Photoconductive Terahertz Emitters and Detectors for the Operation with 1550 nm Pulsed Fiber Lasers

In this thesis, photoconductive terahertz (THz) emitters and detectors suitable for the excitation with femtosecond laser pulses centered on 1550 nm are investigated. The motivation for this study is the development of cost-efficient, flexible and rapid THz time-domain-spectroscopy (TDS) systems for the application in growing fields like non-destructive testing (NDT) and inline process monitoring. In order to achieve this goal, the physics of the generation and detection of THz radiation in photoconductors is investigated. The combination of experimental data with the analytic modeling of the carrier dynamics in THz photoconductors allows for a detailed understanding of the interplay between the growth conditions of the photoconductor and the properties of the fabricated THz device. In this work, three different photoconductive materials were studied as THz emitters and detectors. All these photoconductors contain layers of the ternary semiconductor indium gallium arsenide (InGaAs). When InGaAs is grown lattice matched to an indium phosphide (InP) substrate, the material can be excited by erbium doped femtosecond fiber lasers with a central wavelength around 1550 nm. Therefore, InGaAs is a predestinated absorber in photoconductive THz emitters and detectors. Aside from the common InGaAs layers, the photoconductors investigated in this thesis feature essentially different electrical and optical properties. The reason is that theoretical models and experimental results obtained within the last two decades revealed different demands on photoconductors for THz emitters and detectors. On the detector side, a sub-picosecond electron lifetime is required for the detection of broadband THz radiation with high dynamic range. In contrast, photoconductive materials for THz emitters require high breakdown fields and carrier mobility, whereas the electron lifetime is of minor importance. Therefore, the first part of this work is dedicated to the development of InGaAs-based photoconductors for THz emitters and receivers. Photoconductors with sub-picosecond electron lifetimes were obtained by low-temperature growth of InGaAs with molecular beam epitaxy (MBE). At temperatures below 300 °C the growth is non-stoichiometric and arsenic antisites are incorporated as point defects into the lattice. When these antisites are ionized they serve as fast trapping and recombination centers. In this work, it is shown that the concentration of the (ionized) antisites can be controlled by the growth temperature, by using an additional p-dopant (beryllium), and by the temperature and the duration of a post-growth annealing step. Electron lifetimes as short as 140 fs were obtained. The precise adjustment of all these parameters allowed for the design and the fabrication of THz receivers with a spectral bandwidth of up to 6 THz and a peak dynamic range exceeding 95 dB. For THz emitters, a high mobility, which is generally equivalent to a low defect density, is required in order to enable the efficient acceleration of the photoexcited carriers in the electric field applied to the emitter. Due to the high density of defects, low-temperature-grown (LTG) InGaAs based photoconductors are not the material of choice for THz emitters. Instead, a material comprising almost defect free layers of InGaAs surrounded by InAlAs barriers containing a high density of deep defects was used. These properties were achieved at growth temperatures close to 400 °C in a MBE system. At those temperatures, alloying forms deep defects inside the InAlAs layers, whereas InGaAs grows almost defect free. A THz-power of up to 112 μW ± 7 μW was measured for emitters fabricated from this photoconductor, which is an increase by a factor of 100 compared to emitters made of the LTG material. By combining the optimized photoconductive emitters and receivers compact THz-TDS systems with up to 6 THz bandwidth and 90 dB peak dynamic range were realized. In addition, an all fiber-coupled THz spectrometer with kHz measurement rate as well as a fully fibercoupled near-field imaging system with a lateral resolution of 100 μm was demonstrated with these optimized photoconductive devices. However, a critical disadvantage of individual THz emitter and detector devices appears when THz-TDS measurements are performed in reflection geometry. Since many applications in NDT and in-line process monitoring allow only one side access to the sample under test, reflection measurements are the common use-case of THz-TDS in these fields. In this thesis, a fiber-coupled, monolithically integrated THz transceiver was developed, which combines the emitter and the receiver on a single photoconductive chip. As the photoconductor, Be-doped LTG-InGaAs/InAlAs with 0.5 ps electron lifetime was used in order to enable a broadband detection. The optical coupling of the transceiver was realized with the help of a polymer waveguide chip. With a bandwidth of 4.5 THz and a peak dynamic range larger than 70 dB this THz transceiver showed a significant performance increase compared to previous transceiver concepts (2 THz bandwidth and 50 dB peak dynamic range). In order to further increase the performance of THz transceivers a novel photoconductor had to be developed, which combines the required properties of THz emitters and detectors in the same material. For this purpose, iron (Fe) doped InGaAs grown by MBE was investigated. At growth temperatures close to 400 °C iron could be incorporated homogenously up to concentrations of 5 × 1020 cm-3. The resulting material combined sub-picosecond electron lifetime with high breakdown fields and high mobility. Applied as a photoconductive emitter, 75 μW ± 5 μW of radiated THz power were measured. As a detector, THz pulses with a bandwidth of up to 6 THz and a peak dynamic range of 95 dB were obtained. Hence, Fe-doped InGaAs has not only the potential to replace the relatively complex state-of-the art photoconductors, it also bears great potential for future integrated THz devices. In conclusion, the systematic study of the electrical properties and the carrier dynamics in InGaAs-based photoconductive materials led to significant improvements of individual THz emitter and detector devices. The detectable bandwidth was increased by 50 % from below 4 THz to 6 THz and the emitted THz power was enhanced by a factor of 100. Further, the knowledge from these studies was exploited for the fabrication of a fiber-coupled, monolithically integrated THz transceiver with a 4.5 THz bandwidth and 70 dB peak dynamic range. These results are a significant increase in THz performance compared to previous transceiver concepts (2 THz bandwidth and 50 dB dynamic range). In order to allow for further improvements of THz transceivers and integrated THz devices, Fe-doped InGaAs was investigated as a photoconductive emitter and detector. Due to the unique combination of subpicosecond electron lifetime, high resistivity (> 2 Ω cm) and high mobility (> 900 cm2V-1s-1) Fe-doped InGaAs showed a performance comparable to the optimized THz photoconductors. Hence, the results presented in this work pave the way for compact and integrated THz devices for applications in industrial environments

THz Time-Domain Ellipsometer for Material Characterization and Paint Quality Control with More Than 5 THz Bandwidth

Quality control of car body paint in the automotive industry is a promising industrial application of terahertz technology. Terahertz time-domain spectroscopy in reflection geometry enables accurate, fast, and nondestructive measurement of individual layer thicknesses of multi-layer coatings. For high precision thickness measurements, the frequency dependent complex refractive index of all layers must be calibrated very accurately. THz time-domain ellipsometry is self-referencing and provides reliable, frequency resolved material properties with high signal-to-noise ratio. The method is characterized by a high sensitivity to optical material properties and layer thicknesses. We present characterization results in the frequency range 0.1–6 THz for typical automotive paints and different substrates such as polypropylene (PP), which features a high material anisotropy. We demonstrate that the broadband material properties derived from ellipsometry allow for inline thickness measurements of multi-layer car body paints with high accuracy

Fiber Coupled Transceiver with 6.5 THz Bandwidth for Terahertz Time-Domain Spectroscopy in Reflection Geometry

We present a fiber coupled transceiver head for terahertz (THz) time-domain reflection measurements. The monolithically integrated transceiver chip is based on iron (Fe) doped In0.53Ga0.47As (InGaAs:Fe) grown by molecular beam epitaxy. Due to its ultrashort electron lifetime and high mobility, InGaAs:Fe is very well suited as both THz emitter and receiver. A record THz bandwidth of 6.5 THz and a peak dynamic range of up to 75 dB are achieved. In addition, we present THz imaging in reflection geometry with a spatial resolution as good as 130 µm. Hence, this THz transceiver is a promising device for industrial THz sensing applications

Wireless THz link with optoelectronic transmitter and receiver

Photonics might play a key role in future wireless communication systems that operate at terahertz (THz) carrier frequencies. A prime example is the generation of THz data streams by mixing optical signals in high-speed photodetectors. Over previous years, this concept has enabled a series of wireless transmission experiments at record-high data rates. Reception of THz signals in these experiments, however, still relied on electronic circuits. In this paper, we show that wireless THz receivers can also greatly benefit from optoelectronic signal processing techniques, in particular when carrier frequencies beyond 0.1 THz and wideband tunability over more than an octave is required. Our approach relies on a high-speed photoconductor and a photonic local oscillator for optoelectronic downconversion of THz data signals to an intermediate frequency band that is easily accessible by conventional microelectronics. By tuning the frequency of the photonic local oscillator, we can cover a wide range of carrier frequencies between 0.03 and 0.34 THz. We demonstrate line rates of up to 10 Gbit/s on a single channel and up to 30 Gbit/s on multiple channels transmitted over a distance of 58 m. To the best of our knowledge, our experiments represent the first demonstration of a THz communication link that exploits optoelectronic signal processing techniques both at the transmitter and the receiver

Wireless THz link with optoelectronic transmitter and receiver

Photonics might play a key role in future wireless communication systems that operate at terahertz (THz) carrier frequencies. A prime example is the generation of THz data streams by mixing optical signals in high-speed photodetectors. Over previous years, this concept has enabled a series of wireless transmission experiments at record-high data rates. Reception of THz signals in these experiments, however, still relied on electronic circuits. In this paper, we show that wireless THz receivers can also greatly benefit from optoelectronic signal processing techniques, in particular when carrier frequencies beyond 0.1 THz and wideband tunability over more than an octave is required. Our approach relies on a high-speed photoconductor and a photonic local oscillator for optoelectronic downconversion of THz data signals to an intermediate frequency band that is easily accessible by conventional microelectronics. By tuning the frequency of the photonic local oscillator, we can cover a wide range of carrier frequencies between 0.03 and 0.34 THz. We demonstrate line rates of up to 10 Gbit/s on a single channel and up to 30 Gbit/s on multiple channels transmitted over a distance of 58 m. To the best of our knowledge, our experiments represent the first demonstration of a THz communication link that exploits optoelectronic signal processing techniques both at the transmitter and the receiver

Photoconductive Terahertz Emitters and Detectors for the Operation with 1550 nm Pulsed Fiber Lasers

In this thesis, photoconductive terahertz (THz) emitters and detectors suitable for the excitation with femtosecond laser pulses centered on 1550 nm are investigated. The motivation for this study is the development of cost-efficient, flexible and rapid THz time-domain-spectroscopy (TDS) systems for the application in growing fields like non-destructive testing (NDT) and inline process monitoring. In order to achieve this goal, the physics of the generation and detection of THz radiation in photoconductors is investigated. The combination of experimental data with the analytic modeling of the carrier dynamics in THz photoconductors allows for a detailed understanding of the interplay between the growth conditions of the photoconductor and the properties of the fabricated THz device. In this work, three different photoconductive materials were studied as THz emitters and detectors. All these photoconductors contain layers of the ternary semiconductor indium gallium arsenide (InGaAs). When InGaAs is grown lattice matched to an indium phosphide (InP) substrate, the material can be excited by erbium doped femtosecond fiber lasers with a central wavelength around 1550 nm. Therefore, InGaAs is a predestinated absorber in photoconductive THz emitters and detectors. Aside from the common InGaAs layers, the photoconductors investigated in this thesis feature essentially different electrical and optical properties. The reason is that theoretical models and experimental results obtained within the last two decades revealed different demands on photoconductors for THz emitters and detectors. On the detector side, a sub-picosecond electron lifetime is required for the detection of broadband THz radiation with high dynamic range. In contrast, photoconductive materials for THz emitters require high breakdown fields and carrier mobility, whereas the electron lifetime is of minor importance. Therefore, the first part of this work is dedicated to the development of InGaAs-based photoconductors for THz emitters and receivers. Photoconductors with sub-picosecond electron lifetimes were obtained by low-temperature growth of InGaAs with molecular beam epitaxy (MBE). At temperatures below 300 °C the growth is non-stoichiometric and arsenic antisites are incorporated as point defects into the lattice. When these antisites are ionized they serve as fast trapping and recombination centers. In this work, it is shown that the concentration of the (ionized) antisites can be controlled by the growth temperature, by using an additional p-dopant (beryllium), and by the temperature and the duration of a post-growth annealing step. Electron lifetimes as short as 140 fs were obtained. The precise adjustment of all these parameters allowed for the design and the fabrication of THz receivers with a spectral bandwidth of up to 6 THz and a peak dynamic range exceeding 95 dB. For THz emitters, a high mobility, which is generally equivalent to a low defect density, is required in order to enable the efficient acceleration of the photoexcited carriers in the electric field applied to the emitter. Due to the high density of defects, low-temperature-grown (LTG) InGaAs based photoconductors are not the material of choice for THz emitters. Instead, a material comprising almost defect free layers of InGaAs surrounded by InAlAs barriers containing a high density of deep defects was used. These properties were achieved at growth temperatures close to 400 °C in a MBE system. At those temperatures, alloying forms deep defects inside the InAlAs layers, whereas InGaAs grows almost defect free. A THz-power of up to 112 μW ± 7 μW was measured for emitters fabricated from this photoconductor, which is an increase by a factor of 100 compared to emitters made of the LTG material. By combining the optimized photoconductive emitters and receivers compact THz-TDS systems with up to 6 THz bandwidth and 90 dB peak dynamic range were realized. In addition, an all fiber-coupled THz spectrometer with kHz measurement rate as well as a fully fibercoupled near-field imaging system with a lateral resolution of 100 μm was demonstrated with these optimized photoconductive devices. However, a critical disadvantage of individual THz emitter and detector devices appears when THz-TDS measurements are performed in reflection geometry. Since many applications in NDT and in-line process monitoring allow only one side access to the sample under test, reflection measurements are the common use-case of THz-TDS in these fields. In this thesis, a fiber-coupled, monolithically integrated THz transceiver was developed, which combines the emitter and the receiver on a single photoconductive chip. As the photoconductor, Be-doped LTG-InGaAs/InAlAs with 0.5 ps electron lifetime was used in order to enable a broadband detection. The optical coupling of the transceiver was realized with the help of a polymer waveguide chip. With a bandwidth of 4.5 THz and a peak dynamic range larger than 70 dB this THz transceiver showed a significant performance increase compared to previous transceiver concepts (2 THz bandwidth and 50 dB peak dynamic range). In order to further increase the performance of THz transceivers a novel photoconductor had to be developed, which combines the required properties of THz emitters and detectors in the same material. For this purpose, iron (Fe) doped InGaAs grown by MBE was investigated. At growth temperatures close to 400 °C iron could be incorporated homogenously up to concentrations of 5 × 1020 cm-3. The resulting material combined sub-picosecond electron lifetime with high breakdown fields and high mobility. Applied as a photoconductive emitter, 75 μW ± 5 μW of radiated THz power were measured. As a detector, THz pulses with a bandwidth of up to 6 THz and a peak dynamic range of 95 dB were obtained. Hence, Fe-doped InGaAs has not only the potential to replace the relatively complex state-of-the art photoconductors, it also bears great potential for future integrated THz devices. In conclusion, the systematic study of the electrical properties and the carrier dynamics in InGaAs-based photoconductive materials led to significant improvements of individual THz emitter and detector devices. The detectable bandwidth was increased by 50 % from below 4 THz to 6 THz and the emitted THz power was enhanced by a factor of 100. Further, the knowledge from these studies was exploited for the fabrication of a fiber-coupled, monolithically integrated THz transceiver with a 4.5 THz bandwidth and 70 dB peak dynamic range. These results are a significant increase in THz performance compared to previous transceiver concepts (2 THz bandwidth and 50 dB dynamic range). In order to allow for further improvements of THz transceivers and integrated THz devices, Fe-doped InGaAs was investigated as a photoconductive emitter and detector. Due to the unique combination of subpicosecond electron lifetime, high resistivity (> 2 Ω cm) and high mobility (> 900 cm2V-1s-1) Fe-doped InGaAs showed a performance comparable to the optimized THz photoconductors. Hence, the results presented in this work pave the way for compact and integrated THz devices for applications in industrial environments

Terahertz transceivers

The invention relates to a terahertz transceiver, comprising at least a first and a second antenna (111, 112), wherein the first and/or the second antenna (111, 112) is a dipole antenna comprising a dipole section (113, 114), wherein the dipole section (113, 114) has a gap through which light can be radiated onto the photoconductive material, and wherein a first ending of the dipole section (113, 114) is connected to a first feedline (119a, 120a) and a second ending of the dipole section (113, 114) is connected to a second feedline (119b, 120b), the feedlines (119a, 119b, 120a, 120b) extending with an angle to the dipole section (113, 114). According to the invention, the first and/or the second antenna (111, 112) has an asymmetric design, wherein a first section of at least one of the feedlines (119a, 119b, 120a, 120b) extending on one side of the dipole section (113, 114) is longer than a second section of the at least one feedline (119a, 119b, 120a, 120b) extending on the other side of the dipole section (113, 114) and/or at least one of the feedlines (119a, 119b, 120a, 120b) extends on one side of the dipole section (113, 114), only

Beam Profile Characterisation of an Optoelectronic Silicon Lens-Integrated PIN-PD Emitter between 100 GHz and 1 THz

Knowledge of the beam profiles of terahertz emitters is required for the design of terahertz instruments and applications, and in particular for designing terahertz communications links. We report measurements of beam profiles of an optoelectronic silicon lens-integrated PIN-PD emitter at frequencies between 100 GHz and 1 THz and observe significant deviations from a Gaussian beam profile. The beam profiles were found to differ between the H-plane and the E-plane, and to vary strongly with the emitted frequency. Skewed profiles and irregular side-lobes were observed. Metrological aspects of beam profile measurements are discussed and addressed

Drinking Water Supply in Rural Africa Based on a Mini-Grid Energy System—A Socio-Economic Case Study for Rural Development

Water is an essential resource required for various human activities such as drinking, cooking, growing food, and personal hygiene. As a key infrastructure of public services, access to clean and safe drinking water is an essential factor for local socio-economic development. Despite various national and international efforts, water supply is often not guaranteed, especially in rural areas of Africa. Although many water resources are theoretically available in these areas, bodies of water are often contaminated with dangerous pathogens and pollutants. As a result, people, often women and children, have to travel long distances to collect water from taps and are exposed to dangers such as physical violence and accidents on their way. In this article, we present a socio-economic case study for rural development. We describe a drinking water treatment plant with an annual capacity of 10,950 m3 on Kibumba Island in Lake Victoria (Tanzania). The plant is operated by a photovoltaic mini-grid system with second-life lithium-ion battery storage. We describe the planning, the installation, and the start of operation of the water treatment system. In addition, we estimate the water prices achievable with the proposed system and compare it to existing sources of drinking water on Kibumba Island. Assuming a useful life of 15 years, the installed drinking water system is cost-neutral for the community at a cost price of 0.70 EUR/m3, 22% less than any other source of clean water on Kibumba Island. Access to safe and clean drinking water is a major step forward for the local population. We investigate the socio-economic added value using social and economic key indicators like health, education, and income. Hence, this approach may serve as a role model for community-owned drinking water systems in sub-Saharan Africa