Engineering the dielectric function of plasmonic lattices

Geometrical Dependence on the Onset of Surface Plasmon Polaritons in THz Grid Metasurfaces

The transmission response of metallo-dielectric grid metasurfaces is experimentally investigated through Terahertz Time Domain Spectroscopy and the corresponding effective dielectric function is retrieved. Using a lumped element model we can determine the dependence of the effective plasma frequency (the transition frequency) on the metasurface filling factor F. The change of the transition frequency vs. F spans over one order of magnitude and sets the threshold between the metamaterial (homogeneous) and the photonic crystal (diffraction-like) regime, ruling the onset of two different Surface Plasmon Polaritons, spoof and high order. Field symmetry and spatial extension of such excitations are investigated for the possible applications of THz grid metasurfaces in bio- and chemical sensing and sub-wavelength imaging.

Terahertz magneto-plasmonics using cobalt subwavelength aperture arrays

We characterize the terahertz (THz) magneto-plasmonic response of a cobalt-based periodic aperture array. The bare cobalt surface allows for low loss propagation of surface plasmon-polaritons, as evidenced by comparing the reflection from aperture arrays coated with Au and with Co. When an external magnetic field is applied in a polar Kerr geometry, we observe a maximum polarization rotation of ~0.6° and an ellipticity of ~0.35° from the Co-based array. These values are larger than expected based on existing models that include only interband transitions in ferromagnetic metals. We discuss possible reasons for the difference between experiment and theory.

Optically induced mode coupling and interference in a terahertz parallel plate waveguide

We demonstrate all-optical control of terahertz (THz) wavemode coupling in a silicon-filled parallel plate waveguide. Using an asymmetric photoexcitation of charge carriers on the surface of the silicon slab within the waveguide, the symmetry is broken, and THz light is partially coupled from TEM to higher-order TM modes. The resulting interference between these modes and the residual TEM mode leads to a strong frequency-dependent transmission modulation. This frequency-selective modulation is widely tunable by adjusting the relative modal phases by translating the excitation along the propagation direction. The experimental observations are well described by a numerical and analytic model of modal interference.

Terahertz plasmonic waveguides created via 3D printing

We demonstrate that 3D printing, commonly associated with the manufacture of large objects, allows for the fabrication of high quality terahertz (THz) plasmonic structures. Using a commercial 3D printer, we print a variety of structures that include abrupt out-of-plane bends and continuously varying bends. The waveguides are initially printed in a polymer resin and then sputter deposited with ~500 nm of Au. This thickness of Au is sufficient to support low loss propagation of surface plasmon-polaritons with minimal impact from the underlying layer, thereby demonstrating a useful approach for fabricating a broad range of plasmonic structures that incorporate complex geometries. Using THz time-domain spectroscopy, we measure the guided-wave properties of these devices. We find that the propagation properties of the guided-wave modes are similar to those obtained in similar conventional metal-based waveguides, albeit with slightly higher loss. This additional loss is attributed to roughness associated with limitations that currently exist in commercial 3D printers.

Concentration of broadband terahertz radiation using a periodic array of conically tapered apertures

We describe the optical concentration properties of periodic arrays of conically tapered metallic apertures measured using terahertz (THz) time-domain spectroscopy. As a first step in this process, we optimize the geometrical properties of individual apertures, keeping the output aperture diameter fixed, and find that the optimal taper angle is 30°. A consequence of increasing the taper angle is that the effective cutoff frequency red shifts, which can be readily explained using conventional waveguide theory. We then fabricate and measure the transmission properties of a periodic (hexagonal) array of optimized tapered apertures. In contrast to periodic arrays of subwavelength apertures in thin metal films, which are characterized by narrowband transmission resonances associated with the periodic spacing, here we observe broadband enhanced transmission above the effective cutoff frequency. Further enhancement in the concentration capabilities of the array can be achieved by tilting the apertures towards the array center, although the optical throughput of individual tapered apertures is reduced with increasing tilt angle. Finally, we discuss possible future directions that utilize cascaded structures, as a means for obtaining further enhancement in the amplitude of the transmitted THz radiation.

Measurement of surface plasmon correlation length differences using Fibonacci deterministic hole arrays

Using terahertz (THz) transmission measurements through two-dimensional Fibonacci deterministic subwavelength hole arrays fabricated in metal foils, we find that the surface plasmon-polariton (SPP) correlation lengths for aperiodic resonances are smaller than those associated with the underlying grid. The enhanced transmission spectra associated with these arrays contain two groups of Fano-type resonances: those related to the two-dimensional Fibonacci structure and those related to the underlying hole grid array upon which the aperiodic Fibonacci array is built. For both groups the destructive interference frequencies at which transmission minima occur closely match prominent reciprocal vectors in the hole array (HA) structure-factor in reciprocal space. However the Fibonacci-related transmission resonances are much weaker than both their calculated Fourier intensity in k space and the grid-related resonances. These differences may arise from the complex, multi-fractal dispersion relations and scattering from the underlying grid arrays. We also systematically studied and compared the transmission resonance strength of Fibonacci HA and periodic HA lattices as a function of the number of holes in the array structure. We found that the Fibonacci-related resonance strengths are an order of magnitude weaker than that of the periodic HA, consistent with the smaller SPP correlation length for the aperiodic structure.

Liquid metal-based plasmonics

We demonstrate that liquid metals support surface plasmon-polaritons (SPPs) at terahertz (THz) frequencies, and can thus serve as an attractive material system for a wide variety of plasmonic and metamaterial applications. We use eutectic gallium indium (EGaIn) as the liquid metal injected into a polydimethylsiloxane (PDMS) mold fabricated by soft lithography techniques. Using this approach, we observe enhanced THz transmission through a periodic array of subwavelength apertures. Despite of the fact that the DC conductivity of EGaIn is an order of magnitude smaller than many conventional metals, we clearly observe well-defined transmission resonances. This represents a first step in developing reconfigurable and tunable plasmonic devices that build upon well-developed microfluidic capabilities.

Engineering the properties of terahertz filters using multilayer aperture arrays

We experimentally demonstrate the ability to create additional transmission resonances in a double-layer aperture array by varying the interlayer gap spacing. In the case of periodic aperture arrays, these additional resonances are sharply peaked, while for random aperture arrays the resonances are broad. Surprisingly, these additional resonances only occur when the interlayer gap spacing is greater than half the aperture spacing on a single array. Since there is no corresponding periodicity in the random arrays, these resonances occur regardless of how small the gap spacing is made. This phenomenon can be accurately modeled only if the correct frequency-dependent complex dielectric function of a metal film perforated with subwavelength apertures is used. Using THz time-domain spectroscopy, we are able to directly obtain the complex dielectric response function from the THz experimental transmission measurements. We conclude by demonstrating several passive free-space THz filters using multilayer aperture arrays. Importantly, we show that the magnitude of the lowest order resonance can be approximately maintained, while the background transmission can be significantly suppressed leading to a significant improvement in the optical filter fidelity.

Concentration of terahertz radiation through a conically tapered aperture

We demonstrate that conically tapered cylindrical apertures can be used to efficiently concentrate broadband terahertz (THz) radiation. Keeping the aperture diameter on the input plane fixed, we show that as the diameter of the aperture on the exit plane is decreased, we obtain an increase in the magnitude of the transmitted electric field that varies inversely with the output aperture diameter. Correspondingly, the transmitted THz intensity concentration increases inversely with the square of the output aperture diameter. For the smallest aperture that we fabricated, we obtain a ~50 fold increase in the transmitted THz intensity. We expect further increases in the intensity concentration with smaller output apertures. As the output aperture diameter is decreased with a corresponding increase in the concentration factor, we directly measure an increase in the propagation time delay of a narrowband pulse through the structure. Finally, we demonstrate that further increase in the concentration factor can be achieved by engraving circular grooves around the input aperture.

Efficient homogenization procedure for the calculation of optical properties of 3D nanostructured composites

We present a very efficient recursive method to calculate the effective optical response of metamaterials made up of arbitrarily shaped inclusions arranged in periodic 3D arrays. We apply it to dielectric particles embedded in a metal matrix with a lattice constant much smaller than the wavelength of the incident field, so that we may neglect retardation and factor the geometrical properties from the properties of the materials. If the conducting phase is continuous the low frequency behavior is metallic, and if the conducting paths are thin, the high frequency behavior is dielectric. Thus, extraordinary-transparency bands may develop at intermediate frequencies, whose properties may be tuned by geometrical manipulation.

Resonant infrared transmission and effective medium response of subwavelength H-fractal apertures

The transmission through periodic arrays of subwavelength H-fractal apertures in a gold film at infrared wavelengths is investigated numerically. H-fractal apertures support subwavelength cut-off resonances that are hybridized with surface plasmons along the sidewalls of the aperture. Enhanced transmission occurs at wavelengths that are about ten times the aperture side length. The highly subwavelength size scale of the H-fractal enables an effective medium parameter description for the aperture array, which reveals a lossy plasma permittivity and a diamagnetic permeability.

Extended Fano model of Extraordinary Electromagnetic Transmission through subwavelength hole arrays in the terahertz domain

We developed an extended Fano model describing the Extraordinary Electromagnetic Transmission (EET) through arrays of subwavelength apertures, based on terahertz transmission measurements of arrays of various hole size and shapes. Considering a frequency-dependent coupling between resonant and non-resonant pathways, this model gives access to a simple analytical description of EET, provides good agreement with experimental data, and offers new parameters describing the influence of the hole size and shape on the transmitted signal.