Alastair P. Hibbins

Surface-topography-induced enhanced transmission and directivity of microwave radiation through a subwavelength circular metal aperture

Strongly enhanced transmission of microwave radiation (λ0∼5 mm) is observed through a single subwavelength circular aperture of diameter d=2.5 mm in a metallic plate. The phenomenon is caused by resonant excitation of electromagnetic surface waves supported by four concentric grooves surrounding the aperture on the illuminated side of the sample. It is also shown that similar surface patterning on the output face of the sample results in very strong angular confinement (directivity) of the transmitted beam. A finite element code is used to investigate the electromagnetic fields on both the illuminated and the exit side of the structure, the predictions from which show excellent agreement with the experimental results.

Selective transmission through very deep zero-order metallic gratings at microwave frequencies

Zero-order metal grating structures are found to give extraordinary selective transmission at microwave frequencies through the resonant excitation of coupled surface waves. The metal slat structures with dielectric spacings as small as 250 μm strongly transmit wavelengths of several millimeters. A simple interpretation of these novel results which treats the deep grating structures as “filled” Fabry–Perot cavity systems gives model transmissivities which agree very well with the experimental data.

Gratingless enhanced microwave transmission through a subwavelength aperture in a thick metal plate

Remarkably enhanced transmission of microwave radiation through a single subwavelength slit in a thick metallic substrate surrounded by just a pair of parallel deep and narrow grooves is recorded. By also patterning the output face of the metal slab with two grooves there is strong exit beam confinement. There are no gratings in this structure and, hence, the transmission mechanism is not related to the conventional grating coupling of surface plasmons on the upper and lower surfaces of the substrate. Instead, the slit and the four grooves are all resonant, which is the essence of the functioning of the arrangement. The enhancement is due to the collective excitation of the Fabry–Perot mode in the slit and the cavity modes in the grooves. A finite-element modeling code is used to optimize the response of the structure, and to investigate the electromagnetic fields in the vicinity of the substrate.

The resonant electromagnetic fields of an array of metallic slits acting as Fabry-Perot cavities

Fabry-Perot cavities are perhaps the best known of the optical transmission resonators, with cavity field enhancement accomplished by two parallel and partially reflecting planes. Recently it has been shown that arrays of narrow slits cut into a metal substrate are similarly able to exhibit resonant transmission modes. An analysis of the field solutions and transmission properties of this resonant array is compared to the well-known etalon and dielectric slab geometries, revealing a most elegant illustration of the principles of Maxwell’s electromagnetism. It is demonstrated that the matching of the propagating field to each slit-cavity mode is made possible through strong diffraction at each end. Furthermore, the interface between the slit cavities and semi-infinite space beyond acts as a high-impedance surface on resonance, reflecting the field with a positive reflection-amplitude coefficient. Metallic slit arrays have several advantages over conventional Fabry-Perot resonators with interesting applicat...

Grating-coupled surface plasmons at microwave frequencies

This work presents a detailed investigation of electromagnetic coupling to the surface plasmon-polariton (SPP) at microwave frequencies. We have recorded the wavelength-dependent reflectivity from a metallic sinusoidal diffraction grating of pitch 15 mm. In order to minimize the problems associated with nonplanar incident wavefronts, we have developed an apparatus that collimates the incident beam. We illustrate resonant coupling to the SPP at wavelengths of the order of 10 mm. The wavelength-dependent reflectivities recorded have been successfully fitted using a differential formalism of conical diffraction with a single set of grating parameters describing the grating profile and metal permittivity.

Circuit modeling of the transmissivity of stacked two-dimensional metallic meshes.

This paper presents a simple analytical circuit-like model to study the transmission of electromagnetic waves through stacked two-dimensional (2-D) conducting meshes. When possible the application of this methodology is very convenient since it provides a straightforward rationale to understand the physical mechanisms behind measured and computed transmission spectra of complex geometries. Also, the disposal of closed-form expressions for the circuit parameters makes the computation effort required by this approach almost negligible. The model is tested by proper comparison with previously obtained numerical and experimental results. The experimental results are explained in terms of the behavior of a finite number of strongly coupled Fabry-Pérot resonators. The number of transmission peaks within a transmission band is equal to the number of resonators. The approximate resonance frequencies of the first and last transmission peaks are obtained from the analysis of an infinite structure of periodically stacked resonators, along with the analytical expressions for the lower and upper limits of the pass-band based on the circuit model.

Surface plasmon-polariton study of the optical dielectric function of titanium nitride

Abstract This work presents the first detailed study of the optical dielectric function of optically thick TiN x films using grating coupling of radiation to surface plasmon-polaritons. Angle-dependent reflectivities are obtained in the wavelength range 500–875 nm and by comparison with grating modelling theory, we determine both the imaginary and the real parts of the dielectric function. This method provides an alternative to traditional characterization techniques (e.g. Kramers-Kronig analysis) that may require additional information about film thickness, or the samples optical properties in other parts of the electromagnetic spectrum. We have fitted the determined dielectric function to a model based on a combination of interband absorptions and free-electron response evaluating both the plasma energy and the relaxation time.

Enhanced microwave transmission through a single subwavelength aperture surrounded by concentric grooves

Excitation of bound surface waves on textured metallic structures can lead to strong resonant absorption of incident radiation at frequencies determined by the surface profile. In the present study however, attention is turned to the role of the surface structure in the enhancement of transmission through a circular, subwavelength-diameter aperture. Undertaking the experiment at microwave wavelengths allows for a precision of manufacture and optimization of the surface structure that would be difficult to replicate at optical frequencies, and demonstrates that transmission enhancement may be achieved with near-perfect metals. Further, the use of a finite element method computational model to study the electromagnetic response of the sample allows for the fields associated with transmission enhancement to be examined, thereby obtaining a better understanding of the role of the surface profile in the enhancement mechanism.

Thin resonant structures for angle and polarization independent microwave absorption

We present a microwave absorbing structure comprised of an array of subwavelength radius copper disks, closely spaced from a ground plane by a low loss dielectric. Experiments and accompanying modeling demonstrate that this structure supports electromagnetic standing wave resonances associated with a cylindrical cavity formed by the volume immediately beneath each metal disk. Microwave absorption on resonance of these modes, at wavelengths much greater than the thickness of the structure, is dictated almost entirely by the radius of the disk and permittivity of the dielectric, being largely independent of the incident angle and polarization.

Excitation of remarkably nondispersive surface plasmons on a nondiffracting, dual-pitch metal grating

A nondiffracting metallic lamellar grating formed from three equally spaced grooves per repeat period, with one being slightly shallower than the other two is examined at microwave frequencies. When filled with a slightly lossy dielectric, this structure supports a remarkably nondispersive surface plasmon polariton mode, which exhibits strong selective absorption of incident power. Measured reflectivities show excellent agreement with the results predicted by a rigorous coupled wave theory.