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PCA simulations were conducted using CST Microwave Studio EM simulation software [15]. This software was used to import microantenna designs and model their EM behaviour using a finite element method as metallic objects over semi-conductor substrates. Once the microantenna .gds files were imported, they were scaled appropriately and assigned their metallic properties, for example as highly conductive gold, and an object thickness is chosen. A substrate is then generated with user-defined dimensions and the appropriate material is assigned to it. In these simulations, typically a “lossy” GaAs substrate is used. This means that all the relevant characteristics of a ‘non-perfect’ GaAs substrate were assigned to the block of material used in the simulation such as the relative permittivity, thermal and electrical conductivity and heat capacity. Material parameters for the lossy GaAs substrates used are summarised in Table4.1:

The substrate thickness should represent asaccurately as possible that of the real antenna structures, as this affects the far-field radiation pattern of the THz output signal, for example, amongst other effects.

An excitation signal is then defined, which will be applied to the antenna, and a power “port” can be connected to monitor several relevant parameters such as antenna voltage, current and as such the subsequent signal frequency response of the microantenna-over-semiconductor may be retrieved. Excitation signals can include a variety of energetic inputs such as a single ultrashort electrical pulse or an incoming plane wave at a particular direction and polarisation. Figures4.5–4.8 gives some exemplary simulation results of antenna resonance properties and 3D simulation diagrams for four different microantenna types: (Fig4.5) a 2 mm-long CPS antenna; (Fig4.6) a 100 µm-long dipole antenna; (Fig4.7) an ‘ideal’ tuned 55.4 µm dual-dipole antenna similar to the designs presented in [11]; and (Fig4.8)

a 5 µm-gap log-periodic antenna. Antennas as presented over visible substrate

‘bricks’ in some of these figures, but this is for visual clarification of the simulation setup. Antennas were in fact generally situated over semi-infinite GaAs planes or within a pure GaAs surrounding medium for the purposes of reducing the simulation time and complexity without sacrificing numerical accuracy.

The two main parameters of interest are plotted here: the scattering parameter (S-Parameter) and radiation resistance. The S-Parameter, or more specifically the “reflection coefficient” in this case, is used in the measurement of the return loss of the antenna. This is a common figure of merit in transmission line anal-ysis and can provide information on the passage of the electrical signal through the microantenna. The signal applied across the antenna terminals experiences some loss in transmission and some degree of reflection as it propagates through different material interfaces and impedances. This loss and reflection is gen-erally frequency-dependent, even in ‘broadband’ antenna designs, so simulating this phenomenon can help further characterise the expected frequency-dependent performance of the PCA devices. This was typically sampled at frequency in-tervals of roughly 3 GHz over the 0.05–3 THz range of interest and the sample frequency with the lowest return loss essentially indicates the highest signal reso-nance and gives it’s ‘resonant operating frequency’. The return loss of a resonant antenna, in dB, may be estimated as:

RL_input = 20 log₁₀|S₁₁| , (4.1.3.1) where S₁₁ is the only S-matrix parameter for each simulation as only one input signal port is used here. The radiation resistance is an estimate of the microan-tenna load resistance (V_ant/I_ant) as an EM radiating circuit element, as discussed in Section1.2.4. A range of resonance features appear in all antenna tests, which must be accounted for particularly in the interpretation of experimental results later.

The CPS antenna results shown in Figure 4.5 were essentially dominated by modulation features which were caused by contact length at the low frequencies and contact width at around 1.65 THz, and other than these features there is relatively little enhancement of any specific operating frequencies. The dipole antennas tested in these examples had resonant frequencies at around 0.34 THz for the 100 µm dipole antenna as shown in Figure 4.6. Figure 4.7 indicates at

Figure 4.5: Simulated electrical response of a 50 µm-gap CPS antenna. (a) shows the 3D simulation layout; (b) shows the obtained S-Parameter; and (c) shows the radiation resistance data over the 0.05–3 THz range of interest.

Figure 4.6: Simulated electrical response of a 100 µm-length, 10 µm-gap dipole antenna. (a) shows the 3D simulation layout; (b) shows the obtained S-Parameter; and (c) shows the radiation resistance data over the 0.05–3 THz

range of interest.

Figure 4.7: Simulated electrical response of an ‘ideal’ tuned dual-dipole antenna. (a) shows the 3D simulation layout; (b) shows the obtained S-Parameter; and (c) shows the radiation resistance data over the 0.05–2 THz

range of interest.

Figure 4.8: Simulated electrical response of a broadband, log-periodic toothed antenna. (a) shows the 3D simulation layout; (b) shows the obtained S-Parameter; and (c) shows the radiation resistance data over the 0.05–3 THz

range of interest.

resonant response predominantly located around 0.83 THz for the tuned 55.4 µm dual-dipole antenna, as expected from Equation1.2.4.4. Strong modulating har-monics were also observed at low frequencies and a large response around 1.1 THz due to the geometries of the long lengths and narrow widths of the Au antenna feed lines, but in practice the low frequency harmonics would not give these sharp peaks due to the roughness of the ‘real-world’ etch and surface features. The ra-diation resistance plot for the dual-dipole antenna as shown in Figure 4.7(c) is calculated after the structure has been simplified to remove such features not relevant to the basic design principle and gives an idea of the intended resonance properties of the design. This resonance curve is much more simple and indicates a much clearer resonance enhancement at the design frequency, and is typically observed in real devices [11]. Low-frequency resonance behavior is also observed in the log-periodic antenna as shown in Figure 4.8, where multiple strong reso-nance features were observed which correspond to the different tooth lengths and the inner bow-tie geometry, but a more constant response over the THz range of interest is also observed as expected.