Coherent terahertz laser feedback interferometry for hydration sensing in leaves.

The response of terahertz to the presence of water content makes it an ideal analytical tool for hydration monitoring in agricultural applications. This study reports on the feasibility of terahertz sensing for monitoring the hydration level of freshly harvested leaves of Celtis sinensis by employing a imaging platform based on quantum cascade lasers and laser feedback interferometry. The imaging platform produces wide angle high resolution terahertz amplitude and phase images of the leaves at high frame rates allowing monitoring of dynamic water transport and other changes across the whole leaf. The complementary information in the resulting images was fed to a machine learning model aiming to predict relative water content from a single frame. The model was used to predict the change in hydration level over time. Results of the study suggest that the technique could have substantial potential in agricultural applications.

Comparison of Physical and System Factors Impacting Hydration Sensing in Leaves Using Terahertz Time-Domain and Quantum Cascade Laser Feedback Interferometry Imaging.

To reduce the water footprint in agriculture, the recent push toward precision irrigation management has initiated a sharp rise in photonics-based hydration sensing in plants in a non-contact, non-invasive manner. Here, this aspect of sensing was employed in the terahertz (THz) range for mapping liquid water in the plucked leaves of Bambusa vulgaris and Celtis sinensis. Two complementary techniques, broadband THz time-domain spectroscopic imaging and THz quantum cascade laser-based imaging, were utilized. The resulting hydration maps capture the spatial variations within the leaves as well as the hydration dynamics in various time scales. Although both techniques employed raster scanning to acquire the THz image, the results provide very distinct and different information. Terahertz time-domain spectroscopy provides rich spectral and phase information detailing the dehydration effects on the leaf structure, while THz quantum cascade laser-based laser feedback interferometry gives insight into the fast dynamic variation in dehydration patterns.

Guest Editorial - Enhanced Care: Developing the resource within Acute Medicine.

Critical Care Units provide care to those patients who traditionally need "organ support". There is variation in provision and "admission criteria" across the UK, and although Level 2 admissions have been increasing this often reflects increasing perioperative demand and largely ignores the unmet and often unmeasured medical need.

Enhanced Care Units: Guidance on development and implementation within Acute Medicine.

The Society for Acute Medicine (SAM) and Intensive Care Society (ICS) have produced joint guidance on the standards of care and infrastructure required to deliver enhanced care within Acute Medicine. The cohort of patients this relates to are in the most part already being looked after on the AMU, but co-location and providing enhanced monitoring and nursing input will ensure safe, high-quality care can be delivered to them. We strongly support the development of enhanced care units, whilst clearly acknowledging that they are not a replacement for critical care where that is indicated. Enhanced care and critical care complement each other and will help foster the close working between the two specialties that modern acute care requires. This guidance draws on expertise and existing relevant guidance from the two societies, alongside that from the Faculty of Intensive Care Medicine (FICM), British Thoracic Society (BTS), National Institute for Health and Care Excellence (NICE) and NHS England / Improvement (NHSE/I). We recognise this is an area with limited evidence and so will aim to review it regularly as the knowledge and experience in this area increases.

Resonant two-photon terahertz quantum cascade laser.

Lasers that can emit two photons from a single electron relaxation between two states of the same parity have been discussed since the early days of the laser era. However, such lasers have seen only limited success, mainly due to a lack of suitable gain medium. We propose that terahertz (THz) frequency quantum cascade lasers (QCLs) are an ideal semiconductor structure to realize such two-photon emissions. In this work, we present a THz QCL heterostructure designed to emit two resonant photons from each electronic relaxation between two same-parity states in the active region. We present coupled Maxwell-Bloch equations that describe the dynamics of such a two-photon laser and find analytical solutions for the steady-state light intensity, the steady-state energy-resolved carrier densities, and the total threshold carrier density. Due to the two-photon emission from each excited state relaxation and an increased photon-driven carrier transport rate, our simulations predict a significant enhancement of light intensity in our designed resonant two-photon THz QCL when compared to an exemplar conventional THz QCL structure.

Terahertz quantum cascade laser under optical feedback: effects of laser self-pulsations on self-mixing signals.

In this article, we explore the interplay between the self-pulsations (SPs) and self-mixing (SM) signals generated in terahertz (THz) quantum cascade lasers (QCLs) under optical feedback. We find that optical feedback dynamics in a THz QCL, namely, SPs, modulate the conventional SM interference fringes in a laser feedback interferometry system. The phenomenon of fringe loss in the SM signal - well known in interband diode lasers - was also observed along with pronounced SPs. With an increasing optical feedback strength, SM interference fringes transition from regular fringes at weak feedback (C ≤ 1) to fringes modulated by SPs under moderate feedback (1 < C ≤ 4.6), and then [under strong feedback (C > 4.6)] to a SM waveform with reduced number of fringes modulated by SP, until eventually (under even greater feedback) all the fringes are lost and only SPs are left visible. The transition route described above was identified in simulation when the SM fringes are created either by a moving target or a current modulation of the THz QCL. This SM signal transition route was successfully validated experimentally in a pulsed mode THz QCL with SM fringes created by current modulation during the pulse. The effects of SP dynamics in laser feedback interferometric system investigated in this work not only provides a further understanding of nonlinear dynamics in a THz QCL but also helps to understand the SM waveforms generated in a THz QCLs when they are used for various sensing and imaging applications.

Dual resonance phonon-photon-phonon terahertz quantum-cascade laser: physics of the electron transport and temperature performance optimization.

The state of the art terahertz-frequency quantum cascade lasers have opened a plethora of applications over the past two decades by testing several designs up to the very limit of operating temperature, optical power and lasing frequency performance. The temperature degradation mechanisms have long been under the debate for limiting the operation up to 210 K in pulsed operation in the GaAs/AlGaAs material system. In this work, we review the existing designs and exploit two main temperature degradation mechanisms by presenting a design in which they both prove beneficial to the lasing operation by dual pumping and dual extracting lasing levels. We have applied the density matrix transport model to select potential candidate structures by simulating over two million active region designs. We present several designs which offer better performance than the current record structure.

Laser feedback interferometry in multi-mode terahertz quantum cascade lasers.

The typical modal characteristics arising during laser feedback interferometry (LFI) in multi-mode terahertz (THz) quantum cascade lasers (QCLs) are investigated in this work. To this end, a set of multi-mode reduced rate equations with gain saturation for a general Fabry-Pérot multi-mode THz QCL under optical feedback is developed. Depending on gain bandwidth of the laser and optical feedback level, three different operating regimes are identified, namely a single-mode regime, a multi-mode regime, and a tuneable-mode regime. When the laser operates in the single-mode and multi-mode regimes, the self-mixing signal amplitude (peak to peak value of the self-mixing fringes) is proportional to the feedback coupling rate at each mode frequency. However, this rule no longer holds when the laser enters into the tuneable-mode regime, in which the feedback level becomes sufficiently strong (the boundary value of the feedback level depends on the gain bandwidth). The mapping of the identified feedback regimes of the multi-mode THz QCL in the space of the gain bandwidth and feedback level is investigated. In addition, the dependence of the aforementioned mapping of these three regimes on the linewidth enhancement factor of the laser is also explored, which provides a systematic picture of the potential of LFI in multi-mode THz QCLs for spectroscopic sensing applications.

Photoconductive arrays on insulating substrates for high-field terahertz generation.

We report on the design, fabrication and characterisation of large-area photoconductive THz array structures, consisting of a thin LT-GaAs active region transferred to an insulating substrate using a wafer-scale bonding process. The electrically insulating, transparent substrate reduces the parasitic currents in the devices, allowing peak THz-fields as high as 120 kV cm-1 to be generated over a bandwidth >5 THz. These results are achieved using lower pulse energies than demanded by conventional photoconductive arrays and other popular methods of generating high-field THz radiation. Two device sizes are fully characterised and the emission properties are compared to generation by optical rectification in ZnTe. The device can be operated in an optically saturated regime in order to suppress laser noise.

Sensorimotor maps can be dynamically calibrated using an adaptive-filter model of the cerebellum.

Substantial experimental evidence suggests the cerebellum is involved in calibrating sensorimotor maps. Consistent with this involvement is the well-known, but little understood, massive cerebellar projection to maps in the superior colliculus. Map calibration would be a significant new role for the cerebellum given the ubiquity of map representations in the brain, but how it could perform such a task is unclear. Here we investigated a dynamic method for map calibration, based on electrophysiological recordings from the superior colliculus, that used a standard adaptive-filter cerebellar model. The method proved effective for complex distortions of both unimodal and bimodal maps, and also for predictive map-based tracking of moving targets. These results provide the first computational evidence for a novel role for the cerebellum in dynamic sensorimotor map calibration, of potential importance for coordinate alignment during ongoing motor control, and for map calibration in future biomimetic systems. This computational evidence also provides testable experimental predictions concerning the role of the connections between cerebellum and superior colliculus in previously observed dynamic coordinate transformations.

Coherent imaging using laser feedback interferometry with pulsed-mode terahertz quantum cascade lasers.

We report a coherent terahertz (THz) imaging system that utilises a quantum cascade laser (QCL) operating in pulsed-mode as both the source and detector. The realisation of a short-pulsed THz QCL feedback interferometer permits both high peak powers and improved thermal efficiency, which enables the cryogen-free operation of the system. In this work, we demonstrated pulsed-mode swept-frequency laser feedback interferometry experimentally. Our interferometric detection scheme not only permits the simultaneous creation of both amplitude and phase images, but inherently suppresses unwanted background radiation. We demonstrate that the proposed system utilising microsecond pulses has the potential to achieve 0.25 mega-pixel per second acquisition rates, paving the pathway to video frame rate THz imaging.

Optomechanical response with nanometer resolution in the self-mixing signal of a terahertz quantum cascade laser.

Owing to their intrinsic stability against optical feedback (OF), quantum cascade lasers (QCLs) represent a uniquely versatile source to further improve self-mixing interferometry at mid-infrared and terahertz (THz) frequencies. Here, we show the feasibility of detecting with nanometer precision, the deeply subwavelength ($ \lt \lambda /6000 $<λ/6000) mechanical vibrations of a suspended $ {{\rm Si}_3}{{\rm N}_4} $Si3N4 membrane used as the external element of a THz QCL feedback interferometer. Besides representing an extension of the applicability of vibrometric characterization at THz frequencies, our system can be exploited for the realization of optomechanical applications, such as dynamical switching between different OF regimes and a still-lacking THz master-slave configuration.

Microsporidia: Why Make Nucleotides if You Can Steal Them?

Microsporidia are strict obligate intracellular parasites that infect a wide range of eukaryotes including humans and economically important fish and insects. Surviving and flourishing inside another eukaryotic cell is a very specialised lifestyle that requires evolutionary innovation. Genome sequence analyses show that microsporidia have lost most of the genes needed for making primary metabolites, such as amino acids and nucleotides, and also that they have only a limited capacity for making adenosine triphosphate (ATP). Since microsporidia cannot grow and replicate without the enormous amounts of energy and nucleotide building blocks needed for protein, DNA, and RNA biosynthesis, they must have evolved ways of stealing these substrates from the infected host cell. Providing they can do this, genome analyses suggest that microsporidia have the enzyme repertoire needed to use and regenerate the imported nucleotides efficiently. Recent functional studies suggest that a critical innovation for adapting to intracellular life was the acquisition by lateral gene transfer of nucleotide transport (NTT) proteins that are now present in multiple copies in all microsporidian genomes. These proteins are expressed on the parasite surface and allow microsporidia to steal ATP and other purine nucleotides for energy and biosynthesis from their host. However, it remains unclear how other essential metabolites, such as pyrimidine nucleotides, are acquired. Transcriptomic and experimental studies suggest that microsporidia might manipulate host cell metabolism and cell biological processes to promote nucleotide synthesis and to maximise the potential for ATP and nucleotide import. In this review, we summarise recent genomic and functional data relating to how microsporidia exploit their hosts for energy and building blocks needed for growth and nucleic acid metabolism and we identify some remaining outstanding questions.

Gas spectroscopy with integrated frequency monitoring through self-mixing in a terahertz quantum-cascade laser.

We demonstrate a gas spectroscopy technique, using self-mixing in a 3.4 terahertz quantum-cascade laser (QCL). All previous QCL spectroscopy techniques have required additional terahertz instrumentation (detectors, mixers, or spectrometers) for system pre-calibration or spectral analysis. By contrast, our system self-calibrates the laser frequency (i.e., with no external instrumentation) to a precision of 630 MHz (0.02%) by analyzing QCL voltage perturbations in response to optical feedback within a 0-800 mm round-trip delay line. We demonstrate methanol spectroscopy by introducing a gas cell into the feedback path and show that a limiting absorption coefficient of ∼1×10-4 cm-1 is resolvable.

Quasi-continuous frequency tunable terahertz quantum cascade lasers with coupled cavity and integrated photonic lattice.

We demonstrate quasi-continuous tuning of the emission frequency from coupled cavity terahertz frequency quantum cascade lasers. Such coupled cavity lasers comprise a lasing cavity and a tuning cavity which are optically coupled through a narrow air slit and are operated above and below the lasing threshold current, respectively. The emission frequency of these devices is determined by the Vernier resonance of longitudinal modes in the lasing and the tuning cavities, and can be tuned by applying an index perturbation in the tuning cavity. The spectral coverage of the coupled cavity devices have been increased by reducing the repetition frequency of the Vernier resonance and increasing the ratio of the free spectral ranges of the two cavities. A continuous tuning of the coupled cavity modes has been realized through an index perturbation of the lasing cavity itself by using wide electrical heating pulses at the tuning cavity and exploiting thermal conduction through the monolithic substrate. Single mode emission and discrete frequency tuning over a bandwidth of 100 GHz and a quasi-continuous frequency coverage of 7 GHz at 2.25 THz is demonstrated. An improvement in the side mode suppression and a continuous spectral coverage of 3 GHz is achieved without any degradation of output power by integrating a π-phase shifted photonic lattice in the laser cavity.

Two-dimensional coherent spectroscopy of a THz quantum cascade laser: observation of multiple harmonics.

Two-dimensional spectroscopy is performed on a terahertz (THz) frequency quantum cascade laser (QCL) with two broadband THz pulses. Gain switching is used to amplify the first THz pulse and the second THz pulse is used to probe the system. Fourier transforms are taken with respect to the delay time between the two THz pulses and the sampling time of the THz probe pulse. The two-dimensional spectrum consists of three peaks at (ωτ = 0, ωt = ω0), (ωτ = ω0, ωt = ω0), and (ωτ = 2ω0, ωt = ω0) where ω0 denotes the lasing frequency. The peak at ωτ = 0 represents the response of the probe to the zero-frequency (rectified) component of the instantaneous intensity and can be used to measure the gain recovery.

Multi-spectral terahertz sensing: proposal for a coupled-cavity quantum cascade laser based optical feedback interferometer.

We propose a laser feedback interferometer operating at multiple terahertz (THz) frequency bands by using a pulsed coupled-cavity THz quantum cascade laser (QCL) under optical feedback. A theoretical model that contains multi-mode reduced rate equations and thermal equations is presented, which captures the interplay between electro-optical, thermal, and feedback effects. By using the self-heating effect in both active and passive cavities, self-mixing signal responses at three different THz frequency bands are predicted. A multi-spectral laser feedback interferometry system based on such a coupled-cavity THz QCL will permit ultra-high-speed sensing and spectroscopic applications including material identification.

Terahertz generation mechanism in nano-grating electrode photomixers on Fe-doped InGaAsP.

We report the generation mechanism associated with nano-grating electrode photomixers fabricated on Fe-doped InGaAsP substrates. Two different emitter designs incorporating nano-gratings coupled to the same broadband antenna were characterized in a continuous-wave terahertz (THz) frequency system employing telecommunications wavelength lasers for generation and coherent detection. The current-voltage characteristics and THz emission bandwidth of the emitters is compared for different bias polarities and optical polarisations. The THz output from the emitters is also mapped as a function of the position of the laser excitation spot for both continuous-wave and pulsed excitation. This mapping, together with full-wave simulations of the structures, confirms the generation mechanism to be due to an enhanced optical electric field at the grating tips resulting in increased optical absorption, coinciding with a concentration of the electrostatic field.

Plasma membrane-located purine nucleotide transport proteins are key components for host exploitation by microsporidian intracellular parasites.

Microsporidia are obligate intracellular parasites of most animal groups including humans, but despite their significant economic and medical importance there are major gaps in our understanding of how they exploit infected host cells. We have investigated the evolution, cellular locations and substrate specificities of a family of nucleotide transport (NTT) proteins from Trachipleistophora hominis, a microsporidian isolated from an HIV/AIDS patient. Transport proteins are critical to microsporidian success because they compensate for the dramatic loss of metabolic pathways that is a hallmark of the group. Our data demonstrate that the use of plasma membrane-located nucleotide transport proteins (NTT) is a key strategy adopted by microsporidians to exploit host cells. Acquisition of an ancestral transporter gene at the base of the microsporidian radiation was followed by lineage-specific events of gene duplication, which in the case of T. hominis has generated four paralogous NTT transporters. All four T. hominis NTT proteins are located predominantly to the plasma membrane of replicating intracellular cells where they can mediate transport at the host-parasite interface. In contrast to published data for Encephalitozoon cuniculi, we found no evidence for the location for any of the T. hominis NTT transporters to its minimal mitochondria (mitosomes), consistent with lineage-specific differences in transporter and mitosome evolution. All of the T. hominis NTTs transported radiolabelled purine nucleotides (ATP, ADP, GTP and GDP) when expressed in Escherichia coli, but did not transport radiolabelled pyrimidine nucleotides. Genome analysis suggests that imported purine nucleotides could be used by T. hominis to make all of the critical purine-based building-blocks for DNA and RNA biosynthesis during parasite intracellular replication, as well as providing essential energy for parasite cellular metabolism and protein synthesis.

Visualizing the Translocation and Localization of Bacterial Type III Effector Proteins by Using a Genetically Encoded Reporter System.

Bacterial type III secretion system (T3SS) effector proteins are critical determinants of infection for many animal and plant pathogens. However, monitoring of the translocation and delivery of these important virulence determinants has proved to be technically challenging. Here, we used a genetically engineered LOV (light-oxygen-voltage) sensing domain derivative to monitor the expression, translocation, and localization of bacterial T3SS effectors. We found the Escherichia coli O157:H7 bacterial effector fusion Tir-LOV was functional following its translocation and localized to the host cell membrane in discrete foci, demonstrating that LOV-based reporters can be used to visualize the effector translocation with minimal manipulation and interference. Further evidence for the versatility of the reporter was demonstrated by fusing LOV to the C terminus of the Shigella flexneri effector IpaB. IpaB-LOV localized preferentially at bacterial poles before translocation. We observed the rapid translocation of IpaB-LOV in a T3SS-dependent manner into host cells, where it localized at the bacterial entry site within membrane ruffles.