New Materials for Photoconductive Terahertz Antennas
In this thesis, we have first introduced a new setup for the reliable characterization of photoconductive antennas to be used in THz time-domain spectroscopy. Using this setup one can benchmark THz antennas with high precision. The intra-day reproducibility error is in the range of 1.9% while the r...
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|Summary:||In this thesis, we have first introduced a new setup for the reliable characterization of photoconductive antennas to be used in THz time-domain spectroscopy. Using this setup one can benchmark THz antennas with high precision. The intra-day reproducibility error is in the range of 1.9% while the reproducibility within 9 days is 2.6%. This includes not only absolute power stability but also reproducibility of the spectra by eliminating alignment errors that alter the transfer function from sender to receiver. In order to demonstrate the full capabilities of the system, we investigated samples from five LT-GaAs wafers, grown at temperatures between 200°C and 300°C, in a systematic manner. The obtained results are in good agreement with previous studies on the same material system. These results prove that the system allows for quality control of photoconductors with minimum comparison error. We have also investigated the correlation between THz emission strength and the surface properties of the LT-GaAs photoconductive antenna. The THz characteristics were measured with the highly stable setup mentioned above, which allowed exciting a 10-mm long CPS antenna along the gap without changing the alignment of the optical or THz beam path. The surface properties were quantified regarding roughness and grain size. The roughness was extracted from AFM measurements and the grain size from SEM measurements. A comparison of the THz emission strength in form of the peak-to peak THz amplitude and the surface properties showed a strong nonlinear correlation: a smaller grain size and a smoother surface increase the THz amplitude. These results can be used in the future to optimize the performance of THz antennas. Additionally, we have successfully prepared TiN-nanoparticles using ultrasonic and pulsed laser ablation techniques. The two techniques provide with a different distribution of Zeta-potential and particle size. Within our experimental conditions, pulsed laser ablation can give lower particle size and greater Zeta-potential. TiN-nanoparticles prepared by these techniques have a high and flat absorbance in the spectral range 600 -1000 nm. LT-GaAs covered with dispersed TiNnanoparticles has enhanced THz emission when the average particle size is about 62 nm. More investigations are needed on how to develop preparation and deposition techniques in such a way that control the shape, size, distance between the particles. This may lead to a further improvement of the THz power emitted from such devices. Finally, we demonstrated that coating with MnFe2O4 nanoparticles could be used to improve the performance of photoconductive antennas in the THz region. Our experiments demonstrate that coatings with MnFe2O4−particles provided a new approach to increase the photocurrent density on silicon under CW illumination. In order to understand the effect ofMnFe2O4 nanoparticles on photo-excited silicon, a semiconductor model was proposed to describe this phenomenon. We used this model to calculate the transmission amplitudes of THz pulses transmitted through bare silicon substrates and silicon substrates covered by MnFe2O4 nanoparticles under laser irradiation with different powers. Because the effect of MnFe2O4 nanoparticles on silicon significantly provides an enhanced attenuation of terahertz wave, silicon substrates covered by MnFe2O4 nanoparticles have the potential to be used as an optical modulator in the THz region. This may lead to a costefficient component for THz systems operating in transmission mode. Furthermore, MnFe2O4 nanoparticles could be used for the implementation of novel optical devices.|
|Physical Description:||90 Pages|