Matthew J. Lockyear

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.

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...

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.

Transmission of microwaves through a stepped subwavelength slit

The transmission of normally incident plane wave microwaves through a single stepped subwavelength slit in a thick metal plate is explored. The presence of the step substantially increases the radiation wavelength which may be resonantly transmitted to well beyond twice the plate thickness. Insight into the resonant behavior is provided through analysis of field solutions produced by a finite element model.

Tuning the polarization state of enhanced transmission in gratings

The polarization characteristics of enhanced transmission of lamellar gratings with structural dimensions on the subwavelength scale were studied and experimental results were compared to numerical models. The ability to tune the polarization state of the transmitted beam by varying the grating’s structural parameters is discussed. Gratings were fabricated and tested in the microwave spectral region, and the results were compared to theoretically modeled results. Enhanced transmission produced by cavity modes was experimentally verified for both s-polarized and p-polarized incident beams of light. Applications of these results to photonic devices in the visible, infrared, and microwave spectral regions are discussed.

Angle-independent microwave absorption by ultrathin microcavity arrays

The resonant absorption of microwave radiation by thin, two-dimensional microcavity arrays has been studied. Resonant modes associated with these structures, formed from copper-clad FR4 laminates, exhibit both an azimuthal and polar angle independent electromagnetic response. The experimental data agree well with the predictions of a finite element method computer model, which has been utilized to explore the electromagnetic character of the resonant modes supported.

Low angular-dispersion microwave absorption of a metal dual-period nondiffracting hexagonal grating

The microwave (11.3<λ0<16.7mm) reflectivity response of a nondiffracting dual-period hexagonal grating is explored. In three directions at 60° to each other, the aluminum grating has a repeat period of 7.2mm in which are three equally spaced grooves, one being slightly shallower than the other two. This dual-period (λg and λg∕3) structure exhibits strong microwave absorption at several different frequencies. In addition, some of the absorptions are almost completely independent of the angle of incidence and polarization of the microwave radiation.

Low angular-dispersion microwave absorption of a dual-pitch nondiffracting metal bigrating

The surface plasmon modes supported by a nondiffracting 90° bigrating consisting of three grooves per repeat period with one slightly shallower than the other two are characterized by studying the reflectivity from the structure as a function of the angle of incidence and the incident wavelength (11.3<λ0<16.7 mm). This structure supports two remarkably angle-independent modes plus a further, lower-energy mode which is more dispersive. Experimental reflectivity is compared with that calculated using a finite element model. In addition, to understand the character of each of the modes, the spatial form of the electromagnetic fields at the resonant frequencies are explored.

Resonantly overcoming metal opacity

The near-perfect response of electrons in metals to low-frequency electromagnetic fields makes even a sub-skin-depth film almost completely opaque to microwave radiation. Here, it is experimentally demonstrated that by surrounding a ∼60 nm aluminium film with an array of thin resonant cavities, over 35% of the microwave radiation incident can be transmitted over a discrete set of narrow bands. This represents an enhancement of ∼1000 times over an isolated film and allows for a frequency selective screen with a thickness less than 1/70th of the operating wavelength that may be tuned through choice of resonant geometry.