Resonant Tunnelling Diode Epitaxial Wafer Design, Manufacture and Characterisation

Background

The trends predicted by Moore’s and Edholm’s laws many years ago still represent a challenge for integrated electronics and the rising bandwidth demand. Researcher and industry are facing the lack of technologies, both optical/electrical, that can operate in the THz spectrum. An extensive scientific study in solid-state device is chasing to find solutions for both THz sources and detector for wireless short-range communication and both short/long-range line-of-sight, that are just a first example of beneficiary of this type of technology. Principal limitations of the actual state-of-the-art in this field are the power consumption, the working temperature, the maximum frequency achievable, the tunability and the compact dimension of the entire system (device-oscillator-antenna).

Ph.D. project

In details, this project is focused on the double barrier resonant tunnelling diode (DB-RTD), that is considered a promising solid-state device for the development of compact, room temperature and low power THz technologies. The main purpose of and RTD is becoming part of an oscillator as a THz source, but the device design and the epitaxial quality are crucial to set a trade-off between the maximum power in output and the maximum frequency achievable (also depending on the oscillator specifics). The output power is the principal limitation of this technology that decreases consistently increasing the oscillation frequency. Looking at the RTDs characteristics, the principal figures of merit are the peak-to-valley-current-ratio PVCR, the peak current density, and the peak-to-valley-voltage-ratio.

The goal of the project is then enhancing the output power from high Jpeak -DB-RTD design working on the epitaxial wafer design and the growth quality. The main objective is to increase the PVCR, first optimizing the I peak current and then investigating solution for reducing the valley current. The actual state-of-the-art does not present an epitaxial design that can maximize both the RTD figure of merit and then satisfy all the oscillator specifics. The reason for this absence in the device design is due to the quality of the epitaxial growth. It is important to highlight that the RTD epitaxial structure is sensible to layers imperfection, even in the order of few mono layers ML (1ML=0.293 nm).

Methodology

To achieve these goals the ESR will first developing expertise in  wafer characterization method, especially the non-destructive ones (Low temperature photoluminescence spectroscopy LTPL and LTPL excitation, high resolution X-ray diffraction and reflection HRXRD and XRR) to create a fast characterization scheme and research correlations between the epitaxial characteristics (and defects) and the device performance, quantified by the already mentioned figure of merits.
In first instance, MBE growth methods are going to be explored in order to obtain QW interface perfection and compositional uniformity. In second instance, regrowth by MOVPE will be used to investigate and set the basis for a future low-cost volume manufacture.

Triple Barrier Resonant Tunnelling Diodes for Signal Generation and Detection

Background

THz detectors are key components in enabling THz applications, high-speed and high-sensitivity detectors operating at room temperatures which are also compact, are needed. Direct detection systems utilizing single or multiple diodes have the advantages of broad bandwidth, and simplified circuitry. Furthermore, integrated on planar technology or with an on-chip antenna they can be conveniently used in compact array receiver architectures, for this reasons, solid state diode based detectors are an attractive solution.

Schottky diode based detectors represent due to their high speed and relatively simple fabrication procedure a classical choice to implement solid-state RF detectors. Excellent sensitivity and noise equivalent power performance for III-V semiconductor SBD diodes has been reported in the literature. As a promising alternative, resonant-tunnelling-based devices, provide the most compact and energy efficient semiconductor device operating at THz frequencies. Among those, the Triple Barrier Resonant Tunneling Diode (TB-RTD) exhibits an asymmetrical current-voltage characteristic, with a strong zero-bias directivity. This characteristic makes the TB-RTD a promising device for high-responsivity and low NEP at THz frequencies direct detection.

Research objectives

The final goal of this project is the design and characterisation of solid-state THz detectors based on the Triple-Barrier Resonant Tunneling Diode (TB-RTD) as a semiconductor device operating at zero-bias condition. zero-bias THz detection schemes eliminate the biasing circuitry and the accompanying shot noise, with consequent advantages in terms of reduced complexity and compactness of the circuit.

The complete detector structure is composed of the TB-RTD monolithically integrated into an on-chip antenna, which is in turn placed on an hyper-hemispherical silicon lens at the purpose of collecting the incoming power of the THz signal travelling in the free space. Various types of planar antennas are investigated with the ultimate goal to achieve ultra-wideband low-power signal detection.

Method and early results

The on-chip antenna proprieties at THz frequencies are evaluated through extensive full-wave EM simulations. In particular, the antenna’s radiation proprieties and the matching condition with the TB-RTD are investigated at the purpose of obtaining the best coupling of the incoming THz plane wave. Furthermore, the integrated antenna/RTD rectification proprieties are investigated through non-linear circuit simulations.

The antenna/RTD structure is fabricated by optical lithography on the InP substrate as the last made metallisation layer, after the TB-RTD fabrication. A set of quasi-optical THz lens is used to guide a source signal from a horn antenna to the zero-bias detector placed on the hyper-hemispherical silicon lens. The frequency of the source signal is provided through frequency extenders working at the appropriate measurement frequency bands.

The measured DC voltage drop over the diode is then recorded through a voltmeter, and a broadband detection capability of the TB-RTD device is expected.

Antimony and Indium Arsenide based Resonant Tunnelling Devices for High-Speed and Mid-Infrared Applications

For various practical applications such as THz-radar or THz-imaging, it is of paramount importance to further enhance output power and frequency. Therefore, TeraApps aims to demonstrate RTDs operated as high-frequency oscillators with output powers  mW at fundamental oscillation frequencies of  GHz. For frequencies of  GHz and  THz, the pursued output powers are  mW and  mW, respectively.

The key to achieve these goals lies within an improved heterostructure design as well as an impeccable growth and fabrication of these heterostructures. There are two design approaches: (i) increasing the peak current density enables smaller devices and higher bandwidths. (ii) Maximizing the peak-to-valley current ratio (PVCR) and the voltage span () between peak and valley will increase the output power. It has been and still is a huge challenge to achieve both approaches at the same time, and usually a trade-off needs to be chosen. For example, increasing the peak current density is typically achieved by employing thinner tunneling barriers, which at the same time however reduces the PVCR and leads to decreased reproducibility of the heterostructure across the wafer.

The studied RTDs can be vaguely categorized into three categories depending on the material system used – GaAs-based RTDs, which incorporate a Al(Ga)As/GaAs/Al(Ga)As double barrier structure (DBS), InP-based RTDs, which incorporate an AlAs/InGaAs/AlAs DBS, and (In)GaSb-based RTDs, which incorporate either a Al(As)Sb/InAs/Al(As)Sb or Al(As)Sb/GaSb/Al(As)Sb DBS. InP-based RTDs are mostly explored, where record performances in terms of bandwidth have been demonstrated.

In the project “Antimony and Indium Arsenide based Resonant Tunnelling Devices for High-Speed and Mid-Infrared Applications” we will design, fabricate and characterize Antimony and Indium Arsenide based Resonant Tunnelling Devices. For antimonide based RTDs, in addition to the epitaxial growth challenges, device processing is highly demanding due to the necessary sidewall passivation to reduce sidewall leakage currents.

Optimization of heterostructure design requires accurate control of e.g. the doping profiles, layer thickness and mole fraction of ternary and quaternary alloys. Characterization of these parameters is carried out via different characterization tools such as scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution X-ray diffraction (HR-XRD), time-of-flight secondary-ion mass spectroscopy (TOF-SIMS), and many more. In addition to growth parameter characterizations, opto-electronic characterization techniques, e.g. temperature dependent current-voltage characteristic measurements need to be employed  to test and study different heterostructure design layouts. To achieve the objectives an in-depth understanding of the high-precision epitaxial growth by MBE, material parameters, and proficient nanofabrication needs to be achieved.

The research will focus on the design, fabrication (epitaxial growth and nano-structuring) and characterization of resonant tunneling structures in the InAs and antimonide material system on InAs and GaSb-substrates. For this purpose, the Gottfried-Landwehr-Laboratory for Nanotechnology (GLLN) at University of Würzburg, utilizes several state-of-the-art gas-source and solid-source MBE-systems. The experimental work will be comprised of the epitaxial growth of samples containing resonant tunneling structures, the characterization of the barrier and quantum well layer morphology by use of scanning and transmission electron microscopy and atomic force microscopy, as well as their optical properties by several spectroscopic setups available at the department. Electrical transport measurements will be conducted to characterize the electrical performance of the grown resonant tunneling structures as high-speed and MIR optoelectronic devices.

Room Temperature High-Speed Resonant Tunnelling Diodes for Single Photon Detectors and Optically Controlled Oscillators

Resonant Tunneling Diodes and Resonant Tunneling Diodes-Photodetectors (RTD & RTD-PD) are promising devices for various applications including GHz to THz oscillators and high sensitivity photon detectors due to their ultra-high frequency, ultra-high speed, low power, and high sensitivity characteristic. In this project, our main goal is to obtain high-speed resonant tunnelling diode single photon detectors and optically controlled oscillators at room temperature. The RTD will be a double barrier structure which consists of a few nm thick undoped narrow band gap material layer (the quantum well) that is sandwiched between two undoped large band gap layers (the two barriers). Our RTD structures will be grown on InP and will consists of InGaAs/AlAs. It will be used like RTDs single photon detectors for visible and near infrared wavelengths with low dark count rates and high quantum efficiencies at room temperature.

Our first goal is to obtain excellent epitaxial heterostructure qualities that enable good RTD characteristics with large peak current densities and peak-to-valley current ratios. The samples will be grown by molecular beam epitaxy (MBE). Thanks to this technique, epitaxial layers can be grown and controlled with monolayer precision which is of paramount for resonant tunnelling diodes. During the growth, we will optimise important growth related parameters (substrate temperature, flux ratio, growth rates etc.) and study in detail the influence of the respective parameter on the surface morphology, interface and heterostructure quality.

After obtaining an excellent RTDs heterostructure quality, our second goal is to investigate the electro-optical characteristics of the RTD-PD and to acquire the important figure of merits for photodetectors, i.e. sensitivity, quantum efficiency, noise equivalent power and dark count rates. Those will be studied in detail as a function of external control parameters e.g. temperature, to determine limitations provided by the hole accumulation dynamics that is responsible for the ultra-high gain of RTD-PD. This will enable us get a deeper understanding of InGaAs/AlAs based resonant tunnelling diode photodetectors and oscillators on InP substrates and enable us to significantly push forward the state-of-the art and to realize high-speed resonant tunnelling diode single photon detectors and optically controlled oscillators at room temperature.

Displacement current in quantum devices at THz frequencies

Our main research activity is based on the study of quantum electron devices with nanoscale dimensions working at THz frequencies. Our research activity covers a wide spectrum, from foundational physics till practical engineering.

Within this innovative training TeraApps network, our first goal is giving support to other experimental partners mainly by simulating, designing and modelling Resonant tunneling diodes. We have developed a unique simulation tool, based on time-dependent quantum Monte Carlo simulations using quantum trajectories, especially suited for engineering interpretation of quantum transport at THz frequencies. The outcome of our research activity is expected to be the comparison of experimental results with our simulated data to help in analyzing and understanding the link between quantum physical interpretation and circuit engineering models of THz devices.

The second goal of our research activity is proposing new electron devices, based on displacement current, working at the boundary between electronic and electromagnetic technologies. We argue that at THz frequencies the manipulation of information in nanoscale devices can be performed usign the displacement current, instead of the usual electron dynamics (ransport of energy without transit of charge). We argue that such new type of devices will offer important practical advantages such elimination of joule effects-related phenomena and improvement in device’s speed.

Extensive scientific research in the field of solid−state electronic/optical devices is chasing the continuously improving performance requirements imposed by modern multimedia applications by providing ultra−high speed multi−gigabit wireless communication technology which, according to Edholm’s law and future demand analysis, will have to accommodate, in the near future, data rate demands up to ≈ 100 Gb/s. Particularly interesting application scenarios rely on both long/short−range Line−Of−Sight (LOS)/Non−Line−Of−Sight (NLOS) operation, such as Wireless Local Area Networks (WLAN), Wireless Personal Area Networks (WPAN), wireless connections in data centres, kiosk downloading, wireless back−hauling, chip−to−chip interconnects and nano−cells.

The current state−of−the−art wireless communication systems operating in the low microwave range are limited by the narrow bandwidth, despite several efforts to improve spectral efficiency through advanced modulation and signal processing techniques. Therefore, future candidate technologies will need to operate in the higher bands, with a high interest in the still unregulated terahertz (THz) gap, consisting in a middle ground frequency range between low microwaves (Extremely High Frequency, EHF) and Far−infrared light (FIR), positioned between 300 GHz − 3 THz (with corresponding free−space wavelengths of 1 − 0.1 mm). This allows to take advantage of the wider bandwidth and, in principle, it allows to achieve higher data rates.

Current artificial THz sources (regardless huge−size systems such as gyrotrons and synchrotrons or other kinds of sources such as Backward Wave Oscillator (BWO) and Schottky diode/varicap multipliers) are typically photonic−based, such as organic gas Far−Infrared Lasers (FIR), Free−Electron Lasers (FEL) and Quantum Cascade Lasers (QCL). However, these systems are usually complex, cumbersome and some need proper cooling (QCLs work at T < 100 K in the THz regime), which makes them not the best candidates for compact wireless communication systems. Therefore, the key point consists in designing compact, integrated and room temperature THz sources (and detectors) which will allow to bring THz wireless communications technology to a widespread consumer marketplace.

From the semiconductor devices side, various types of sources have been investigated with electronic and optical devices. In the optical devices side, p−type germanium lasers and Quantum Cascade Lasers have been studied. Continuous−Wave photomixer sources have also extensively studied. Electron devices have also been intensively investigated from the millimetre (sub−THz) and sub−millimetre (THz) wave side. In particular, transistors including Heterojunction Bipolar Transistors (HBTs), High Electron Mobility Transistors (HEMTs) and silicon Complementary Metal−Oxide Semiconductor (Si-CMOS) transistors are making significant progress in operation frequency due to the easiness of integration, even though they are usually narrow−band (scaling is not straightforward due to short−channel effects, ...) and have high−phase noise. Research studies on Tunneling Transit−Time (TUNNETT) diodes (wide bandwidth, low phase noise, low−power), Impact ionization Avalanche Transit−Time (IMPATT) diodes (high−power, limited cut−off frequency, high phase noise) and Gunn diodes (lower power than IMPATT at the same frequency) have also been reported, even if they are all characterized by low DC−RF conversion efficiency and they are incompatible with monolithic integration (monolithic microwave integrated circuit technology, MMIC).

Born as an improvement of Tunnel Diodes (TDs), Resonant Tunneling Diodes (RTDs) are the fastest demonstrated pure solid−state electronic devices operating at room temperature (contrarily to QCLs), with frequencies which are approaching 2 THz. This is because they take advantage of the extremely low time−scale of quantum mechanical tunneling processes (≈ fs, even though the concept of tunneling time is still a controversial issue) through heterostructure−based nanoscale barriers, which widens the intrinsic bandwidth of the device up to the THz range, making it possible to accommodate ultra−high data rate requirements. Moreover, they are fully compatible with MMIC technology, have moderate phase noise and rely on a simple and low−cost fabrication based on standard microelectronic processes, such as optical lithography (or EBL), lift−off and wet/dry etching. For these reasons, RTDs are considered the most promising candidate for compact and low−cost THz sources. Moreover, due to the strong non−linearities in the IV characteristic, they are being investigated as THz detectors. The highest demonstrated fundamental oscillation frequency for a single device oscillator at room temperature at the date of this review is ≈ 1.98 THz and was obtained by Prof. Dr. M. Asada’s group at Tokyo Institute of Technology, while the highest output power was reported by Prof. Dr. E. Wasige’s group at the University of Glasgow, featuring ≈ 1 mW at 260 GHz (single device) and ≈ 2 mW at 84 GHz (two parallelized devices).

The major limit of these devices consists in the low output power. Therefore, most of the research effort on this kind of technology is nowadays focused on pushing the power performances up.

The main goal of the Ph.D. project will consist in leveraging the current InP RTD oscillator design knowledge existing at the University of Glasgow (UGLA) to achieve the targeted high power and high efficiency devices. In particular, the work to be carried out will build upon the present design approach, which is based on low peak current density J_p and large area (≈ 16 μm²) devices in a power combining circuit topology (that employs 2−4 parallel RTDs) and has allowed the achievement of J−band (220−325 GHz) oscillators in both microstrip and CPW technologies, with ≈ 1 mW record output power (single device) and ≈ 2 mW (two parallel devices) in W−band (75−110 GHz) in un−matched 50−Ohm loads. With advanced epitaxial design and impedance matching to the load, extremely high efficiency RTD−based sources with up to a 40 % DC−RF power conversion efficiency are expected, implementing state−of−the−art low frequency stabilization schemes developed at UGLA. Output powers around 10 mW at 100 GHz, 5 mW at 300 GHz, 2 mW at 500 GHz and 1 mW at 1 THz are expected. These sources will form the baseline for compact THz demonstrators in typical applications.

In order to successfully reach this goal, the ESR will develop expertise in the field of RTD-based oscillators design, cleanroom fabrication and high-frequency characterization. The ESR will moreover attend workshops, training courses, summer schools, secondments and conferences which will help him in deepen his knowledge in the field of high-frequency electron devices and introduce himself to the related worldwide engineering community”.

Traveling-wave RTD oscillators

Our goal is the development and investigation of a new type of RTD oscillators, which is based on the traveling wave principle. Such devices could solve the major problem of insufficient output power in RTD oscillators and they could be used in diverse THz and sub-THz applications. In the travelling-wave geometry, the RTD is integrated into a very thin wave-guide (with the thickness of around a micron), which behaves as a resonator structure. This wave-guide is then coupled to an antenna to emit the radiation. Different wave-guide geometries and antenna designs have to be investigated in order to get a sub-THz oscillator (300GHz or higher). The traveling wave geometry might also possess better characteristics compared to other oscillator designs regarding higher emitted output power. The major difficulties are the heat dissipation – due to high current density the semiconductor structure might overheat – and the out-coupling of the radiation from the device. Our work involves electromagnetic simulation, fabrication of the proposed devices with nano-lithographical techniques performed in the CEITEC research institute in Brno and characterization of the working devices at TU Wien.

Antenna-in-Antenna Subharmonic Injection Locking for Resonant Tunnelling

In RTD oscillators, because of parasitic parameters and fabrication uncertainty, there usually exists a shift in oscillation frequency compared to the designed value. Besides, it is hard to design the phase control element in the THz band like e.g. a phase locked loop. Therefore, we propose to utilise the lower frequency subharmonic injection signal to control the phase and oscillation frequency of triple-barrier resonant-tunnelling diode (TB-RTD) oscillators for operation frequencies starting from 300GHz up to the THz range.

The research activities are mainly focused on compact oscillator design concepts in regarding to the technologies that are used in the project. This includes on-chip antennas-in-antenna topologies for both, THz emission and mm-wave injection frequencies and the mixed RF circuit and electromagnetic modelling of the overall injection locking and oscillation mechanism. Special emphasis is given to the proper wireless injection-locking via electromagnetic field coupling.

Resonant tunneling diode based optoelectronic receivers and transmitters

Project description

Resonant tunneling diodes (RTDs) have shown great potential in high frequency electronic and optical circuits. This project builds upon previous knowledge gathered on the integration of RTD double barrier quantum-well structures with a photo-detective layer to make high performance optical receivers operating at the low loss windows of the fiber optic spectrum (1300nm and 1550nm). The focus will be on understanding the roles played by each layer of the RTD-PD device, and to use these results to produce epi-layer designs which optimize the device’s performance. The aim will be to increase the responsively, sensitivity, bandwidth and speed. With enough data on various devices we also are aiming at producing simulations which will mimic actual physical device performance. These simulations will act a base, upon which more advanced applications can be investigated. The integration of RTD-PD devices with light sources (like laser diodes) to produce optical transmitters in another area of research. The fabricated devices will be integrated with laser diodes in a hybrid scheme for study purposes. The goal eventually will be to mathematically model the operation of such hybrid configuration. These transmitters show high potential in being used as RF over fiber optic transceivers, which can operate at THz frequencies. Finally, the improvement which is to be achieved in sensitivity will be geared towards the use of such devices as single photon detectors which should give cheaper alternatives to current PIN and Avalanche photodiodes in the field of single photon detection. This project is expected to deliver high quality research results which will be published in high impact journals and conferences and should contribute and add upon existing literature. We are hoping that the results obtained will find their practical use in the industry, especially in the telecommunications and medical sectors, and help in achieving the vision of Horizon 2020.

Novel terahertz (THz) detectors/emitters based on different two-dimensional Dirac systems

Research Objectives

The research project is devoted to design, fabrication and characterization of novel terahertz (THz) detectors/emitters based on different two-dimensional Dirac systems. It is planned to investigate devices made from graphene and graphene-like materials, e.g. HgCdTe and InAs/GaSb. The unique properties of these materials give rise to fascinating new phenomena, which may be used to efficiently detect or emit THz radiation.

Scientific Background

In condensed matter physics the term Dirac matter commonly refers to a group of materials that can be characterized using the Dirac equation. This includes fascinating materials such as Graphene, Topological Insulators or Wyle semimetals with unique properties, e.g. huge scattering times even at room temperature or protected spin polarized edge channels. The recent advances in controlled fabrication and processing of these materials has shifted them into the focus of potential applications. We plan on investigating miscellaneous phenomena occurring in these systems like THz cyclotron emission or specific transitions between Landau levels to verify their usability as terahertz detectors/emitters.

Methodology

As a first step we want to analyze the usability and limitations of the selected materials. Therefore, we are currently developing a new characterization setup for multiple optical and electrical measurements including low temperatures and high magnetic fields. The setup consists of a four window magneto-optical cryostat operating between 1.6K and 325K with a magnetic field up to 6T. For characterization measurements it is possible to use different THz sources producing monochromatic or broadband radiation in the pulsed and continuous wave regime. It is additionally planned to implement THz emitting devices, e.g. resonant tunneling diodes, manufactured by associates of the TeraApps project into the characterization setup. Secondly we want to improve the design, fabrication and processing of the samples at the facilities available in our own University as well as by intense collaboration within the TeraApps network.

Expected Outcome

The targeted breakthrough of the project is to prove, that the investigated samples can indeed be used as detectors with good signal-to-noise ratio or as tunable THz sources.

Development of Room-Temperature Terahertz Nano-Detectors

Brief overview and description:

One-dimensional (1D) nanostructure devices are at the frontline of studies on future electronics, although issues like massive parallelization, doping control, surface effects, and compatibility with silicon industrial requirements are still open challenges.

The recent progresses in atomic to nanometer scale control of materials morphology, size, and composition including the growth of axial, radial, and branched nanowire (NW)-based heterostructures make the NW an ideal building block for implementing rectifying diodes or detectors that could be well operated into the Terahertz (THz), thanks to their typical achievable attofarad-order capacitance.

Semiconductor nanowire field effect transistors (NW-FETs) as plasma wave or thermoelectric THz detectors will be developed during the project. Here, InAs, InSb or heterostructured InAs/InSb nanowires will be used as active channel material to design an array of 1D NW-FETs exploiting low effective mass and high mobilities materials, as well as direct coupling of the oscillating radiation field to one longitudinal plasmon mode. This work will also include the design and realisation of novel 2D material based multi-grating-gate FET structures with asymmetric unit cells to achieve plasmonic THz detection.

Research objectives:

1) Development of semiconductor (InAs, InSb and InAs/InAs) 1D nanowire field effect transistors (FETs) as plasma wave or thermoelectric THz detectors;

2) Development of THz plasmonic detectors based on new layered materials

Research skills and techniques:

• Training in specific new areas, or technical expertise etc:

- Nanofabrication trainings:

1. Cleanroom
2. Elctron beam lithography
3. Scanning electron microscopy
4. Reactive ion etching
5. Thermal evaporation
6. Optical lithography
7. Wet chemical etching
8. Atomic layer deposition

- THz photonics:

1. Fourier traform infrared spectroscopy
2. Time domain spectroscopy
3. Transport and optical tests

Expected Results:

1) NW FET responsivities > 100V/W; noise equivalent power <10-10W/√Hz, detectivities < 108cm√Hz/W; bandwidth 0.3 – 2 THz.

2) Plasmonic detectors, 1-4 THz range, increased detectivity (few orders of magnitude)

Terahertz imaging with extended functionality by tailored optical metasurfaces

Research Objectives

The major goal of this project is the design, development and characterisation of a compact imaging system with extended functionalities in the Terahertz range (ca. 0.1 – 10 THz). Optical metamaterials offer hereby the potential for ultra-compact & efficient optics due to their ability to superimpose multiple optical phase-functions, hence generating highly integrated optics for beam shaping of both phase and polarization. Furthermore, previously unknown phenomena based on geometric phase, negative permittivity/refractive index etc. open up completely new opportunities for functional imaging. This project strives to adapt the unique traits of metamaterials to innovate THz optics, with specific focus on functional THz imaging to obtain extended information on the imaged sample (e.g. 3-D reconstruction, dichroic imaging, etc.).

Scientific Background

The limiting factor for many optical applications of THz technology to-date is the low ratio of achievable source power to the detector’s sensitivity. Particularly more complex imaging setups that aim to exploit the unique traits of THz radiation often produce blurred images close to the noise floor due to high losses of the beamforming optics. Therefore, advanced THz imaging systems would greatly benefit from both optics based on highly efficient dielectric metasurfaces (70-90 % in most cases) as well as from the reduced number of optical components through combined functionalities, resulting in fewer reflective & absorptive losses. Consequently, this project aims to improve the performance of THz optics, while other ESRs work to improve the surrounding source and detector technology.

Methodology

As a first step, the desired optical response of our beam forming optics needs to be defined depending on its intended function. Then, the outlined setup can be simplified by integrating and combining optical components into optical metamaterials, which are consequently designed via FDTD simulations. These are then manufactured by (photo-)lithographic patterning and tested for their intended performance by high-power and broadband THz sources. Once their correct operation is confirmed, they will be employed for THz imaging with extended functionality.

Expected Outcome

The targeted output of this project is consequently a very compact, easy to handle/assemble THz imaging system (at video-rate). Particularly extended functionalities and/or imaging modes that increase the versatility of our setup by providing additional valuable information on the imaged sample are envisioned.

Non-destructive testing of pharmaceutical tablets using THz sensing

The pharmaceutical industry is a particularly promising target area for THz applications, both as a spectroscopic tool and for purposes of condition monitoring (CM) and non-destructive testing (NDT). There are three main reasons for this. First, pharmaceutical materials are (semi-) transparent at THz frequencies and possess characteristic spectral signatures, whereas they are opaque in the visible and near-infrared. Second, high-precision quality monitoring is of immense importance in pharmaceutical production. And third, pharmaceuticals are high-value products where increased product consistency and reduced wastage bring significant cost benefits, therefore justifying investment in relatively expensive new technologies that improve CM and NDT.

The work is focused on improving the dissolution of pharmaceutical tablets by monitoring their granularity and porosity using THz transmission measurements. Pharmaceutical tablets are the most popular and widespread form to administer a drug to a patient due to their cost effectiveness, ease of use and patient compliance. The physical and chemical properties of both API (active pharmaceutical ingredient) and excipient particles dominate tableting and delivery functionality of the dosage form. The formulation and manufacturing strategy is driven by the type of drug delivery device: immediate- or modified-release. The majority of pharmaceutical tablets are designed to immediately release the drug, where the tablet microstructure and the disintegration process play a pivotal role in product performance. This becomes even more important for future drug products, as about 70% of all leading compounds that emerge from drug discovery have shown poor aqueous solubility and so were dropped out of the development pipeline.

Disintegration of pharmaceutical tablets is controlled by their chemical and mechanical properties. During the compression of a tablet, particles are consolidated to form interparticulate bonds and pores. The pores in a tablet – their size and connectivity – directly affect the rate at which the physiological fluids enter the tablet, leading to swelling of the particles and eventually causing the break-up of the compact into smaller agglomerates. The size of the disintegrated particles then drives the dissolution rate of the drug. These mechanisms are strongly interconnected, as the swelling of particles dynamically changes the internal pore structure which influences the liquid imbibition process. The performance of a tablet can thus be predicted and optimized by understanding the relationship between dissolution rate and the granularity and porosity of the tablet material. Despite the widely recognized importance of monitoring the tablet porosity during the manufacturing process, there are no continuous in-line non-destructive techniques for achieving this. THz technology is a highly promising solution to this urgent problem.

The project has two aims:

1) The ESR will use THz transmission measurements to characterize THz optical scattering by tablet material (frequency and angular dependence). The results will then be used to quantify the relationship between THz scattering, porosity and/or granularity, and tablet dissolution rate.

2) In the second phase of the project, the ESR will develop a demonstrator system suitable for inline measurements on a pharmaceutical production line, and will carry out a field demonstration.

Research Objectives

The research project is devoted to the investigation of the long-range electrodynamics interactions between biomolecules. Our perspective stems from the observation that it is hardly conceivable that the astonishingly high efficiency, rapidity and robustness against environmental disturbances of the complex networks of biochemical reactions in living cells be only driven by random encounters between cognate partners.

Methodology

The research methodology consist essentially in an integrated interplay of samples preparation of different biomolecules in water solution, experiments performed using specially developed THz spectroscopy setups and theoretical models and numerical simulations for the interpretation of the obtained results. An experimental configuration consists in a suitable terahertz transmission experiment based on a tuneable and continuous-wave primary source whose emitted beam is then focalised on the biological samples eventually in the presence of an additional optical excitation providing the necessary energy for the molecules to enter in a collective resonant state. The detection, based on specially designed near-field THz probes connected to a waveguide, will enable the direct measurement of the THz field. The output of the experiments is the transmission spectra providing resonant features depending or not on the presence of the external excitation.

Expected Outcome

The targeted breakthrough of the project is to prove the activation, under physico-chemical conditions typical of cell cytoplasm, of long-distance, resonant electrodynamic forces selectively acting to attract cognate molecular partners of biochemical reactions occurring inside living cells. The detection, measurement and physical interpretation of these long-range interactions among biomolecules would represent a significative change in our understanding of how living matter evolves complex hierarchies of collective order and would open the possibility to modulate biochemical reactions from the outside through externally applied electromagnetic fields.

Terahertz metrology for on wafer S-parameters measurements

For over two decades, mm-wave electronics has been a worldwide research subject. Numerous applications are being developed in this frequency range, and several are starting to be commercialized, including non-destructive testing, automotive radars, and wide bandwidth wireless. The E-band frequencies (60 - 90GHz) are already being widely used by the industry. At higher frequencies, however, applications are still limited, mainly because it remains difficult to develop direct sources and detectors for terahertz radiation. Low power and high uncertainty in the measurements done to characterize circuits prevents for realizing more complex designs, which could in fact be applicable in many new fields (e.g. medical, tomography...).

It is in this regard that the raw performance of the measurement systems becomes as important as the calibration process to assure repeatable and accurate modeling of those device behavior. On wafer S-parameters measurements over 110GHz suffer from a lack of traceability. This is due to the growth of influencing factors, in number and magnitude. It is therefore important to improve and develop new calibration flows and new methods to quantify and reduce uncertainties.

The main objective of this project will be to develop better on-wafer calibration structures for S-parameters measurements up to 500GHz thanks to uncertainty quantification. With this first step, various geometries can be screened for their performance, with the aim of improving accuracy, ultimately leading to better device knowledge.