Chapter 6. Further Investigation of THz Antennas from Substrate Effect and CAD
6.3. A Novel Simulation Method for THz Photoconductive Antenna
6.3.2. Parametric Study and Analysis
In this section, the effect of several parameters on the detected THz signal is investigated. Variations in these parameters affect the photocurrent, the radiated THz field and consequently the detected THz signal. In this study, it is assumed that parameters in the detector side are the same as the emitter side unless it is stated.
First, the laser pulse duration of the optical excitation signal is varied. This parameter affects the rise time of the photocurrent shown in Fig. 6.13. As illustrated in Fig. 6.14, changes in the laser pulse duration do not influence the peak detected frequency but at larger laser pulse durations, the signal spectrum is smaller. This implies that for a larger bandwidth, optical sources with shorter laser pulses are required. The trend of real measurement setups is the use of optical sources with shorter laser pulse durations for broader detected signals [72] and this simulation results match well with that.
Fig. 6.14 Spectral variation of the detected THz signal for the emitter antenna with H = 100 μm and carrier lifetime of 1 ps
Next, the impact of carrier lifetime of photoconductive material is investigated. According to Fig. 6.15a, by increasing the carrier lifetime, the amplitude of the detected signal is increased. However, spectral range of the signal with a shorter carrier lifetime is larger as shown in Fig. 6.15b. In other words, devices with a short carrier lifetime have larger amplitudes at higher
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frequencies. Therefore, there is a compromise between having a large signal amplitude and wide bandwidth in THz antennas.
(a)
(b)
Fig. 6.15 (a) amplitude of THz signal THz signal for spectral coverage comparison for H = 100 μm and laser pulse duration of 120 fs (b) normalised amplitude of the calculated, “em” stands for emitter and “rec” stands for the detector
In practical cases, usually photoconductive materials with large carrier lifetimes are used in the emitter side and materials with short lifetimes are employed in receiver. To study this case when the carrier lifetime in the emitter is long and in the receiver is short, the amplitude of the signal and its spectral range is moderate as shown in Fig. 6.15. In other words, these characteristics are in between the two cases where both the emitter and receiver have large or
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short carrier lifetime. Moreover, by varying the carrier lifetime, the peak frequency of the detected signal shifts. This shows that the peak frequency of the THz signal from the antenna is not only dependent on the antenna geometry, but also it is influenced by the carrier lifetime. This resonance shift in detected signal was reported in measurement results in [7, 69].
The effect of antenna gap length change is considered as shown in Fig. 6.16 where the optical laser power is assumed to be the same, by increasing the antenna gap length, the amplitude of the detected signal decreases considerably. The reason is attributed to the reduction in the optical power density in the antenna gap, which leads to smaller creation of free carriers in the antenna gap and photocurrent.
Fig. 6.16 Detected THz signal amplitude for dipole antenna with a constant 10 μm gap width, laser pulse duration of 120 fs and carrier lifetime of 1 ps
The impact of the antenna gap width is studied next ensuring the optical power is kept fixed for all situations. When the antenna gap length is constant, as illustrated in Fig. 6.17, enlarging the antenna width has a consequence of reducing the amplitude of the detected signal.
The investigations on the gap geometry of the antenna demonstrate that to have a larger detected THz signal the antenna gap length and width should be kept small. It is good to mention that there are some practical limitations; such as device breakdown threshold; which limit the antenna gap area in relation to the input optical power.
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Fig. 6.17 Amplitude of THz signal dipole antenna with a fixed gap length of 5 μm, H = 100, laser pulse duration of 120 fs and carrier lifetime of 1 ps
In order to validate this procedure, the simulated result is compared with measurement results from literature for different dipole lengths [192, 237]. Fig. 6.18 shows a very good agreement with the measurement result, with an increased length of dipole resulting in an enlarged peak detected signal and the peak frequency shifted downwards. Moreover, numerical results presented in [192, 237] deviate considerably from their measurement (Fig. 6.18b); however, the simulation results through this method matches better with the real measured results as shown in Fig. 6.18.
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Fig. 6.18 (a) Simulation of the detected THz signal for various dipole antenna length (b) corresponding experimental results [237]
6.4.
Summary
Two other dissimilarities of THz antennas with RF/MW antennas, i.e. substrate thickness and numerical simulation of the antenna, were discussed in this chapter. Investigation on the effect of the substrate thickness on the THz antenna performance showed that when the substrate thickness is set larger than one tenth of wavelength, (i.e. > λsub/10) the efficiency drops severely and starts to oscillate notably. Such an oscillation provides an opportunity to tune the antenna for the local maxima efficiency in peaks. The analytical study was compared with simulation result and it showed a good agreement. Also, a new computational simulation method for characterizing the detected signal from THz photoconductive antennas was developed. The approach was based on employing both optoelectronic and full-wave EM properties of the antenna. Effects of several parameters, related to the excitation and geometry of the device, were studied. The detected THz signal for different dipole antenna lengths from this method matched very well with measurement results reported in the literature. This simulation method is an easier procedure as compared to the full numerical method and it enables prediction of the antenna performance more accurately compared to the other full numerical methods used in [192, 237]. Thus, it is now possible to more
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accurately examine the THz antenna performance and understand the influence of different parameters before the antenna is fabricated.