This paper discusses the application of on-chip terahertz (THz) filters attached to waveguides that can act as sensor elements, including for scanned imaging applications. Our work presents a comparative numerical study of several different geometries (comprising five split-ring resonator geometries and a quarter-wavelength stub resonator, the latter being well established as a sensor at THz frequencies and therefore able to act as a benchmark). We designed each structure to have a resonant frequency of 500 GHz, allowing the impact of resonator geometry on sensing performance to be isolated; the performance was quantified by assessing each design using four figures of merit: resonance quality factor, sensitivity (relative frequency shift under dielectric loading), responsivity (sensitivity weighted by resonance sharpness), and the electric field confinement area. Simulations were conducted using Ansys HFSS using the properties of a commercially available photoresist (Shipley 1813) as a dielectric load to assess performance under conditions comparable to previous experimental studies. The analysis showed that while sensitivity remained broadly similar across geometries, responsivity and quality factor differed substantially between resonators. Furthermore, the spatial distribution of the electric field and current density, particularly in rotated configurations, was found to significantly impact coupling efficiency between the resonator and transmission line. Our findings provide guidance for the general design of systems employing THz sensors while establishing a framework with which to benchmark future sensor geometries.

We demonstrate detection of single microplastic particle using 3D-printed terahertz metamaterials with a vertical gap structure. These 3D metamaterials show higher dielectric sensitivity and enhanced electric field concentration compared to conventional 2D and 3D metamaterials without a vertical gap. Simulation results show that these 3D metamaterials with a vertical gap exhibit the largest resonant frequency shift, indicating their superior detection performance.

The photogalvanic effect can probe the surface states of topological insulators. At optical photon energies surface states near the Dirac point are excited into bulk bands, whereas at energies commensurate with THz excitation, surface states are excited within the Dirac cone leading to a potential enhancement in sensitivity to topological surface states (TSS) and Berry phase. Here, we demonstrate measurements of the circular photogalvanic effect (CPGE) in the topological insulator Bi2Se3 with both optical and THz frequencies at room temperature. Furthermore, we also probe this effect at millikelvin temperatures with THz light. We find the CPGE increases rapidly below 2 K, and is potentially correlated with the weak antilocalization, which itself is dependent on phase coherence lengths. This work lays the foundation for studying both TI and TSS using CPGE at ultra-low temperatures.

We demonstrate that asynchronous optical sampling (ASOPS) can be used to measure the propagation of terahertz (THz) bandwidth pulses in a coplanar waveguide device with integrated photoconductive switches used for signal excitation and detection. We assess the performance of the ASOPS technique as a function of measurement duration, showing the ability to acquire full THz time-domain traces at rates up to 100 Hz. We observe a peak dynamic range of 40 dB for the shortest measurement duration of 10 ms, increasing to 88 dB with a measurement time of 500 s. Our work opens a route to real-time video-rate imaging via modalities using scanned THz waveguides, as well as real-time THz sensing of small volume analytes; we benchmark our on-chip ASOPS measurements against previously published simulations of scanning THz sensor devices, demonstrating sufficient dynamic range to underpin future video-rate THz spectroscopy measurements with these devices.

Surface plasmon polaritons (SPPs) are electromagnetic waves that have attracted significant interest owing to their subwavelength confinement and the strong field enhancement that they provide. Yet in the terahertz (THz) frequency region of the spectrum, which is well below the plasma frequency of metals, these surface waves are characterized by extremely weak confinement that has severely limited their exploitation for information processing and sensing. One means to circumvent this limitation is through subwavelength structuring of a metallic surface, which can thereby be engineered to support the propagation of spoof surface plasmon polaritons (SSPPs) that closely mimic the properties of SPPs. In this work, we report the design and experimental characterization of an ultra-thin metamaterial planar waveguide that supports SSPPs at THz frequencies. Finite-element method simulations are shown to predict the excitation of SSPPs on the surface of our devices under free-space illumination at 3.45 THz. We investigate these structures experimentally using THz scattering-type scanning near-field microscopy (THz-s-SNOM) to map directly the out-of-plane electric field associated with the propagation of SSPPs on the surface of the waveguides. Our work paves the way for the future development of plasmonic integrated circuit technologies and components operating in the THz frequency band.

We present a terahertz (THz) sensing chip capable of detecting trace biomolecules. Its operating bandwidth stretches from 0.2 to 1.35 THz, capable of signal strengths exceeding 38 dB above noise levels, achieved by the mitigation of mode competition issues. The high sensitivity of the sensing chip is used to observe the absorption fingerprints of alpha-lactose monohydrate, L-tyrosine, and L-histidine powders at their characteristic frequencies for sample masses as low as 0.3 mg. Using a solution titration technique, we demonstrate detection of the absorption fingerprint of alpha-lactose monohydrate for concentrations of 72 mu g/mu l, adding just 57.6 mu g of the powder onto the chip after evaporation of the ethanol solvent, representing a twofold advancement over prior work by providing a significantly broader operating spectrum and higher sensitivity. The relatively easy manufacturability, low cost, and scalability of the sensing chip paves the way for studies of enhanced light-matter interaction between THz waves and trace analytes, while also unlocking innovative potential applications in biomolecular sensing, medical diagnostics, and environmental monitoring.

Magnetic skyrmions in thin films with perpendicular magnetic anisotropy are promising candidates for magnetic memory and logic devices, making the development of ways to transport skyrmions efficiently in a desired trajectory of significant interest. Here, we investigate the transport of skyrmions by surface acoustic waves (SAWs) via several modalities using micromagnetic simulations. We show skyrmion pinning sites created by standing SAWs at anti-nodes and skyrmion Hall-like motion without pinning driven by traveling SAWs. We also show how orthogonal SAWs formed by combining a longitudinal traveling SAW and a transverse standing SAW can be used for the 2D positioning of skyrmions. Our results also suggest SAWs offer a viable approach to the transport of multiple skyrmions along a multichannel racetrack.

We have performed an investigation into the optical conductivity and magnetotransport properties of top-gated devices patterned on the topological insulator Bi 2 Se 3 in order to determine the relative effects of the different carrier species that exist within these novel materials. We find that the topologically protected surfaces within our samples are partially screened from the action of the gate by trivial band-bending states formed at the top surface of the topological insulator. Despite this, the mobility of the topological surface carriers is significantly affected by the application of an external gate bias. Additionally, we find that the optical conductivity response is dominated by the topologically protected surface states, and that the optical conductivity is particularly sensitive to the scattering caused by the topological surfaces coupling to trivial states, arising from the bulk or band-bending induced surface states. These results will have interesting applications to the design of future plasmonic devices that incorporate topological materials.

In this paper we report an improved method of coherent sensing through the use of a generalized phase-stepping algorithm to extract magnitude and phase information from interferometric fringes acquired by laser feedback interferometry (LFI). Our approach allows for significantly reduced optical sampling and acquisition times whilst also avoiding the need for fitting to complex models of lasers under optical feedback in post-processing. We investigate theoretically the applicability of this method under different levels of optical feedback, different laser parameters, and for different sampling conditions. We furthermore validate its use experimentally for LFI-based sensing using a terahertz (THz)-frequency laser in both far-field and near-field sensing configurations. Finally we demonstrate our approach for two-dimensional nanoscale imaging of the out-of-plane field supported by individual micro-resonators at THz frequencies. Our results show that fully coherent sensing can be achieved reliably with as little as 4 sampling points per imaging pixel, opening up opportunities for fast coherent sensing not only at THz frequencies but across the visible and infra-red spectrum.

Millimeter and terahertz wave imaging has emerged as a powerful tool for applications such as security screening, biomedical imaging, and material analysis. However, intensity images alone are often insufficient for detecting variations in the dielectric constant of a sample, and extraction of material properties without additional phase information requires extensive prior knowledge of the sample. Digital holography provides a means for intensity-only detectors to reconstruct both amplitude and phase images. Here we utilize a commercially available source and detector array, both operating at room temperature, to perform digital holography in real-time for the first time in the mm-wave band (at 290 GHz). We compare the off-axis and phase-shifting approaches to digital holography and discuss their trade-offs and practical challenges in this regime. Owing to the low pixel count, we find phase-shifting holography to be the most practical and high fidelity approach for such commercial mm-wave cameras even under real-time operational requirements.

Previously we demonstrated integration of a THz quantum cascade laser (QCL) within a dry dilution refrigerator, directing its output using a hollow metal waveguide (HMWG) into the sample space [1]. Here, we show that the waveguide can be replaced successfully by free-space optics within the refrigerator; this allows polarization of the QCL output to be maintained along the way to the sample. We also show active control of polarization using a mechanical filter wheel able to insert different optical components into the beam path during experiments. Our system overcomes waveguide dispersion and losses intrinsic to prior system designs. The results pave the wave to THz polarization control of radiation from THz QCLs at sub-Kelvin temperatures.

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.

Surface acoustic waves (SAWs) have significant potential for energy-efficient control of magnetic domain walls (DWs) owing to the magnetoelastic coupling effect. However, the dissipation of radiofrequency (rf) power in a SAW device can result in heating, which can also affect the DW motion. In this work, the heating of a SAW device consisting of a Pt/Co/Ta thin film with perpendicular magnetic anisotropy in between two interdigitated transducers is measured in situ with use of an on-chip Pt film as a thermometer within the SAW beam path. The application of SAWs at a center frequency of 48 MHz and a total rf power of 21 dBm results in a temperature increase of approximately 10 K within the SAW beam path owing to rf-power dissipation. DW velocity in a Pt/Co/Ta thin film is evaluated separately with use of Kerr microscopy at various temperatures or in the presence of SAWs. With a 10-K increase in temperature only, the DW velocity is found to increase from 33 & PLUSMN; 3 & mu;m/s (at room temperature) to 104 & PLUSMN; 8 & mu;m/s under an external magnetic field of 65 Oe. Traveling-SAW-assisted DW velocity (116 & PLUSMN; 3 & mu;m/s) is slightly higher than that with a 10-K temperature increase alone, suggesting that the heating plays the major role in promoting DW motion, whereas the DW motion is significantly enhanced (418 & PLUSMN; 8 & mu;m/s) in the presence of standing SAWs, indicating that magnetoelastic coupling is more important than heating in this scenario.

Wide bandwidth THz pulses can be used to record the distinctive spectral fingerprints related to the vibrational or rotational modes of polycrystalline biomolecules, and can be used to resolve the time-dependent dynamics of such systems. Waveguides, owing to their tight spatial confinement of the electromagnetic fields and the longer interaction distance, are promising platforms with which to study small volumes of such systems. The efficient input of sub-ps THz pulses into waveguides is challenging owing to the wide bandwidth of the THz signal. Here, we propose a sensing chip comprised of a pair of back-to-back Vivaldi antennas feeding into, and out from, a 90° bent slotline waveguide to overcome this problem. The effective operating bandwidth of the sensing chip ranges from 0.2 to 1.15 THz, and the free-space to on-chip coupling efficiency is as high as 51% at 0.44 THz. Over the entire band, the THz signal is ∼42 dB above the noise level at room temperature, with a peak of ∼73 dB above the noise. In order to demonstrate the use of the chip, we have measured the characteristic fingerprint of α-lactose monohydrate, and its sharp absorption peak at ∼0.53 THz was successfully observed, demonstrating the promise of our technique. The chip has the merits of efficient in-plane coupling, ultra-wide bandwidth, ease-of-integration, and simple fabrication. It has the potential for large-scale manufacture, and can be a strong candidate for integration into other THz light-matter interaction platforms.

We present theoretical methods for the analysis of acoustic phonon modes in superlattice structures, and terahertz-frequency quantum-cascade lasers (THz QCLs). Our generalized numerical solution of the acoustic-wave equation provides good agreement with experimental pump-probe measurements of the acoustic resonances in a THz QCL. We predict that the detailed layer structure in THz QCLs imprints up to similar to 2 GHz detuning of the acoustic mode spacing, which cannot be seen in analytical models. This effect is strongest in devices with large and abrupt acoustic mismatch between layers. We use an acoustic deformation potential within a density-matrix approach to analyze electron transport induced in a range of the most common THz QCL active-region design schemes. We conclude that acoustic modes up to <^>200 GHz are capable of significantly perturbing QCL transport, highlighting their potential for ultrafast modulation of laser emission.

Many quantum technologies rely heavily on propagation of RF and microwave signals through devices at cryogenic temperatures, and detailed understanding of materials and signal propagation is therefore key to improving the performance of quantum circuits. The properties of dielectric substrate materials used for transmission lines (TLs) such as their permittivity need to be precisely determined to design high performance quantum integrated circuits. In this paper, we discuss a measurement technique for determining the effective permittivity of a TL at mK temperatures. The technique utilizes S-parameter measurements of multiple TLs to reliably extract the effective permittivity of the TL implemented in a substrate material. The technique is demonstrated using measured S-parameters of grounded co-planar waveguide (GCPW) at 296 K and 15 mK. The effective permittivity of the TL at 296 K and 15 mK are determined from measurements and compared. We observed the effective permittivity determinations at 15 mK to be approximately frequency independent and calculated the relative permittivity of Rogers RO4350B material at 15 mK to be 3.64. There is no significant deviation from this relative permittivity value with respect to manufacturer data and from measured data at 296 K.

Surface acoustic waves (SAWs) have significant potential for the energy-efficient control of magnetic domain walls (DWs). This study investigates the influence of SAW frequency (50, 100, and 200 MHz) on DW dynamics in magnetic thin films. Micromagnetic simulations are performed to examine the effects of SAWs on DW velocity. The results demonstrate that SAWs enhance DW motion by promoting the depinning of DWs from pinning sites through SAW-induced spin rotation. This spin rotation exhibits the same frequency as the applied SAWs. The impact of SAW frequency varies depending on the level of anisotropy disorder in thin films. In films with 3% anisotropy disorder, the DW velocity increases with SAW frequency, highlighting the amplifying effect of spin rotation and enhanced DW depinning. Conversely, in thin films with 1% anisotropy disorder, the DW velocity decreases with SAW frequency owing to significant SAW-induced energy dissipation via spin rotation. These findings underscore the intricate interplay between SAWs, spin rotation, and DW dynamics, emphasizing the role of anisotropy disorder in governing the response of DWs to SAWs. The study contributes to the understanding of SAW-assisted DW motion and provides insights into the optimization of and energy-efficient control of DWs in spintronic applications using SAWs.

We report the use of a generalized phase-extraction algorithm to extract amplitude and phase data in THz scattering-type scanning near-field optical microscopy (s-SNOM) employing laser feedback interferometry in a THz quantum cascade laser. Our approach allows for only a small number of measurements of the self-mixing voltage, thus greatly improving acquisition rates. We investigate the accuracy of this method experimentally and demonstrate its use for fully coherent imaging of the out-of-plane field supported by an individual micro-scale resonator. Furthermore we report a dual-modulation approach that allows direct electronic measurement of the complex scattered field without recourse to fringe fitting.

We demonstrate a substantial enhancement to the integration of a quantum cascade laser into a dilution refrigerator via waveguides by the inclusion of a multi-mesh 6THz low-pass filter to block IR radiation, and a Winston cone to focus light output from the waveguide. These alterations allow us to lower the base temperatures to $\sim 114$ mK, and the sample temperature to $\sim 160$ mK while detecting a cyclotron resonance (CR) response in a 2DEG illuminated within the refrigerator. For comparison, before these changes we achieve a base temperature of 206mK and an effective sample temperature of 430mK while detecting CR [1].