

Annihilating optical angular momentum and realizing a meta-waveplate with anomalous functionalities.

Manipulating the polarization states of electromagnetic waves, a fundamental issue in optics, has attracted intense attention. However, most of the reported devices are either so bulky or with specific functionalities. Here we propose a conceptually new approach to design an ultra-thin meta-waveplate (MWP) with anomalous functionalities. By elaborately designing the structural units of the metasurface, the incident right circular polarized (CP) light carrying spin angular momentum can be coupled into two surface plasmon modes with opposite orbital angular momenta which interaction with each other in the near-field, degenerating to a linear polarized (LP) light in the far-filed. The incoming spin angular momentum is annihilated and the designed MWP can function as a quarter-waveplate. However, compared with the conventional quarter-waveplates, our designed MWP owns the unidirectional function (only converting CP light to LP light) with a certain output polarization angle, which provides an extra degree of freedoms in controlling the polarization. Moreover, the designed MWP can function as a chiral material and exhibiting optical rotation properties within a broad bandwidth.

Polarization-independent metalens constructed of antennas without rotational invariance.

We propose a novel approach to designing an ultrathin polarization-independent metalens (PIM) by utilizing antennas without rotational invariance. Two arrays of nanoblocks are elaborately designed to form the super cell of the PIM, which are capable of focusing right-handed circularly polarized and left-handed circularly polarized lights. With such a strategy, the PIM is able to achieve polarization-independent focusing, since the light with any polarization can be treated as a combination of the two orthogonal ones. A theoretical analysis based on the Jones vector is proposed to detailedly explore the underlying physics. The polarization-independent characteristic of the designed PIM is also demonstrated by utilizing finite difference time domain simulations. Moreover, polarization-independent focusing can be achieved within a wavelength range of 400 nm. These results can deepen our understanding of polarization-independent focusing and provide a new method for designing ultrathin polarization-independent devices.

Plasmon-induced transparency in a single multimode stub resonator.

We investigate electromagnetically induced transparency (EIT)-like effect in a metal-dielectric-metal (MDM) waveguide coupled to a single multimode stub resonator. Adjusting the geometrical parameters of the stub resonator, we can realize single or double plasmon-induced transparency (PIT) windows in the plasmonic structure. Moreover, the consistency between analytical results and finite difference time domain (FDTD) simulations reveals that the PIT results from the destructive interference between resonance modes in the stub resonator. Compared with previous EIT-like scheme based on MDM waveguide, the plasmonic system takes the advantages of easy fabrication and compactness. The results may open up avenues for the control of light in highly integrated optical circuits.

Combined theoretical analysis for plasmon-induced transparency in waveguide systems.

We propose a novel combination of a radiation field model and the transfer matrix method (TMM) to demonstrate plasmon-induced transparency (PIT) in bright-dark mode waveguide structures. This radiation field model is more effective and convenient for describing direct coupling in bright-dark mode resonators, and is promoted to describe transmission spectra and scattering parameters quantitatively in infinite element structures by combining it with the TMM. We verify the correctness of this novel combined method through numerical simulation of the metal-dielectric-metal (MDM) waveguide side-coupled with typical bright-dark mode, H-shaped resonators; the large group index can be achieved in these periodic H-shaped resonators. These results may provide a guideline for the control of light in highly integrated optical circuits.

Uniform theoretical description of plasmon-induced transparency in plasmonic stub waveguide.

We investigate a classic analog of electromagnetically induced transparency (EIT) in a metal-dielectric-metal (MDM) bus waveguide coupled to two stub resonators. A uniform theoretical model, for both direct and indirect couplings between the two stubs, is established to study spectral features in the plasmonic stub waveguide, and the theoretical results agree well with the finite difference time domain simulations. Adjusting phase difference and coupling strength of the interaction, one can realize the EIT-like phenomena and achieve the required slow light effect. The theoretical results may provide a guideline for the control of light in highly integrated optical circuits.

Theoretical analysis and applications on nano-block loaded rectangular ring.

We propose compact and switchable optical filters based on nano-block loaded rectangular rings, and investigate the selection property numerically and theoretically. A simple and convenient phase model is established for the theoretical analysis. The dependent factors, such as the number, size, and positions of the loaded blocks, are discussed in detail. It is found that a longer wavelength can be obtained without increasing the device dimension, and the selected wave is more sensitive to the length of the loaded blocks. The loading positions play key roles in the realization of separating the second-order modes. Finally, applications of this proposed structure are discussed simply. We find that the loaded filter device provides a more compact size than the unloaded one for the same properties, and a tunable plasmon induced transparency based switch effect is also achieved. These findings suggest potential applications in compact filters, tunable slow light devices, and sensor fields.

Formation and evolution mechanisms of plasmon-induced transparency in MDM waveguide with two stub resonators.

We demonstrate the realization of plasmonic analog of electromagnetically induced transparency (EIT) in a system composing of two stub resonators side-coupled to metal-dielectric-metal (MDM) waveguide. Based on the coupled mode theory (CMT) and Fabry-Perot (FP) model, respectively, the formation and evolution mechanisms of plasmon-induced transparency by direct and indirect couplings are exactly analyzed. For the direct coupling between the two stub resonators, the FWHM and group index of transparent window to the inter-space are more sensitive than to the width of one cut, and the high group index of up to 60 can be achieved. For the indirect coupling, the formation of transparency window is determined by the resonance detuning, but the evolution of transparency is mainly attributed to the change of coupling distance. The consistence between the analytical solution and finite-difference time-domain (FDTD) simulations verifies the feasibility of the plasmon-induced transparency system. It is also interesting to notice that the scheme is easy to be fabricated and may pave the way to highly integrated optical circuits.

Recent Progress on Optical Micro/Nanomanipulations: Structured Forces, Structured Particles, and Synergetic Applications.

Optical manipulation has achieved great success in the fields of biology, micro/nano robotics and physical sciences in the past few decades. To date, the optical manipulation is still witnessing substantial progress powered by the growing accessibility of the complex light field, advanced nanofabrication and developed understandings of light-matter interactions. In this perspective, we highlight recent advancements of optical micro/nanomanipulations in cutting-edge applications, which can be fostered by structured optical forces enabled with diverse auxiliary multiphysical field/forces and structured particles. We conclude with our vision of ongoing and futuristic directions, including heat-avoided and heat-utilized manipulation, nonlinearity-mediated trapping and manipulation, metasurface/two-dimensional material based optical manipulation, as well as interface-based optical manipulation.

Lifetime and nonlinearity of modulated surface plasmon for black phosphorus sensing application.

Black phosphorus surface plasmon (BPSP) is a new promising candidate material for electromagnetic field confinement at the subwavelength scale. Here, we theoretically investigated the light confinement, second-order nonlinearity and lifetimes of tunable surface plasmons in nanostructured black phosphorus nanoflakes with superstrates. The grating structure can enhance the local optical field of the fundamental wave (FW) and second harmonic wave (SHW) due to the surface plasmon resonance. Based on the coupled mode theory (CMT), a theoretical model for the nanostructured black phosphorus was established to study the spectrum features of FW. The lifetimes of the plasmonic resonant modes were investigated with the finite difference time domain (FDTD) simulations and CMT. Since the permittivity of black phosphorus depends on its Fermi energy and electron scattering rate, the lifetimes of plasmonic absorption modes are tunable with both the Fermi energy and scattering rate. The intensity, wavelengths and spectral width of BPSP resonance modes and their lifetimes can be precisely controlled with the Fermi energy, scattering rate, side length and refractive index of the superstrate. The sensitivity is described by varying the refractive index of the superstrate such as an aqueous solution. We have introduced a second-order nonlinear source to investigate the SHW of nanostructured black phosphorus. This paper presents the corner/edge energy distribution and the tunable lifetime of BPSP as well as their unprecedented capability of photon manipulation for second-order nonlinearity within the deep subwavelength scale. The configuration and method are useful for research of the absorption, lifetime of light and nonlinear optical processes in black phosphorus-based optoelectronic devices, especially the modulation and sensing applications.

High efficiency focusing vortex generation and detection with polarization-insensitive dielectric metasurfaces.

The optical vortex beam with an orbital angular momentum, featuring a doughnut intensity distribution and a helically structured wavefront, has received extensive attention due to its applications in nanoparticle manipulation and optical communications. In this paper, we propose high-efficiency polarization-independent vortex beam generators which are capable of transforming the arbitrarily polarized plane wave into a focusing optical vortex beam and an abruptly focusing airy vortex beam. Besides, based on holographic metasurfaces, we provide a general design scheme for detecting the topological charges. With such a design strategy, multichannel topological charge resolved devices are demonstrated, which successfully implement the detection of the topological charges from -2 to 2. The metasurfaces designed with a simple and effective method in light manipulation promise photonic applications in secure communications and other related areas.

Tunable and high-sensitivity sensing based on Fano resonance with coupled plasmonic cavities.

Tunable and high-sensitivity sensing based on Fano resonance is analytically and numerically investigated in coupled plasmonic cavities structure. To analyze and manipulate the Fano line shape, the coupled cavities are taken as a composite cavity that supports at least two resonance modes. A theoretical model is newly-established, and its results agree well with the finite difference time domain (FDTD) simulations for the plasmonic stub-pair structure. The detection sensitivity factor in coupled cavities approaches 6.541 × 107 m-1, which is an order of magnitude larger than single stub case. In addition, the wavelengths of resonant modes in the plasmonic stub-pair structure can be adjusted independently, which paves a new way for improving detection sensitivity. These discoveries hold potential applications for realizing tunable and highly integrated photonic devices.

Reflective metalens with sub-diffraction-limited and multifunctional focusing.

We propose an ultra-thin planar reflective metalens with sub-diffraction-limited and multifunctional focusing. Based on the equal optical path principle, the specific phase distributions for multifunction focusing are derived. Following the formulas, on-center focusing with the characteristics of sub-diffraction-limited, high focusing efficiency (85%) and broadband focusing is investigated in detail. To demonstrate the flexibility of the reflective metalens, off-center and dual spots focusing (at the horizontal and longitudinal directions) are demonstrated. Note that all these focusings are sub-diffraction-limited due to the evanescent-filed enhancement mechanism in our elaborately designed structure. The designed reflective metalens will find important applications in super-resolution imaging, microscopes, and spectroscopic designs.