Ultra-thin Semiconductor/Metal Resonant Superabsorbers

The design of thin-film semiconductor absorbers is a long-sought-after goal of crucial importance for optoelectronic devices. We propose a new strategy that achieves multi-band optical absorption in an ultra-thin semiconductor-insulator-metal nanostructure. The whole thickness of the absorber is just 60 nm, which is less than λ/12. The ultra-thin semiconductor resonators are used as the photonic coupling elements. The plasmonic metal layer with the thickness about 15 nm simultaneously acts as the transmission cancel layer and the plasmon source for resonant coupling with the optical near-field energy. The combined semiconductor resonators and the thin metal film produce strong electromagnetic field coupling and confinement effects, which mainly contribute to the efficient light trapping for the multi-band strong light absorption. The semiconductors such as Si, GaAs, and Ge are confirmed with the capability to show high light absorption via this simple hybrid metal-semiconductor resonant system. These features pave new insight on ultra-thin semiconductor absorbers and hold potential applications for optoelectronics such as nonlinear optics, hot-electron excitation and extraction, and the related devices.

     

A Facile Strategy for All-Optical Controlling Platform by Using Plasmonic Perfect Absorbers

Dual-band light absorption with the maximal absorptivity up to 99.7% and the minimal spectral bandwidth down to 3 nm is obtained in the plasmonic absorbers consisting of triple-layer plasmonic crystal-nonlinear medium cavity-metal substrate structure, where the intercalated dielectric material is chosen to be a Kerr medium cavity. Efficient all-optical controlling with high spectral intensity change ratios and detecting signal-to-noise is achieved for the system after a slight increase of pumping intensity. These impressive results mainly result from the strong plasmonic resonant field confinement in the middle nonlinear Kerr medium cavity and the near-perfect relative intensity change response by the ultra-sharp anti-reflection spectrum. This work can lay a foundation for advanced all-optical devices by exploiting light perfect absorption behavior and resonant optical field enhancement.     

All-Optical Switching and Routing with a Nonlinear Metamaterial

We report the dynamic control characteristics of electromagnetic wave propagation in a nonlinear metamaterial by an applied electric field, which is constructed by an array of metallic nanowires embedded into a nonlinear dielectric. Numerical results show that the composite structure can appear three kinds of interesting interconversion characteristics among positive refraction, negative refraction, and cut-off states by adjusting the intensity of the applied electric field. Consequently, we can switch all-optically light states between the total reflection state (OFF state) and the total transmission state (ON state), as well as control light propagation route dynamically. Moreover, we also elaborate on the dependency of the refraction angles of energy flow and wave vector, and Brewster angle on the applied electric field and the orientation angle φ. These properties open up an avenue for potential applications of nonlinear metamaterials in nanophotonic devices such as all-optical switches, routers, and wave cut-off devices.     

All-Optical Strong Coupling Switches Based on a Coupled Meta-atom and MIM Nanocavity Configuration

Based on a coupled meta-atom and metal-nonlinear dielectric-metal nanocavity, nonlinear all-optical strong coupling switches are proposed and numerically investigated. In the absence of the external pumping light, the resonances of the meta-atom are continuously tuned across the one of the nanocavity by changing the size of the meta-atom. The meta-atomic electric dipole and quadrupole interaction with the plasmonic nanocavity is obtained. The characteristic anticrossing behaviors manifest the occurrence of the strong coupling. With the resonance of the meta-atom being tuned to the one of the nanocavity, we dynamically tune the coupled strength of the system by changing intensity (power) of the pumping light and realize the transition from the strong coupling regime to the weak one. This means that this system can be used as an on/off switch in which the strong coupling can be on/off with an external control light, and the on/off states correspond to strong/weak coupling regime, respectively. Such a strong coupling all-optical switching is of considerable interest for applications in nanoscale plasmonic circuits.     

Common Metal-Dielectric-Metal Nanocavities for Multispectral Narrowband Light Absorption

We present a method to theoretically achieve multispectral narrowband light absorption in common metal-dielectric-metal nanocavities. Polarization-independent and wide-angle multi-band light absorption with absorbance up to 99 % is achieved owing to the excitation of multiple localized plasmon cavity modes. Strong interactions between plasmon resonances and photonic modes are further introduced for achieving sharp absorption spectrum with sub-10-nm bandwidth via a high-index dielectric spacer with a thickness exceeding λ/2. These findings can offer new perspectives for multispectral nano-optics devices including perfect light absorbers and subtractive polychromatic filters.     

Large and Ultrafast Optical Response of a One-Dimensional Plasmonic–Photonic Cavity

The resonant coupling of a localized surface plasmon mode and a cavity mode in a photonic crystal has been recently shown to strongly tailor the stationary optical response of gold nanoparticles. Here, we demonstrate that this can be further exploited for controlling light on an ultrashort time scale. The stationary and ultrafast optical responses of such a plasmonic–photonic cavity are investigated numerically. We show that the transient photo-induced change of the optical transmittance of a bare nanocomposite thin film can be amplified up to 60 times once resonantly coupled to the cavity mode in the hybrid device, despite the degradation of this mode due to absorption losses. In addition, different all-optical, ultrafast, efficient, and reversible photonic functions (increase or decrease of the signal intensity, transient spectral shift of the cavity mode) can be achieved depending on the spectral position of the transmitted mode tuned by varying the angle of incidence. The transient modification of the signal intensity is predicted to reach about 300 % after a subpicosecond rise time when the defect mode matches the plasmon resonance.     

Broadband Plasmon-Induced Transparency in Plasmonic Metasurfaces Based on Bright-Dark-Bright Mode Coupling

Plasmon-induced transparency (PIT), an analog of electromagnetically induced transparency, originates from destructive interference of plasmonic resonators with different quality factors and brings about the extreme dispersion within the narrow transparency window, promising remarkable potential for slow light, nonlinear optics and biochemical sensors. However, sometimes a broad transmission frequency band is more desirable for other applications such as bandpass filters. In general, strong coupling between bright and dark plasmon modes in coupled resonant systems leads to wide transparency bandwidth at the PIT resonance. Based on multi-oscillator coupling theory, a metasurface structure consisting of three perpendicularly connected metallic nanobars is purposefully designed and numerically demonstrated to support broadband PIT spectral response. The near-field patterns indicate that the broadening of the transparent band results from the constructive interference of dual excitations of the single non-radiative (dark) resonator by the two radiative (bright) antennas. These results show that this scheme of bright-dark-bright mode coupling is significantly beneficial for designing filters operating over a broad frequency range.

     

Magnetic Coupling Effect on Nonlinear Properties in Negative Index Metamaterials with a Kerr Nonlinearity

We investigate magnetic coupling effect on nonlinear electromagnetic properties in a three-dimensional negative index metamaterial constituted by arrays of conducting wires and split-ring resonators embedded into a Kerr nonlinear dielectric. Numerical results show that the switches of nonlinear electromagnetic properties between right-handed and left-handed properties depend closely on magnetic coupling strength, which can be divided into several different coupling regions according to the angular frequency of incident light and the nonlinear types (focused or defocused) of the dielectric. These properties may be instructive for designing optimizely composite metamaterials with negative refraction and provide various routes to manipulating light.     

Influence of Anisotropy on Optical Bistability in Plasmonic Nanoparticles with Cylindrical Symmetry

In this paper, the effect of radial anisotropy on optical bistability in the cylindrical nanoshells is theoretically investigated within the quasi-static approximation. We consider two cases: when the shell is anisotropic and the core is nonlinear metal and when the core is anisotropic and the shell is a nonlinear metal. The dependence of optical bistability on the size of the nonlinear/anisotropic shell or core, the embedding medium, the anisotropy parameter, and the type of noble metals as candidates for plasmonics is studied and demonstrated. We show that by changing the type of the plasmonic metal, the switching threshold field changes can be used to design nanoparticle-based all-optical sensors. It is also shown that significant optical bistability and all-optical switching behavior can be obtained in the cylindrical nanoshells due to nonlinearity enhancement via the plasmonic structure.     

Tunable Scattering Cancellation of Light Using Anisotropic Cylindrical Cavities

Engineered core-shell cylinders are good candidates for applications in invisibility and cloaking. In particular, hyperbolic nanotubes demonstrate tunable ultra-low scattering cross section in the visible spectral range. In this work, we investigate the limits of validity of the condition for invisibility, which was shown to rely on reaching an epsilon near zero in one of the components of the effective permittivity tensor of the anisotropic metamaterial cavity. For incident light polarized perpendicularly to the scatterer axis, critical deviations are found in low-birefringent arrangements and also with high-index cores. We suggest that the ability of anisotropic metallodielectric nanocavities to dramatically reduce the scattered light is associated with a multiple Fano-resonance phenomenon. We extensively explore such resonant effect to identify tunable windows of invisibility.     

Tailoring Plasmon Lifetime in Suspended Nanoantenna Arrays for High-Performance Plasmon Sensing

We demonstrate significantly longer plasmon lifetime and stronger electric field enhancement by lifting the nanoantenna arrays above the substrate by dielectric nanopillars. The role of the pillar is to offer a more homogeneous dielectric background allowing stronger diffraction coupling among plasmonic nanoantennas leading to a Fanolike asymmetric lineshape. It is found that the electric fields around the nanoantennas can be greatly enhanced when the Fanolike resonance is excited, and a 4.2 times enhancement is achieved compared with the pure resonance in individual nanoantennas. Furthermore, only a collective surface mode with its electric fields of the same direction as the induced electric moment in the nanoantennas could mediate the excitation of such a Fanolike resonance. More importantly, the sensitivity and the figure of merit (FOM) of this plasmonic structure can reach as high as 900 nm/RIU and 53, respectively. Our study offers a new, simple, and efficient way to design the plasmonic systems with desired electric field enhancement and spectral lineshape for different applications.     

A Tunable Slow Light Device with Multiple Channels Based on Plasmon-Induced Transparency

Slow light devices with buffering capability play a critical role in all-optical signal processing. In this paper, multiple slow light phenomena are implemented based on plasmon-induced transparency (PIT) in our device. The device mainly consists of dual tooth cavities coupled with stub resonators, respectively. Temporal coupled-mode theory model illustrates that the triple PIT phenomena can be achieved based on different formation mechanisms. The simulation results calculated by the finite-difference time-domain method reveal that significant slow light response occurs at two wavelength regions. In addition, the parameters of structure have an important influence on PIT response and slow light characteristics. Moreover, the separate manipulation of wavelength, transmission and group index at transparency peak can be achieved in different slow light channels by adjusting the structural parameters. This plasmonic device is of great significance for the design of optical networks on chips.

     

Polarized fluorescent emission from probes near dielectric interfaces

The electromagnetic field surrounding and emitted by a dipolar molecular probe very near to a dielectric interface is the sum of the real dipole field and the field of the image dipole induced inside the dielectric interface. The total charge distribution, made up of the real and image dipoles in close proximity to each other, approximates a quadrupole distribution and emits a light intensity pattern similar to that of an oscillating electric quadrupole. The electromagnetic field emitted by this system contains information that can be directly related to the spatial and orientational distributions of the dipole near the interface. Experimental methods are discussed that utilize this system for determining the spatial and orientational distribution of fluorescent probes in biological material.     

Resonant TE Transmission Through a Continuous Metal Film: Perspectives for Low-Loss Plasmonic Elements

Whereas resonant transverse magnetic transmission across an undulated continuous metal film is achieved with the mediation of plasmon modes excited by the undulation, it is shown here that transverse electric (TE) resonant transmission through a continuous metal film can also be achieved with the mediation of the second-order TE₁ mode of a dielectric slab waveguide having the metal film sandwiched at its middle. The demonstration is made by using the materials currently used in the domain of optical security and counterfeit deterrence: ZnS is shown to possibly be a lossless interface/adhesion layer between a polymer and a noble metal for plasmonic resonant elements.     

Development of Whispering Gallery Mode Polymeric Micro-optical Electric Field Sensors

Optical modes of dielectric micro-cavities have received significant attention in recent years for their potential in a broad range of applications. The optical modes are frequently referred to as "whispering gallery modes" (WGM) or "morphology dependent resonances" (MDR) and exhibit high optical quality factors. Some proposed applications of micro-cavity optical resonators are in spectroscopy¹, micro-cavity laser technology², optical communications^3-6 as well as sensor technology. The WGM-based sensor applications include those in biology⁷, trace gas detection⁸, and impurity detection in liquids⁹. Mechanical sensors based on microsphere resonators have also been proposed, including those for force^10,11, pressure¹², acceleration¹³ and wall shear stress¹⁴. In the present, we demonstrate a WGM-based electric field sensor, which builds on our previous studies^15,16. A candidate application of this sensor is in the detection of neuronal action potential.The electric field sensor is based on polymeric multi-layered dielectric microspheres. The external electric field induces surface and body forces on the spheres (electrostriction effect) leading to elastic deformation. This change in the morphology of the spheres, leads to shifts in the WGM. The electric field-induced WGM shifts are interrogated by exciting the optical modes of the spheres by laser light. Light from a distributed feedback (DFB) laser (nominal wavelength of ~ 1.3 μm) is side-coupled into the microspheres using a tapered section of a single mode optical fiber. The base material of the spheres is polydimethylsiloxane (PDMS). Three microsphere geometries are used: (1) PDMS sphere with a 60:1 volumetric ratio of base-to-curing agent mixture, (2) multi layer sphere with 60:1 PDMS core, in order to increase the dielectric constant of the sphere, a middle layer of 60:1 PDMS that is mixed with varying amounts (2% to 10% by volume) of barium titanate and an outer layer of 60:1 PDMS and (3) solid silica sphere coated with a thin layer of uncured PDMS base. In each type of sensor, laser light from the tapered fiber is coupled into the outermost layer that provides high optical quality factor WGM (Q ~ 10⁶). The microspheres are poled for several hours at electric fields of ~ 1 MV/m to increase their sensitivity to electric field.     

Magnetic Excitations in Silver Nanocrescents at Visible and Ultraviolet Frequencies

We study magnetic excitations at optical frequencies in disordered crescent-form split-ring resonators made of silver. The resonators that are less than 100 nm in diameter are fabricated on a 4 in. quartz wafer by using a simple, fast, and inexpensive fabrication technique. The measured transmission and polarization-rotation spectra of the resonators reveal the excitation of circulating electric currents that give rise to magnetic dipole moments in the structures at visible and ultraviolet light frequencies. These frequencies have higher values than the limiting magnetic-resonance frequency predicted by the conventional LC resonance model presumably due to the plasmonic nature of the excitations.     

MICROTUBULES IN BIOLOGICAL CELLS AS CIRCULAR WAVEGUIDES AND RESONATORS

The microtubules in the cellular cytoskeleton have a fundamental role in the living processes of biological cells. They are hollow cylinders which resemble circular waveguides or cylindrical resonators. The cutoff and resonant frequencies of the transverse magnetic and transverse electric modes of the microtubule cavities are in the band of soft x-rays. This suggests the possibility of interaction of electromagnetic cavity modes with inner electrons in atoms (e.g., in carbon, nitrogen, and oxygen). Biological cells (e.g., the yeast cells of spherical shape) may also represent cavity resonators. In this case, the resonant frequencies may be in the infrared region.     

Metal-Dielectric-Graphene Hybrid Heterostructures with Enhanced Surface Plasmon Resonance Sensitivity Based on Amplitude and Phase Measurements

Metal-dielectric-graphene hybrid heterostructures based on oxides Al₂O₃, HfO₂, and ZrO₂ as well as on complementary metal–oxide–semiconductor compatible dielectric Si₃N₄ covering plasmonic metals Cu and Ag have been fabricated and studied. We show that the characteristics of these heterostructures are important for surface plasmon resonance biosensing (such as minimum reflectivity, sharp phase changes, resonance full width at half minimum and resonance sensitivity to refractive index unit (RIU) changes) can be significantly improved by adding dielectric/graphene layers. We demonstrate maximum plasmon resonance spectral sensitivity of more than 30,000 nm/RIU for Cu/Al₂O₃ (ZrO₂, Si₃N₄), Ag/Si₃N₄ bilayers and Cu/dielectric/graphene three-layers for near-infrared wavelengths. The sensitivities of the fabricated heterostructures were?~?5–8 times higher than those of bare Cu or Ag thin films. We also found that the width of the plasmon resonance reflectivity curves can be reduced by adding dielectric/graphene layers. An unexpected blueshift of the plasmon resonance spectral position was observed after covering noble metals with high-index dielectric/graphene heterostructures. We suggest that the observed blueshift and a large enhancement of surface plasmon resonance sensitivity in metal-dielectric-graphene hybrid heterostructures are produced by stationary surface dipoles which generate a strong electric field concentrated at the very thin top dielectric/graphene layer.

     

Effective Coupling of Incident Light Through an Air Region into an S-Shape Plasmonic Ag Nanowire Waveguide with Relatively Long Propagation Length

Coupling of incident light through an air region into an S-shape silver (Ag) plasmonic nanowire waveguide (SSAPNW) is a highly difficult challenge of light guiding on the surface of metal nanowire. In this paper, we numerically analyze the coupling effect of an SSAPNW which is covered by a dielectric medium using a finite element method. The coupling effect can be modulated by adjusting the Ag nanowire diameter and the covering dielectric medium width and wavelength of incident light, and the propagation length of surface plasmon (SP) coupling can be maximized. Simulation results reveal that the field confinement can be significantly improved and the majority of the electric field can be carried on the surface of a bending Ag nanowire. The effect of electric field transport along an SSAPNW due to SP coupling and Fabry-Perot resonance is investigated for different dimensions and lengths. Accordingly, long propagation lengths of about 41.5 μm for 10?×?SSAPNW at an incident wavelength of 810 nm and longer propagation length can be achieved if more sections of an SSAPNW are used. Simulation results offer an efficient method for optimizing SP coupling into bending metal nanowire waveguides and promote the realization of highly integrated plasmonic devices.     

Tunable Triple-Band Plasmonically Induced Transparency Effects Based on Double π-Shaped Metamaterial Resonators

Tunable triple-peaks with the transmission intensity of more than 90% plasmonically induced transparency metamaterial resonator based on nested double π-shaped metallic structure is proposed at the terahertz frequency region, which is consisted of three sets of gold nanorods with different sizes placed on a dielectric substrate of SiO₂. The coupling effect of localized electric field between different parts of the proposed structure can be used to explain the physical mechanism of three transparent windows. The finite-difference time-domain (FDTD) is used to study the spectral properties of the proposed structure, and the influence of the size of the nanorods and the relative distance between them on the spectral characteristics are also discussed. It can be seen that some obvious shift phenomena occur in the spectra with the change of these nanorods. These results indicate that the proposed structure opens up new avenues in many related applications, especially for multi-channel filters, optical switches, and sensors.