THz waves generated from the optical beats between longitudinal mode of a multimode cw laser daide. Efficiency of optical beats in a chaotically oscillating la-ser is confirmed comparing that of free running CW laser using a highly efficient plasmonic photomixer. The great potential of chaotically oscillating lasers is verified for THz systems.

Although advances in THz radiation sources have resulted in a wide variety of applications in communications, security scanning, medical imaging, spectroscopy, etc., an absence of viable sources has stalled the transition of THz technologies from the laboratory into the real world. This has in turn led to a resounding lack of commercially available THz components. Typical computer numerical control (CNC) machining tolerances are on the order of 100 µm, which is much higher than the submicron accuracy that would be ideal for the fabrication of THz components. Here, we present a twisted waveguide designed to rotate the electric field polarisation of free space THz radiation from 1 THz to 2.5 THz by 90° with near perfect efficiency, fabricated using two-photon lithography. We envision such methods being utilised for the quick prototyping of various passive THz components in both the laboratory and commercial settings.

Terahertz Time-Domain Spectroscopy (THz-TDS) uses ultrafast lasers to emit and detect broadband, picosecond pulses with excellent signal-to-noise ratios and rapid data acquisition. Commercial spectrometers have become available and there is now great access to the technology. However, the data analysis remains complex and prone to errors due to multiple processing steps and variation in experimental setups, hindering its true breakout into industry. Machine learning, particularly the training of artificial neural networks with simulated data, has proven effective in various spectroscopic techniques, including refractive index extraction with THz-TDS. This approach allows controlled inclusion of analytical and experimental errors, enabling performant networks that are easier to characterize. We explore the use of deep neural networks for complex refractive index prediction that account for experimental and analytical errors, such as laser drift, compensating for imperfect experimental data and potentially superseding current extraction methods.

In this study, we developed THz time-of-flight imaging based on all-reflective optics to preserve the high-frequency components of a THz antenna for inline applications. In particular, asynchronous optical sampling enables rapid scanning at 200 Hz per pixel with a full time-delay of 10 ns. This configuration enabled us to obtain phase and amplitude images with a spatial resolution of 0.5 mm for non-destructive testing applications such as finding defects in packaged chips. Also, we introduced conformal mapping techniques for maintaining high spatial resolution, applicable for objects with variable heights.

Our recent THz imaging system performs full-frame high-speed imaging (12000 fps) by exploiting efficient THz-to-optical conversion in an excited Caesium atomic vapour. Structured Illumination Microscopy (SIM) has revolutionized optical microscopy pushing beyond the diffraction limit. At THz frequencies the diffraction limit is measured on the sub-mm scale therefore would benefit from improvement. Implementing Structured Illumination Super Resolution at the THz regime has previously been unattractive due to the long acquisition times of conventional THz detectors, compounded by the requirement of multiple images for super-resolution image reconstruction. Using our high-speed THz imaging system, we investigate the application of Structured Illumination Super Resolution Imaging as a method to improve spatial resolution, while maintaining the high penetrating properties and high detection sensitivity at 0.55THz.

Mueller matrix polarimetry allows for sophisticated analysis of polarization-sensitive information and enhanced contrast in imaging but is particularly cumbersome and slow in the case of terahertz (THz) frequency measurements. We have recently debuted our THz Portable HAndheld Spectral Reflection (PHASR) Scanner as a versatile tool for high-speed broadband time-domain spectroscopic imaging of 1 inch scenery in about 6-8 seconds. We present the design of the polarization-sensitive THz PHASR Scanner version 3.0 and describe a calibration method for capturing accurate spectral Stokes vector images with just one acquisition. Extraction of Muller matrix images of different non-destructive testing targets will then be presented.

The egg is world famous for its role in the evolution of life, religious traditions, culture and breakfast. With the latter, the chicken egg is the primary choice for which the production depends on careful inspection of each egg to ensure high quality. To make the inspection efficient, non-destructive inspection (NDI) techniques, which are of high speed, and that can potentially be implemented in the line of production, are in demand. In this work, we present the first mid-infrared (MIR) optical coherence tomography (OCT) study of eggs. We apply both near-infrared- (NIR) and MIR OCT systems, of respective centre wavelengths of 1.3 µm and 4 µm. We inspect a quail egg and two chicken eggs, brown and white. The quail and chicken eggs present two different kinds of shells, both seen in structure and thickness. Funding: Horizon Europe, Grant Agreement No. 101058054 (TURBO) and No. 101057404 (ZDZW). VILLUM Fonden (2021 Villum Investigator project no. 00037822: Table-Top Synchrotrons).

We present optical beatnote detection from a THz QCL comb operating at 80 K in a small nitrogen-cooled dewar. The 21.7 GHz comb beatnote is detected by downconversion, directly mixing free-space signals from the QCL and a microwave synthesizer onto an NbN HEB optimized for RF frequencies. The setup constitutes a very convenient platform for the study of QCL-based optical frequency combs and a building block for compact, portable frequency comb fast spectrometers.

By using plasmonic nanoantennas that can act as light concentrators in infrared band and terahertz antennas in terahertz band, we developed a terahertz focal-plane array (THz-FPA) that can be used in terahertz pulsed imaging systems. The detector array consists of 64 pixels and can scan a line width of 5 cm. By offering high scan speed with large field-of-view, the THz-FPA can transform terahertz pulsed imaging systems from a metrology tool to a high-throughput instrument that can be used in industrial settings for various non-destructive evaluation applications.

In this study, we designed and fabricated GaAs AP-SBDs for a 300 GHz subharmonic mixer. To enhance the reliability of electromagnetic simulations, we measured and calculated the optical parameters of the material in the terahertz range and designed a component with minimal impedance variation across the frequency range of 140-300 GHz. GaAs AP-SBDs were fabricated using MOCVD and an i-line stepper, followed by electrical characterization. Finally, a subharmonic mixer was constructed using WR3.4/WR6.5 waveguides, achieving an single side band conversion loss of 9.72 dB and a 3 dB bandwidth of 40 GHz.

High power, high-speed photodiodes are pivotal in reshaping RF systems, offering potential replacement of conventional RF cabling with more efficient optical fiber. This presentation offers an overview of novel design concepts underlying these high-performance photodiodes, highlighting advancements in high-performance photodiodes for analog photonic links. We will present details of such detectors at Freedom Photonics, as well as an overview of new literature.

We demonstrate an electro-optic imaging system for measuring millimeter-wave propagation along a coplanar waveguide (CPW). A polarization-resolved microscope images small electro-optic effects due to millimeter-wave voltages between the signal and ground of the CPW. Dual electro-optic frequency combs demonstrate time-domain waveform imaging on-wafer with >100 GHz of bandwidth. A second configuration demonstrates continuous wave optically-derived vector network analysis with measurement planes on-wafer, filling the role of multiple electronic network analyzers but with instantaneous bandwidth and time-domain capability that cannot be achieved otherwise.

We show a way to gain a precise understanding how a MEMS hotplate as a mid-infrared emitter works. We used experimental methods to measure its main thermo-electric properties but also FEM simulation for gaining insight into not-so-easily accessible physical quantities. Those are for instance the current density field on a microscopic level or the amount of heat dissipation by convection. Since at the start of modelling the electrical and thermal conductivities of some materials were unknown at elevated temperatures, temperature characteristics were adjusted to fit the measured data. We discuss different ways to find matching combinations of characteristics, such as a) direct search in the calculated parameter set, b) a self-normalizing artificial neural network trained for regression, and c) physical intuition. When the influences of the packaging and of the surrounding air were properly included into the model, it reproduced the measured data quite well in vacuum and normal ambient conditions. We conclude, this modelling technique leads to models which are quantitatively verified in a large part of the applicable parameter space and cover all relevant physical effects.

Terahertz time-domain spectroscopy (THz-TDS) is a useful technology that has a wide range of applications. However, the conventional THz-TDS system is very time-consuming for taking time domain measurements due to the usage of the mechanical stepper. While the existing rapid scan method introduces greater complexity to the system. We developed a rapid scan system for real-time THz acquisition using a rapid shaker as the delay line and utilizing a Michelson interferometer to achieve high-accuracy position tracking of the delay line, which enables real-time time-domain measurement for high-frequency THz electrical fields.

This presentation describes a high-sensitivity terahertz detection approach that utilizes photomixing to convert incoherent terahertz radiation to the radio frequency (RF) regime, at which high-performance RF electronics are available. We describe how utilizing plasmonic nanostructures provide a significantly enhanced wavelength conversion efficiency over a broad terahertz frequency range. Having a versatile design, this incoherent radiation detection scheme has broad applicability to quantum optics, chemical sensing, biological studies, medical diagnosis, high data-rate communication, as well as astronomy and atmospheric studies.

Si optical modulator, one of the key components in Si photonic integrated circuits (PIC), should have good linearity performance so that the microwave photonic systems based on Si PIC can satisfy requirements in many applications. The linearity performance of Si optical modulators, including Si Mach-Zehnder modulators (MZM), is typically evaluated by third-order intermodulation distortion (IMD3) and spurious-free dynamic range (SFDR). In this presentation, we propose a new methodology to determine the IMD3 and SFDR values of Si MZMs accurately. Our model includes several characteristics of Si MZMs and, therefore, the model parameters should be extracted by various methods. Using the parameters, we simulate the values of the IMD3 and SFDR for a sample Si MZM. Examples of design optimization will be also presented based on this model to maximize the linearity performance of Si MZMs.

We stabilized the timing of a 9.92-GHz microcomb using a 250-MHz mode-locked laser as a reference. To ensure precise timing linkage between two optical pulse sources, the electro-optic sampling-based timing detector (EOS-TD), which can detect the timing difference between microwave and optical pulse train with attoseconds precision, was employed. The 9.92-GHz microwave phase noise of the micro-comb was suppressed to –100 dBc/Hz at 10 kHz offset frequency, which is suppressed by >25 dB compared to the free-running case.

We fabricated functional hollow waveguide (WG) devices, specifically straight waveguides (WGs), bandpass filter (BPF) WGs and twisted WGs, for electromagnetic waves at 200–400 GHz using our UV-curable resin type 3D printer, RECILS together with metal plating. These devices mainly comprise 25.4-mm-long hollow structures with a cross section of 0.86×0.43mm^2. RECILS is featured by its ability to produce palm-sized objects with a high resolution (20-30 µm) at high speed (100 cm^3/hour). The confinement of electromagnetic waves is realized by forming a copper layer plated on surfaces of hollow structures. BPF WGs consist of 5 coupled cavities formed in a WG. Twisted WGs are composed of a waveguide in which a 6-mm region of a WG is twisted by 90 degrees. These functional WG devices exhibit performance comparable to that of bulk metal WGs. This technology can fabricate THz WG devices with arbitrarily shapes in a short time at low cost.

Many optical devices that can be used in THz systems are continuously being researched and developed. Among them, liquid crystal (LC)-based polarizers and phase shifting devices have also been developed. The LC devices used in the THz system are fabricated in the form of a cell. The substrate of the cell should be transparent with low absorption in the THz frequency range. In the THz frequency band, research and development using various glass materials such as BK7 glass, slide glass, and quartz as substrates have been reported. In this paper, we report the measurement results of the refractive indices and birefringence of glass materials in the THz band and the possibility of using them as substrates for LC cells. In addition, based on these results, we report the results of measuring refractive indices, absorption coefficient, and birefringence in the THz band after fabricating a LC cell.

Wind turbine blades are an important part of green wind energy production, but the blades are subject to manufacturing defects that lead to material erosion during their operation. Optical coherence tomography (OCT) is a useful tool for sub-surface non-destructive imaging in the few millimetre range. We present non-destructive inspection (NDI) characterisation of wind turbine blade samples with a MIR OCT system employing a supercontinuum laser source with central wavelength around 4 µm. In our inspection of the sub-surface of wind turbine blades to detect defects inside the coating, we hope to improve understanding in blade manufacturing and support the industry in achieving zero waste production. The project was funded by the Horizon Europe, Grant Agreement No. 101058054 (TURBO) and No. 101057404 (ZDZW), and by VILLUM Fonden (2021 Villum Investigator project no. 00037822: Table-Top Synchrotrons).

Terahertz dynamics in molecular crystals are being increasingly shown to map out important phenomena in both the spatial and frequency regimes. In this work, the role of hindered rotational dynamics in organic solids will be shown to be directly related to a number of material properties ranging from the efficiency of organic electronics, thermal energy storage, and pharmaceutical stability.

This conference presentation was prepared for SPIE Photonics West, 2024.

Deep learning (DL) has enabled the development of deep inverse models (DIMs) to solve inverse problems in artificial electromagnetic materials (AEMs). DIMs often outperform conventional optimization approaches, but their performance has not been thoroughly compared. We evaluated eight state-of-the-art DIMs on three unique AEM design problems, quantitatively comparing their solution time and accuracy. We found that modern DIMs can be decomposed into independent modules, and that interchanging these modules can create novel higher performing DIMs. We taxonomized the unique modules of current state-of-the-art DIMs into three categories: initializers, filters, and optimizers. We conclude by discussing some important outstanding issues of deep inverse design of AEMs, and presenting an outlook of this exciting field.

Carcinogenesis involves DNA methylation which is a primary alteration in DNA in the development of cancer before genetic mutation. Because the abnormal DNA methylation is found in most cancer cells, the assessment of DNA methylation using terahertz radiation can be a novel optical method to detect and control cancer. The methylation has been directly observed by terahertz time-domain spectroscopy and this epigenetic chemical change could be manipulated to the state of demethylation using resonant terahertz radiation. Demethylation of cancer cells is a key issue in epigenetic cancer therapy and our results may lead to the treatment of cancer using electromagnetic waves.

We present a diffractive terahertz sensor using a single-pixel detector to rapidly sense hidden defects within a target sample volume. Leveraging multiple spatially-engineered diffractive layers optimized via deep learning, this diffractive sensor can all-optically process the sample scattered waves and generate an output spectrum encoding information for indicating the presence/absence of hidden defects. We experimentally validated this framework using a single-pixel terahertz time-domain spectroscopy set-up and 3D-printed diffractive layers, successfully detecting unknown hidden defects within silicon samples. By circumventing raster scanning and digital image formation/reconstruction, this framework holds vast potential for various applications requiring high-throughput, non-destructive defect detection.

In this work we try to optimize an IR-MEMS-emitter as a source for fast NDIR (Non-dispersive infrared) measurements. NDIR sensors are state of the art for measuring gas concentrations in the Mid-infrared spectrum. However, current IR-light sources such as micro light bulbs and MIR-LEDs cannot satisfy all the requirements for modern NDIR sensors. Therefore MEMS-IR-emitters are the only suitable source. Time dependent measurements require a pulsed sensor operation. Industrial users expect measurement frequencies of 20 Hz to 100 Hz. Current commercially available MEMS-IR-emitters already reach their usability limit at 10 to 20 Hz. The aim of our studies is to optimize an IR-MEMS-emitter and its operating mode for high signal modulation at pulse rates about 100 Hz. After initial tests with hybrid assembled arrays of different internally manufactured emitters, more complex emitter geometrics are simulated with Comsol Multiphysics within the range of our manufacturing possibilities.

Terahertz (THz) light that resonates with the vibrational frequency of hydrated water molecules, can alter functional expression of proteins. Previous studies have shown that high-intensity THz light promotes actin fiber formation, but such mechanism is not comprehensively investigated. Here, we aim to clarify this mechanism by irradiating actin solution with THz pulsed light of lower average power and higher electric field intensity to suppress thermal effects. We compare the degree of fiber formation with and without irradiation. The result shows the contribution of electric field intensity in the accelerated actin fiber formation and clarifies the existence of non-thermal effects.

In this paper, Continuous Wave Terahertz system is utilized to image freshly excised oral and breast tissues. The THz images show significant contrast between tumour and adjacent normal/fat tissues in both breast and oral cancer. The obtained images are compared with the histopathology images for the confirmation. Advanced Artificial Intelligence algorithm is developed in which the THz images at each pixel is labelled based on overlapping of THz and pathology images. The results demonstrate the potential of low frequency THz imaging to differentiate benign and malignant tissue in freshly excised samples.