Tutorial: Terahertz beamforming, from concepts to realizations

The terahertz range possesses significant untapped potential for applications including high-volume wireless communications, noninvasive medical imaging, sensing, and safe security screening. However, due to the unique characteristics and constraints of terahertz waves, the vast majority of these applications are entirely dependent upon the availability of beam control techniques. Thus, the development of advanced terahertz-range beam control techniques yields a range of useful and unparalleled applications. This article provides an overview and tutorial on terahertz beam control. The underlying principles of wavefront engineering include array antenna theory and diffraction optics, which are drawn from the neighboring microwave and optical regimes, respectively. As both principles are applicable across the electromagnetic spectrum, they are reconciled in this overview. This provides a useful foundation for investigations into beam control in the terahertz range, which lies between microwaves and infrared light. Thereafter, noteworthy experimental demonstrations of beam control in the terahertz range are discussed, and these include geometric optics, phased array devices, leaky-wave antennas, reflectarrays, and transmitarrays. These techniques are compared and contrasted for their suitability in applications of terahertz waves.Daniel Headland, Yasuaki Monnai, Derek Abbott, Christophe Fumeaux, and Withawat Withayachumnanku

Highly-Sensitive Thin Film THz Detector Based on Edge Metal-Semiconductor-Metal Junction

Terahertz (THz) detectors have been extensively studied for various applications such as security, wireless communication, and medical imaging. In case of metal-insulator-metal (MIM) tunnel junction THz detector, a small junction area is desirable because the detector response time can be shortened by reducing it. An edge metal-semiconductor-metal (EMSM) junction has been developed with a small junction area controlled precisely by the thicknesses of metal and semiconductor films. The voltage response of the EMSM THz detector shows the clear dependence on the polarization angle of incident THz wave and the responsivity is found to be very high (similar to 2,169 V/W) at 0.4 THz without any antenna and signal amplifier. The EMSM junction structure can be a new and efficient way of fabricating the nonlinear device THz detector with high cut-off frequency relying on extremely small junction area

Fluid challenges in intensive care: the FENICE study A global inception cohort study

Fluid challenges (FCs) are one of the most commonly used therapies in critically ill patients and represent the cornerstone of hemodynamic management in intensive care units. There are clear benefits and harms from fluid therapy. Limited data on the indication, type, amount and rate of an FC in critically ill patients exist in the literature. The primary aim was to evaluate how physicians conduct FCs in terms of type, volume, and rate of given fluid; the secondary aim was to evaluate variables used to trigger an FC and to compare the proportion of patients receiving further fluid administration based on the response to the FC.This was an observational study conducted in ICUs around the world. Each participating unit entered a maximum of 20 patients with one FC.2213 patients were enrolled and analyzed in the study. The median [interquartile range] amount of fluid given during an FC was 500 ml (500-1000). The median time was 24 min (40-60 min), and the median rate of FC was 1000 [500-1333] ml/h. The main indication for FC was hypotension in 1211 (59 %, CI 57-61 %). In 43 % (CI 41-45 %) of the cases no hemodynamic variable was used. Static markers of preload were used in 785 of 2213 cases (36 %, CI 34-37 %). Dynamic indices of preload responsiveness were used in 483 of 2213 cases (22 %, CI 20-24 %). No safety variable for the FC was used in 72 % (CI 70-74 %) of the cases. There was no statistically significant difference in the proportion of patients who received further fluids after the FC between those with a positive, with an uncertain or with a negatively judged response.The current practice and evaluation of FC in critically ill patients are highly variable. Prediction of fluid responsiveness is not used routinely, safety limits are rarely used, and information from previous failed FCs is not always taken into account

Planar array of electric-LC resonators with broadband tunability

A planar array of microwave electric- LC (ELC) resonators with broadband tunability of the resonance frequency is presented in this letter. An ELC resonator is typically composed of inductive loops and a capacitive gap, resonant at a wavelength much larger than its physical dimension. Here, the original resonator is modified to accommodate a varactor and its accompanying dc bias enabling resonance tunability. The wideband operation can be achieved through strategic placement of the varactor. The robustness of the response for a large array containing hundreds of the varactor-loaded resonators is considered via a sensitivity analysis. The numerical and experimental results show that the fabricated array possesses a wide tuning range of nearly 32% with no significant resonance broadening, despite the tolerance in varactor characteristics.Withayachumnankul W., Fumeaux C. and Abbott, D

Near-field interactions in electric inductive-capacitive resonators for metamaterials

Extent: 7p.Near-field interactions in an array of electric inductive–capacitive (ELC) resonators are investigated analytically, numerically and experimentally. The measurement and simulation results show that inter-cell coupling plays an important role in determining the response of metamaterials. A quasistatic dipole–dipole interaction model, together with a Lagrangian formalism, quantitatively explains the interplay between the electric and magnetic couplings in the resonator array. Depending on the alignment of the resonators, the couplings can cause resonance shifting and/or splitting. The knowledge obtained from this study is crucial in designing metamaterials with ELC resonators.Withawat Withayachumnankul, Christophe Fumeaux and Derek Abbot

Spatial response of infrared antennas

We present measurements on the spatial-response profiles of nanometer-scale thin-film Ni-NiO-Ni diodes integrated with infrared dipole and bow-tie antennas. Antennas are usually tested for their angular response using collimated radiation. However, in this study, focused radiation with a wavelength of 10.6 micrometer is scanned across the receiving area of the detector. This permits determination of the effective collection area of an individual infrared antenna. The width of the collection area parallel to the antenna axis is shown to scale with the physical length of the antenna. The determination of the effective collection area permits a characterization of the fringe fields surrounding the antenna and can be used to investigate the cross talk between adjacent antennas. It allows calculation on the power collected by an infrared antenna for a given irradiance of the illuminating beam. The spatial response also gives insight into the current-wave modes propagating on the antenna. Fast infrared detectors have dimensions considerably smaller than the wavelength of the incident radiation. Their performance is enhanced with the aid of wire or planar antennas having dimensions comparable with the wavelength. The efficiency of infrared lithographic antennas for detection at wavelengths near 10 micrometer was demonstrated with various types of detectors, including thin film metal-oxide-metal diodes (MOM or MIM) and Nb microbolometers. To investigate the mechanism of infrared antennas, we determine the spatial response of various dipole and bow-tie antennas at 10.6 micrometer wavelength. For this purpose, we scan tightly focused radiation at normal incidence across the receiving area of the detector in two orthogonal directions. The effective receiving area is a relevant parameter for infrared antennas. It first permits a radiometric characterization of the detector based on received power for a given incident irradiance. It also defines the spacing of individual detectors required for construction of an area receiver, and in a two-dimensional array allows calculation of cross-talk level between adjacent devices. The spatial response of infrared antennas on a substrate also contributes to the understanding of the resonant modes propagating on the structure. ©2003 Copyright SPIE - The International Society for Optical Engineering

Tunable polarization response of a planar asymmetric-spiral infrared antenna

We present measurements at 10.6 μm that demonstrate electronic tuning of the polarization response of asymmetric-spiral infrared antennas connected to Ni-NiO-Ni diodes. Continuous variation of the bias voltage applied to the diode results in a rotation of the principal axis of the polarization ellipse of the spiral antenna. A 90° tuning range is measured for a bias voltage that varies from -160 to +160 mV. This effect is caused by a small asymmetry of the deposited diode contact or by a variation of the detector capacitance with the applied bias voltage. © 1998 Optical Society of America

Polarization response of asymmetric-spiral infrared antennas

We present measurements on the polarization response of Ni-NiO-Ni diodes coupled to asymmetric spiral antennas. Our data are for the wavelength dependence of the orientation of the major axis of the polarization ellipse over the wavelength range 10.2-10.7 μm. The data are well fit by a two-wire antenna model. We find that the modes excited on the antenna are a combination of the balanced and unbalanced modes of a two-wire lossy transmission line. © 1997 Optical Society of America

Terahertz reflectarray for bidirectional beam splitting

A beam splitter based on the concept of a reflectarray is designed for polarization-dependent bidirectional deflection at 1 THz. A unit cell is composed of two sets of orthogonally oriented dipole resonators arranged in an interlaced triangular lattice. A subarray is constructed from multiple unit cells to create two independent progressive phase distributions to respond to the TE and TM polarized incident waves. Numerical results for the near- and far-field distributions of the reflectarray demonstrate that the proposed structure can separate the polarization components of an incident beam by deflecting them into two different directions, i.e. -48:6 ◦ and 30 °, in plane.Tiaoming Niu, Withawat Withayachumnankul, Derek Abbott, and Christophe Fumeau