requirements for accelerating electrons to specific kinetic energies, and the footprint occupied by ancillary systems,

limited size reductions are possible for e-beam sources. One notable development, however, uses modified electron

optics based on a scanning electron microscope design in order to produce a THz laser for medical applications [3].

More recent source technologies have applied down-conversion of optical frequencies to obtain lower frequency THz.

One method employs photomixing to generate radiation at a lower difference frequency. Another optical method

involves use of a short pulse laser to trigger the discharge of a charged microantenna that is fabricated on the THz scale.

Quantum cascade lasers have shown promise, but have drawbacks in the need for cryocooling, and the difficulty in

reaching the low THz range. Hence, the shortcomings of optical techniques have constrained them, to a large degree, to

the laboratory or permanent installations.

B. Metamaterials

The radiative component in our design is based upon an array of subwavelength structures termed metamaterial

or negative index material. NIM originated with the consideration of a hypothetical material defined by a negative

permittivity and negative permeability at a given frequency. Theoretical treatments of the material's interaction with an

electromagnetic field gave rise to counterintuitive and controversial results [4]. Recent experiments in electromagnetic

materials corroborated early theoretical predictions, notably, sub-wavelength imaging [5, 6]. These engineered materials

are variously named NIM, left-handed materials, or metamaterials. The negative permittivity and permeability of NIM

yield a negative index of refraction at a specific frequency so that light incident upon a NIM interface is refracted

towards the normal axis rather than away. Left-handed materials are so labeled because power propagation is in the

opposite direction indicated by the right-hand rule.

One way to create a metamaterial is through uniformly mixing nanoparticles in a liquid substrate, e.g. epoxy or

a thermoset compound. Nanoparticles are typically metallic spheres or air voids, with diameters several magnitudes

smaller than a particular wavelength of interest, i.e. λ0. The inclusions are distributed throughout the host substrate so

that the composite matrix has a lattice spacing considerably less than λ0. Thus, radiation at λ0 interacts macroscopically

with the metamaterial, without regard for its microscopic structure. In this manner, a composite material may be

engineered to manifest optical effects for a particular range of frequencies. The term metamaterial encompasses

materials with subwavelength structures whose aggregate interacts in a macroscopic manner with radiation of a specific

wavelength(s). The split ring resonator (SRR) geometry discussed in this paper falls under this heading and is developed

in greater detail in the following section. Metamaterials that serve as THz active elements, i.e. source or detector, are

well established and promise to yield new technologies that are smaller, cheaper, and more efficient than previous

methods [7].

2. DESIGN

The design presented in this paper employs a hybrid approach that draws upon technologies from both

microwave and photonics, with a metamaterial comprised of THz resonators combined with a lens array, to obtain a

highly-directive THz active element in a monolithic element. This design was developed to enable fabrication via

standard optical lithography and silicon manufacturing protocols. Although the design is based upon a nominal

frequency of 1 THz, the concept may be applied with dimensional modifications to a wide range of operating

frequencies, due to the extensibility of electricity and magnetism. The metrics used to quantify performance are the gain

and half power beam width.

The design and assembly for the APELC device consists of three components--driver circuitry, source array,

and directive optics. The driver circuit conditions the input from the power source through conversion of low-frequency

signal to high-frequency microwave using commercial off-the-shelf (COTS) components initially then transitioning to

custom geometries. The THz source is a grid of SRR elements driven in a manner that optimizes constructive addition

of the resulting radiation. The directive element in this design is a helical antenna array constructed with THz scale

elements, i.e. 1 mm to 10 micron for 0.3 to 30 THz.

2.1. Driver circuitry

The driver electronics are the circuits that source the array of THz resonators. The circuit is also designed to

enable the system to function as both a transmitter and receiver by supporting dual mode operation of the SRR and