Via

Substrate

Metal

Dielectric

Figure 3. SRR from figure 2 fabricated in a two metal process. First metal provides input and ground traces. The SRR

is fabricated by patterning second metal.

2.3. THz Helical antenna array

The helix is an antenna that yields higher gain with more windings. The resonance of the helical antenna occurs

at the wavelength that matches the circumference of the circular cross-section. The operational frequencies of the

antenna are determined by several parameters, including the precision of the windings with respect to the desired

wavelength, the spacing between turns, and the total number of turns. Optimal design parameters for this antenna are

given in comprehensive detail by its inventor, J.D. Kraus, in [15]. A significant benefit of the helical antenna is that the

conductor is wound in a circular manner rather than printed as a linear structure, so that its cross-sectional footprint is

reduced by a factor of 1/π ( ≈ 32%) in comparison to a trace of length λ. This allows higher integration density in the

present implementation. Second, because the antenna is a resonant device, its center frequency may be closely matched

to that of the SRR array. Moreover, the Q of this antenna can be controlled through a detuning of the winding

circumference to support a wider bandwidth if necessary. Finally, the helical geometry is extensible across the range of

electromagnetic frequencies and has been applied with success to THz radiation [16]. To create the monolithic active

element, the array of helical elements is fabricated over the source array. The coupling of the SRR fabric to free space is

bi-directional, as the antenna array may function as both a transmitter and receiver. To characterize the scale of the

geometry, antenna dimensions are tabulated for several frequencies in the THz regime.

Table 2. THz frequencies and wavelengths and helix outer diameter (OD)

Frequency

Wavelength

Helix OD

THz

micron

micron

0.50

600.00

190.99

1.00

300.00

95.49

2.50

120.00

38.20

5.00

60.00

19.10

As seen in the table, feature dimensions for 5 to 0.5 THz vary from 60 to 600 microns. In the current state of fabrication

and photolithography, these dimensions are quite large and are easily within reach of legacy exposure tools.

In keeping with the goal of maintaining standardized processing, the three-dimensional helix is fabricated with a

stack of two-dimensional structures, through a sequence of deposition, patterning, and etch steps. The first structure

fabricated is a partial turn (i.e. winding). In the present implementation, the winding geometry is hexagonal when

viewed end-on. The next step adds a metal contact to provide connectivity between layers. The contact is fabricated at

angle as shown in figure 4, to complete a single turn in the winding geometry.