Rajind Mendis

Undistorted guided-wave propagation of subpicosecond terahertz pulses

We report efficient quasi-optic coupling of a freely propagating beam of terahertz (THz) pulses into a parallel-plate copper waveguide (with a plate separation of 108mum) and subsequent low-loss, single-TEM-mode propagation with virtually no group-velocity dispersion. Undistorted, low-loss propagation of the incoming 0.3-ps FWHM THz pulses was observed within the bandwidth from 0.1 to 4 THz for a length of 24.4 mm. We compare experimentally derived values for the absorption and phase velocity with theory to show consistency. This demonstration is direct proof of the excellent performance of the parallel-plate waveguide as a wideband THz interconnect.

Plastic ribbon THz waveguides

We report an experimental and theoretical study of single mode, guided wave propagation in plastic ribbons. Using cylindrical THz optics, we obtain excellent quasioptic coupling between freely propagating THz waves and a single TM0 waveguide mode. Dispersive, low-loss propagation is observed within the experimental bandwidth of 0.1–3.5 THz for 2-cm-wide, high-density polyethylene ribbon waveguides with dimensions of 150 μm thick by 10 mm long, and 120 μm thick by 20 mm long. Such ribbons show promise as a flexible THz interconnect with a controllable group velocity dispersion.

THz interconnect with low-loss and low-group velocity dispersion

We report the demonstration of a physically flexible, practicable THz interconnect with minimal pulse distortion and loss. The interconnect is a parallel-plate waveguide, with the TEM mode excited, constructed of two thin copper strips. The incoming 0.22 ps THz pulse broadens to 0.39 ps after propagating 250 mm in the waveguide and is also attenuated by a factor of ten. We show that this attenuation is mainly due to the finite conductivity of copper with some additional loss caused by the beam spread in the unguided dimension. The pulse broadening is due to the frequency-dependent loss since the group velocity dispersion is negligible.

Nature of subpicosecond terahertz pulse propagation in practical dielectric-filled parallel-plate waveguides

It is analytically shown that the presence of submicrometer-sized air gaps between the dielectric and metal contact surfaces in a dielectric-filled metallic parallel-plate waveguide can have a dramatic effect on the guided-wave propagation of subpicosecond terahertz pulses. Through the use of metal-evaporated dielectric surfaces to overcome the imperfect contact problem, and a special air-dielectric-air cascaded waveguide geometry to avoid multimode excitation, undistorted subpicosecond terahertz pulse propagation via the single-TEM mode is demonstrated, for what is believed to be the first time, in a silicon-filled PPWG.

Spectral characterization of broadband THz antennas by photoconductive mixing: toward optimal antenna design

The spectral characterization of a broadband antenna using a pump-probe photomixing continuous-wave (CW) terahertz (THz) system is presented. The high dynamic range of the system, comparable to or better than that of similar pump-probe systems reported in the literature, provides an accurate means of antenna characterization. The planar antenna exhibits a log-periodic behavior at low frequencies, a bow-tie behavior at high frequencies, and a resonance characteristic in between, well in agreement with the antenna geometry. It is predicted that an improved geometry that extends the log-periodic behavior to higher frequencies would contribute significantly in enhancing the broadband performance of antenna-coupled photomixers.

Strong terahertz emission from (100) p-type InAs

Terahertz emission has been observed from (100) Zn-acceptor-doped InAs under illumination by fs pulses of near-infrared radiation. Turning the crystal about the surface normal produces two maxima per rotation, whether the angle of incidence is 45° or 75°, in contrast to (111) p-InAs, where three maxima per rotation have been reported. The emitted terahertz power has a quadratic variation with the pump power and decreases with increasing temperature in the range 20–300K. This behavior is consistent with a photocurrent surge being the dominant terahertz generating mechanism at low excitation fluences. The p-type InAs generates about two orders of magnitude more power than the standard unbiased terahertz emitter, 1mm thick ZnTe.

THz transmission characteristics of dielectric-filled parallel-plate waveguides

THz transmission characteristics of dielectric-filled (DF) metallic parallel-plate waveguides (PPWGs) are experimentally and theoretically investigated via the propagation of sub-ps THz pulses. It is found that a PPWG having a high-resistivity (>10 kΩ cm) silicon filling can, in fact, exhibit a higher transmission loss compared to one having a polythene filling, despite the significantly lower THz absorption in the bulk silicon. Consequently, it is shown that the key to achieving low-loss transmission in DF-PPWGs is to use a dielectric filling having not only a low absorption, but also a low refractive index. The application of this guided-wave geometry for THz spectroscopy of minute (film) substances is also predicted.

Terahertz mirage: Deflecting terahertz beams in an inhomogeneous artificial dielectric based on a parallel-plate waveguide

The field of metamaterials and the formalism of transformation optics have provided a prescription for constructing artificial dielectrics with unique properties such as light trapping and cloaking. Here, we describe a different approach to creating an inhomogeneous artificial medium, based on waveguide techniques, which does not rely on engineered subwavelength-scale components. We demonstrate a mirage effect in which an object several times larger than the selected wavelength is rendered invisible by bending a beam around it.

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.

Terahertz emission from (100)p-InAs

Terahertz emission from (100) p-type InAs illuminated by ultrafast near-infrared pulses is investigated. A two-fold rotational symmetry was observed when rotated about the surface normal. A quadratic relationship was found for the emission dependence on optical pump power. These suggest the presence of photocarrier transport and optical rectification mechanisms. The InAs emission was found to exceed that of a blackbody radiator for frequencies below 1 THz for nominal input power levels. The generated power was found to be roughly two orders of magnitude greater than a 1mm ZnTe emitter.