Binary THz modulator based on silicon Schottky-metasurface.

We propose a metasurface THz modulator based on split-ring resonators (SRRs) formed by four interconnected horizontal Si-Au Schottky diodes. The equivalent junction capacitance of each SRR in the proposed modulator is much smaller than that of the previously reported metasurface counterparts with vertical Schottky junctions, leading to a higher modulation speed. To modulate a THz incident signal by the proposed metasurface, we vary the bias voltage externally applied to the Schottky junctions. Applying a reverse bias of VA = - 5 V to the Au gate, two LC resonances at 0.48 THz, and 0.95 THz are excited in the metasurface. Switching the applied voltage to VA = + 0.49 V, we diminish the oscillator strengths of the LC resonances, creating one dipole resonance at 0.73 THz in the transmission spectrum of the metasurface modulator. The modulation depths at these resonances are more than 45%, reaching 87% at 0.95 THz. The phase modulation for this THz modulator is about 1.12 rad at 0.86 THz. Furthermore, due to the particular design of the meta-atoms, the modulation speed of this device is estimated up to approximately several hundred GHz, which makes this device an appropriate candidate for high-speed applications in wireless communications systems based on external modulators.

Mitochondrial Permeability Transition Pore Sealing Agents and Antioxidants Protect Oxidative Stress and Mitochondrial Dysfunction Induced by Naproxen, Diclofenac and Celecoxib.

Nonsteroidal anti-inflammatory drugs (NSAIDs) like naproxen, diclofenac and celecoxib used to reduce pain. Many of these drugs have been associated with an increased risk of cardiovascular disease (CVD). The molecular mechanism(s) by which NSAIDs induce CVD up to now is unknown. We investigated the effects of naproxen, diclofenac and celecoxib with different structures and mechanism action on isolated rat heart mitochondria. All tested NSAIDs increased reactive oxygen species (ROS) formation, mitochondrial membrane collapse (MMP), mitochondrial swelling, lipid peroxidation, and glutathione and ATP depletion, which all of them play important roles in developing cardiotoxicity. We reported that mitochondrial permeability transition (MPT) pore sealing agents and antioxidants have the capacity to significantly prevent mitochondrial toxicity. Therefore, the inhibition of mitochondrial oxidative stress and mitochondrial dysfunction by MPT pore sealing agents and antioxidants can double confirm NSAID-induced cardiomyocytes toxicity is resulted from induction of apoptosis signaling trough ROS-mediated mitochondrial permeability transition.

Reprogrammable Graphene-based Metasurface Mirror with Adaptive Focal Point for THz Imaging.

Recent emergence of metasurfaces has enabled the development of ultra-thin flat optical components through different wavefront shaping techniques at various wavelengths. However, due to the non-adaptive nature of conventional metasurfaces, the focal point of the resulting optics needs to be fixed at the design stage, thus severely limiting its reconfigurability and applicability. In this paper, we aim to overcome such constraint by presenting a flat reflective component that can be reprogrammed to focus terahertz waves at a desired point in the near-field region. To this end, we first propose a graphene-based unit cell with phase reconfigurability, and then employ the coding metasurface approach to draw the phase profile required to set the focus on the target point. Our results show that the proposed component can operate close to the diffraction limit with high focusing range and low focusing error. We also demonstrate that, through appropriate automation, the reprogrammability of the metamirror could be leveraged to develop compact terahertz scanning and imaging systems, as well as novel reconfigurable components for terahertz wireless communications.

Improvement of Terahertz Photoconductive Antenna using Optical Antenna Array of ZnO Nanorods.

An efficient terahertz (THz) photoconductive antenna (PCA), as a major constituent for the generation or detection of THz waves, plays an essential role in bridging microwave-to-photonic gaps. Here, we propose an impressive approach comprising the use of arrayed zinc oxide nanorods (ZnO NRs) as an optical nanoantenna over an anti-reflective layer (silicon nitride) in the antenna gap to boost the photocurrent and consequently the THz signal. The numerical approach applied in investigating the optical behavior of the structure, demonstrates a significant field enhancement within the LT-GaAs layer due to the optical antenna performing simultaneously as a concentrator and an antireflector which behaves as a graded-refractive index layer. ZnO NRs have been fabricated on the PCA gap using the hydrothermal method as a simple, low cost and production compatible fabrication method compared to other complex methods used for the optical nanoantennas. Compared to the conventional PCA with a traditional antireflection coating, the measured THz power by time domain spectroscopy (TDS) is increased more than 4 times on average over the 0.1-1.2 THz range.

Reconfigurable THz Plasmonic Antenna Based on Few-Layer Graphene with High Radiation Efficiency.

Graphene plasmonic antennas possess two significant features that render them appealing for short-range wireless communications, notably, inherent tunability and miniaturization due to the unique frequency dispersion of graphene and its support for surface plasmon waves in the terahertz band. In this letter, dipole-like antennas using few-layer graphene are proposed to achieve a better trade-off between miniaturization and radiation efficiency than current monolayer graphene antennas. The characteristics of few-layer graphene antennas are evaluated and then compared with those of antennas based on monolayer graphene and graphene stacks, which could also provide such improvements. To this end, first, the propagation properties of one-dimensional and two-dimensional plasmonic waveguides based on the aforementioned graphene structures are obtained by transfer matrix theory and finite-element simulation, respectively. Second, the antennas are investigated as three-dimensional structures using a full-wave solver. Results show that the highest radiation efficiency among the compared designs is achieved with the few-layer graphene, while the highest miniaturization is obtained with the even mode of the graphene stack antenna.

Nanoslit cavity plasmonic modes and built-in fields enhance the CW THz radiation in an unbiased antennaless photomixers array.

A new generation unbiased antennaless CW terahertz (THz) photomixer emitters array made of asymmetric metal-semiconductor-metal (MSM) gratings with a subwavelength pitch, operating in the optical near-field regime, is proposed. We take advantage of size effects in near-field optics and electrostatics to demonstrate the possibility of enhancing the THz power by 4 orders of magnitude, compared to a similar unbiased antennaless array of the same size that operates in the far-field regime. We show that, with the appropriate choice of grating parameters in such THz sources, the first plasmonic resonant cavity mode in the nanoslit between two adjacent MSMs can enhance the optical near-field absorption and, hence, the generation of photocarriers under the slit in the active medium. These photocarriers, on the other hand, are accelerated by the large built-in electric field sustained under the nanoslits by two dissimilar Schottky barriers to create the desired large THz power that is mainly radiated downward. The proposed structure can be tuned in a broadband frequency range of 0.1-3 THz, with output power increasing with frequency.

Unbiased continuous wave terahertz photomixer emitters with dis-similar Schottky barriers.

We are introducing a new bias free CW terahertz photomixer emitter array. Each emitter consists of an asymmetric metal-semiconductor-metal (MSM) that is made of two side by side dis-similar Schottky contacts, on a thin layer of low temperature grown (LTG) GaAs, with barrier heights of difference (ΔΦ(B)) and a finite lateral spacing (s). Simulations show that when an appropriately designed structure is irradiated by two coherent optical beams of different center wavelengths, whose frequency difference (∆f) falls in a desired THz band, the built-in field between the two dis-similar potential barriers can accelerate the photogenerated carriers that are modulated by ∆ω, making each pitch in the array to act as a CW THz emitter, effectively. We also show the permissible values of s and ΔΦ(B) pairs, for which the strengths of the built-in electric field maxima fall below that of the critical of 50 V/µm- i.e., the breakdown limit for the LTG-GaAs layer. Moreover, we calculate the THz radiation power per emitter in an array. Among many potential applications for these bias free THz emitters their use in endoscopic imaging without a need for hazardous external biasing circuitry that reduces the patient health risk, could be the most important one. A hybrid numerical simulation method is used to design an optimum emitter pitch, radiating at 0.5 THz.

A hybrid analysis method for plasmonic enhanced terahertz photomixer sources.

A hybrid analysis of a continuous-wave terahertz photomixer source structure with plasmonic nano-grating electrodes is presented. Using the hybrid analysis, the enhancement of the optical power absorption due to the presence of the one-dimensional metallic nano-grating is investigated by defining an absorption enhancement factor. We show that the proposed absorption enhancement factor can be used as a design tool, whose maximization provides the optimum geometrical parameters of the nano-grating. Based on drift-diffusion model, the photocurrent enhancement due to the nano-grating electrodes is studied under three different bias configurations. Moreover, the dependence of the photocurrent on the physical parameters of the photomixer is analyzed.

Terahertz time-domain spectroscopic ellipsometry: instrumentation and calibration.

We present a new instrumentation and calibration procedure for terahertz time-domain spectroscopic ellipsometry (THz-TDSE) that is a newly established characterization technique. The experimental setup is capable of providing arbitrary angle of incidence in the range of 15°-85° in the reflection geometry, and with no need for realignment. The setup is also configurable easily into transmission geometry. For this setup, we successfully used hollow core photonic band gap fiber with no pre-chirping in order to deliver a femtosecond laser into a THz photoconductive antenna detector, which is the first demonstration of this kind. The proposed calibration scheme can compensate for the non-ideality of the polarization response of the THz photoconductive antenna detector as well as that of wire grid polarizers used in the setup. In the calibration scheme, the ellipsometric parameters are obtained through a regression algorithm which we have adapted from the conventional regression calibration method developed for rotating element optical ellipsometers, and used here for the first time for THz-TDSE. As a proof-of-principle demonstration, results are presented for a high resistivity silicon substrate as well as an opaque Si substrate with a high phosphorus concentration. We also demonstrate the capacity to measure a few micron thick grown thermal oxide on top of Si. Each sample was characterized by THz-TDSE in reflection geometry with different angle of incidence.