Based on parallel-plate waveguides (PWWGs), we have developed both passive and active terahertz (THz) sensors, filters, splitters, and switches. We utilize the PPWG since it has excellent waveguiding properties with a simple geometry that allows for incorporation of unique electromagnetic functionalities. A few passive devices are discussed while the focus remains on active devices. The active control is enabled by our innovation to incorporate liquid metals, which can be relocated by application of a small electrical voltage (< 4V), as part of the waveguide geometry itself. The device geometry directly affects the device performance (e.g., frequency band of operation, channel output power). Therefore, by moving the liquid metal, the geometry is reconfigured which results in altering the device function. To show the practicality of these devices to be used for THz wireless communications, we demonstrate the ability of these devices to support a 1 Gb/s data stream on a THz carrier wave and show that this signal can be successfully switched with high modulation depths of ±40 dB. These results show the strong promise of these components to form important signal processing building blocks in the future infrastructure of THz wireless communications.
Various Luneburg-lens geometries are used in the microwaves industry as radar reflectors and omnidirectional antennas. Here, we implement a two-dimensional Luneburg lens for the THz frequency region using a waveguide-based artificial-dielectric technology. The cylindrical device has a parabolic shaped top surface and a flat bottom surface. The substrate material of the lens is ultra-pure Teflon, with the top and bottom surfaces coated with high-conductivity silver paint to form a quasi-parallel-plate waveguide. Our experimental results show that the lens can focus an approximately 2-cm diameter input beam to a spot size of 3.4 mm at the diametrically opposite edge, at an operating frequency of 0.162 THz. This work demonstrates the versatility of this artificial-dielectric technology to design and fabricate inhomogeneous, gradient-index devices for the THz region.
The newly discovered atomically thin and layered materials which host electronic system that respond to longwavelength light in extraordinary manner can lead to a major breakthrough in the field of terahertz (THz) optics and photonics. However, their low conductivities due to either low densities or low mobility make it challenging to characterize their basic THz properties with the standard spectroscopic method. Here, we develop a THz spectroscopic technique based on parallel plate waveguide (PPWG) to overcome the limitations of the conventional THz time domain spectroscopy (TDS) technique. The present method is particularly suitable to ultrathin conductive materials with low carrier density. We report in details the derivation of the dispersion equations of the terahertz wave propagation in a PPWG loaded by a thin conductive materials with zero-thickness. These dispersion equations for transverse magnetic (TM) and transverse electric (TE) waveguide modes are the core of the optical parameters extraction algorithm in the THz-PPWG-TDS analysis. We demonstrate the effectiveness of the waveguide approach by characterizing low conductive CVD graphene. The high sensitivity of THz-PPWG-TDS technique enables us to study the carrier dynamics in graphene with Drude and Drude-Smith model.
We theoretically and experimentally investigate the optical properties of a subwavelength hole in a thin metallic
film. The microscopic origin of the hole plasmon resonance is a collective state formed by propagating thin film
surface plasmons of wavelengths equal to integer fractions of the hole diameter. We show that the plasmon
resonance depend strongly on the polarization of the incident light. We also, using time-domain terahertz
spectroscopy, demonstrate the first experimental observation of the optical coupling between antibonding film
plasmon modes and perpendicularly polarized light to the film surface.
The detection of trace quantities of aromatic compounds is important to defense and security
applications, including the detection of CB agents, explosives, and other substances. These pose
threats to forces and the environment. This paper explores an approach to the detection and
identification of quantities as little as single molecules of explosives. It can in principle provide
Apertureless near-field scanning optical microscopy (ANSOM) is one of several promising
methods for obtaining spatial resolution below the diffraction limit at various wavelengths,
including in the terahertz regime. By scattering incident light off the junction between a probe
with a sub-wavelength tip and the surface of a sample, spatial resolution on the order of the tip
size can be obtained. For terahertz time-domain spectroscopy where the wavelength-limited
resolution is ~1 millimeter, this is a significant advantage.
In the case of a sufficiently small probe tip and a thin metallic substrate, plasmonic interaction
between the tip and sample provides an enhancement of the near-field in the junction. This
effect is dramatically enhanced for nanometer-scale metal layers, since surface plasmon states
from both sides of the film can contribute to the overall field enhancement.
We present preliminary results of THz plasmonic field enhancements, using a thin (500 nm) gold
film evaporated on glass. We observe an enhancement in the scattered THz wave, which we
attribute to the large density of plasmonic states extending throughout the THz range. This result
indicates a route to single-molecule spectroscopy at terahertz frequencies.
Dielectric mirrors are widely used in optical setups for spectral regions such as UV, visible, as well as IR. Yet, for the rapidly growing field of terahertz spectroscopy dielectric multilayer optics are sparsely utilized. But with low-loss materials high quality THz optics can be obtained. We present two approaches for the realization of highly effective dielectric THz mirrors. First, four thin slices of high-resistivity silicon and five common polypropylene (PP) foils were alternately stacked together to obtain a broad reflection band. This stop-band blueshifts with increasing angles of incidence. But due to the high index step between Si and PP a band from 0.32 to 0.375 THz always remains the stopband for all incidence angles and both the s- and p-polarization. The measurement data obtained in reflection and transmission geometry are reproduced well by numerical simulations. With a minor change of the layer sequence a microresonator is obtained which reveals a sharp transmission peak at around 0.3 THz within the reflection band. The second material system consists of ceramic laminates of alumina (A) and alumina-zirconia (AZ). Measurements on 12.5 pairs of A/AZ layers yield a strong stop-band from 0.3 to 0.37 THz at normal incidence, which again match numerical simulations. The big advantage of the ceramic mirror is the rugged, quasimonolithic design of the sintered multilayer structure.
We describe two different ideas for novel architectures based on photonic crystals of sub-micron colloids. The first involves the formation of photonic superlattices from colloidal photonic crystals. The superlattice periodicity induces the formation of minibands due to folding of the photonic band structure. This represents the first instance in which mid-gap states have been incorporated into a colloidal photonic crystal via a specifically engineered structural modification. The second idea involves applying the superprism concept to three-dimensionally periodic structures. Near a photonic band edge, the diffraction angle is extremely sensitive to the wavelength and propagation direction of the incident light. We analyze this effect in the context of macroporous polymer thin films formed from colloidal crystal templates.
We report on the fabrication and optical characterization of photonic superlattices formed of colloidal photonic crystals. The superlattice periodicity induces the formation of minibands due to folding of the photonic band structure. This represents the first instance in which mid-gap states have been incorporated into a colloidal photonic crystal via a specifically engineered structural modification.
Recently, a real-time imaging system based on terahertz (THz) time-domain spectroscopy has been developed. This technique offers a range of unique imaging modalities due to the broad bandwidth, sub-picosecond duration, and phase-sensitive detection of the THz pulses. This paper provides a brief introduction of the state-of-the-art in THz imaging. It also focuses on expanding the potential of this new and exciting field through two major efforts. The first concentrates on improving the experimental sensitivity of the system. We are exploring an interferometric arrangement to provide a background-free reflection imaging geometry. The second applies novel digital signal processing algorithms to extract useful information from the THz pulses. The possibility exists to combine spectroscopic characterization and/or identification with pixel-by-pixel imaging. We describe a new parameterization algorithm for both high and low refractive index materials.
The mesoscopic structure of water has long been a subject of discussion. We postulate that, on the mesoscopic scale, liquid water forms nm-size ice-like crystals and that his structure is responsible for absorption in the THz-frequency range. However, until the recent development of Thz-time domain spectroscopy (THz-TDS), it was difficult to determine the optical constants in this frequency range with a good signal-to-noise ratio and hence to study the absorption properties of water. Here we report on the optical properties of water in the frequency range 0.05-1.4 THz and discuss the mesoscopic structure of water. We use THz-TDS based on photoconductive dipole antennas gated by a 150 femtosecond laser pulses to generate and detect the THz- frequency pulses. A new theoretical approach is also presented which were use to explain the absorption behavior in the measured THz frequency range. In this theory, molecular plasma oscillations of H<SUB>3</SUB>O<SUB>2</SUB> complexes, that are distinctly separate from the H<SUB>5</SUB>O<SUB>2</SUB><SUP>+</SUP> complexes which form an underlying crystalline lattice, are assumed to be responsible for absorption in the THz- frequency range. This model provides good agreement to our data.
The ultrafast dynamics of solid C<SUB>60</SUB> following optical excitation are discussed. Excitation into the lowest optical band using pulses 12 fs in duration centered at 620 nm results in coherent vibrational motion as well as nonexponential relaxation dynamics dominated by interactions between photoexcitations. Excitation at 500 nm into the next highest band reveals complex relaxation dynamics indicative of fast energy relaxation from the higher electronic state.