Metallic Subwavelength Apertures Research Articles

Evolution dynamics of optical angular momentum and torque through a birefringent metallic subwavelength aperture

The interaction of light and matter results in the transfer of linear momentum and angular momentum (AM) which induces optical force and torque, respectively. It is revealed that the linear momentum and AM are conserved for a system of matter and electromagnetic field. In this paper, we present a waveguide-mode analysis to show the dynamic evolution rules for both longitudinal and transverse angular momenta in a metallic subwavelength aperture with different symmetries. In addition, we demonstrate the relationship between AM and optical torque that is based on the AM conservation of light and the metallic subwavelength aperture. Accompanying with the change of AM is the existence of non-zero optical torque, which is promising for dynamic optical manipulation in nanoscale. We believe that unveiling such phenomenon is conducive to the further development of the angular-momentum-based optical mechanics in subwavelength aperture.

Microwave Bandpass Filter Based on Mie-Resonance Extraordinary Transmission.

Microwave bandpass filter structure has been designed and fabricated by filling the periodically metallic apertures with dielectric particles. The microwave cannot transmit through the metallic subwavelength apertures. By filling the metallic apertures with dielectric particles, a transmission passband with insertion loss 2 dB appears at the frequency of 10–12 GHz. Both simulated and experimental results show that the passband is induced by the Mie resonance of the dielectric particles. In addition, the passband frequency can be tuned by the size and the permittivity of the dielectric particles. This approach is suitable to fabricate the microwave bandpass filters.

Hollow waveguides as polarization converting elements: a theoretical study

Subwavelength apertures in a metallic film act as hollow waveguides. By using a non-quadratic cross-section, an anisotropic transmission behaviour results for the two polarization states. Thus, an array of metallic subwavelength apertures may be used as polarization converter, e.g., as a half-wave plate. By varying orientation and shape of the cross-sections locally, one can design polarization shifting elements for complex wave fields. Here, we present a theoretical consideration on the physical properties and compare with dielectric form birefringence.

Total broadband transmission of microwaves through a subwavelength aperture by localized E-field coupling of split-ring resonators.

Resonance coupling of two resonators with the same resonant frequency is a highly efficient energy transfer approach in physics. Here we report total broadband transmission of microwaves through a metallic subwavelength aperture using the coupled resonances of the strongly localized electric fields at the gaps of two split-ring resonators (SRRs) placed on either side of the aperture. At the center frequency of the broad band, the phase difference between the two localized time-varying electric fields is 90°, which is consistent with the critical coupling state that is a sufficient condition for the two-resonator system to realize total transmission if the resonators are assumed to be lossless.

Molecular recognition with nanostructures fabricated by photopolymerization within metallic subwavelength apertures.

The first demonstration of fabrication of submicron lateral resolution molecularly imprinted polymer (MIP) patterns by photoinduced local polymerization within metal subwavelength apertures is reported. The size of the photopolymerized MIP features is finely tuned by the dose of 532 nm radiation. Rhodamine 123 (R123) has been selected as a fluorescent model template to prove the recognition capability of the MIP nanostructures, which has been evaluated by fluorescence lifetime imaging microscopy (FLIM) with single photon timing measurements. The binding selectivity provided by the imprinting effect has been confirmed in the presence of compounds structurally related to R123. These results pave the way to the development of nanomaterial architectures with biomimetic artificial recognition properties for environmental, clinical and food testing.

Doubly resonant Ag–LiNbO_3 embedded coaxial nanostructure for high second-order nonlinear conversion

Coaxial nanopatterned lithium niobate embedded in Ag enables a large electromagnetic confinement in small-volume cavities. The nonlinear material filling these cavities is at the origin of high second-order nonlinear conversion, where no phase matching is needed. A doubly resonant optical spectrum of the linear response is required in order to boost this phenomenon, providing a promising and stable second-harmonic (SH) device tailored for any desired wavelength. The structure is fabricated by using electron-beam lithography, dry etching, and chemical mechanical polishing. We report a second-harmonic generation (SHG) enhancement factor of 26 compared to unpatterned x-cut lithium niobate wafer at λpump=1550  nm. The enhancement, which comes exclusively from the nanostructured lithium niobate, is experimentally and theoretically demonstrated. A comparison with three types of metallic subwavelength apertures is shown. The embedded structure shows the strongest SH signal. Analysis of the strong dependence of the incident polarization on the SHG intensity with a homemade 3D–NL–FDTD algorithm shows good agreement with experimental data.

Plasmon-Assisted Optical Curtains

We predict an optical curtain effect, i.e., formation of a spatially invariant light field as light emerges from a set of periodic metallic nano-objects. The underlying physical mechanism of generation of this unique optical curtain can be explained in both the spatial domain and the wave-vector domain. In particular, in each period, we use one metallic nanostrip to equate the amplitudes of lights impinging on the openings of two metallic nanoslits and also shift their phases by π difference. We elaborate the influence on the output effect from some geometrical parameters like the periodicity, the slit height, and so on. By controlling the light illuminated on metallic subwavelength apertures, it is practical to generate optical curtains of arbitrary forms, which may open new routes of plasmonic nanolithography.

Field enhancement in metallic subwavelength aperture arrays probed by erbium upconversion luminescence

Upconversion luminescence from erbium ions placed in the near field of subwavelength aperture arrays is used to investigate field enhancement of incident near-infrared light in such nanostructures. We study field enhancement due to the excitation of both propagating and localized surface plasmon resonances in arrays of square and annular apertures in a Au film. The conversion of 1480 nm excitation light to 980 nm emission is shown to be enhanced up to a factor 450 through a subwavelength hole array. The effects of array periodicity and aperture size are investigated. It is shown that a Fano model can describe both far-field transmission and near-field intensity. The upconversion enhancement reveals the wavelength and linewidth of the surface plasmon modes that are responsible for extraordinary transmission in such arrays. Angle-dependent measurements on annular aperture arrays prove that the field enhancement due to localized resonances is independent of the incident angle. These experiments provide insight in the mechanisms responsible for extraordinary transmission and are important for applications that aim to exploit near-field enhancement in nanostructured metal films.

Generation of an Axially Asymmetric Bessel-Like Beam from a Metallic Subwavelength Aperture

An electromagnetic nondiffractive Bessel-like beam from a subwavelength aperture is generated by placing a metallic circular gratinglike structure in front of the aperture. When the incident wave is linearly polarized, the beam is axially asymmetric. The beam possesses fluctuating, but approximately uniform, intensity distribution along its longitudinal axis. The full width at half maximum of the beam remains less than two wavelengths over nearly ten wavelengths. Our experimental results are in good agreement with the simulation results and analytical results.

Off-axis beaming from subwavelength apertures

Photonic crystal waveguides and metallic subwavelength apertures are promising tools for light manipulation. It is possible to obtain enhanced directional beams by using these structures via coupling to surface waves. In addition, these apertures can be designed to steer such directional beams by introducing asymmetrical gratings on the output surface. In the present paper, we report directional yet off-axis beaming from subwavelength apertures at microwave frequencies. The full width at half maximum of the beam is 10° while the beaming angle is 15°. Our results show that it is possible to steer the beam by the appropriate modification of the output surface.

Improved fabrication of zero-mode waveguides for single-molecule detection

Metallic subwavelength apertures can be used in epi-illumination fluorescence to achieve focal volume confinement. Because of the near field components inherent to small metallic structures, observation volumes are formed that are much smaller than the conventional diffraction limited volume attainable by high numerical aperture far field optics (circa a femtoliter). Observation volumes in the range of 10−4fl have been reported previously. Such apertures can be used for single-molecule detection at relatively high concentrations (up to 20μM) of fluorophores. Here, we present a novel fabrication of metallic subwavelength apertures in the visible range. Using a new electron beam lithography process, uniform arrays of such apertures can be manufactured efficiently in large numbers with diameters in the range of 60–100nm. The apertures were characterized by scanning electron microscopy, optical microscopy, focused ion beam cross sections/transmission electron microscopy, and fluorescence correlation spectroscopy measurements, which confirmed their geometry and optical confinement. Process throughput can be further increased using deep ultraviolet photolithography to replace electron beam lithography. This enables the production of aperture arrays in a high volume manufacturing environment.

Enhanced transmission and directivity from metallic subwavelength apertures with nonuniform and nonperiodic grooves

Nonuniform and nonperiodic grooves are used to enhance the transmission and directivity of emissions from a single metallic subwavelength aperture. By using nonuniform and nonperiodic grooves, the amplitude and phase of the diffracted power flow from each groove can be adjusted properly. As a result, the transmission and emission directivity can be further improved when compared to apertures with uniform and periodic grooves. Our experimental results are in good agreement with the finite difference time domain simulation results.