Artificial dielectric beam-scanning prism for the terahertz region.

We design and fabricate an artificial dielectric prism that can steer a terahertz beam in space and experimentally investigate its behavior. The artificial dielectric medium consists of a uniformly spaced stack of metal plates, electromagnetically equivalent to an array of parallel-plate waveguides operating in tandem. At an operating frequency of 0.3 THz, we observe a maximum beam deflection of 29°, limited by the precision of the available spacers. Spring-loading the spacers between the plates allow us to scan the beam continuously and dynamically over a range of 5°. The measured beam intensity maps at the input and output of the device reveal very good Gaussian beam quality and an estimated power efficiency of 71%. As a possible real-world application, we integrate the prism into the path of a free-space terahertz communication link and demonstrate unimpaired performance.

Broadband wide-angle terahertz antenna based on the application of transformation optics to a Luneburg lens.

The design of antennas for terahertz systems remains a significant challenge. These antennas must provide very high gain to overcome significant free-space path loss, which limits their ability to broadcast or receive a beam over a wide angular range. To circumvent this limitation, here we describe a new device concept, based on the application of quasi-conformal transformation optics to the traditional Luneburg lens. This device offers the possibility for wide-angle beam steering and beam reception over a broad bandwidth, scalable to any frequency band in the THz range.

Real-time object tracking using a leaky THz waveguide.

We demonstrate a 2D radar system for the THz region using a leaky parallel-plate waveguide, which can support real-time object tracking. The system can track a target within 200 ms with an accuracy of 1 mm in range and 1.4° in angle. Because the system is based on broadband excitation, it can locate multiple objects simultaneously. The broadband excitation also enables sensing of objects for which there is no direct line-of-sight path to the waveguide, via detection of a non-line-of-sight path.

Characteristics of resonance-induced optical vortices and spatial reshaping.

The spatial profile of a beam can experience complicated reshaping after interacting with a planar resonator near resonant conditions. Previously, this phenomenon was recognized as the Goos-Hänchen effect, which only partially explains the experimental observations. In this Letter, we introduce a 2D model that can fully describe the resonance-induced spatial reshaping. The model predicts several general features of the output beam profile and suggests that optical phase or polarization vortices can be generated and manipulated by an arbitrary planar resonator. We validate our theoretical results with experimental measurements using terahertz spectroscopy.

Electrically reconfigurable terahertz signal processing devices using liquid metal components.

Many applications of terahertz (THz) technology require the ability to actively manipulate a free space THz beam. Yet, although there have been many reports on the development of devices for THz signal processing, few of these include the possibility of electrical control of the functionality, and novel ideas are needed for active and reconfigurable THz devices. Here, we introduce a new approach, based on the integration of electrically actuated liquid metal components in THz waveguides. This versatile platform offers many possibilities for control of THz spectral content, wave fron"ts, polarization, and power flow. We demonstrate two illustrative examples: the first active power-splitting switch, and the first channel add-drop filter. We show that both of these devices can be used to electrically switch THz communication signals while preserving the information in a high bit-rate-modulated data stream.

Extraordinary optical reflection resonances and bound states in the continuum from a periodic array of thin metal plates.

The creation of artificial structures with very narrow spectral features in the terahertz range has been a long-standing goal, as they can enable many important applications. Unlike in the visible and infrared, where compact dielectric resonators can readily achieve a quality factor (Q) of 106, terahertz resonators with a Q of 103 are considered heroic. Here, we describe a new approach to this challenging problem, inspired by the phenomenon of extraordinary optical transmission (EOT) in 1D structures. In the well-studied EOT problem, a complex spectrum of resonances can be observed in transmission through a mostly solid metal structure. However, these EOT resonances can hardly exhibit extremely high Q, even in a perfect structure with lossless components. In contrast, we show that the inverse structure, a periodic array of very thin metal plates separated by air gaps, can exhibit non-trivial bound states in the continuum (BICs) reflection resonances, with arbitrarily high Q, and with peak reflectivity approaching 100% even for a vanishingly small metal filling fraction. Our analytical predictions are supported by numerical simulations, and also agree well with our experimental measurements. This configuration offers a new approach to achieving ultra-narrow optical resonances in the terahertz range, as well as a new experimentally accessible configuration for studying BICs.

Artificial dielectric stepped-refractive-index lens for the terahertz region.

In this paper we theoretically and experimentally demonstrate a stepped-refractive-index convergent lens made of a parallel stack of metallic plates for terahertz frequencies based on artificial dielectrics. The lens consist of a non-uniformly spaced stack of metallic plates, forming a mirror-symmetric array of parallel-plate waveguides (PPWGs). The operation of the device is based on the TE1 mode of the PPWG. The effective refractive index of the TE1 mode is a function of the frequency of operation and the spacing between the plates of the PPWG. By varying the spacing between the plates, we can modify the local refractive index of the structure in every individual PPWG that constitutes the lens producing a stepped refractive index profile across the multi stack structure. The theoretical and experimental results show that this structure is capable of focusing a 1 cm diameter beam to a line focus of less than 4 mm for the design frequency of 0.18 THz. This structure shows that this artificial-dielectric concept is an important technology for the fabrication of next generation terahertz devices.

Artificial dielectric polarizing-beamsplitter and isolator for the terahertz region.

We demonstrate a simple and effective strategy for implementing a polarizing beamsplitter for the terahertz spectral region, based on an artificial dielectric medium that is scalable to a range of desired frequencies. The artificial dielectric medium consists of a uniformly spaced stack of metal plates, which is electromagnetically equivalent to a stacked array of parallel-plate waveguides. The operation of the device relies on both the lowest-order, transverse-electric and transverse-magnetic modes of the parallel-plate waveguide. This is in contrast to previous work that relied solely on the transverse-electric mode. The fabricated polarizing beamsplitter exhibits extinction ratios as high as 42 dB along with insertion losses as low as 0.18 dB. Building on the same idea, we also demonstrate an isolator with non-reciprocal transmission, providing high isolation and low insertion loss at a select design frequency. The performance of our isolator far exceeds that of other experimentally demonstrated terahertz isolators, and indeed, even rivals that of commercially available isolators for optical wavelengths. Because these waveguide-based artificial dielectrics are low loss, inexpensive, and easy to fabricate, this approach offers a promising new route for polarization control of free-space terahertz beams.

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.

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.

Terahertz Artificial Dielectric Lens.

We have designed, fabricated, and experimentally characterized a lens for the THz regime based on artificial dielectrics. These are man-made media that mimic properties of naturally occurring dielectric media, or even manifest properties that cannot generally occur in nature. For example, the well-known dielectric property, the refractive index, which usually has a value greater than unity, can have a value less than unity in an artificial dielectric. For our lens, the artificial-dielectric medium is made up of a parallel stack of 100 µm thick metal plates that form an array of parallel-plate waveguides. The convergent lens has a plano-concave geometry, in contrast to conventional dielectric lenses. Our results demonstrate that this lens is capable of focusing a 2 cm diameter beam to a spot size of 4 mm, at the design frequency of 0.17 THz. The results further demonstrate that the overall power transmission of the lens can be better than certain conventional dielectric lenses commonly used in the THz regime. Intriguingly, we also observe that under certain conditions, the lens boundary demarcated by the discontinuous plate edges actually resembles a smooth continuous surface. These results highlight the importance of this artificial-dielectric technology for the development of future THz-wave devices.

Focused terahertz waves generated by a phase velocity gradient in a parallel-plate waveguide.

We demonstrate the focusing of a free-space THz beam emerging from a leaky parallel-plate waveguide (PPWG). Focusing is accomplished by grading the launch angle of the leaky wave using a PPWG with gradient plate separation. Inside the PPWG, the phase velocity of the guided TE1 mode exceeds the vacuum light speed, allowing the wave to leak into free space from a slit cut along the top plate. Since the leaky wave angle changes as the plate separation decreases, the beam divergence can be controlled by grading the plate separation along the propagation axis. We experimentally demonstrate focusing of the leaky wave at a selected location at frequencies of 100 GHz and 170 GHz, and compare our measurements with numerical simulations. The proposed concept can be valuable for implementing a flat and wide-aperture beam-former for THz communications systems.

Inhibiting the TE1-mode diffraction losses in terahertz parallel-plate waveguides using concave plates.

We present numerical and experimental results on inhibiting diffraction losses associated with the lowest order transverse electric (TE1) mode of a terahertz (THz) parallel-plate waveguide (PPWG) via the use of slightly concave plates. We find that there is an optimal radius of curvature that inhibits the diffraction for a given waveguide operating at a given frequency. We also find that introducing this curvature does not introduce any additional group-velocity dispersion. These results support the possibility of realizing long range transport of THz radiation using the TE1 mode of the PPWG.

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.

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.

Analysis of rectangular resonant cavities in terahertz parallel-plate waveguides.

We describe an experimental and theoretical characterization of rectangular resonant cavities integrated into parallel-plate waveguides, using terahertz pulses. When the waveguide is excited with the lowest-order transverse-electric mode, these cavities exhibit resonances with narrow linewidths. Broadband transmission spectra are compared with the results of mode-matching calculations, for various cavity dimensions.

Superfocusing terahertz waves below lambda/250 using plasmonic parallel-plate waveguides.

We experimentally demonstrate complete two-dimensional (2-D) confinement of terahertz (THz) energy in finite-width parallel-plate waveguides, defying conventional wisdom in the century-old field of microwave waveguide technology. We find that the degree of energy confinement increases exponentially with decreasing plate separation. We propose that this 2-D confinement is mediated by the mutual coupling of plasmonic edge modes, analogous to that observed in slot waveguides at optical wavelengths. By adiabatically tapering the width and the separation, we focus THz waves down to a size of 10 microm (approximately lambda/260) by 18 microm ( approximately lambda/145), which corresponds to a mode area of only 2.6 x 10(-5) lambda(2).

Bending and coupling losses in terahertz wire waveguides.

We present an experimental study of several common perturbations of wire waveguides for terahertz pulses. Sommerfeld waves retain significant signal strength and bandwidth even with large gaps in the wire, exhibiting more efficient recoupling at higher frequencies. We also describe a detailed study of bending losses. For a given turn angle, we observe an optimum radius of curvature that minimizes the overall propagation loss. These results emphasize the impact of the distortion of the spatial mode on the radiative bend loss.

Comparison of the lowest-order transverse-electric (TE1) and transverse-magnetic (TEM) modes of the parallel-plate waveguide for terahertz pulse applications.

We present a comprehensive experimental study comparing the propagation characteristics of the virtually unknown TE(1) mode to the well-known TEM mode of the parallel-plate waveguide (PPWG), for THz pulse applications. We demonstrate that it is possible to overcome the undesirable effects caused by the TE(1) mode's inherent low-frequency cutoff, making it a viable THz wave-guiding option, and that for certain applications, the TE(1) mode may even be more desirable than the TEM mode. This study presents a whole new dimension to the THz technological capabilities offered by the PPWG, via the possible use of the TE(1) mode.

Comment on "Low-loss terahertz ribbon waveguides".

We point out an important fundamental limitation associated with the dielectric ribbon waveguide described by Yeh [Appl. Opt.44, 5937 (2005)], which would restrict its practical use in ultrahigh- or terahertz-speed guided-wave applications, contrary to their underlying prediction, and we additionally point out a noteworthy inaccuracy in their introductory discussion.