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 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

Enhanced Terahertz Radiation Generation of Photoconductive Antennas Based on Manganese Ferrite Nanoparticles

This paper presents a significant effect of manganese ferrite nanoparticles (MnFe2O4 NPs) on the increase of the surface photoconductivity of semiconductors. Herein, the optical characterization of photo-excited carriers of silicon coated with MnFe2O4 NPs was studied by using THz time-domain spectroscopy (THz-TDs). We observed that silicon coated with MnFe2O4 NPs provided a significantly enhanced attenuation of THz radiation in comparison with bare silicon substrates under laser irradiation. The experimental results were assessed in the context of a surface band structure model of semiconductors. In addition, photoconductive antennas coated with MnFe2O4 NPs significantly improved the efficiency of THz radiation generation and signal to noise ratio of the THz signal. This work demonstrates that coating with MnFe2O4 NPs could improve the overall performance of THz systems, and MnFe2O4 NPs could be further used for the implementation of novel optical devicesQ.Z. acknowledges a fellowship from the Chinese Scholarship Council. Part of the project was funded by the European Commission (grant Future NanoNeeds to WJP). Financial support from MINECO (MAT2015–74381-JIN to B.P., RYC-2014–16962 to P.dP.), the Consellería de Cultura, Educación e Ordenación Universitaria (Centro singular de investigación de Galicia accreditation 2016–2019, ED431G/09), and the European Regional Development Fund (ERDF) is gratefully acknowledgedS

THz-TDS for Detecting Glycol Contamination in Engine Oil

There continues to be a need for an in-situ sensor system to monitor the engine oil of internal combustion engines. Engine oil needs to be monitored for contaminants and depletion of additives. While various sensor systems have been designed and evaluated, there is still a need to develop and evaluate new sensing technologies. This study evaluated Terahertz time-domain spectroscopy (THz-TDS) for the identification and estimation of the glycol contamination of automotive engine oil. Glycol contamination is a result of a gasket or seal leak allowing coolant to enter an engine and mix with the engine oil. An engine oil intended for use in both diesel and gasoline engines was obtained. Fresh engine oil samples were contaminated with four levels of glycol (0 ppm, 150 ppm, 300 ppm, and 500 ppm). The samples were analyzed with THz-TDS and converted to frequency domain parameters of refractive index and absorption coefficient. While both parameters showed potential, the absorption coefficient had the best potential and was able to statistically discriminate among the four contamination levels

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 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

High accuracy terahertz time-domain system for reliable characterization of photoconducting antennas

We report on a terahertz (THz) time-domain system to characterize photoconductive THz emitters and detectors, that is designed for highest reproducibility. This system is excellently suited for studying the performance of THz sources and detectors in a systematic manner, either by varying the substrate materials or the geometrical parameters of metallic antenna contacts building a photoconductive switch. After confirming the reproducibility and stability of the system with errors of only 1.9% (over 3 h) and 2.6% (over 9 days), we use the system to compare the performance of five low temperature grown (LT) GaAs wafers with growth temperatures between 200°C and 300°C

THz-TDS for Detecting Glycol Contamination in Engine Oil

There continues to be a need for an in-situ sensor system to monitor the engine oil of internal combustion engines. Engine oil needs to be monitored for contaminants and depletion of additives. While various sensor systems have been designed and evaluated, there is still a need to develop and evaluate new sensing technologies. This study evaluated Terahertz time-domain spectroscopy (THz-TDS) for the identification and estimation of the glycol contamination of automotive engine oil. Glycol contamination is a result of a gasket or seal leak allowing coolant to enter an engine and mix with the engine oil. An engine oil intended for use in both diesel and gasoline engines was obtained. Fresh engine oil samples were contaminated with four levels of glycol (0 ppm, 150 ppm, 300 ppm, and 500 ppm). The samples were analyzed with THz-TDS and converted to frequency domain parameters of refractive index and absorption coefficient. While both parameters showed potential, the absorption coefficient had the best potential and was able to statistically discriminate among the four contamination levels