Kimberly S. Reichel

High-Contrast Terahertz Wave Modulation by Gated Graphene Enhanced by Extraordinary Transmission through Ring Apertures

Gate-controllable transmission of terahertz (THz) radiation makes graphene a promising material for making high-speed THz wave modulators. However, to date, graphene-based THz modulators have exhibited only small on/off ratios due to small THz absorption in single-layer graphene. Here we demonstrate a ∼50% amplitude modulation of THz waves with gated single-layer graphene by the use of extraordinary transmission through metallic ring apertures placed right above the graphene layer. The extraordinary transmission induced ∼7 times near-filed enhancement of THz absorption in graphene. These results promise complementary metal-oxide-semiconductor compatible THz modulators with tailored operation frequencies, large on/off ratios, and high speeds, ideal for applications in THz communications, imaging, and sensing.

An electrically driven terahertz metamaterial diffractive modulator with more than 20 dB of dynamic range

We design and experimentally demonstrate a switchable diffraction grating for terahertz modulation based on planar active metamaterials, where a Schottky gate structure is implemented to tune the metamaterial resonances in real-time via the application of an external voltage bias. The diffraction grating is formed by grouping the active split-ring resonators into an array of independent columns with alternate columns biased. We observe off-axis diffraction over a wide frequency band in contrast to the narrow-band resonances, which permits operation of the device as a relatively high-speed, wide-bandwidth, high-contrast modulator, with more than 20 dB of dynamic range.

A Broadband Terahertz Waveguide T-Junction Variable Power Splitter

In order for the promise of terahertz (THz) wireless communications to become a reality, many new devices need to be developed, such as those for routing THz waves. We demonstrate a power splitting router based on a parallel-plate waveguide (PPWG) T-junction excited by the TE1 waveguide mode. By integrating a small triangular septum into the waveguide plate, we are able to direct the THz light down either one of the two output channels with precise control over the ratio between waveguide outputs. We find good agreement between experiment and simulation in both amplitude and phase. We show that the ratio between waveguide outputs varies exponentially with septum translation offset and that nearly 100% transmission can be achieved. The splitter operates over almost the entire range in which the waveguide is single mode, providing a sensitive and broadband method for THz power splitting.

High-Q terahertz Fano resonance with extraordinary transmission in concentric ring apertures

We experimentally demonstrate a polarization-independent terahertz Fano resonance with extraordinary transmission when light passes through two concentric subwavelength ring apertures in the metal film. The Fano resonance is enabled by the coupling between a high-Q dark mode and a low-Q bright mode. We find the Q factor of the dark mode ranges from 23 to 40, which is 3~6 times higher than Q of bright mode. We show the Fano resonance can be tuned by varying the geometry and dimension of the structures. We also demonstrate a polarization dependent Fano resonance in a modified structure of concentric ring apertures.

In situ spectroscopic characterization of a terahertz resonant cavity

In many cases, the characterization of the frequency-dependent electric field profile inside a narrowband resonator is challenging, either due to limited optical access or to the perturbative effects of invasive probes. An isolated groove inside a terahertz parallel-plate waveguide provides an opportunity to overcome these challenges, as it forms a narrowband resonator and also offers direct access to the resonant cavity via the open sides of the waveguide. We characterize the spatially varying spectral response of such a resonator using a noninvasive probe. We observe a frequency-dependent field enhancement, which varies depending on the location of the probe within the cavity. This spectral dependence cannot be observed using conventional far-field measurements.

A mode-matching analysis of dielectric-filled resonant cavities coupled to terahertz parallel-plate waveguides

We use the mode-matching technique to study parallel-plate waveguide resonant cavities that are filled with a dielectric. We apply the generalized scattering matrix theory to calculate the power transmission through the waveguide-cavities. We compare the analytical results to experimental data to confirm the validity of this approach.

Extraordinary optical transmission inside a waveguide: spatial mode dependence

We study the influence of the input spatial mode on the extraordinary optical transmission (EOT) effect. By placing a metal screen with a 1D array of subwavelength holes inside a terahertz (THz) parallel-plate waveguide (PPWG), we can directly compare the transmission spectra with different input waveguide modes. We observe that the transmitted spectrum depends strongly on the input mode. A conventional description of EOT based on the excitation of surface plasmons is not predictive in all cases. Instead, we utilize a formalism based on impedance matching, which accurately predicts the spectral resonances for both TEM and non-TEM input modes.

Evanescent wave coupling in terahertz waveguide arrays

We experimentally study THz evanescent wave coupling in an array of parallel-plate waveguides in close proximity. We observe stronger coupling with larger plate separations and longer propagation paths.

Terahertz Microfluidic Sensing Using a Parallel-plate Waveguide Sensor

Refractive index (RI) sensing is a powerful noninvasive and label-free sensing technique for the identification, detection and monitoring of microfluidic samples with a wide range of possible sensor designs such as interferometers and resonators. Most of the existing RI sensing applications focus on biological materials in aqueous solutions in visible and IR frequencies, such as DNA hybridization and genome sequencing. At terahertz frequencies, applications include quality control, monitoring of industrial processes and sensing and detection applications involving nonpolar materials. Several potential designs for refractive index sensors in the terahertz regime exist, including photonic crystal waveguides, asymmetric split-ring resonators, and photonic band gap structures integrated into parallel-plate waveguides. Many of these designs are based on optical resonators such as rings or cavities. The resonant frequencies of these structures are dependent on the refractive index of the material in or around the resonator. By monitoring the shifts in resonant frequency the refractive index of a sample can be accurately measured and this in turn can be used to identify a material, monitor contamination or dilution, etc. The sensor design we use here is based on a simple parallel-plate waveguide. A rectangular groove machined into one face acts as a resonant cavity (Figures 1 and 2). When terahertz radiation is coupled into the waveguide and propagates in the lowest-order transverse-electric (TE1) mode, the result is a single strong resonant feature with a tunable resonant frequency that is dependent on the geometry of the groove. This groove can be filled with nonpolar liquid microfluidic samples which cause a shift in the observed resonant frequency that depends on the amount of liquid in the groove and its refractive index. Our technique has an advantage over other terahertz techniques in its simplicity, both in fabrication and implementation, since the procedure can be accomplished with standard laboratory equipment without the need for a clean room or any special fabrication or experimental techniques. It can also be easily expanded to multichannel operation by the incorporation of multiple grooves. In this video we will describe our complete experimental procedure, from the design of the sensor to the data analysis and determination of the sample refractive index.

Waveguide Devices for Terahertz Signal Processing

We introduce two waveguide based devices for signal processing in future terahertz wireless communications systems: a leaky-wave antenna for frequency multiplexing and a Tjunction waveguide for broadband power splitting.