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

Open-path dual-frequency comb spectroscopy of methane from livestock production

Doctor of PhilosophyDepartment of PhysicsBrett D DePaolaBrian R WashburnThis project focuses on providing outdoor open-path spectroscopic measurements for detection of methane and other agriculturally significant gases over long periods of time in an agricultural setting. The use of dual-comb spectroscopy for remote sensing on agricultural sites has led to an aptly named system, the Agrocombs. The decomposition and fermentation of food performed by microbes in the stomach of ruminants, known as enteric fermentation, is one of the largest sources of anthropogenic methane emissions in the US due in large part by the dense population of livestock such as cattle. Several long-range open-path remote sensing techniques, such as Fourier transform infrared spectroscopy or tunable laser absorption spectroscopy, could be considered to detect and identify methane in an agricultural setting, but limitations in these techniques prove to be deterrents in their applications. Dual-comb spectroscopy provides a unique advantage of simultaneously measuring several significant gases (such as CH4, NH3, CO2, and H2O) with no external reference system or large structural support. Applying this method to agricultural processes began the journey of the Agrocombs system, a dual-comb pulsed laser system designed to be mobile and noninvasive enough to be placed on a site without disruption of operations, while performing long-term measurements of significant agricultural gases to result in concentration data. To prove the merit of this system, the Agrocombs research group performed a 2019 measurement at a KSU operated beef stocker site in parallel to a closed-path cavity ring-down system commonly used for trace gas measurements. The results of this experiment show an agreement between the two systems of 6% for methane, with the Agrocombs system providing a concentration precision of 1.25 ppm·m at 900 s. Additionally, the Agrocombs system was able to record concentrations for carbon dioxide, ammonia, and water vapor simultaneously without additional equipment. After a successful measurement in a feedlot system, where the cattle are confined in pens and present in large numbers, the next step has been to move towards a pasture to capture measurements of cattle in another important lifestyle, grazing. Grazing cattle in a pasture system provide a unique measurement potential for the Agrocombs system due to the low animal density and the presence of methane sinks that can detract from overall methane production and discharge, otherwise known as emissions. Traditional models for cattle emissions tend to lean towards the assumption that cattle contribute uniformly based on number of cattle in a system, but often neglect the complexity of a system’s additional factors to the gas cycle. Pastures provide more area for less animals, allowing for free roaming and independent grazing, which differs greatly from our previous measurement. Additionally, microbial activity in the soil may prove to act as a methane sink in native grasslands, reducing the overall contributions of the grazing system. While feedlot emissions were found to be approximately 137±86 µg/m2 /s, we expect that the contributing factors of less cattle in a larger area of interest, combined with the methane sink of microbial activity in the soil, will garner us a net methane emission in a pasture of an order of magnitude less than the feedlot. In order to measure emissions from a pasture, the Agrocombs project must achieve a precision of approximately 0.2 parts per billion (ppb), significantly smaller than the approximate 3 ppb in our feedlot measurement, determined through simulation. To test our precision and work towards this goal, we will conduct a controlled release experiment to mimic cattle in a pasture. This also allows for testing a newly packaged system and its accompanying equipment, as well as techniques to handle the ever-moving cattle and their large area of mobility. This thesis details the beginnings and preliminary results of a controlled release in a pasture, as well as the steps taken to achieve such precision needed for this difficult sensing measurement

Optimization of photomixers and antennas for continuous-wave terahertz emission

We have studied terahertz emission from interdigitated finger photomixers coupled to planar antenna structures. Using both pulsed and continuous-wave excitation, polarization measurements reveal that the antenna design dominates the properties of the radiated output at frequencies below 0.6 THz, while the efficiency at higher frequencies is additionally dependent on the design of the photomixer fingers. We have produced terahertz maps of the device, characterizing the photomixer by measuring the generated power as a function of the excitation position. Together, these measurements have allowed us to understand better the distinct roles of the photomixer and antenna in emission at different fre

Modern Applications in Optics and Photonics: From Sensing and Analytics to Communication

Optics and photonics are among the key technologies of the 21st century, and offer potential for novel applications in areas such as sensing and spectroscopy, analytics, monitoring, biomedical imaging/diagnostics, and optical communication technology. The high degree of control over light fields, together with the capabilities of modern processing and integration technology, enables new optical measurement systems with enhanced functionality and sensitivity. They are attractive for a range of applications that were previously inaccessible. This Special Issue aims to provide an overview of some of the most advanced application areas in optics and photonics and indicate the broad potential for the future