Photoconductive Metasurfaces for Terahertz Applications

Photoconductive antennas (PCAs) are widely used as terahertz (THz) detectors for spectroscopy and imaging. However, their relatively low efficiency and sensitivity often limits the signal-to-noise and measurement capabilities of experimental systems. By replacing the photoconductive region with an all-dielectric, fully absorbing metasurface the efficiency and sensitivity of PCAs is substantially improved. This thesis describes the design, modelling and experimental testing of highly absorbing metasurfaces made for the purpose of improving PCAs. Perfect absorption is achieved through the degenerate critical coupling of Mie modes. By simple modifications of the metasurface geometry, perfect absorption is obtained across the wavelength range of near-infrared ultrafast lasers commonly used for PCA excitation. When used as PCA detectors, high signal-to-noise is achieved at unprecendently low excitation powers, and extremely low dark resistance enables high sensitivity detection. Furthermore, when integrated with near-field aperture probes, the ultra-thin design of such metasurface PCAs could significantly enhance the spatial resolution and spectral sensitivity of THz near-field systems. In addition to PCA detectors, this thesis investigates whether GaAs metasurfaces could be used for THz emission via ultrafast charge carrier dynamics. Perfect absorption is demonstrated when the metasurface is excited at oblique angles, as necessary for THz emission and out-coupling. This work suggests the possibility of efficient, adaptable and integrable THz sources which do not require external bias for operation. Finally, this thesis explores whether metasurfaces can enhance absorption in low-temperature materials at sub-bandgap energies. It is shown that substantial improvements in absorption are possible using degenerate critical coupling, even for materials with very low absorption coefficients. This finding invites the future development of efficient PCA detectors that use convenient, turn-key operated fiber lasers for excitation - enabling cheaper, more functional THz spectroscopy and imaging systems

Near-field spectroscopy of Dirac plasmons in Bi2Se3 ribbon arrays

Plasmons supported in the massless electron surface states of topological insulators (TIs), known as Dirac plasmons, have great potential in next generation optoelectronics. However, their inherent confinement to the surface makes the investigation of Dirac plasmons challenging. Near-field techniques provide the ideal platform to directly probe Dirac plasmons due to the sensitivity to evanescent fields at the surface. Here, we demonstrate the use of aperture near-field spectroscopy for the investigation of localized terahertz (THz) Dirac plasmon resonances in Bi2Se3 ribbon arrays with widths ranging from 10 to 40 µm. Unlike scattering THz near-field techniques, the aperture method is most sensitive to plasmons with the relevant lower-momenta corresponding to plasmon wavelengths on the scale of ∼20 µm. The combination of THz time-domain spectroscopy and aperture near-field microscopy enables sampling of localized Dirac plasmons in the near-field zone in the 0.5–2.5 THz range. We map the plasmon dispersion, which reveals a coupled plasmon–phonon polariton interaction. The near-field spectra show a higher contrast of the upper polariton branch in comparison with far-field observations. The information revealed by aperture near-field spectroscopy could deepen our understanding of the behavior of Dirac plasmons, leading to the potential development of real-world TI devices

Highly efficient terahertz photoconductive metasurface detectors operating at microwatt-level gate powers

Despite their wide use in terahertz (THz) research and technology, the application spectra of photoconductive antenna (PCA) THz detectors are severely limited due to the relatively high optical gating power requirement. This originates from poor conversion efficiency of optical gate beam photons to photocurrent in materials with subpicosecond carrier lifetimes. Here we show that using an ultra-thin (160 nm), perfectly absorbing low-temperature grown GaAs metasurface as the photoconductive channel drastically improves the efficiency of THz PCA detectors. This is achieved through perfect absorption of the gate beam in a significantly reduced photoconductive volume, enabled by the metasurface. This Letter demonstrates that sensitive THz PCA detection is possible using optical gate powers as low as 5 μW-three orders of magnitude lower than gating powers used for conventionalPCAdetectors.We show that significantly higher optical gate powers are not necessary for optimal operation, as they do not improve the sensitivity to the THz field. This class of efficient PCA THz detectors opens doors for THz applications with low gate power requirements

Near-Field Spectroscopy of Individual Asymmetric Split-Ring Terahertz Resonators

Metamaterial resonators have become an efficient and versatile platform in the terahertz frequency range, finding applications in integrated optical devices, such as active modulators and detectors, and in fundamental research, e.g., ultrastrong light–matter investigations. Despite their growing use, characterization of modes supported by these subwavelength elements has proven to be challenging and it still relies on indirect observation of the collective far-field transmission/reflection properties of resonator arrays. Here, we present a broadband time-domain spectroscopic investigation of individual metamaterial resonators via a THz aperture scanning near-field microscope (a-SNOM). The time-domain a-SNOM allows the mapping and quantitative analysis of strongly confined modes supported by the resonators. In particular, a cross-polarized configuration presented here allows an investigation of weakly radiative modes. These results hold great potential to advance future metamaterial-based optoelectronic platforms for fundamental research in THz photonics.

Terahertz Pulse Generation from GaAs Metasurfaces

Ultrafast optical excitation of select materials gives rise to the generation of broadband terahertz (THz) pulses. This effect has enabled the field of THz time-domain spectroscopy and led to the discovery of many physical mechanisms behind THz generation. However, only a few materials possess the required properties to generate THz radiation efficiently. Optical metasurfaces can relax stringent material requirements by shifting the focus onto the engineering of local electromagnetic fields to boost THz generation. Here we demonstrate the generation of THz pulses in a 160 nm thick nanostructured GaAs metasurface. Despite the drastically reduced volume, the metasurface emits THz radiation with efficiency comparable to that of a thick GaAs crystal. We reveal that along with classical second-order volume nonlinearity, an additional mechanism contributes strongly to THz generation in the metasurface, which we attribute to surface nonlinearity. Our results lay the foundation for engineering of semiconductor metasurfaces for efficient and versatile THz radiation emitters

Near-Field Spectroscopy of Individual Asymmetric Split-Ring Terahertz Resonators

Metamaterial resonators have become an efficient and versatile platform in the terahertz frequency range, finding applications in integrated optical devices, such as active modulators and detectors, and in fundamental research, e.g., ultrastrong light–matter investigations. Despite their growing use, characterization of modes supported by these subwavelength elements has proven to be challenging and it still relies on indirect observation of the collective far-field transmission/reflection properties of resonator arrays. Here, we present a broadband time-domain spectroscopic investigation of individual metamaterial resonators via a THz aperture scanning near-field microscope (a-SNOM). The time-domain a-SNOM allows the mapping and quantitative analysis of strongly confined modes supported by the resonators. In particular, a cross-polarized configuration presented here allows an investigation of weakly radiative modes. These results hold great potential to advance future metamaterial-based optoelectronic platforms for fundamental research in THz photonics

IRF5 promotes influenza-induced inflammatory responses in human iPSC-derived myeloid cells and murine models.

Recognition of Influenza A virus (IAV) by the innate immune system triggers pathways that restrict viral replication, activates innate immune cells, and regulates adaptive immunity. However, excessive innate immune activation can exaggerate disease. The pathways promoting excessive activation are incompletely understood, with limited experimental models to investigate mechanisms driving influenza-induced inflammation in humans. Interferon regulatory factor (IRF5) is a transcription factor that plays important roles in induction of cytokines after viral sensing. In an in vivo model of IAV infection, IRF5 deficiency reduced IAV-driven immune pathology and associated inflammatory cytokine production, specifically reducing cytokine-producing myeloid cell populations in Irf5-/- mice, but not impacting type 1 IFN production or virus replication. Using cytometry by time-of-flight (CyTOF), we identified that human lung IRF5 expression was highest in cells of the myeloid lineage. To investigate the role of IRF5 in mediating human inflammatory responses by myeloid cells to IAV, we employed human induced pluripotent stem cells (hIPSCs) with biallelic mutations in IRF5, demonstrating for the first time iPS-derived dendritic cells (iPS-DCs) with biallelic mutations can be used to investigate regulation of human virus-induced immune responses. Using this technology, we reveal that IRF5 deficiency in human DCs, or macrophages, corresponded with reduced virus-induced inflammatory cytokine production, with IRF5 acting downstream of TLR7 and, possibly, RIG-I after viral sensing. Thus, IRF5 acts as a regulator of myeloid cell inflammatory cytokine production during IAV infection in mice and humans, and drives immune-mediated viral pathogenesis independently of type 1 IFN and virus replication.ImportanceThe inflammatory response to Influenza A virus (IAV) participates in infection control but contributes to disease severity. After viral detection intracellular pathways are activated, initiating cytokine production, but these pathways are incompletely understood. We show that interferon regulatory factor 5 (IRF5) mediates IAV-induced inflammation and, in mice, drives pathology. This was independent of antiviral type 1 IFN and virus replication, implying that IRF5 could be specifically targeted to treat influenza-induced inflammation. We show for the first time that human iPSC technology can be exploited in genetic studies of virus-induced immune responses. Using this technology, we deleted IRF5 in human myeloid cells. These IRF5-deficient cells exhibited impaired influenza-induced cytokine production and revealed that IRF5 acts downstream of Toll-like receptor 7 and possibly retinoic acid-inducible gene-I. Our data demonstrate the importance of IRF5 in influenza-induced inflammation, suggesting genetic variation in the IRF5 gene may influence host susceptibility to viral diseases.This work was supported by The Wellcome Trust. This work was funded by a Wellcome 641 Trust Senior Research Fellowship to Ian Humphreys (207503/Z/17/Z); Medical Research 642 Council, United Kingdom (MR/L018942/1 and MRC Human Immunology Unit Core); 643 Chinese Academy of Medical Sciences (CAMS) Innovation Fund for Medical Sciences 644 (CIFMS), China (grant number: 2018-I2M-2-002). The Wellcome Trust Sanger Institute was 645 the source of the Kolf2 human induced pluripotent cell line which was generated under the 646 Human Induced Pluripotent Stem Cell Initiative funded by a grant from the Wellcome Trust Downloaded from http://jvi.asm.org/ on March 2, 2020 at CAMBRIDGE UNIV27 and Medical Research Council, supported 647 by the Wellcome Trust (WT098051) and the 648 NIHR/Wellcome Trust Clinical Research Facility, and Life Science Technologies 649 Corporation provided Cytotune for reprogramming. We thank the Wellcome Trust Sanger Institute Gene editing pipeline for generation of IRF5-/- 650 iPSCs and the Mass spectrometry 651 Facility at the Weatherall Institute of Molecular Medicine for help with CyTOF experiments

Modern optical astronomy: technology and impact of interferometry

The present `state of the art' and the path to future progress in high spatial resolution imaging interferometry is reviewed. The review begins with a treatment of the fundamentals of stellar optical interferometry, the origin, properties, optical effects of turbulence in the Earth's atmosphere, the passive methods that are applied on a single telescope to overcome atmospheric image degradation such as speckle interferometry, and various other techniques. These topics include differential speckle interferometry, speckle spectroscopy and polarimetry, phase diversity, wavefront shearing interferometry, phase-closure methods, dark speckle imaging, as well as the limitations imposed by the detectors on the performance of speckle imaging. A brief account is given of the technological innovation of adaptive-optics (AO) to compensate such atmospheric effects on the image in real time. A major advancement involves the transition from single-aperture to the dilute-aperture interferometry using multiple telescopes. Therefore, the review deals with recent developments involving ground-based, and space-based optical arrays. Emphasis is placed on the problems specific to delay-lines, beam recombination, polarization, dispersion, fringe-tracking, bootstrapping, coherencing and cophasing, and recovery of the visibility functions. The role of AO in enhancing visibilities is also discussed. The applications of interferometry, such as imaging, astrometry, and nulling are described. The mathematical intricacies of the various `post-detection' image-processing techniques are examined critically. The review concludes with a discussion of the astrophysical importance and the perspectives of interferometry.Comment: 65 pages LaTeX file including 23 figures. Reviews of Modern Physics, 2002, to appear in April issu

Population genomics of the Wolbachia endosymbiont in Drosophila melanogaster

Wolbachia are maternally-inherited symbiotic bacteria commonly found in arthropods, which are able to manipulate the reproduction of their host in order to maximise their transmission. Here we use whole genome resequencing data from 290 lines of Drosophila melanogaster from North America, Europe and Africa to predict Wolbachia infection status, estimate cytoplasmic genome copy number, and reconstruct Wolbachia and mtDNA genome sequences. Complete Wolbachia and mitochondrial genomes show congruent phylogenies, consistent with strict vertical transmission through the maternal cytoplasm and imperfect transmission of Wolbachia. Bayesian phylogenetic analysis reveals that the most recent common ancestor of all Wolbachia and mitochondrial genomes in D. melanogaster dates to around 8,000 years ago. We find evidence for a recent incomplete global replacement of ancestral Wolbachia and mtDNA lineages, which is likely to be one of several similar incomplete replacement events that have occurred since the out-of-Africa migration that allowed D. melanogaster to colonize worldwide habitats.Comment: 41 pages, 5 figure