Metrology State-of-the-Art and Challenges in Broadband Phase-Sensitive Terahertz Measurements

The two main modalities for making broadband phase-sensitive measurements at terahertz (THz) frequencies are vector network analyzers (VNA) and time-domain spectrometers (TDS). These measuring instruments have separate and fundamentally different operating principles and methodologies, and they serve very different application spaces. The different architectures give rise to different measurement challenges and metrological solutions. This article reviews these two measurement techniques and discusses the different issues involved in making measurements using these systems. Calibration, verification, and measurement traceability issues are reviewed, along with other major challenges facing these instrument architectures in the years to come. The differences in, and similarities between, the two measurement methods are discussed and analyzed. Finally, the operating principles of electro-optic sampling (EOS) are briefly discussed. This technique has some similarities to TDS and shares application space with the VNA

Ultrafast electrooptic dual-comb interferometry

The femtosecond laser frequency comb has enabled the 21st century revolution in optical synthesis and metrology. A particularly compelling technique that relies on the broadband coherence of two laser frequency combs is dual-comb interferometry. This method is rapidly advancing the field of optical spectroscopy and empowering new applications, from nonlinear microscopy to laser ranging. Up to now, most dual-comb interferometers were based on modelocked lasers, whose repetition rates have restricted the measurement speed to ~ kHz. Here we demonstrate a novel dual-comb interferometer that is based on electrooptic frequency comb technology and measures consecutive complex spectra at a record-high refresh rate of 25 MHz. These results pave the way for novel scientific and metrology applications of frequency comb generators beyond the realm of molecular spectroscopy, where the measurement of ultrabroadband waveforms is of paramount relevance

Silicon Doping Profile Measurement Using Terahertz Time Domain Spectroscopy

Doping profiles in silicon greatly determine electrical performances of microelectronic devices and are frequently engineered to manipulate device properties. To support engineering studies afterward, essential information is usually required for physically characterized doping profiles. Secondary ion mass spectrometry (SIMS), spreading resistance profiling (SRP) and electrochemical capacitance voltage (ECV) profiling are mainstream techniques for now to measure doping profiles destructively. SIMS produces a chemical doping profile through the ion sputtering process and owns a better characterization resolution. ECV and SPR, on the other hand, gauge an electrical doping profile from the free carrier detection in microelectronic devices. The major discrepancy between chemical and electrical profiles is at heavily doped (\u3e1020 atoms / cm3) regions. At the profile region over the solubility limit, inactive dopants induce a flat plateau and only being detected by electrical measurements. Destructive techniques are usually designed as stand-alone systems for the remote usage. For an in-situ process control purpose, non-contact approaches, such as non-contact capacitance-voltage (CV) and ellipsometry techniques, are currently under developing. In this dissertation, novel terahertz time domain spectroscopy (THz-TDS) is adopted to achieve an electrical doping profile measurement in both destructive and non-contact manners. For this brand new application, everything has been studied from bottom-up. Firstly, the measurement uncertainty from the change of a bulk wafer thickness and the recognition of the doping profile dissimilarity were proven experimentally. The phosphorus refractive index from 1.2×1015 cm-3 to 1.8×1020 cm-3 levels was then generated physically for the modeling of the complex THz transmission and its shift to the Drude Model prediction is explained two scientific mechanisms. Through the experimental demonstrated of the proactical degeneracy, relative strategies were proposed to shrink or break it. The doping profile measurement was finally performed by both methods. We conclude that THz-TDS can be designed as either an either in-situ or stand-alone system to estimate a doping profile in semiconductor materials

Terahertz Components and Systems : Metamaterials, Measurement Techniques and Applications

THz technology has been a promising, yet problematic field in science for a long time. Up until two decades ago, the lack of fundamental components and materials operating at THz frequencies constrained its use mostly to astronomy, with very little commercial focus. Today, the field has grown remarkably, with both scientific and industrial applications pushing the development of new devices and systems to control THz radiation. Further work is necessary to overcome the region’s fundamental challenges and advance the technology on par with the rest of the electromagnetic spectrum. This thesis aims to address new applications for THz spectroscopy, both in the frequency and time domain, as well as enhancement of THz device performance. A new design approach for THz resonant metamaterials is proposed that aims to improve their resonant response, irrespective of individual resonator geometries. The new approach can be applied to a wide range of already existing structures without altering the individual resonator design and relies on metamaterial cell symmetry and substrate dimensions. The design approach is used to create split-ring optical modulators, demonstrating their response is strong enough to be actuated with an LED lamp as a light source alone. The development of a multiple-angle-of-incidence, multi-wavelength THz ellipsometry system is also presented. The utility of the system for material characterisation is demonstrated, extracting complex optical parameters of composite materials, as well as non-homogeneous, anisotropic and highly absorptive materials in the THz range, which can be otherwise problematic to characterise. The use of the ellipsometry system as an imaging tool for visualising and measuring internal material stresses is introduced. Finally, the application of THz-TDS in conjunction with machine learning for waste oil quality control is investigated, introducing a new potential field of application for THz spectroscopy

Terahertz for subsurface imaging and metrology applications

In the area of metrology and non-destructive testing, Terahertz wavelengths have been widely researched and used. However, the lack of 2D detectors working at room temperature and high power sources prevent the widespread application of Terahertz in industry. In that context, research on the development of new Terahertz equipment is moving at a fast pace. Within the scope of this thesis, applications of newly developed Terahertz technologies were explored using the scanning of single point detectors with the objective to establish the feasibility for their full-field applications in readiness for future 2D detectors. For the first time, a frequency tuneable, all-optical Terahertz source was implemented in multi-wavelength interferometry to overcome one wavelength ambiguity in precise thickness/distance measurements with sub-millimetre resolution. Phase-shifting digital holography is another interferometry technique which allows us to reconstruct not only the amplitude of one object, but also the phase and the depth of it, using existing mathematical algorithms. Digital holography was performed successfully at Terahertz wavelengths using a multiplier/mixer Terahertz source coupled with a single point pyroelectric detector for the applications of non-destructive testing and depth measurements. The novelty is that the phase-stepping technique for digital holography was implemented in THz frequencies for the first time to remove unwanted terms in the reconstructed image in order to improve image quality compare to conventional holography. In the current experiments, recording time for one set of phase-shifting holograms (4 holograms for 4 phase-steps algorithm) was 6 hours. When the technology is ready for 2D detectors, recording time of holograms could be reduced considerably, and the technique will play an important role in full-field applications in industry metrology and/or non-destructive testing and evaluation.EPSR

The 2023 terahertz science and technology roadmap

Terahertz (THz) radiation encompasses a wide spectral range within the electromagnetic spectrum that extends from microwaves to the far infrared (100 GHz–∼30 THz). Within its frequency boundaries exist a broad variety of scientific disciplines that have presented, and continue to present, technical challenges to researchers. During the past 50 years, for instance, the demands of the scientific community have substantially evolved and with a need for advanced instrumentation to support radio astronomy, Earth observation, weather forecasting, security imaging, telecommunications, non-destructive device testing and much more. Furthermore, applications have required an emergence of technology from the laboratory environment to production-scale supply and in-the-field deployments ranging from harsh ground-based locations to deep space. In addressing these requirements, the research and development community has advanced related technology and bridged the transition between electronics and photonics that high frequency operation demands. The multidisciplinary nature of THz work was our stimulus for creating the 2017 THz Science and Technology Roadmap (Dhillon et al 2017 J. Phys. D: Appl. Phys. 50 043001). As one might envisage, though, there remains much to explore both scientifically and technically and the field has continued to develop and expand rapidly. It is timely, therefore, to revise our previous roadmap and in this 2023 version we both provide an update on key developments in established technical areas that have important scientific and public benefit, and highlight new and emerging areas that show particular promise. The developments that we describe thus span from fundamental scientific research, such as THz astronomy and the emergent area of THz quantum optics, to highly applied and commercially and societally impactful subjects that include 6G THz communications, medical imaging, and climate monitoring and prediction. Our Roadmap vision draws upon the expertise and perspective of multiple international specialists that together provide an overview of past developments and the likely challenges facing the field of THz science and technology in future decades. The document is written in a form that is accessible to policy makers who wish to gain an overview of the current state of the THz art, and for the non-specialist and curious who wish to understand available technology and challenges. A such, our experts deliver a 'snapshot' introduction to the current status of the field and provide suggestions for exciting future technical development directions. Ultimately, we intend the Roadmap to portray the advantages and benefits of the THz domain and to stimulate further exploration of the field in support of scientific research and commercial realisation

The 2023 terahertz science and technology roadmap

Terahertz (THz) radiation encompasses a wide spectral range within the electromagnetic spectrum that extends from microwaves to the far infrared (100 GHz-∼30 THz). Within its frequency boundaries exist a broad variety of scientific disciplines that have presented, and continue to present, technical challenges to researchers. During the past 50 years, for instance, the demands of the scientific community have substantially evolved and with a need for advanced instrumentation to support radio astronomy, Earth observation, weather forecasting, security imaging, telecommunications, non-destructive device testing and much more. Furthermore, applications have required an emergence of technology from the laboratory environment to production-scale supply and in-the-field deployments ranging from harsh ground-based locations to deep space. In addressing these requirements, the research and development community has advanced related technology and bridged the transition between electronics and photonics that high frequency operation demands. The multidisciplinary nature of THz work was our stimulus for creating the 2017 THz Science and Technology Roadmap (Dhillon et al 2017 J. Phys. D: Appl. Phys. 50 043001). As one might envisage, though, there remains much to explore both scientifically and technically and the field has continued to develop and expand rapidly. It is timely, therefore, to revise our previous roadmap and in this 2023 version we both provide an update on key developments in established technical areas that have important scientific and public benefit, and highlight new and emerging areas that show particular promise. The developments that we describe thus span from fundamental scientific research, such as THz astronomy and the emergent area of THz quantum optics, to highly applied and commercially and societally impactful subjects that include 6G THz communications, medical imaging, and climate monitoring and prediction. Our Roadmap vision draws upon the expertise and perspective of multiple international specialists that together provide an overview of past developments and the likely challenges facing the field of THz science and technology in future decades. The document is written in a form that is accessible to policy makers who wish to gain an overview of the current state of the THz art, and for the non-specialist and curious who wish to understand available technology and challenges. A such, our experts deliver a ‘snapshot’ introduction to the current status of the field and provide suggestions for exciting future technical development directions. Ultimately, we intend the Roadmap to portray the advantages and benefits of the THz domain and to stimulate further exploration of the field in support of scientific research and commercial realisation

High Dynamic Range, Heterogeneous, Terahertz Quantum Cascade Lasers Featuring Thermally Tunable Frequency Comb Operation over a Broad Current Range

We report on the engineering of broadband quantum cascade lasers (QCLs) emitting at Terahertz (THz) frequencies, which exploit a heterogeneous active region scheme and have a current density dynamic range (Jdr) of 3.2, significantly larger than the state-of-the-art, over a 1.3 THz bandwidth. We demonstrate that the devised broadband lasers operate as THz optical frequency comb synthesizers, in continuous-wave, with a maximum optical output power of 4 mW (0.73 mW in the comb regime). Measurement of the intermode beatnote map reveals a clear dispersion-compensated frequency comb regime extending over a continuous 106 mA current range (current density dynamic range of 1.24), significantly broader than the state-of-the-art at similar geometries, with a corresponding emission bandwidth of ≈1.05 THz and a stable and narrow (4.15 kHz) beatnote detected with a signal-to-noise ratio of 34 dB. Analysis of the electrical and thermal beatnote tuning reveals a current-tuning coefficient ranging between 5 and 2.1 MHz/mA and a temperature-tuning coefficient of −4 MHz/K. The ability to tune the THz QCL combs over their full operating dynamic range, by temperature and current, paves the way for their use as a powerful spectroscopy tool that can provide broad frequency coverage combined with high precision spectral accuracy

Time-resolved nonlinear ghost imaging: route to hyperspectral single-pixel reconstruction of complex samples at THz frequencies

Terahertz (THz) is an innovative form of electromagnetic radiation providing unique spectroscopy capabilities in critical fields, ranging from biology to material science and security. The limited availability of high-resolution imaging devices, however, constitutes a major limitation in this field. In this work, we tackle this challenge by proposing an innovative type of time-space nonlinear Ghost-Imaging (GI) methodology that conceptually outperforms established single-pixel imaging protocols. Our methodology combines nonlinear pattern generation with time-resolved single-pixel measurements, as enabled by the state-of-the-art Time-Domain Spectroscopy (TDS) technique. This approach is potentially applicable to any wave-domain in which the field is a measurable quantity. The full knowledge of the temporal evolution of the transmitted field enables devising a new form of full-wave reconstruction process. This gives access not only to the morphological features of the sample with deeply subwavelength resolution but also to its local spectrum (hyperspectral imaging). As a target application, we consider hyperspectral THz imaging of a disordered inhomogeneous sample