Comparative efficiency and power assessment of optical photoconductive material-based terahertz sources for wireless communication systems

Electronic version of an article published as [Journal of Circuits, Systems and Computers, vol. 28, num. 1, 2018] [https://doi.org/10.1142/S0218126619500051] © [copyright World Scientific Publishing Company] [https://www.worldscientific.com/worldscinet/jcsc]Terahertz band has recently attracted the attention of the communication society due to its huge bandwidth and very high-speed wireless communications capability. It has been utilized in a variety of disciplines including physics, biology and astronomy for years; and the main concerns have always been obtaining highly efficient and high-power terahertz sources. Today, these problems are still the most important issues in establishing an operable wireless terahertz communication link. In this paper, recent studies in the field of terahertz source design are investigated based on the terahertz output power and efficiency. Solid-state sources and optical sources were comparatively reviewed with optical photoconductive material (OPM)-based methods which are combined with the terahertz antennas in the design phase generally. For wireless communication, the most suitable frequencies are between 0.3THz and 1THz due to the attenuation profile of the atmosphere. For this reason, based on the recently published studies, it has been observed that OPM and resonant tunneling diode-based sources are the most promising terahertz sources in terms of efficiency and power. Key issues and the main problems of terahertz photoconductive antennas which are the base of OPM method were also discussed in this paper.Peer ReviewedPostprint (author's final draft

Doping Profile Measurements in Silicon Using Terahertz Domain Spectroscopy (THz-TDS) Via Electrochemical Anodic Oxidation

Doping profiles are engineered to manipulate device properties and to determine electrical performances of microelectronic devices frequently. To support engineering studies afterward, essential information is usually required from physically characterized doping profiles. Secondary Ion Mass Spectrometry (SIMS), Spreading Resistance Profiling (SRP) and Electrochemical Capacitance Voltage (ECV) profiling are standard techniques for now to map profile. SIMS yields a chemical doping profile via ion sputtering process and owns a better resolution, whereas ECV and SRP produce an electrical doping profile detecting free carriers in microelectronic devices. The major difference between electrical and chemical doping profiles is at heavily doped regions greater than 1020 atoms/cm3. At the profile region over the solubility limit, inactive dopants induce a flat plateau and detected by electrical measurements only. Destructive techniques are usually designed as stand-alone systems to study impurities. For an in-situ process control purpose, non-contact methods, such as ellipsometry and non-contact capacitance voltage (CV) techniques are current under development. In this theses work, terahertz time domain spectroscopy (THz-TDS) is utilized to achieve electrical doping profile in both destructive and non-contact manners. In recent years the Terahertz group at Rochester Institute Technology developed several techniques that use terahertz pulses to non-destructively map doping profiles. In this thesis, we study a destructive but potentially higher resolution version of the terahertz based approach to map the profile of activated dopants and augment the non-destructive approaches already developed. The basic idea of the profile mapping approach developed in this MS thesis is to anodize, and thus oxidize to silicon dioxide, thin layers (down to below 10 nm) of the wafer with the doping profile to be mapped. Since the dopants atoms and any free carriers in the silicon oxide thin film are invisible to the terahertz probe this anodization step very effectively removes a ‘thin slice’ from the doping profile to be mapped. By iterating between anodization and terahertz measurements that detect only the ‘remaining’ non-oxidized portion of the doping profile one can re-construct the doping profile with significantly higher precision compared to what is possible by only a single non-destructive measurement of the un-anodized profile as used in the non-destructive version of our technique. In this MS thesis we explore all aspects of this anodization based variation of doping profile mapping using free space terahertz pulses. This includes a study of silicon dioxide thin film growth using a room temperature electrochemical oxidation process. Etching procedures providing the option to remove between successive anodization and terahertz measurement steps. THz-TDS measurements of successively anodized profiles will be compared with sheet resistance and SIMS measurements to benchmark and improve the new technique

Design, Analysis, and Applications of Optically-Activated Antennas and Dielectric Lenses Using Photosensitive Semiconducting Materials

PhDThe primary objective of the research is to investigate photosensitive semiconducting materials, mainly organic, and utilise them in antenna front-end systems and dynamic lenses for sub-THz applications. Mechanisms such as phase-shifting and photo-conductive switching are introduced in EM-devices to alter the antenna performance and behaviour. Using such mechanisms the devices are able to attain frequency, radiation pattern and polarisation reconfigurability. The common inorganic semiconductor, Si, and organic semiconductors such as poly 3-hexylthiophene (P3HT), [6,6]-Phenyl C61 butyric acid methyl ester (PCBM) have been extensively studied and used in the exemplar EM-devices developed for this thesis. In this research, novelty is deployed with the use of photosensitive semiconductors as a means of ‘tuning’ dielectrics to control the propagation of the emerging beam-field of an antenna. Both organic and inorganic photosensitive semiconductors have been implemented in this work. The research begins by exploring the physical properties of such photosensitive semiconductors at microwave frequencies. Medium-resistivity Si was characterised using a conventional microstrip transmission line and the conductivity of the Si piece in dark and active states were estimated by matching its transmission characteristics with the modelled Si in CST Studio Suite. Thereafter the modelled Si was used in an antenna design to estimate the reconfigurability of the device. However, inorganic semiconductors are being replaced with organic semiconductors because of their inflexibility in device fabrication. Organic polymers, on the other hand, are light in weight, can be cast onto any surface, when blended with an organic solvent, and also photo-excited using white light. Organic polymer heterojunction 95% P3HT: 5% PCBM was characterised and changes in the real and imaginary parts of the complex dielectric constant of the organic blend are measured in the range of –0.05 to –0.55 and +0.01 to +0.52 respectively, over the sub-THz frequency-domain. In order to demonstrate EM-control of a wave using a photo-sensitive material, a two-element patch antenna array using organic polymer P3HT-PCBM is fabricated and the functionality for antenna beam steering examined. Non-optimum illumination of the organic layer on the antenna patches, led to an asymmetric and perturbed beam steer. Hence, a novel optically triggered antenna has been proposed at S-Band (2 – 4 GHz), where sodalime glass is being used as lower substrate, ITO (Indium Tin Oxide), transparent to white-light, as the ground plane and transmission lines along with patches are modelled onto the upper substrate layer (P3HT:PCBM). The estimates of the dielectric changes in the organic polymer blend due to optical excitation were used as inputs in the modelled device to show the proof-of-concept for beam steering with such a phase-shifting device. In addition, the antenna design also demonstrated that with a small change in the real part of the permittivity of the substrate it is possible to generate a maximum beam steer of 5°, using an effective phase-shifting design in CST Studio Suite. At millimetre-wave or sub-terahertz frequencies, small changes in the dielectric with excitation-region depth comparable to the wavelength are plenty to manipulate the emerging wave of an antenna or lens. Hence, an optically-activated dynamic lens is proposed and designed to dynamically control millimetre-wave transmission using optical illumination. The lens acts as a graduated gateway for phase transmission by adjusting the spatial permittivity across the lens. A nearfield measurement system is used to analyse the performance of the lens over the WR-10 (75 – 110 GHz) waveband. The phase distribution of the electric field across the face of the plane organic lens shows a similar pattern in the spatial phase-distribution of the lens plane in the active state as that projected by the illuminating source, allowing for projection-angle-induced cosine errors. Hence the dynamic operation of the lens can be beneficial for beam controlling applications in imaging, surveillance and remote sensing in the mm-wave frequency-domain.Queen Mary Doctoral College Postgraduate Research Initiative Fun

Novel Materials and Devices for Terahertz Detection and Emission for Sensing, Imaging and Communication

Technical advancement is required to attain a high data transmission rate, which entails expanding beyond the currently available bandwidth and establishing a new standard for the highest data rates, which mandates a higher frequency range and larger bandwidth. The THz spectrum (0.1-10 THz) has been considered as an emerging next frontier for the future 5G and beyond technology. THz frequencies also offer unique characteristics, such as penetrating most dielectric materials like fabric, plastic, and leather, making them appealing for imaging and sensing applications. Therefore, employing a high-power room temperature, tunable THz emitters, and a high responsivity THz detector is essential. Dyakonov-theory Shur\u27s was applied in this dissertation to achieve tunable THz detection and emission by plasma waves in high carrier density channels of field-effect devices. The first major contribution of this dissertation is developing graphene-based THz plasmonics detector with high responsivity. An upside-down free-standing graphene in a field effect transistor based resonant room temperature THz detector device with significantly improved mobility and gate control has been presented. The highest achieved responsivity is ~3.1kV/W, which is more than 10 times higher than any THz detector reported till now. The active region is predominantly single-layer graphene with multi-grains, even though the fabricated graphene THz detector has the highest responsivity. The challenges encountered during the fabrication and measurement of the graphene-based detector have been described, along with a strategy to overcome them while preserving high graphene mobility. In our new design, a monolayer of hBN underneath the graphene layer has been deposited to increase the mobility and electron concentration rate further. We also investigated the diamond-based FETs for their potential characteristics as a THz emitters and detectors. Diamond\u27s wide bandgap, high breakdown field, and high thermal conductivity attributes make it a potential semiconductor material for high voltage, high power, and high-temperature operation. Diamond is a good choice for THz and sub-THz applications because of its high optical phonon scattering and high momentum relaxation time. Numerical and analytical studies of diamond materials, including p-diamond and n-diamond materials, are presented, indicating their effectiveness as a prospective contender for high temperature and high power-based terahertz applications These detectors are expected to be a strong competitor for future THz on-chip applications due to their high sensitivity, low noise, tunability, compact size, mobility, faster response time, room temperature operation, and lower cost. Furthermore, when plasma wave instabilities are induced with the proper biasing, the same devices can be employed as THz emitters, which are expected to have a higher emission power. Another key contribution is developing a method for detecting counterfeit, damaged, forged, or defective ICs has been devised utilizing a new non-destructive and unobtrusive terahertz testing approach to address the crucial point of hardware cybersecurity and system reliability. The response of MMICs, VLSI, and ULSIC to incident terahertz and sub-terahertz radiation at the circuit pins are measured and analyzed using deep learning. More sophisticated terahertz response profiles and signatures of specific ICs can be created by measuring a more significant number of pins under different frequencies, polarizations, and depth of focus. The proposed method has no effect on ICs operation and could provide precise ICs signatures. The classification process between the secure and unsecure ICs images has been explained using data augmentation and transfer learning-based convolution neural network with ~98% accuracy. A planar nanomatryoshka type core-shell resonator with hybrid toroidal moments is shown both experimentally and analytically, allowing unique characteristics to be explored. This resonator may be utilized for accurate sensing, immunobiosensing, quick switching, narrow-band filters, and other applications

Pulsed Free Space Photonic Vector Network Analyzers

Terahertz (THz) radiation (0.1–10 THz) has demonstrated great significance in a wide range of interdisciplinary applications due to its unique properties such as the capacity to penetrate optically opaque materials without ionizing effect, superior spatial resolution as compared to the microwave domain for imaging or ability to identify a vast array of molecules using THz fingerprinting. Advancements in generation and detection techniques, as well as the necessities of application-driven research and industry, have created a substantial demand for THz-range devices and components. However, progress in the development of THz components is hampered by a lack of efficient and affordable characterization systems, resulting in limited development in THz science and technology. Vector Network Analyzers (VNAs) are highly sophisticated well-established characterization instruments in the microwave bands, which are now employed in the lower end of the THz spectrum (up to 1.5 THz) using frequency extender modules. These modules are extremely expensive, and due to the implementation of hollow metallic waveguides for their configuration, they are narrowband, requiring at least six modules to achieve a frequency coverage of 0.2–1.5 THz. Moreover, they are susceptible to problems like material losses, manufacturing and alignment tolerances etc., making them less than ideal for fast, broadband investigation. The main objective of this thesis is to design a robust but cost-effective characterization system based on a photonic method that can characterize THz components up to several THz in a single configuration. To achieve this, we design architectures for the Photonic Vector Network Analyzer (PVNA) concept, incorporating ErAs:In(Al)GaAs-based photoconductive sources and ErAs:InGaAs-based photoconductive receivers, driven with a femtosecond pulsed laser operating at 1550 nm. The broadband photonic devices replace narrowband electronic ones in order to record the Scattering (S)-parameters in a free space configuration. Corresponding calibration and data evaluation methods are also developed. Then the PVNAs are configured, and their capabilities are validated by characterizing various THz components, including a THz isolator, a distributed Bragg Reflector, a Split-Ring Resonator array and a Crossed-Dipole Resonator (CDR) array, in terms of their S-parameters. The PVNAs are also implemented to determine the complex refractive index or dielectric permittivity and physical thickness of several materials in the THz range. Finally, we develop an ErAs:In(Al)GaAs-based THz transceiver and implement it in a PVNA configuration, resulting in a more compact setup that is useful for industrial applications. The feasibility of such systems is also verified by characterizing several THz components. The configured systems achieve a bandwidth of more than 2.5 THz, exceeding the maximum attainable frequency of the commercial Electronic Vector Network Analyzer (EVNA) extender modules. For the 1.1-1.5 THz band, the dynamic range of 47-35 dB (Equivalent Noise Bandwidth (ENBW) = 9.196 Hz) achieved with the PVNA is comparable to the dynamic range of 45-25 dB (ENBW = 10 Hz) of the EVNA. Both amplitude and phase of the S-parameters, determined by the configured PVNAs, are compared with simulations or theoretical models and showed excellent agreement. The PVNA could discern multi-peak and narrow resonance characteristics despite its lower spectral resolution (∼3-7 GHz) compared to the EVNA. By accurately determining the S-parameters of multiple THz components, the transceiver-based PVNA also demonstrated its exceptional competence. With huge bandwidth and simpler calibration techniques, the PVNA provides a potential solution to bridge the existing technological gap in THz-range characterization systems and offers a solid platform for THz component development, paving the way for more widespread application of THz technologies in research and industry

Design, Fabrication and Measurement of a Plasmonic Enhanced Terahertz Photoconductive Antenna

Generation of broadband terahertz (THz) pulses from ultrafast photoconductive antennas (PCAs) is an attractive method for THz spectroscopy and imaging. This provides a wide frequency bandwidth (0.1-4 THz) as well as the straightforward recovery of both the magnitude and phase of the transmitted and/or reflected signals. The achieved output THz power is low, approximately a few microwatts. This is due to the poor conversion of the femtosecond laser used as the optical pump to useable current inside the antenna semiconducting material. The majority of THz power comes from the photocarriers generated within ~ 100 nm distance from the antenna electrodes. However, the optical beam covers larger spot size, therefore much of the absorbed optical photons do not contribute to the THz power. The goal of this work is to advance the design, fabrication, and measurement of THz-PCAs to generate significantly improved output power. This work proposed a plasmonic enhanced thin-film photoconductive antenna to enhance optical carrier generation in the PCA. The electromagnetic wave equations were solved in order to compute the enhanced plasmonic field in the semiconductor. The Poisson’s and the drift-diffusion equations were solved in order to compute the carrier dynamics inside of the semiconductor. A parametric optimization was implemented in order to design the plasmonic nanodisks and the thickness of the ultrathin photoconductive layer. These solutions and optimizations were achieved using the commercial package COMSOL® Multiphysics model. The PCAs’ fabrication was accomplished using the electron beam lithography for patterning the plasmonic nanostructures, the molecular beam epitaxy for the sample growth, the lapping/selective etching for the epitaxial liftoff, and standard microfabrication practices for patterning the antenna and device packaging. The PCA was characterized utilizing a tunable pulsed laser system with a 100 fs pulse width for the optical excitation and a Gentec-EO pyroelectric power detector for measurement of the output THz power. Also, the spectral characterization of the PCA was conducted, in collaboration with Teraview LTD in their site at UK, using a THz time-domain spectroscopy experimental set-up. The results demonstrate the enhancement in the output THz power of the plasmonic thin-film PCAs in comparison with conventional THz-PCAs

The Design, Fabrication and Characterization of Integrated Photoconductive Antennas for On-Chip Terahertz Wave Radiation and Detection

Terahertz (THz) wave (between 0.1 and 10 THz) is attracting a lot of attention due to its unique properties that are favorable to various applications. These include non-ionizing radiation, better resolution than a microwave, unique spectral absorption, and an ability to propagate through many types of materials. It has been intensively researched in sensing and imaging technology for a wide range of applications in areas such as biology, pharmaceutical, food and drug control, medical science, and security screening. Driven by mostly scientific research interests, the majority of THz systems are more focused on system performance rather than system size, integration, and cost. Many THz applications aforementioned would be benefit from the compact integration of THz devices and other types of functional devices. This dissertation research focuses on developing a THz source based on heterogeneous thin film device integration. The demonstration shows a cost-effective integration approach and a feasibility to develop a THz integrated system that utilizes separately optimized LTG-GaAs based THz devices with other types of Si-based devices. The key aspect of the integration lies in the thin-film format of LTG-GaAs based THz devices, which allows their seamless integration on a final integration substrate and subsequent fabrication processes on the top of the THz devices. Using this approach, THz devices can be integrated on any host substrate (including organic and inorganic substrates), which gives a design freedom to enhance THz integrated system performances. Based on post-integration approach, the demonstrated method does not require significant modification of a host substrate technology. This allows THz functional devices to be integrated on various integration platforms including microfluidics, optics, and digital electronics. Intimate integration of THz devices with other functional devices will benefit a broad range of applications, which has limitations due to the current bulky THz systems

Generation of terahertz-modulated optical signals using AlGaAs/GaAs laser diodes

The Thesis reports on the research activities carried out under the Semiconductor-Laser Terahertz-Frequency Converters Project at the Department of Electronics and Electrical Engineering, University of Glasgow. The Thesis presents the work leading to the demonstration of reproducible harmonic modelocked operation from a novel design of monolithic semiconductor laser, comprising a compound cavity formed by a 1-D photonic-bandgap (PBG) mirror. Modelocking was achieved at a harmonic of the fundamental round-trip frequency with pulse repetition rates from 131 GHz up to a record-high frequency of 2.1 THz. The devices were fabricated from GaAs/AlGaAs material emitting at a wavelength of 860 nm and incorporated two gain sections with an etched PBG reflector between them, and a saturable absorber section. Autocorrelation studies are reported, which allow the device behaviour for different modelocking frequencies, compound cavity ratios, and type and number of intra-cavity reflectors to be analyzed. The highly reflective PBG microstructures are shown to be essential for subharmonic-free modelocking operation of the high-frequency devices. It was also demonstrated that the multi-slot PBG reflector can be replaced with two separate slots with smaller reflectivity. Some work was also done on the realisation of a dual-wavelength source using a broad-area laser diode in an external grating-loaded cavity. However, the source failed to deliver the spectrally-narrow lines required for optical heterodyning applications. Photomixer devices incorporating a terahertz antenna for optical-to microwave down-conversion were fabricated, however, no down-conversion experiments were attempted. Finally, novel device designs are proposed that exploit the remarkable spectral and modelocking properties of compound-cavity lasers. The ultrafast laser diodes demonstrated in this Project can be developed for applications in terahertz imaging, medicine, ultrafast optical links and atmospheric sensing

Cellular effects of terahertz waves

Significance: An increasing interest in the area of biological effects at exposure of tissues and cells to the terahertz (THz) radiation is driven by a rapid progress in THz biophotonics, observed during the past decades. Despite the attractiveness of THz technology for medical diagnosis and therapy, there is still quite limited knowledge about safe limits of THz exposure. Different modes of THz exposure of tissues and cells, including continuous-wave versus pulsed radiation, various powers, and number and duration of exposure cycles, ought to be systematically studied. Aim: We provide an overview of recent research results in the area of biological effects at exposure of tissues and cells to THz waves. Approach: We start with a brief overview of general features of the THz-wave–tissue interactions, as well as modern THz emitters, with an emphasis on those that are reliable for studying the biological effects of THz waves. Then, we consider three levels of biological system organization, at which the exposure effects are considered: (i) solutions of biological molecules;(ii) cultures of cells, individual cells, and cell structures; and (iii) entire organs or organisms; special attention is devoted to the cellular level. We distinguish thermal and nonthermal mechanisms of THz-wave–cell interactions and discuss a problem of adequate estimation of the THz biological effects’ specificity. The problem of experimental data reproducibility, caused by rareness of the THz experimental setups and an absence of unitary protocols, is also considered. Results: The summarized data demonstrate the current stage of the research activity and knowledge about the THz exposure on living objects. Conclusions: This review helps the biomedical optics community to summarize up-to-date knowledge in the area of cell exposure to THz radiation, and paves the ways for the development of THz safety standards and THz therapeutic applications