A. Currie, J. Liu, J.M. Woolley, and J. Lloyd-Hughes, J. Phys. Chem. Lett. - - (Apr 2026)
Phonon engineering has improved charge transport in semiconducting organic molecules by introducing high mass side chains, which ameliorate harmful transient localization. The influence of these side chains on thermal energy transfer and dynamic disorder has not been fully explored. In this work, we first use low temperature X-ray diffraction to probe thermally induced structural changes in functionalized acene molecular semiconductors, combined with low temperature IR spectroscopy to track changes to their vibrational energy landscape. Furthermore, time-resolved IR pump–IR probe spectroscopy is employed to measure IR absorption kinetics associated with the high mass side chains of molecules, revealing intramolecular vibrational energy redistribution pathways. Alkyne groups in the side chains are shown to act as vibrational energy traps, remaining hot for time scales of >2 ns. The results reveal nonequilibrium vibrational pathways associated with side chains that may influence the phonon manifold relevant to dynamic disorder.
H. Ou, E. Pickwell-MacPherson and J. Lloyd-Hughes J. Appl. Phys. 139, 123101 (Mar 2026)
Generalized ellipsometry can uncover the optical properties of anisotropic materials, in which the light–matter interaction alters the polarization state. In the terahertz frequency range, generalized ellipsometry has been infrequently realized due to the challenge of rapidly controlling and measuring THz polarization. Here, we report the development and calibration of a high-efficiency terahertz time-domain generalized ellipsometer based on two dual-channel photoconductive antennas. The fiber-coupled multi-pixel devices act as source and detector and achieve a high data throughput with four independent terahertz pulses in a single optical delay scan, without the need for any bulky or slow polarization elements such as polarizers. Following a one-off system calibration, an accurate optical characterization of a uniaxially birefringent Al2O3 crystal serves to validate the method. Our technique benefits from efficient data throughput, full polarization-resolved capability, and reduced system complexity, paving the way to in situ and real-time monitoring applications.
Terahertz (THz) sensing has gained significant attention as a non-ionizing modality capable of probing the superficial layers of biological tissue with high sensitivity to water content, structural changes, and biochemical composition. Its strong interaction with water makes it uniquely suited for investigating dermatological applications such as hydration assessment, wound and burn monitoring, and the detection and characterization of skin cancers. However, the shallow penetration depth of THz light, combined with the sensitivity of measurements to probe-skin coupling, contact pressure, and motion artifacts, poses persistent challenges for in vivo use. Overcoming these limitations requires carefully engineered hardware, robust measurement protocols, and advanced computational techniques tailored to biological variability. This article reviews the evolution of THz instrumentation for in vivo skin sensing, spanning early laboratory systems to emerging compact, handheld, robotic, and computationally enhanced platforms. Key developments in probe miniaturization, ergonomic design, and automated control of probe orientation and pressure are described, as these advances directly influence measurement reproducibility and clinical usability. We highlight recent progress in single-pixel imaging, which offers video-rate capabilities, the emergence of waveguide-integrated metamaterial sensors that push spatial resolution far below the diffraction limit, and THz ellipsometry that gives complementary contrast. In addition, we discuss the growing role of data-driven processing, compressed-sensing algorithms, and biophysical modeling in extracting clinically relevant parameters from THz measurements. Although promising, many of the emerging systems remain untested in in vivo dermatological scenarios, and challenges including motion artifacts, calibration stability, and inter-subject variability must be addressed. Continued interdisciplinary work between engineers, physicists, and clinicians is essential for transitioning THz systems toward practical, patient-compliant diagnostic tools.
S. Mou, R.I. Stantchev, S. Saxena, H. Ou, S. Rane, S. Pain, J.D. Murphy, E. Hendry, J. Lloyd-Hughes and E. Pickwell-MacPherson
Nature Communications 17 1571 (Jan 2026)
Real-time, non-invasive imaging techniques are essential for advancing biomedical diagnostics and material analysis, yet existing terahertz (THz) systems often suffer from limited speed, bulky designs, and poor adaptability to in situ environments. Addressing these challenges, we present a fully fibre-coupled THz attenuated total internal reflection single-pixel imaging system, offering a compact, flexible, and robust platform for non-destructive spectroscopy and in vivo imaging. This all-fibre architecture enables seamless integration for in situ biomedical applications, including measurements directly on patients. Central to our design is a THz spatial light modulator based on an unpassivated silicon wafer, facilitating high-speed modulation and enabling video-rate imaging with a spatial resolution down to 360 渭m. Despite being in the reflection geometry and using fibre-coupled light, our system achieves an imaging throughput exceeding 30,000 pixels per second for 64-by-64 images - over five-fold higher than the state of the art - representing a substantial improvement in real-time THz imaging capabilities.
Jake D. Hutchinson, Marcin Giza, Nathaniel P. Gallop, Benjamin Vella, Edward Butler-Caddle, Shaoyang Wang, James Lloyd-Hughes, Pablo Docampo and Rebecca L Milot
Adv. Optical Materials 13 e02011 (Dec 2025)
The incorporation of Ruddlesden–Popper (RP)/3D perovskite heterostructures into photovoltaic cells has been shown to increase both the efficiency and stability of the devices. Here, a series of methylammonium lead triiodide (MAPbI3) thin films treated with varying 2-phenylethylammonium (PEA) concentrations are investigated with static and ultrafast spectroscopic techniques to reveal the mechanisms of the observed performance benefits. Transient absorption spectroscopy is employed to elucidate the effect of a surface RP layer on the excited state of the MAPbI3 films and reveal that several different RP structures are formed and participate in the charge-carrier dynamics. The passivation effects of PEA are investigated with optical pump–terahertz probe (OPTP) experiments using a variety of excitation conditions to simultaneously probe the surface and bulk recombination dynamics. Fitting models to the OPTP data for each excitation scheme allows the material parameters that govern the ultrafast dynamics to be quantified. It is found that as the PEA concentration increases, the surface recombination velocity exhibits a monotonic decrease, suggesting the RP layer is effective at passivating surface traps. Furthermore, the bulk monomolecular recombination rate is also found to decrease with the addition of PEA, indicating that the benefits of this passivation approach are not limited to the upper surface of the MAPbI3 films.
T.J. Keat, J. Zhao, J.M. Woolley, P. Malakar, G.M. Greetham, X. Wu, J.P. Goss, R.J. Cruddace, C.B. Hartland, M.W. Dale, V.G. Stavros, M.E. Newton and J. Lloyd-Hughes, Phys. Rev. Lett. 135 216902 (Nov 2025)
We investigated ultrafast defect-lattice dynamics in diamond using the Ns:贬鈭扖0 defect, an analog of bond-centered hydrogen in semiconductors. Combining synthesis, ultrafast vibrational spectroscopy, and ab initio calculations, we show that excitation of the defect鈥檚 stretch mode leads to the generation of localized phonons and the formation of a hot ground state, where the interatomic potential is transiently modified. Our results reveal unexpected nonequilibrium phonon effects despite diamond鈥檚 exceptionally high thermal conductivity, with implications for quantum defect engineering.
J.J. Young, A. Agarwal, B.G. Page, A. Dogra, A.I. Hernandez-Serrano, J. Hardwicke and E. Pickwell-MacPherson J. IR mm THz waves 46, 36 (May 2025)
In this work, we demonstrate significant modifications to our robotically controlled terahertz (THz) sensing system, the 鈥淧icoBot,鈥� enabling it to perform in vivo imaging of skin rather than limiting it to single-point measurements. By integrating a robotic arm equipped with force-sensitive feedback control, we maintain consistent contact pressure between the probe and the skin surface throughout imaging. In conjunction with this hardware advancement, we introduce an accompanying image analysis pipeline that reduces noise and enhances repeatability across scans. These improvements allow for reliable intra- and inter-subject comparisons, a critical step toward the clinical utility of THz imaging. Our ultimate aim is to use THz imaging to detect skin cancer margins: this paper highlights progress towards this goal and skin evaluation in general.
J. Lloyd-Hughes, N. Chopra, J. Deveikis, R. Pandya and J. Woolley J. Infrared mm THz 46 34 (May 2025)
Electro-optic sampling allows the electric field of THz, mid-infrared and visible light pulses to be measured directly as a function of time, with data analysis often performed in the frequency domain after fast Fourier transform. Here, we review aspects of Fourier theory relevant to the frequency-domain analysis of light pulses recorded in the time domain. We describe a 鈥渂est practise鈥� approach to using the discrete Fourier transform that ensures consistency with analytical results from the continuous Fourier transform. We summarise a phenomenological time-domain model of THz pulses, based on carrier and envelope waves, and show that it can reproduce a wide variety of experimental single- to multi-cycle THz pulses, with exemplary data from lab-based sources (photoconductive antennae, optical rectification, spintronic emitters) and a THz free-electron laser. A quantitative comparison of the spectral energy density of these distinct sources is enabled by the amplitude-accurate discrete Fourier transform. We describe a method that ensures the accurate calculation of the absolute spectral phase (valid for arbitrary sampling windows in the time domain) and summarise how the carrier-envelope phase, pulse arrival time, and chirp can be obtained from the phase. Our aim with this overview of THz pulse analysis is to highlight algorithms and concepts that are useful to newcomers to time-domain spectroscopy and experts, alike.
B.G. Page, J.J. Young, A.I. Hernandez-Serrano, J. Hardwicke and E. Pickwell-MacPherson IEEE Trans. THz Sci. Tech. 15, 573 (May 2025)
Eczema and psoriasis are two of the most prevalent skin conditions in the United Kingdom. Typically, diagnosis and treatment assessment are evaluated based on qualitative measures such as skin appearance and patient feedback. Thus, there is a need for a robust and quantitative method of assessing treatment efficacy and monitoring improvements in skin condition. Terahertz (THz) sensing is a promising candidate, owing to its nonionising properties and high water sensitivity. However, THz sensing is heavily reliant upon signal processing techniques, in particular deconvolution methods used to extract skin-dependent parameters. Conventionally, double Gaussian deconvolution has been used, however this has been shown to oversmooth data and cause unnecessary signal suppression. In this study, an alternative approach, known as frequency-wavelet domain deconvolution, has been applied to in vivo THz skin measurements obtained in a clinical study. It is shown that this deconvolution method significantly enhances the usable signal, and improves the accuracy of the extracted impulse response function.
R. Degli'Innocenti, Y. Lu, A.M. Zaman, J.M. Woolley, N. Chopra, W. Tadbier, W. Michailow, S. Hofmann and J. Lloyd-Hughes APL Photonics 10 056101 (May 2025)
We report on the terahertz (THz) harmonic generation in an active graphene/metamaterial device by using powerful ultrafast table-top THz time-domain spectroscopic systems. Complex nonlinear transmission spectra, comprising even and odd harmonics, emerge when the devices are tested with intense ultrafast THz pulses with peak electric fields in the range of 1–150 kV/cm. The odd and even harmonic features show a positive correlation with carrier concentration, allowing for efficient frequency tuning on top of tunable group delay dispersion. Interestingly, before the onset of saturation, observed at 20–25 kV/cm, the main resonance and the harmonic features exhibit an anti-crossing trend that can be further exploited for active frequency tuning. These results report a key milestone for the fundamental investigation of the nonlinearity of 2D materials. At the same time, they represent an important advance in the design of future integrated THz optoelectronics by providing novel functionalities for THz light generation and manipulation.
A. Ali, V. Cascos, F.G. Pritchard, J.R.R. Verlet, N.R. Sutherland, J. Woolley, J. Lloyd-Hughes, M. Avdeev, A. Beeny, S.J. Clark and E.E. McCabe J. Mater. Chem. A 13 23073 (Apr 2025)
The Dion–Jacobson phases are increasingly gaining attention for their photoactivity and potential applications as photocatalysts and sensors, in addition to their polar structures. The structure – composition – property relationships for the n = 3 phases (of general formula A鈥睞2B3O10e.g., A鈥� = Rb, Cs; A = Ca, Sr, Ba; B = Nb, Ta) are not well understood because of the lack of reliable structural models for this series. Our combined experimental and computational study addresses this by determining and explaining the complex structural chemistry of these materials, and provides a guide to allow structures and properties of new materials to be predicted and understood. We find that both A鈥� and A cations determine the tilts of BO6 octahedra and hence small A2+ ions tend to give materials with larger band gaps. The polar structures observed for A = Ca phases result from 鈥減roper鈥� mechanisms and the role of the second-order Jahn–Teller effect of the B = Nb, Ta cations is discussed.
A. Dogra, D. Jones, A.I. Hernandez-Serrano, S. Chakraborty, J.J. Young, B.G. Page, J. Hardwicke, P. Valdastri and E. Pickwell-MacPherson Scientific Reports 15, 4568 (Feb 2025)
Terahertz (THz) light has the unique properties of being very sensitive to water, non-ionizing, and having sub-millimeter depth resolution, making it suitable for medical imaging. Skin conditions including eczema, psoriasis and skin cancer affect a high percentage of the population and we have been developing a THz probe to help with their diagnosis, treatment and management. Our in vivo studies have been using a handheld THz probe, but this has been prone to positional errors through sensorimotor perturbations and tremors, giving spatially imprecise measurements and significant variations in contact pressure. As the operator tires through extended device use, these errors are further exacerbated. A robotic system is therefore needed to tune the critical parameters and achieve accurate and repeatable measurements of skin. This paper proposes an autonomous robotic THz acquisition system, the PicoBot, designed for non-invasive diagnosis of healthy and diseased skin conditions, based on hydration levels in the skin. The PicoBot can 3D scan and segment out the region of interest on the skin鈥檚 surface, precisely position (卤 0.5/1 mm/degrees) the probe normal to the surface, and apply a desired amount of force (卤 0.1N) to maintain firm contact for the required 60 s during THz data acquisition. The robotic automation improves the stability of the acquired THz signals, reducing the standard deviation of amplitude fluctuations by over a factor of four at 1 THz compared to hand-held mode. We show THz results for skin measurements of volunteers with healthy and dry skin conditions on various parts of the body such as the volar forearm, forehead, cheeks, and hands. The tests conducted validate the preclinical feasibility of the concept along with the robustness and advantages of using the PicoBot, compared to a manual measurement setup.
N. Chopra and J. Lloyd-Hughes ACS Photonics 12 917 (Jan 2025)
A dual-beam THz spectrometer is reported that substantially reduces the influence of systematic errors in THz time-domain spectroscopy such as those caused by variations in femtosecond laser power or the environmental temperature and humidity. Dual THz beams with single-cycle waveforms were generated simultaneously using a dual-pixel interdigitated photoconductive antenna, allowing the simultaneous acquisition of sample and reference data in the spectrometer using the same optical components. A low-aberration optical geometry ensured diffraction-limited spatial profiles for both beams despite their off-axis propagation and was validated experimentally by measuring frequency-dependent beam profiles and theoretically via physical optics calculations. Although the experimental amplitudes and absolute phase spectra of both beams were very similar, we further provided a correction procedure to eliminate these small differences. The robustness of the dual-beam spectrometer design was evaluated by measuring the complex transmission of a thin plastic sheet after intentionally introducing a change in the relative humidity of the THz beam path. The dual-beam THz spectrometer was effective at removing systematic errors in the amplitude and phase by simultaneously measuring the two THz beams under the same conditions.
G. Costa, E.M. Brogden, J.J. Young, H. Ou, A.I. Hernandez-Serrano, R. I. Stantchev, Stefan A.F. Bon and E. Pickwell-MacPherson Langmuir 40, 25023 (Nov 2024)
The dynamics of the drying process of polymer latexes after casting as a wet film onto a substrate are important to track as they influence the physical and mechanical properties and performance of the dried polymer films. Current methods used to follow this drying process include gravimetric analysis, coupled with advanced techniques like GARField-NMR or optical coherence tomography. The latter two methods provide height and spatial information in the z-direction, normal to the substrate, and occasionally in the xz- or yz-planes. Terahertz time-domain spectroscopy (THz-TDS) is a welcome addition as it provides both the structural and spectroscopic information in the parallel xy-plane, filling the geometric gap. Herein, we utilize THz-TDS to study the drying and film formation process of various polymer latexes with a broad range of glass transition temperatures. We showcase the applicability of this technique in obtaining 2D parallel hydration maps of the drying dispersions, in the form of droplets, using latex-dependent calibration lines. Our findings display known phenomena arising from the drying of the colloidal dispersions.
X. Romain, P.R. Wilshaw, R.I. Stantchev, T. Miao, S. Mou T. Niewelt, S. McNab, S.L. Pain, N.E. Grant, R.S. Bonilla, E. Pickwell-MacPherson and J.D. Murphy IEEE Trans. THz Sci. Tech. 15, 76 (Oct 2024)
Silicon-based terahertz (THz) photomodulators suffer from a modulation speed limited by the lifetime of the charge carriers photoexcited in the silicon. We report a silicon-based THz photomodulator scheme offering real-time reconfiguration of the switching behavior by manipulation of effective charge carrier lifetime. Atomic layer deposition was used to coat silicon samples with dielectric layers to passivate the surfaces with a conductive polymer subsequently deposited to enable electrical gating over the whole surface. The resulting gated photomodulators are characterized using photoconductance decay and photoluminescence imaging. A gated photomodulator with HfO2 passivation is then implemented into a THz time domain spectroscopy setup to demonstrate the potential for live photomodulation optimization during a single-pixel imaging experiment. We use the device to achieve a real-time improvement of the signal-to-noise ratio of the images by a factor of 8.
X. Ding, A.I. Hernandez-Serrano, J.J. Young and E. Pickwell-MacPherson Biomedical Optics Express 15, 5180 (Aug 2024)
The skin, being the body鈥檚 largest organ, plays a pivotal role in protecting the body against dangerous external factors. The maintenance of adequate hydration levels is essential for the skin to fulfill this protective function. However, skin hydration depends upon different biophysical factors and lifestyles, such as ethnicity, sex, age, water consumption, and many more. Consequently, methods to assess skin hydration in a precise and non-invasive manner are in high demand. In this paper, using a portable and handheld terahertz (THz) probe, we systematically examine the correlation between diverse biophysical factors and skin hydration profile in a population exceeding 300 participants. Through comparative analysis of THz light reflected from the skin against a dielectric model, we successfully extracted the thickness and hydration percentage of the outermost layer of the epidermis, the stratum corneum (SC). Our findings indicate that SC hydration and thickness are associated with variables such as daily water consumption, age, drinking coffee, and exercise. Additionally, our measurements reveal distinctions in the skin鈥檚 hydration properties concerning susceptibility to UV-induced effects by bringing in the Fitzpatrick skin types. This THz-based technique holds the potential for facile integration into clinical settings for the evaluation and diagnosis of various skin-related conditions.
E. Butler-Caddle, K.D.G.I. Jayawardena, A. Wijesekara, R.L. Milot and J. Lloyd-Hughes Phys. Rev. Applied 22024103 (Aug 2024)
In perovskite solar cells, photovoltaic action is created by charge transport layers (CTLs) either side of the light-absorbing metal halide perovskite semiconductor. Hence, the rates for desirable charge extraction and unwanted interfacial recombination at the perovskite-CTL interfaces play a critical role for device efficiency. Here, the electrical properties of perovskite-CTL bilayer heterostructures are obtained using ultrafast terahertz and optical studies of the charge carrier dynamics after pulsed photoexcitation, combined with a physical model of charge carrier transport that includes the prominent Coulombic forces that arise after selective charge extraction into a CTL, and cross-interfacial recombination. The charge extraction velocity at the interface and the ambipolar diffusion coefficient within the perovskite are determined from the experimental decay profiles for heterostructures with three of the highest-performing CTLs, namely C60, PCBM and Spiro-OMeTAD. Definitive targets for the further improvement of devices are deduced: fullerenes deliver fast electron extraction, but suffer from a large rate constant for cross-interface recombination or hole extraction. Conversely, Spiro-OMeTAD exhibits slow hole extraction but does not increase the perovskite鈥檚 surface recombination rate, likely contributing to its success in solar cell devices.
Justas Deveikis, Marcin Giza, David Walker, Jie Liu, Claire Wilson, Nathaniel P. Gallop, Pablo Docampo, James Lloyd-Hughes and Rebecca L. Milot J. Phys. Chem. C 128 13108(July 2024)
Improved knowledge of the influence of temperature upon layered perovskites is essential to enable perovskite-based devices to operate over a broad temperature range and to elucidate the impact of structural changes upon the optoelectronic properties. We examined the Ruddlesden–Popper layered perovskite 2-thiophenemethylammonium lead iodide (ThMA2PbI4) and observed a structural phase transition between a high- and a low-temperature phase at 220 K using temperature-dependent X-ray diffraction, UV–visible absorption, and photoluminescence (PL) spectroscopy. The structural phase transition altered the tilt pattern of the inorganic octahedra layer, modifying the absorption and PL spectra. Further, we found a narrow and intense additional PL peak in the low-temperature phase, which we assigned to radiative emission from a defect-bound exciton state. In both phases we determined the thermal expansion coefficient and found values similar to those of cubic 3D perovskites, i.e., larger than those of typical substrates such as glass. These results demonstrate that the organic spacer plays a critical role in controlling the temperature-dependent structural and optoelectronic properties of layered perovskites and suggests more widely that strain management strategies may be needed to fully utilize layered perovskites in device applications.
J.J. Young, A.I. Hernandez-Serrano, J. Hardwicke and E. Pickwell-MacPherson Scientific Reports 14, 17546 (July 2024)
In this study we present the first in vivo clinical study of patients with eczema and psoriasis using terahertz (THz) sensing. Eczema and psoriasis patients were measured using a handheld THz scanner, both before and after the application of moisturiser. We show that THz sensing can distinguish between dry and healthy skin in different regions of the body. Furthermore, the impact of applying moisturiser on the skin can also be observed and potentially evaluated using THz light.
BFM Healy, SL Pain, J. Lloyd-Hughes, NE Grant and JD Murphy Adv. Mater. Interfaces 112400305 (Jul 2024)
Monolayer molybdenum disulfide (1L MoS2), a promising optoelectronic material, emits strong visible photoluminescence (PL). Systematic control of the intensity, energy, and spectral width of PL from 1L MoS2 on silicon dioxide/silicon (SiO2/Si) is demonstrated via simple external treatments. Treating MoS2 with solutions formed from the superacid bis-(trifluoromethanesulfonyl)amide (TFSA) enhances, blueshifts, and sharpens the PL. Treatments with solutions from structurally analogous chemicals that lack sulfur, in the case of bis(trifluoroacetamide) (BTFA), or lack fluorine, in the case of methanesulfonamide (MSA), show the same trend, suggesting a two-component mechanism for TFSA involving the presence of electronegative species and sulfur vacancy passivation. Up to 鈮�100脳 enhancement of the PL intensity is achieved, with the peak blueshifted by 鈮�30 meV and the spectral linewidth halved. Conversely, direct thermal atomic layer deposition (ALD) of aluminum oxide (Al2O3) or hafnium oxide (HfO2) is found to suppress the PL by up to a factor of 鈮�3, redshift by up to 鈮�70 meV, and broaden by 鈮�3脳. Single-spot and mapping Raman/PL techniques are combined in a robust characterization process to associate changes in the PL character to charge doping. This work demonstrates the convenient tunability of the optical behavior of 1L MoS2 by varying the electron density.
A.I. Hernandez-Serrano, X. Ding, G. Costa, G. Nurumbetov, D.M. Haddleton and E. Pickwell-MacPherson Biomedical Optics Express 15, 3064 (May 2024)
Transdermal drug delivery patches are a good alternative to hypodermic drug injection. The drug delivery efficiency depends strongly on the hydration of the skin under treatment, and therefore, it is essential to study the effects on the skin induced by the application of these medical-grade patches. Terahertz (THz) spectroscopy shows great promise for non-invasive skin evaluation due to its high sensitivity to subtle changes in water content, low power and non-ionizing properties. In this work, we study the effects of transdermal drug delivery patches (three fully occlusive and three partially occlusive) applied on the upper arms of ten volunteers for a maximum period of 28 h. Three different levels of propylene glycol (0 %, 3 % and 6 %) are added to the patches as excipient. By performing multilayer analysis, we successfully retrieve the water content of the stratum corneum (SC) which is the outermost layer of skin, as well as its thickness at different times before and after applying the patches. This study demonstrates the potential of using THz sensing for non invasive skin monitoring and has wide applications for skin evaluation as well as the development of skin products.
A.I. Hernandez-Serrano, X. Ding, J. Young, G. Costa, A. Dogra, J. Hardwicke and E. Pickwell-MacPherson Advanced Photonics Nexus 3, 016012 (Feb 2024)
This study introduces a handheld terahertz (THz) scanner designed to quantitatively evaluate human skin hydration levels and thickness. This device, through the incorporation of force sensors, demonstrates enhanced repeatability and accuracy over traditional fixed THz systems. The scanner was evaluated in the largest THz skin study to date, assessing 314 volunteers, successfully differentiating between individuals with dry skin and hydrated skin using a numerical stratified skin model. The scanner measures and displays skin hydration dynamics within a quarter of a second, indicating its potential for real-time, noninvasive examinations, opening up opportunities for in vivo and ex vivo diagnosis during patient consultations. Furthermore, the portability and ease of use of our scanner enable its widespread application for in vivo and ex vivo diagnosis during patient consultations, potentially allowing in situ biopsy evaluation and elimination of histopathology processing wait times, thereby improving patient outcomes by facilitating simultaneous tumor diagnosis and removal.
H. Ou, R.I. Stantchev, X. Chen, T. Blu, M. Semtsiv, W.T. Masselink, A. Hernandez Serrano, G. Costa, J. Young, N. Chopra, J. Lloyd-Hughes, and E. Pickwell-MacPherson Optics Express 32, 5567 (Feb 2024)
We propose a polarization sensitive terahertz time-domain spectrometer that can record orthogonally polarized terahertz fields simultaneously, using fibre-coupled photoconductive antennas and a scheme that modulated the emitter鈥檚 polarization. The s and p channels of the multi-pixel terahertz emitter were modulated at different frequencies, thereby allowing orthogonal waveforms to be demultiplexed from the recorded signal in post-processing. The performance of the multi-pixel emitter used in this multiplexing scheme was comparable to that of a commercial single-polarization H-dipole antenna. The approach allowed two orthogonally polarized terahertz pulses to be recorded with good signal to noise (>1000:1) within half a second. We verified the capability of the spectrometer by characterizing a birefringent crystal and by imaging a polarization-sensitive metamaterial. This work has significant potential to improve the speed of terahertz polarization sensitive applications, such as ellipsometry and imaging.
Folusho Helen Balogun, Nathaniel P Gallop, Dumitru Sirbu, Jake D Hutchinson, Nathan Hill, Jack M Woolley, David Walker, Stephen York, Pablo Docampo and Rebecca L Milot
Mater. Res. Express 11 025503 (Feb 2024)
Layered hybrid perovskites (LPKs) are promising as alternatives or additives to 3D metal halide perovskites for optoelectronic applications including photovoltaic cells, LEDs and lasers due to their increased stability. However, high exciton binding energies in these materials mean that excitons are the majority species under the operating conditions of many devices. Although the efficiency of devices that incorporate LPKs has been increasing, much is still unknown about the interplay of excitons and free charge-carriers in these materials, which is vital information for understanding how optoelectronic properties dictate device efficiency. In this work, we employ optical pump/THz probe spectroscopy (OPTP) and visible transient absorption spectroscopy (TAS) to analyse the optoelectronic properties and charge-carrier dynamics of phenylethylammonium lead iodide (PEA)2PbI4. By combining these techniques, we are able to disentangle the contributions from excitons and free charge-carriers. We observe fast cooling of free charge-carriers and exciton formation on a timescale of 鈭�400 fs followed by slower bimolecular recombination of residual free charge-carriers with a rate constant k2 鈭� 109 cm3s鈭�1. Excitons recombine via two monomolecular processes with lifetimes t1 鈭� 11 ps and t2 鈭� 83 ps. Furthermore, we detect signatures of exciton–phonon coupling in the transient absorption kinetic traces. These findings provide new insight into the interplay between free charge-carriers and excitons as well as a possible mechanism to further understand the charge-carrier dynamics in LPKs.
BFM Healy, SL Pain, J. Lloyd-Hughes, NE Grant and JD Murphy Materials Research Express 11 015002 (Jan 2024)
Monolayer molybdenum disulfide (MoS2) is a promising candidate for inclusion in optoelectronic technologies, owing to its two-dimensional (2D) nature and resultant novel photoluminescence (PL). Chemical vapour deposition (CVD) is an important method for the preparation of large-area films of monolayer MoS2. The PL character of as-prepared monolayer MoS2 must be well understood to facilitate detailed evaluation of any process-induced effects during device fabrication. We comparatively explore the PL emission from four different commercially available CVD-grown MoS2 monolayer films. We characterize the samples via Raman and PL spectroscopy, using both single-spot and mapping techniques, while atomic force microscopy (AFM) is applied to map the surface structure. Via multipeak fitting, we decompose the PL spectra into constituent exciton and trion contributions, enabling an assessment of the quality of the MoS2 monolayers. We find that the PL character varies significantly from sample to sample. We also reveal substantial inhomogeneity of the PL signal across each individual MoS2 film. We attribute the PL variation to non-uniform MoS2 film morphologies that result from the nucleation and coalescence processes during the CVD film development. Understanding the large variability in starting PL behaviour is vital to optimize the optoelectronic properties for MoS2-based devices.
T.J. Keat, D. J. L. Coxon, R.J. Cruddace, V. G. Stavros, M. E. Newton, and J. Lloyd-Hughes Diamond and Related Materials 141110661 (Jan 2024)
The dynamics of the 3237 cm鈭�1 local vibrational mode in diamond, associated with an unknown defect, was investigated using ultrafast infrared pump-probe spectroscopy. When pumped at 3237 cm鈭�1, a degenerate probe was used to study the ground state's recovery, while a non-degenerate probe tracked excited state absorption at 3029 cm鈭�1, corresponding to the 1 鈫� 2 vibrational state transition. The similar population lifetimes for the ground state recovery and excited state absorption suggests a single population decay pathway, with a lifetime of T1=2.2+-0.1ps. Perturbed free induction decay signals observed in negative time delays gave the dephasing time of the coherent state between the 0 and 1 vibrational states, and further predicted the 3029 cm鈭�1 transition. Images from FTIR microscopy show that the 3237 cm鈭�1 feature and the 3107 cm鈭�1 absorption line from the N3VH0 defect are not correlated, and our pump-probe study shows the 3237 cm鈭�1 feature does not share a common ground state with the N3VH0 defect, both of which suggest that this local vibrational mode does not originate from the N3VH0 defect. A calibration factor was obtained via a Morse potential model constrained by the observed transition energies, which relates the concentration of the defect producing the 3237 cm鈭�1 feature to its absorption coefficient measured by FTIR spectroscopy. Based on FTIR absorption spectroscopy under uniaxial stress, we further assign a trigonal symmetry character to the defect that gives the 3237 cm鈭�1 feature. The results presented are consistent with the theory that the 3237 cm鈭�1 feature originates from the N4VH defect, the quantification of which allows better tracking of the nitrogen content in diamond.
N. Chopra and J. Lloyd-Hughes J Infrared Milli Terahz Waves 44, 981 (Nov 2023)
Off-axis parabolic mirrors (OAPMs) are widely used in the THz and mm-wave communities for spectroscopy and imaging applications, as a result of their broadband, low-loss operation and high numerical apertures. However, the aspherical shape of an OAPM creates significant geometric aberrations: these make achieving diffraction-limited performance a challenge, and lower the peak electric field strength in the focal plane. Here, we quantify the impact of geometric aberrations on the performance of the most widely used spectrometer designs, by using ray tracing and physical optics calculations to investigate whether diffraction-limited performance can be achieved in both the sample and the detector plane. We identify simple rules, based on marginal ray propagation, that allow spectrometers to be designed that are more robust to misalignment errors, and which have minimal aberrations for THz beams. For a given source, this allows the design of optical paths that give the smallest THz beam focal spot, with the highest THz electric field strength possible. This is desirable for improved THz imaging, for better signal-to-noise ratios in linear THz spectroscopy and optical-pump THz-probe spectroscopy, and to achieve higher electric field strengths in non-linear THz spectroscopy.
Nathaniel P. Gallop, Dumitru Sirbu, David Walker, James Lloyd-Hughes, Pablo Docampo and Rebecca L. Milot ACS Photonics 104022(October 2023)
We report on the emission of high-intensity pulsed terahertz radiation from the metal-free halide perovskite single crystal methyl-DABCO ammonium iodide (MDNI) under femtosecond illumination. The power and angular dependence of the THz output implicate optical rectification of the 800 nm pump as the mechanism of THz generation. Further characterization finds that, for certain crystal orientations, the angular dependence of THz emission is modulated by phonon resonances attributable to the motion of the methyl-DABCO moiety. At maximum, the THz emission spectrum of MDNI is free from significant phonon resonances, resulting in THz pulses with a temporal width of <900 fs and a peak-to-peak electric field strength of approximately 0.8 kV cm–1鈹�2 orders of magnitude higher than any other reported halide perovskite emitters. Our results point toward metal-free perovskites as a promising new class of THz emitters that brings to bear many of the advantages enjoyed by other halide perovskite materials. In particular, the broad tunability of optoelectronic properties and ease of fabrication of perovskite materials opens up the possibility of further optimizing the THz emission properties within this material class.
A. Ren, H. Wang, L. Dai, J. Xia, E. Butler-Caddle, J.A. Smith, ... S.A. Hindmarsh, A.M. Sanchez, J. Lloyd-Hughes, S. J Sweeney, ... and Wei Zhang Nature Photonics17, 798–805 (July 2023)
Light-emitting diodes (LEDs) are ubiquitous in modern society, with applications spanning from lighting and displays to medical diagnostics and data communications. Metal-halide perovskites are promising materials for LEDs because of their excellent optoelectronic properties and solution processability. Although research has progressed substantially in optimizing their external quantum efficiency, the modulation characteristics of perovskite LEDs remain unclear. Here we report a holistic approach for realizing fast perovskite photonic sources on silicon based on tailoring alkylammonium cations in perovskite systems. We reveal the recombination behaviour of charged species at various carrier density regimes relevant for their modulation performance. By integrating a Fabry–P茅rot microcavity on silicon, we demonstrate perovskite devices with efficient light outcoupling. We achieve device modulation bandwidths of up to 42.6 MHz and data rates above 50 Mbps, with further analysis suggesting that the bandwidth may exceed gigahertz levels. The principles developed here will support the development of perovskite light sources for next-generation data-communication architectures. The demonstration of solution-processed perovskite emitters on silicon substrates also opens up the possibility of integration with micro-electronics platforms.
Jake D. Hutchinson, Edoardo Ruggeri, Jack M. Woolley, G茅raud Delport, Samuel D. Stranks, Rebecca L. Milot Advanced Functional Materials2305736, (August 2023)
Mixed 2D/3D perovskite materials are of particular interest to the photovoltaics and light- emitting diode (LED) communities due to their impressive opto-electronic properties alongside improved moisture stability compared to conventional 3D perovskite absorbers. Here, a mixed lead-tin perovskite containing distinct, self-assembled domains of either 3D structures or highly phase-pure Ruddlesden–Popper 2D structures is studied. The complex energy landscape of the material is revealed with ultrafast optical transient absorption measurements. It is shown that charge transfer between these microscale domains only occurs on nanosecond timescales, consistent with the large size of the domains. Using optical pump-terahertz probe spectroscopy, the effective charge-carrier mobility is shown to be an intermediary between analogous pure 2D and 3D perovskites. Furthermore, detailed analysis of the free carrier recombination dynamics is presented. By combining results from a range of excitation wavelengths within a full dynamic model of the photoexcited carrier population, it is shown that the 2D domains in the film exhibit remarkably similar carrier dynamics to the 3D domains, suggesting that long-range charge-transport should not be impeded by the heterogeneous structure of the material.
A. Whittock, X. Ding, X. Ramirez Barker, N. Auckloo, R. Sellers, J.M. Woolley V. Krishnan, C. Marine, C. Corre, E. Pickwell-MacPherson and V.G. Stavros Chem. Sci. 14, 6763 (June 2023)
Biomimicry has become a key player in researching new materials for a whole range of applications. In this study, we have taken a crude extract from the red algae Palmaria palmata containing mycosporine-like amino acids – a photoprotective family of molecules. We have applied the crude extract onto a surface to assess if photoprotection, and more broadly, light-to-heat conversion, is retained; we found it is. Considering sunscreens as a specific application, we have performed transmission and reflection terahertz spectroscopy of the extract and glycerol to demonstrate how one can monitor stability in real-world applications.
M.G. Burdanova, A.P. Tsapenko, S. Ahmad, E.I. Kauppinen and J. Lloyd-Hughes Nanotechnology 34405203, (June 2023)
Mixed-nanomaterial composites can combine the excellent properties of well-known low-dimensional nanomaterials. Here we highlight the potential of one-dimensional single-walled carbon nanotubes interfaced with two-dimensional graphene by exploring the composite's ac conductivity and photoconductivity, and the influence of HAuCl4 doping. In the composite, the equilibrium terahertz conductivity from free carrier motion was boosted, while the localised plasmon peak shifted towards higher frequencies, which we attribute to shorter conductivity pathways in the composite. A negative terahertz photoconductivity was observed for all samples under 410nm optical excitation and was reproduced by a simple model, where the Drude spectral weight and the momentum scattering rate were both lowered under photoexcitation. The composite had an enhanced modulation depth in comparison to reference carbon nanotube films, while retaining their characteristically fast (picosecond) response time. The results show that CNT-graphene composites offer new opportunities in devices by controlling charge carrier transport and tuning their optoelectronic properties.
A. Leitenstorfer, ..., E. Pickwell-MacPherson, ... and J. Cunningham J. Phys. D: Appl. Phys. 56, 223001 (April 2023)
Terahertz (THz) radiation encompasses a wide spectral range within the electromagnetic spectrum that extends from microwaves to the far infrared (100 GHz–鈭�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.
X. Ding, G. Costa, A. I. Hernandez-Serrano, R.I. Stantchev, G. Nurumbetov, D.M. Haddleton, and E. Pickwell-MacPherson Bio. Opt. Express 14, 1146 (Feb 2023)
Transdermal drug delivery (TDD) has been widely used in medical treatments due to various advantages, including delivering drugs at a consistent rate. However, variations in skin hydration can have a significant effect on the permeability of chemicals. Therefore, it is essential to study the changes in skin hydration induced by TDD patches for better control of the delivery rate. In this work, in vivo terahertz (THz) spectroscopy is conducted to quantitatively monitor human skin after the application of patches with different backing materials and propylene glycol concentrations. Changes in skin hydration and skin response to occlusion induced by other patches are investigated and compared. Our work demonstrates the potential application of in vivo THz measurements in label-free, non-invasive evaluation of transdermal patches on human skin and further reveals the mechanism behind the effect.
Z. Hu, B. Breeze, M. Walker, E. Faulques, J. Sloan and J. Lloyd-Hughes ACS Appl. Nano Mater. 6, 2883 (Feb 2023)
Studying the optical performance of carbon nanotubes (CNTs) filled with guest materials can reveal the fundamental photochemical nature of ultrathin one-dimensional (1D) nanosystems, which are attractive for applications including photocatalysis. Here, we report comprehensive spectroscopic studies of how infiltrated HgTe nanowires (NWs) alter the optical properties of small-diameter (dt < 1 nm) single-walled carbon nanotubes (SWCNTs) in different environments: isolated in solution, suspended in a gelatin matrix, and heavily bundled in network-like thin films. Temperature-dependent Raman and photoluminescence measurements revealed that the HgTe NW filling can alter the stiffness of SWCNTs and therefore modify their vibrational and optical modes. Results from optical absorption and X-ray photoelectron spectroscopy demonstrated that the semiconducting HgTe NWs did not provide substantial charge transfer to or from the SWCNTs. Transient absorption spectroscopy further highlighted that the filling-induced nanotube distortion can alter the temporal evolution of excitons and their transient spectra. In contrast to previous studies on functionalized CNTs, where electronic or chemical doping often drove changes to the optical spectra, we highlight structural distortion as playing an important role.
N. Chopra,J. Deveikis and J. Lloyd-Hughes Appl. Phys. Lett. 122061102 (Feb 2023)
The spatial profile of a beam of pulsed terahertz (THz) radiation is controlled electrically using a multi-pixel photoconductive emitter, which consists of an array of interdigitated electrodes fabricated on semi-insulating GaAs. Activating individual pixels allows the transverse position of the THz beam's focus to be varied off-axis, as verified by spatial beam profiles. Enabling multiple pixels simultaneously permits non-Gaussian beam shapes to be created. The diffraction-limited performance of the system is established by comparison with the Abb茅 and Sparrow criteria, and a condition for effective beam steering using this design is derived. The spatial resolution of the approach is linked to the frequency of the THz radiation and the f-number of the collection optic.
G.M. Katyba, N.I. Raginov, E.M. Khabushev, V.A. Zhelnov, A. Gorodetsky, D.A. Ghazaryan, M.S. Mironov, D.V. Krasnikov, Y.G. Gladush, J. Lloyd-Hughes, A.G. Nasibulin, A.V. Arsenin, V.S. Volkov, K.I. Zaytsev, and M.G. Burdanova Optica 10, 53 (Jan 2023)
Tunable optoelectronics have attracted a lot of attention in recent years because of their variety of applications in next-generation devices. Among the potential uses for tuning optical elements, those allowing consistent parameter control stand out. We present an approach for the creation of mechanically tunable zone plate lenses in the THz range. Our devices comprise single-walled carbon nanotube (SWCNT) thin films of predetermined design integrated with stretchable polymer films. These offer high-performance and in situ tunability of focal length up to 50%. We studied the focusing properties of our lenses using the backward-wave oscillator THz imaging technique, supported by numerical simulations based on the finite element frequency domain method. Our approach may further enable the integration of SWCNT films into photonic and optoelectronic applications and could be of use for the creation of a variety of flexible and stretchable THz optical elements.
E. Butler-Caddle, N.E. Grant, S.L. Pain, J.D. Murphy, K.D.G.I. Jayawardena and J. Lloyd-Hughes Appl. Phys. Lett. 122012101 (Jan 2023)
Optical pump terahertz probe spectroscopy (OPTP) is a versatile non-contact technique that measures transient photoconductance decays with femtosecond temporal resolution. However, its maximum temporal range is limited to only a few nanoseconds by the mechanical delay lines used. We extended the temporal range of OPTP to milliseconds and longer while retaining sub-nanosecond resolution. A separate pump laser was electrically synchronized to the probe pulses, allowing the pump–probe delay to be controlled with an electronic delay generator. We demonstrated the capabilities of this technique by examining the photoconductance decays of semiconductors with lifetimes ranging over six orders of magnitude: III-Vs, metal halide perovskites, germanium, and silicon. A direct comparison of results on silicon from OPTP and inductively coupled photoconductance decay highlighted the higher spatial and temporal resolution of OPTP, which allowed in-plane and out-of-plane carrier diffusion to be studied.
T.J. Keat , D. J. L. Coxon, M. Staniforth, M. W. Dale, V. G. Stavros, M. E. Newton, and J. Lloyd-Hughes Phys. Rev. Lett. 129237401 (Nov 2022)
The perturbed free induction decay (PFID) observed in ultrafast infrared spectroscopy was used to unveil the rates at which different vibrational modes of the same atomic-scale defect can interact with their environment. The N3VH0 defect in diamond provided a model system, allowing a comparison of stretch and bend vibrational modes within different crystal lattice environments. The observed bend mode (first overtone) exhibited dephasing times T2=2.8(1) ps, while the fundamental stretch mode had surprisingly faster dynamics T2<1.7 ps driven by its more direct perturbation of the crystal lattice, with increased phonon coupling. Further, at high defect concentrations the stretch mode鈥檚 dephasing rate was enhanced. The ability to reliably measure T2 via PFID provides vital insights into how vibrational systems interact with their local environment.
J. Deveikis and J. Lloyd-Hughes Optics Express 3043293 (Nov 2022)
A multi-pixel photoconductive emitter is reported that generates THz beams with either azimuthal, radial or linear polarization states. Switching between the different polarization states was purely electrical, via the bias voltage applied, circumventing the need for mechanical polarization optics or different THz emitters to change the polarization. Dipole array modelling was performed to validate emitter array designs, and to explore their optimal bias configuration, while spatially-resolved electro-optic detection of the generated beams confirmed that cylindrical-vector beams were produced. We further demonstrate that the spatial beam profile was optimized by adjusting the bias level on particular pixels, improving the polarization purity of the beam.
X. Chen, H. Lindley-Hatcher, R. I. Stantchev, J. Wang, K. Li, A. I. Hernandez-Serrano, Z. D. Taylor, E. Castro-Camus and E. Pickwell-MacPherson Chem. Phys. Rev. 3, 011311 (June 2022)
Terahertz (THz) technology has experienced rapid development in the past two decades. Growing numbers of interdisciplinary applications are emerging, including materials science, physics, communications, and security as well as biomedicine. THz biophotonics involves studies applying THz photonic technology in biomedicine, which has attracted attention due to the unique features of THz waves, such as the high sensitivity to water, resonance with biomolecules, favorable spatial resolution, capacity to probe the water–biomolecule interactions, and nonionizing photon energy. Despite the great potential, THz biophotonics is still at an early stage of development. There is a lack of standards for instrumentation, measurement protocols, and data analysis, which makes it difficult to make comparisons among all the work published. In this article, we give a comprehensive review of the key findings that have underpinned research into biomedical applications of THz technology. In particular, we will focus on the advances made in general THz instrumentation and specific THz-based instruments for biomedical applications. We will also discuss the theories describing the interaction between THz light and biomedical samples. We aim to provide an overview of both basic biomedical research as well as pre-clinical and clinical applications under investigation. The paper aims to provide a clear picture of the achievements, challenges, and future perspectives of THz biophotonics.
X. Ding, A. I. Hernandez-Serrano, H. Lindley-Hatcher, R. I. Stantchev, J. Zhou and E. Pickwell-MacPherson Optics Express 30, 18079 (May 2022)
Terahertz time-domain spectroscopy (THz-TDS) has shown promise in biomedical sample characterization and high characterization sensitivity is in demand due to the thin-film (TF) feature of the sample. This paper proposes an optimized multilayer structure for sensitive characterization of TF aqueous solutions in reflection THz-TDS. Theoretical simulations are conducted for structural optimization and the 75 渭m window-sample-mirror structure displays the best sensitivity compared to other sandwich structures and traditional THz measurement geometries. 0-20% TF glucose solutions are then measured; and a spectral peak introduced by the proposed structure is observed to result in the high sensitivity. Our work provides a new way of customizing multilayer structure for THz thin-film characterization.
X. E. Ramirez Barker, R. I. Stantchev, A. I. Hernandez-Serrano and E. Pickwell-MacPherson Sensors 22, 3387 (April 2022)
Recently, finite rate of innovation methods have been successfully applied to achieve low sampling rates in many areas, such as for ultrasound and radio signals. However, to the best of our knowledge, there are no journal publications applying this to real terahertz signals. In this work, we mathematically describe a finite rate of innovation method applied specifically to terahertz signals both experimentally and in simulation. To demonstrate our method, we applied it to randomized simulated signals with and without the presence of noise and to simple experimental measurements. We found excellent agreement between the simulated signals and those recreated based on results from our method, with this success also being replicated experimentally. These results were obtained at relatively low sampling rates, compared to standard methods, which is a key advantage to using a finite rate of innovation method as it allows for faster data acquisition and signal processing.
Z. Hu, B. Breeze, R.J. Kashtiban, J. Sloan and J. Lloyd-Hughes ACS Nano 16 6789 (Apr 2022)
Atomically thin nanowires (NWs) can be synthesized inside single-walled carbon nanotubes (SWCNTs) and feature unique crystal structures. Here we show that HgTe nanowires formed inside small-diameter (<1 nm) SWCNTs can advantageously alter the optical and electronic properties of the SWCNTs. Metallic purification of the filled SWCNTs was achieved by a gel column chromatography method, leading to an efficient extraction of the semiconducting and metallic portions with known chiralities. Electron microscopic imaging revealed that zigzag HgTe chains were the dominant NW geometry in both the semiconducting and metallic species. Equilibrium-state and ultrafast spectroscopy demonstrated that the coupled electron–phonon system was modified by the encapsulated HgTe NWs, in a way that varied with the chirality. For semiconducting SWCNTs with HgTe NWs, Auger relaxation processes were suppressed, leading to enhanced photoluminescence emission. In contrast, HgTe NWs enhanced the Auger relaxation rate of metallic SWCNTs and created faster phonon relaxation, providing experimental evidence that encapsulated atomic chains can suppress hot carrier effects and therefore boost electronic transport.
H. Hempel, T.J. Savenjie, M. Stolterfoht, J. Neu, M. Failla, V.C. Paingad, P. Ku啪el, E.J. Heilweil, J.A. Spies, M. Schleuning, J. Zhao, D. Friedrich, K. Schwarzburg, L.D.A. Siebbeles, P. D枚rflinger, V. Dyakonov, R. Katoh, M.J. Hong, J.G. Labram, M. Monti, E. Butler-Caddle, J. Lloyd-Hughes, M.M. Taheri, J.B. Baxter, T.J. Magnanelli, S. Luo, J.M. Cardon, S. Ardo, T. Unold Advanced Energy Materials 2102776 (Feb 2022)
Bringing together the expertise from fifteen laboratories the current-voltage characteristics of a solar cell are modeled using contactless terahertz and microwave measurements. To this end, the impact of measurement conditions, alternate interpretations, and experimental inter-laboratory variations are discussed. For a neat (Cs,FA,MA)Pb(I,Br)3 thin film, the implied resistance-free JV-curve and the fill factor losses by its finite mobility are revealed.
M.G. Burdanova, A.P. Tsapenko, M.V. Kharlamova, E.I. Kauppinen, B.P. Gorshunov, J. Kono and J. Lloyd-Hughes
Advanced Optical Materials 2101042 (Sept 2021)
Terahertz (THz) spectroscopy is an ideal non-contact and non-destructive technique that probes the electrical conductivity of nanomaterials. This review presents the current status of research in the THz properties of quasi-1D materials, such as nanotubes (NTs) and NT heterostructures. The detailed description of THz experimental methods (THz time-domain spectroscopy, optical pump-THz probe spectroscopy) and conductivity extraction methods are presented along with the physical models (Drude, plasmon, effective medium theories, etc.) supporting them. Optoelectronic applications, such as optical modulators, switches, and shielding devices, are discussed and illustrate a bright future for these materials.
M.G. Burdanova, M. Liu, M. Staniforth, Y. Zheng, R. Xiang, S. Chiashi, A. Anisimov, E. I. Kauppinen, S. Maruyama and J. Lloyd-Hughes Advanced Functional Materials 2104969 (Sept 2021)
Strong intertube excitonic coupling is demonstrated in 1D van der Waals heterostructures by examining the ultrafast response of radial C/BN/MoS2 core/shell/skin nanotubes to femtosecond infrared light pulses. Remarkably, infrared excitation of excitons in the semiconducting carbon nanotubes (CNTs) creates a prominent excitonic response in the visible range from the MoS2 skin, even with infrared photons at energies well below the bandgap of MoS2. Via classical analogies and a quantum model of the light–matter interaction these findings are assigned to intertube excitonic correlations. Dipole–dipole Coulomb interactions in the coherent regime produce intertube biexcitons, which persist for tens of femtoseconds, while on longer timescales (>100 ps) hole tunneling—from the CNT core, through the BN tunnel barrier, to the MoS2 skin—creates intertube excitons. Charge transfer and dipole–dipole interactions thus play prominent roles on different timescales, and establish new possibilities for the multi-functional use of these new nanoscale coaxial cables.
C.D.W. Mosley, J. Deveikis and J. Lloyd-Hughes Appl. Phys. Lett. 119121105 (Sep 2021)
Full control of the ellipticity of broadband pulses of THz radiation, from linear to left- or right-handed circular polarization, was demonstrated via a four-pixel photoconductive emitter with an integrated achromatic waveplate. Excellent polarization purity and accuracy were achieved, with Stokes parameters exceeding 97% for linear and circular polarization, via a robust scheme that corrected electrically for polarization changes caused by imperfect optical elements. Furthermore, to assess the speed and precision of measurements of the THz polarization, we introduced a figure of merit, the standard error after one second of measurement, found to be 0.047掳 for the polarization angle.
R.J. Kashtiban, M.G. Burdanova, A. Vasylenko, J. Wynn, P.V.C. Medeiros, Q. Ramasse, A.J. Morris, D. Quigley, J. Lloyd-Hughes and J. Sloan
ACS Nano 1513389 (Aug 2021)
One-dimensional (1D) atomic chains of CsI were previously reported in double-walled carbon nanotubes with 鈭�0.8 nm inner diameter. Here, we demonstrate that, while 1D CsI chains form within narrow 鈭�0.73 nm diameter single-walled carbon nanotubes (SWCNTs), wider SWCNT tubules (鈭�0.8–1.1 nm) promote the formation of helical chains of CsI 2 脳 1 atoms in cross-section. These CsI helices create complementary oval distortions in encapsulating SWCNTs with highly strained helices formed from strained Cs2I2 parallelogram units in narrow tubes to lower strain Cs2I2 units in wider tubes. The observed structural changes and charge distribution were analyzed by density-functional theory and Bader analysis. CsI chains also produce conformation-selective changes to the electronic structure and optical properties of the encapsulating tubules. The observed defects are an interesting variation from defects commonly observed in alkali halides as these are normally associated with the Schottky and Frenkel type. The energetics of CsI 2 脳 1 helix formation in SWCNTs suggests how these could be controllably formed.
J. Lloyd-Hughes, P.M. Oppeneer, T. Pereira dos Santos, A. Schleife, S. Meng, M.A. Sentef, M. Ruggenthaler, A. Rubio, I. Radu, M. Murnane, X. Shi, H. Kapteyn, B. Stadtm眉ller, K.M. Dani, F.H. da Jornada, E. Prinz, M. Aeschlimann, R.L. Milot, M. Burdanova, J. Boland, T. Cocker and F. Hegmann
J. Phys.: Cond. Matt. 33 353001 (July 2021)
In the 60 years since the invention of the laser, the scientific community has developed numerous fields of research based on these bright, coherent light sources, including the areas of imaging, spectroscopy, materials processing and communications. Ultrafast spectroscopy and imaging techniques are at the forefront of research into the light–matter interaction at the shortest times accessible to experiments, ranging from a few attoseconds to nanoseconds. Light pulses provide a crucial probe of the dynamical motion of charges, spins, and atoms on picosecond, femtosecond, and down to attosecond timescales, none of which are accessible even with the fastest electronic devices. Furthermore, strong light pulses can drive materials into unusual phases, with exotic properties. In this roadmap we describe the current state-of-the-art in experimental and theoretical studies of condensed matter using ultrafast probes. In each contribution, the authors also use their extensive knowledge to highlight challenges and predict future trends.
