Breaking the limitations with sparse inputs by variational frameworks (BLIss) in terahertz super-resolution 3D reconstruction.

Data acquisition, image processing, and image quality are the long-lasting issues for terahertz (THz) 3D reconstructed imaging. Existing methods are primarily designed for 2D scenarios, given the challenges associated with obtaining super-resolution (SR) data and the absence of an efficient SR 3D reconstruction framework in conventional computed tomography (CT). Here, we demonstrate BLIss, a new approach for THz SR 3D reconstruction with sparse 2D data input. BLIss seamlessly integrates conventional CT techniques and variational framework with the core of the adapted Euler-Elastica-based model. The quantitative 3D image evaluation metrics, including the standard deviation of Gaussian, mean curvatures, and the multi-scale structural similarity index measure (MS-SSIM), validate the superior smoothness and fidelity achieved with our variational framework approach compared with conventional THz CT modal. Beyond its contributions to advancing THz SR 3D reconstruction, BLIss demonstrates potential applicability in other imaging modalities, such as X-ray and MRI. This suggests extensive impacts on the broader field of imaging applications.

DaISy: diffuser-aided sub-THz imaging system.

Sub-terahertz (Sub-THz) waves possess exceptional attributes, capable of penetrating non-metallic and non-polarized materials while ensuring bio-safety. However, their practicality in imaging is marred by the emergence of troublesome speckle artifacts, primarily due to diffraction effects caused by wavelengths comparable to object dimensions. In addressing this limitation, we present the Diffuser-aided sub-THz Imaging System (DaISy), which utilizes a diffuser and a focusing lens to convert coherent waves into incoherent counterparts. The cornerstone of our progress lies in a coherence theory-based theoretical framework, pivotal for designing and validating the THz diffuser, and systematically evaluating speckle phenomena. Our experimental results utilizing DaISy reveal substantial improvements in imaging quality and nearly diffraction-limited spatial resolution. Moreover, we demonstrate a tangible application of DaISy in the scenario of security scanning, highlighting the versatile potential of sub-THz waves in miscellaneous fields.

Terahertz deep learning fusion computed tomography.

Terahertz (THz) tomographic imaging based on time-resolved THz signals has raised significant attention due to its non-invasive, non-destructive, non-ionizing, material-classification, and ultrafast-frame-rate nature for object exploration and inspection. However, the material and geometric information of the tested objects is inherently embedded in the highly distorted THz time-domain signals, leading to substantial computational complexity and the necessity for intricate multi-physics models to extract the desired information. To address this challenge, we present a THz multi-dimensional tomographic framework and multi-scale spatio-spectral fusion Unet (MS3-Unet), capable of fusing and collaborating the THz signals across diverse signal domains. MS3-Unet employs multi-scale branches to extract spatio-spectral features, which are subsequently processed through element-wise adaptive filters and fused to achieve high-quality THz image restoration. Evaluated by geometry-variant objects, MS3-Unet outperforms other peer methods in PSNR and SSIM. In addition to the superior performance, the proposed framework additionally provides high scalable, adjustable, and accessible interface to collaborate with different user-defined models or methods.

Making the Invisible Visible: Toward High-Quality Terahertz Tomographic Imaging via Physics-Guided Restoration.

Terahertz (THz) tomographic imaging has recently attracted significant attention thanks to its non-invasive, non-destructive, non-ionizing, material-classification, and ultra-fast nature for object exploration and inspection. However, its strong water absorption nature and low noise tolerance lead to undesired blurs and distortions of reconstructed THz images. The diffraction-limited THz signals highly constrain the performances of existing restoration methods. To address the problem, we propose a novel multi-view Subspace-Attention-guided Restoration Network (SARNet) that fuses multi-view and multi-spectral features of THz images for effective image restoration and 3D tomographic reconstruction. To this end, SARNet uses multi-scale branches to extract intra-view spatio-spectral amplitude and phase features and fuse them via shared subspace projection and self-attention guidance. We then perform inter-view fusion to further improve the restoration of individual views by leveraging the redundancies between neighboring views. Here, we experimentally construct a THz time-domain spectroscopy (THz-TDS) system covering a broad frequency range from 0.1 to 4 THz for building up a temporal/spectral/spatial/material THz database of hidden 3D objects. Complementary to a quantitative evaluation, we demonstrate the effectiveness of our SARNet model on 3D THz tomographic reconstruction applications. Supplementary Information: The online version contains supplementary material available at 10.1007/s11263-023-01812-y.

Thermal evaporated group IV Ge(Sn)-on-Si terahertz photoconductive antenna.

We have experimentally demonstrated thermal evaporated group IV Ge1-xSnx-on-Si terahertz (THz) photoconductive antennas (PCA) pumped by an Er-doped femtosecond laser for broadband THz generation. The Ge1-xSnx THz PCAs, free from material epitaxial growth methods, can offer comparable material properties in photocarrier generation, transportation, recombination, and the collection as group III-V THz PCAs. At the optical pumping power of 90 mW and a bias voltage of 40V, the Ge1-xSnx THz PCAs have achieved a broadband spectrum over 1.5 THz with a 40 dB signal-to-noise ratio (SNR). This CMOS-compatible group IV THz source can be monolithically integrated on the Si photonic platform, paving the way toward THz system-on-chip (SoC) for many on-site applications in the non-destructive evaluation, biomedical imaging, and industrial inspections.

Terahertz spatio-temporal deep learning computed tomography.

Terahertz computed tomography (THz CT) has drawn significant attention because of its unique capability to bring multi-dimensional object information from invisible to visible. However, current physics-model-based THz CT modalities present low data use efficiency on time-resolved THz signals and low model fusion extensibility, limiting their application fields' practical use. In this paper, we propose a supervised THz deep learning computed tomography (THz DL-CT) framework based on time-domain information. THz DL-CT restores superior THz tomographic images of 3D objects by extracting features from spatio-temporal THz signals without any prior material information. Compared with conventional and machine learning based methods, THz DL-CT delivers at least 50.2%, and 52.6% superior in root mean square error (RMSE) and structural similarity index (SSIM), respectively. Additionally, we have experimentally demonstrated that the pretrained THz DL-CT model can generalize to reconstruct multi-material systems with no prerequisite information. THz CT through the DL data fusion approach provides a new pathway for non-invasive functional imaging in object investigation.

Group IV THz large area emitter based on GeSn alloy.

THz photoconductive emitters based on III-V materials have demonstrated excellent THz radiation properties, enabling many unique applications. However, the incompatibility with the complementary-metal-oxide-semiconductor (CMOS) foundry fabrication process and the challenging growth condition hampers THz photoconductive emitters from large-scale production. To address this limitation, we proposed the GeSn alloy as the photoconductive material candidate through the CMOS-compatible epitaxy instrument. The GeSn photoconductor features a 518 cm2/V-s mobility and a 7187 cm-1 absorption coefficient at the wavelength of 1560 nm, resulting in sufficiently ultrafast photocurrent generation for THz radiation. As a result, the GeSn THz emitter provides over a bandwidth of 2 THz and a 40 dB signal-to-noise ratio, which shows its potential in realizing mass-producible, cost-effective THz integrated systems with CMOS technology.

Performance and complexity analysis using a sparse deep learning method for indoor terahertz transmission.

In this Letter, we propose and experimentally validate a sparse deep learning method (SDLM) for terahertz indoor wireless-over-fiber by transmitting a 16-quadrature amplitude modulation (QAM) orthogonal frequency-division multiplexing (OFDM) signal over a 15-km single-mode fiber (SMF) and a wireless link distance of 60 cm at 135 GHz through a cost-effective intensity-modulated direct detection (IM-DD) communications system. The proposed SDLM imposes the L1-regularized mechanism on the cost function, which not only improves performance but also reduces complexity when compared with traditional Volterra nonlinear equalizer (VNLE), sparse VNLE, and conventional DLM. Our experimental findings show that the proposed SDLM provides viable options for successfully mitigating nonlinear distortions and outperforms conventional VNLE, conventional DLM, and SVNLE with a 76%, 72%, and 61% complexity reduction, respectively, for 8-QAM without losing signal integrity.

Spectral characteristics of terahertz radiation from plasmonic photomixers.

We present a comprehensive analysis of spectral characteristics of terahertz radiation from plasmonic photomixers. We fabricate plasmonic photomixer prototypes with plasmonic contact electrode gratings on a low temperature grown GaAs substrate and characterize the spectral properties of the generated terahertz radiation by use of a heterodyne detection scheme. Our analysis shows that linewidth, stability, and frequency tuning range of the generated terahertz radiation are directly determined by linewidth, stability, and wavelength tuning range of optical pump beam and not affected by device geometry, substrate properties, optical pump power level and other operational settings. Our study indicates the crucial role of optical sources in realizing high performance terahertz spectroscopy and wireless communication systems based on plasmonic photomixers.

Influence of point mutations on PR65 conformational adaptability: Insights from molecular simulations and nanoaperture optical tweezers.

PR65 is the HEAT repeat scaffold subunit of the heterotrimeric protein phosphatase 2A (PP2A) and an archetypal tandem repeat protein. Its conformational mechanics plays a crucial role in PP2A function by opening/closing substrate binding/catalysis interface. Using in silico saturation mutagenesis, we identified PR65 "hinge" residues whose substitutions could alter its conformational adaptability and thereby PP2A function, and selected six mutations that were verified to be expressed and soluble. Molecular simulations and nanoaperture optical tweezers revealed consistent results on the specific effects of the mutations on the structure and dynamics of PR65. Two mutants observed in simulations to stabilize extended/open conformations exhibited higher corner frequencies and lower translational scattering in experiments, indicating a shift toward extended conformations, whereas another displayed the opposite features, confirmed by both simulations and experiments. The study highlights the power of single-molecule nanoaperture-based tweezers integrated with in silico approaches for exploring the effect of mutations on protein structure and dynamics.

Enhanced light-matter interaction at nanoscale by utilizing high-aspect-ratio metallic gratings.

We report the design, fabrication, and experimental characterization of high-aspect-ratio metallic gratings integrated with nanoscale semiconductor structures, which enable efficient light-matter interaction at the nanoscale over interaction lengths as long as two times of the effective optical wavelength. The efficient light-matter interaction at the nanoscale is enabled by excitation of the guided modes of subwavelength slab waveguides formed by the high-aspect-ratio metallic gratings. By controlling the height of the high-aspect-ratio gratings, the wavelength of the guided modes through the nanoscale semiconductor structures is determined.

Cooperative mechanics of PR65 scaffold underlies the allosteric regulation of the phosphatase PP2A.

PR65, a horseshoe-shaped scaffold composed of 15 HEAT (observed in Huntingtin, elongation factor 3, protein phosphatase 2A, and the yeast kinase TOR1) repeats, forms, together with catalytic and regulatory subunits, the heterotrimeric protein phosphatase PP2A. We examined the role of PR65 in enabling PP2A enzymatic activity with computations at various levels of complexity, including hybrid approaches that combine full-atomic and elastic network models. Our study points to the high flexibility of this scaffold allowing for end-to-end distance fluctuations of 40-50 Å between compact and extended conformations. Notably, the intrinsic dynamics of PR65 facilitates complexation with the catalytic subunit and is retained in the PP2A complex enabling PR65 to engage the two domains of the catalytic subunit and provide the mechanical framework for enzymatic activity, with support from the regulatory subunit. In particular, the intra-repeat coils at the C-terminal arm play an important role in allosterically mediating the collective dynamics of PP2A, pointing to target sites for modulating PR65 function.

Influence of Point Mutations on PR65 Conformational Adaptability: Insights from Nanoaperture Optical Tweezer Experiments and Molecular Simulations.

PR65 is the HEAT-repeat scaffold subunit of the heterotrimeric protein phosphatase 2A (PP2A) and an archetypal tandem-repeat protein, forming a spring-like architecture. PR65 conformational mechanics play a crucial role in PP2A function by opening/closing the substrate-binding/catalysis interface. Using in-silico saturation mutagenesis we identified "hinge" residues of PR65, whose substitutions are predicted to restrict its conformational adaptability and thereby disrupt PP2A function. Molecular simulations revealed that a subset of hinge mutations stabilized the extended/open conformation, whereas another had the opposite effect. By trapping in nanoaperture optical tweezer, we characterized PR65 motion and showed that the former mutants exhibited higher corner frequencies and lower translational scattering, indicating a shift towards extended conformations, whereas the latter showed the opposite behavior. Thus, experiments confirm the conformations predicted computationally. The study highlights the utility of nanoaperture-based tweezers for exploring structure and dynamics, and the power of integrating this single-molecule method with in silico approaches.

Optical characteristics of a quantum-dot laser with a metallic waveguide.

We report the optical characteristics of a quantum-dot laser with a metal-coated waveguide, which shows a large group index of 4.2 compared to 3.2 for an uncoated laser. Temperature-dependent measurements reveal a high characteristic temperature in the temperature range of 7 degrees C-25 degrees C. The optical gain, refractive index change, and linewidth enhancement factor are extracted from the measured Fabry-Perot amplified spontaneous emission spectra. We use the pulse measurements to eliminate the thermal effect and obtain a low linewidth enhancement factor of 0.35.

Glyceraldehyde-3-phosphate dehydrogenase (GAPDH) interaction with 3' ends of Japanese encephalitis virus RNA and colocalization with the viral NS5 protein.

Replication of the Japanese encephalitis virus (JEV) genome depends on host factors for successfully completing their life cycles; to do this, host factors have been recruited and/or relocated to the site of viral replication. Glyceraldehyde-3-phosphate dehydrogenase (GAPDH), a cellular metabolic protein, was found to colocalize with viral RNA-dependent RNA polymerase (NS5) in JEV-infected cells. Subcellular fractionation further indicated that GAPDH remained relatively constant in the cytosol, while increasing at 12 to 24 hours postinfection (hpi) and decreasing at 36 hpi in the nuclear fraction of infected cells. In contrast, the redistribution patterns of GAPDH were not observed in the uninfected cells. Co-immunoprecipitation of GAPDH and JEV NS5 protein revealed no direct protein-protein interaction; instead, GAPDH binds to the 3' termini of plus- and minus-strand RNAs of JEV by electrophoretic mobility shift assays. Accordingly, GAPDH binds to the minus strand more efficiently than to the plus strand of JEV RNAs. This study highlights the findings that infection of JEV changes subcellular localization of GAPDH suggesting that this metabolic enzyme may play a role in JEV replication.

Electronically-Controlled Beam-Steering through Vanadium Dioxide Metasurfaces.

Engineered metamaterials offer unique functionalities for manipulating the spectral and spatial properties of electromagnetic waves in unconventional ways. Here, we report a novel approach for making reconfigurable metasurfaces capable of deflecting electromagnetic waves in an electronically controllable fashion. This is accomplished by tilting the phase front of waves through a two-dimensional array of resonant metasurface unit-cells with electronically-controlled phase-change materials embedded inside. Such metasurfaces can be placed at the output facet of any electromagnetic radiation source to deflect electromagnetic waves at a desired frequency, ranging from millimeter-wave to far-infrared frequencies. Our design does not use any mechanical elements, external light sources, or reflectarrays, creating, for the first time, a highly robust and fully-integrated beam-steering device solution. We demonstrate a proof-of-concept beam-steering metasurface optimized for operation at 100 GHz, offering up to 44° beam deflection in both horizontal and vertical directions. Dynamic control of electromagnetic wave propagation direction through this unique platform could be transformative for various imaging, sensing, and communication applications, among others.