Enhanced Optical Transmission Assisted Near-Infrared Plasmonic Optical Filter via Hybrid Subwavelength Structures

Plasmonics - Enhanced optical transmission (EOT), which results from the incident light interacting with the subwavelength nanostructures patterned in a metallic film, showcases the excitation of...     

Extremely High Transmittance at Visible Wavelength Induced by Magnetic Resonance

A new quasi-3D structure composed of stacked double-layer subwavelength metal gratings is designed for magnetic resonance in the visible region. The coupling of two-layer gratings induces a type of magnetic plasmon propagation mode characterized by extraordinary optical transmission (EOT) with extremely high transmittance of up to 0.94 for transverse magnetic polarization. The results show that magnetic resonance is an effective method to enhance the transmittance and avoid much energy loss, one of the barriers for application in the visible region. The magnetic resonance or EOT is strongly dependent on the wavelength which can simply be tuned by the period of gratings. This work paves a way to designing metallic metamaterials that are magnetically active in the visible spectral ranges. In addition, the proposed structure can be easily constructed using nanofabrication.     

Chirality Switching Via Rotation of Bilayer Fourfold Meta-Structure

Chiral effects have been observed from the interaction of chiral plasmonics nanostructures with light. Such nanostructures enhance the chiral response of molecules and provide an ideal platform for biological and chemical sensing. Here, we investigate the chiral switching effects of an array of subwavelength nanostructures with a unit cell composed of four double-layered nanostrips arranged to be rotationally symmetric. We observe chiral switching leading to a change in circular dichroism (CD) signature when the mutual angle between the first and second layer increases from 0° to 90° with respect to each other. This mutual angle can be manipulated to switch the handedness of the nanostructure and cause a change in the outgoing light. We also investigated the field distribution of each mode when circularly polarized light is normally incident into the structure. These modes can be categorized into longitudinal and transverse modes depending on the orientation of their dipole moments. The mode studies clearly show the nature of each plasmonics mode.     

A Manipulated Extraordinary Optical Transmission Filter Composed with Subwavelength Hole Complex Arrays

We present a new style extraordinary optical transmission (EOT) nano optical filter combined by two kinds of subwavelength holes array on a gold film. In the design, a square array of non-penetrating holes (hollow holes) inlays into another square array of penetrating holes ordered by a central arrange mode. We numerically calculated the transmission spectra of the patterned gold films by finite-difference time-domain (FDTD) method. Results show that the transmission of the filter can be manipulated by changing the depth of non-penetrating holes. The (1, 1) peak can be enhanced when the incident light normally illuminates one side of the filter with the hollow holes, yet the (1, 1) peak can be suppressed when the light illuminates the other side without hollow holes. It also depicts that the hollow hole array results in energy level splitting of (1, 0) mode propagating on the surface of the filter. What’s more, the splitting can be eliminated by modulating the depth of the hollow holes. Our study further reveals the role of suface plasmon effect in the EOT.     

Extraordinary Transmission of Three-Dimensional Crescent-like Holes Arrays

We developed a method to fabricate a periodic array of three-dimensional crescent-like holes (3DCLH) via an inverted hemispherical colloidal lithography. It is found that there exists an extraordinary optical transmission in this non-planar perforated periodic array of 3DCLH when the electric field of the incident light is perpendicular to the cross-line of the crescent-like hole. This extraordinary optical peak is insensitive with the incident angles and sensitive with the angle between the electric field of the incident light to the cross-line of the 3DCLH. Numerical simulation based on finite-difference time-domain method reveals that this peak is caused by an asymmetric localized surface plasmon resonance. This structure might be useful for the optical sensing and optical-integrated circuits.     

Compact Color Filter and Polarizer of Bilayer Metallic Nanowire Grating Based on Surface Plasmon Resonances

Surface plasmon resonances on bilayer aluminum nanowire gratings are studied in both theory and experiment. It is found that there are two kinds of surface plasmon on the bilayer metallic gating: longitudinal aluminum/dielectric/aluminum slit and lateral aluminum/dielectric interface waveguide mode. The surface plasmon waveguide mode resonance in the slits makes the grating act as a transverse magnetic (TM)-passing polarizer. With the lateral waveguide mode resonance, certain wavelengths of the incident TM light are translated to aluminum/air or aluminum/substrate waveguide light, and the grating acts as a color filter. With both resonances, the bilayer nanowire grating can be a compact-integrated polarizer and color filter.     

Plasmonics Resonance Enhance Magneto-Optical Effects Through Metallic Sub-wavelength Grating with Bismuth Iron Garnet Slab

We theoretically investigated an enhancement of the magneto-optical Faraday rotations along with large transmittance in two multilayer structures. The shifts of the Faraday rotation peaks are more obvious than the transmittance peaks when the grating period is changed, which is beneficial to acquire the large Faraday rotation and transmittance. The Faraday rotation of tri-layer system is five times larger than the bilayer system, and the Faraday resonance peak can be mediated by changing the refractive index or thickness of the additional nonmagnetic dielectric layer (NDL) layer. These results are important for applications in highly integrated optoelectronic and magneto-optical devices.     

Nonlinear Nanowire Close to a Truncated Metallic Film: a Step Toward Nanosized Light Sources

A controllable nanosized light source based on nonlinear interaction of light and a semiconductor nanowire is proposed. Surface plasmon polariton (SPP) waves with different frequencies propagate along the upper and lower surfaces of a truncated metallic film and are scattered at its end face. A nanowire, in that vicinity, is pumped by the scattered light, and new harmonics are generated via second-order nonlinear optical effects. Green's function surface integral equation method is exploited to numerically calculate the electric field, the magnetic field, and the power of the generated frequency components. Results show that the power of the generated harmonics depends on the position and radius of the nanowire, thickness of the metallic film, as well as the wavelength of the incident SPP waves. On the other hand, by controlling the phase difference between incident SPP waves having the same frequencies, it is possible to manipulate the electric field pattern and also to change the power of the generated harmonics.     

Simultaneous Excitation of Surface Plasmon Polaritons on Both Interfaces in an Asymmetric Dielectric-Metal Corrugated Structure

The simultaneous excitation of plasmon polaritons on both surfaces of metal film was studied for asymmetric dielectric-metal-dielectric corrugated structures. Due to the small resonant absorption of the incident light on the transmission side of the structure, we investigated the enhancement of the surface plasmon polaritons on the mentioned side by controlling the structure parameters. When the illuminate light changes from normal incidence to non-normal incidence, the resonant absorption peak splits into a doublet. The simultaneous excitation of surface plasmon polaritons on both surfaces of the metal film can be achieved by controlling the incident angle. Since the wave vector matching condition is not satisfied, there is no coupling between the plasmon polaritons modes on the two surfaces of the corrugated metallic film. The excitation and control of the non-coupled surface plasmon polartions simultaneously propagating on the different interfaces of one metallic film have potential applications for designing novel compact and tunable nano-photonic devices at visible frequency.     

Effect of Boundary Condition and Periodical Extension on Transmission Characteristics of Terahertz Filters with Periodical Hole Array Structure Fabricated on Aluminum Slab

Terahertz (THz) filters based on extraordinary optical transmission from periodical hole array structures fabricated on aluminum slab have been experimentally investigated by using THz time-domain spectroscopy. The incident THz pulses with frequency from 0.1 to 2.7 THz could be partly filtered, and the central peak was at ～0.26. The high frequency signal could be observed to decrease, especially for the frequency above ～1 THz. Moreover, the transmission peak from small-size sample with less hole arrays shifts to high frequency at ～0.53 THz due to both the effects of boundary condition and insufficient periodical extension. Furthermore, finite element method with surface plasmon polariton theory is employed to analyze this extraordinary optical transmission and filter phenomena.     

A New Angle on Second Harmonic Generation from a Small Hole in a Metal Film

Plasmonics - Bethe’s theory treats a subwavelength aperture in a metal film as the combination of a parallel magnetic dipole and transverse electric dipole. For linear optics, this gives the...     

覆膜开孔土壤蒸发实验研究

为获得灌后覆膜开孔蒸发过程中土壤含水率分布的动态资料,研制了室内垂直一维入渗-覆膜开孔蒸发动态观测实验系统,并进行了不同覆膜开孔率和不同入渗定额下的覆膜开孔蒸发实验.根据实测资料分析了土表覆膜开孔率和入渗定额不同条件下,蒸发土壤含水率剖面的时间变化和蒸发结束的含水率剖面特征.累积蒸发量随时间变化趋势表明,两者关系符合Gardner理论关系,且该理论关系系数与不同覆膜开孔率和入渗定额存在进一步的函数关系,从而得出覆膜开孔率和入渗定额影响下的累积蒸发量随时间变化多因子模型.同时,采用类似的方法分析了覆膜开孔率和入渗定额对相对累积蒸发量和单位膜孔面积累积蒸发量的影响,得出了定量函数关系,其拟合的相关系数均很高.     

Plasmon Enhanced Faraday Rotation in Fe3O4/Ag Ferrofluids for Magneto Optical Sensing Applications

The magnetic/metallic nanocomposites are special intelligent materials and find many applications in photonics, magneto-optical devices, and biosensors. This paper introduces the synthesis and performance of Fe₃O₄/Ag ferrofluids using a facile one-step modified co-precipitation method. The nanocomposites were characterized using X-ray diffraction, scanning electron microscopy, XPS, and Fourier transform infrared spectra for structure, morphology, and component investigation, respectively. The viscosity, stability, magnetic, and magneto-optical properties of the ferrofluids were measured through the vibrating sample magnetometer, Faraday rotation optical bench, and viscosity tester. Fe₃O₄/Ag-based ferrofluids demonstrated good stability (did not agglomerate for 120 h), big magnetic field-dependent-viscosity, surface plasmon resonance effect, superpagamagnetic nature, and improved Faraday effect; these characters are good for MO sensing application. The Verdet constant of ferrofluids at 633 nm increased from 5.66 to 8.53 × 10⁻² deg/G cm after Ag coating on Fe₃O₄.

     

Optical Fiber-Based Surface-Enhanced Raman Scattering Sensor Using Au Nanovoid Arrays

Plasmonic properties of gold nanovoid array substrates for fiber-based surface-enhanced Raman scattering (SERS) sensing are studied numerically and experimentally. In the nanovoid arrays, each void has openings on both sides, bottom hole facing the fiber tip for introducing incident light and collecting scattered light and the top hole exposed to the analyte solution for interrogating analyte molecules in the voids. Electromagnetic field modes are confined strongly in and around these nanovoids, acting as localized plasmon resonators. The enhanced electric field extends hundreds of nanometers into the voids, resulting in a large SERS-active zone several orders of magnitude larger than nanoparticle-based structures. The effect of structural parameters of the nanovoid arrays, including void diameter, Au film thickness, and bottom hole diameter, on electric field confinement in the voids is investigated using three-dimensional finite difference time domain simulation. Au nanovoid arrays are fabricated using a scalable, inexpensive nanosphere lithography fabrication method. The largest SERS signal is realized by tuning the localized plasmon resonance peak of Au nanovoid arrays to the laser excitation wavelength. Multiplexed detection capability with the fiber-based SERS sensor using Au nanovoid arrays is demonstrated by measuring the Raman spectrum of a mixture solution of diethylthiatricarbocyanine and crystal violet.     

Characterization of Grating Coupled Surface Plasmon Polaritons Using Diffracted Rays Transmittance

A method to sense the excitation of surface plasmon polariton (SPP) on metallic grating device using the transmitted signal will be presented. The grating transmittance signal will be fully characterized varying the light incident angle and azimuthal grating orientation by means of the SPP vector model and rigorous coupled-wave analysis simulation. Simulation results will be compared with experimental measurements obtained with a 635 nm wavelength laser in the transverse magnetic polarization mode. The laser will light grating devices in contact with either air or water through a customized microfluidic chamber. A characterization of the diffracted rays will show the relationship between the grating coupling configuration and the Kretschmann one. In fact, the diffracted ray affected by SPP resonance is transmitted with an output angle which is the same incident angle that should be used to excite SPP in Kretschmann configuration. Lastly, the grating parameters (amplitude and metal thickness) impact on transmittance signal will be analyzed with respect to the order zero reflectance signal.     

Optical Absorption Enhancement in Freestanding GaAs Thin Film Nanopyramid Arrays

Although III–V compound semiconductor multi‐junction cells show the highest efficiency among all types of solar cells, their cost is quite high due to expensive substrates, long epitaxial growth and complex balance of system components. To reduce the cost, ultra‐thin films with advanced light management are desired. Here effective light trapping in freestanding thin film nanopyramid arrays is demonstrated and multiple‐times light path enhancement is realized, where only 160 nm thick GaAs with nanopyramid structures is equivalent to a 1 μm thick planar film. The GaAs nanopyramids are fabricated using a combination of nanosphere lithography, nanopyramid metal organic chemical vapor deposition (MOCVD) growth, and gas‐phase substrate removal processes. Excellent optical absorption is demonstrated over a broad range of wavelengths, at various incident angles and at large‐curvature bending. Compared to an equally thick planar control film, the overall number of photons absorbed is increased by about 100% at various incident angles due to significant antireflection and light trapping effects. By implementing these nanopyramid structures, III–V material usage and deposition time can be significantly reduced to produce high‐efficiency, low‐cost thin film III–V solar cells.     

Multiple-Wavelength Focusing and Demultiplexing Plasmonic Lens Based on Asymmetric Nanoslit Arrays

A multiple-wavelength focusing and demultiplexing plasmonic lens based on asymmetric nanoslit arrays is designed. The nanoslit arrays are perforated in a gold film and act as metal–insulator–metal plasmonic waveguides. By manipulating the widths of the slit arrays, the plasmonic lens can concentrate two incident plane wave beams to two separated focal points corresponding to their wavelengths. The full wave simulation is performed to verify the designed lens. This work provides a way to design more compact and integrated wavelength-division multiplexing plasmonic devices for nanophotonic communication and spectral imaging.     

Optical Response of Plasmonic Nanohole Arrays: Comparison of Square and Hexagonal Lattices

Nanohole arrays in metal films allow extraordinary optical transmission (EOT); the phenomenon is highly advantageous for biosensing applications. In this article, we theoretically investigate the performance of refractive index sensors, utilizing square and hexagonal arrays of nanoholes, that can monitor the spectral position of EOT signals. We present near- and far-field characteristics of the aperture arrays and investigate the influence of geometrical device parameters in detail. We numerically compare the refractive index sensitivities of the two lattice geometries and show that the hexagonal array supports larger figure-of-merit values due to its sharper EOT response. Furthermore, the presence of a thin dielectric film that covers the gold surface and mimics a biomolecular layer causes larger spectral shifts within the EOT resonance for the hexagonal array. We also investigate the dependence of the transmission responses on hole radius and demonstrate that hexagonal lattice is highly promising for applications demanding strong light transmission.     

Spatial Mode Selection by the Phase Modulation of Subwavelength Plasmonic Grating

The extraordinary transmission of the subwavelength gold grating has been investigated by the rigorous coupled-wave analysis and verified by the metal–insulator–metal plasmonic waveguide method. The physical mechanisms of the extraordinary transmission are characterized as the excitation of the surface plasmon polariton modes. The subwavelength grating integrated with the distributed Bragg reflector is proposed to modulate the phase to realize spatial mode selection, which is prospected to be applied for transverse mode selection in the vertical cavity surface-emitting laser.     

Design Method for Light Absorption Enhancement in Ultra-Thin Film Organic Solar Cells with the Metallic Nanoparticles

In this paper, a method is presented for designing the parameters of metallic nanoparticles introduced into ultra-thin film organic solar cells (OSCs) to improve the light absorption. On the basis of Mie theory, a relationship is setup between the scattering efficiency of localized surface plasmon resonance and the size parameter of metallic nanoparticles, by which metallic nanoparticles with optimal size can be designed to realize the highest ratio of resonant scattering to resonant absorption, thus light absorption enhancement of OSCs is maximized. By taking spherical Ag nanoparticles into an OSC system with an active layer of poly(3-hexylthiophene) and [6, 6]-phenyl-C61-butyric acid methyl ester as subject, light absorption increase of 26 % at an average wavelength of incident light is demonstrated. This design method is also applicable to other types of OSCs.