Mohammed Reza M. Hashemi

Significant performance enhancement in photoconductive terahertz optoelectronics by incorporating plasmonic contact electrodes

Even though the terahertz spectrum is well suited for chemical identification, material characterization, biological sensing and medical imaging, practical development of these applications has been hindered by attributes of existing terahertz optoelectronics. Here we demonstrate that the use of plasmonic contact electrodes can significantly mitigate the low-quantum efficiency performance of photoconductive terahertz optoelectronics. The use of plasmonic contact electrodes offers nanoscale carrier transport path lengths for the majority of photocarriers, increasing the number of collected photocarriers in a subpicosecond timescale and, thus, enhancing the optical-to-terahertz conversion efficiency of photoconductive terahertz emitters and the detection sensitivity of photoconductive terahertz detectors. We experimentally demonstrate 50 times higher terahertz radiation powers from a plasmonic photoconductive emitter in comparison with a similar photoconductive emitter with non-plasmonic contact electrodes, as well as 30 times higher terahertz detection sensitivities from a plasmonic photoconductive detector in comparison with a similar photoconductive detector with non-plasmonic contact electrodes.

Switchable Scattering Meta-Surfaces for Broadband Terahertz Modulation

Active tuning and switching of electromagnetic properties of materials is of great importance for controlling their interaction with electromagnetic waves. In spite of their great promise, previously demonstrated reconfigurable metamaterials are limited in their operation bandwidth due to their resonant nature. Here, we demonstrate a new class of meta-surfaces that exhibit electrically-induced switching in their scattering parameters at room temperature and over a broad range of frequencies. Structural configuration of the subwavelength meta-molecules determines their electromagnetic response to an incident electromagnetic radiation. By reconfiguration of the meta-molecule structure, the strength of the induced electric field and magnetic field in the opposite direction to the incident fields are varied and the scattering parameters of the meta-surface are altered, consequently. We demonstrate a custom-designed meta-surface with switchable scattering parameters at a broad range of terahertz frequencies, enabling terahertz intensity modulation with record high modulation depths and modulation bandwidths through a fully integrated, voltage-controlled device platform at room temperature.

Evolution of Composite Right/Left-Handed Leaky-Wave Antennas

In this paper, the evolution of the composite right/left-handed (CRLH) leaky-wave antennas (LWAs) yielding several novel CRLH LWA is reviewed. Starting with the concept of CRLH LWAs, the concave and convex passive CRLH LWAs are discussed along with the conformation effects in each case and a dispersion engineering method is introduced to compensate for the conformation effect to refocus the main radiation beam. Next, planar and conformal varactor-based electronically controlled CRLH LWAs are reviewed. In the planar case electronic beam steering and beamwidth controlling are discussed while in the case of conformal version electronic radiation aperture selectivity as well as electronic beam focusing is discussed. Finally, a dual polarized CRLH LWA is presented which is a balanced four-port coupled CRLH LW-TL that conserves its CRLH nature under both common-mode and differential-mode excitations but with two orthogonal E-planes resulting in two orthogonal polarizations for the radiated fields.

Dispersion engineered metamaterial-based transmission line for conformal surface application

In this paper, a novel conformal metamaterial-based transmission line (TL) is introduced and the effect of conformation is investigated. The presented structure is a 25 unit-cell composite right/left-handed (CRLH) TL that is conformed on a cylindrical object with a radius of 20.0 cm. The effect of conformation on this CRLH TL is investigated for two situations: slow-wave mode operation and fast-wave mode operation. It is shown that conformation has insignificant effect on the structure’s performance in terms of S-parameters when the structure is operating in the slow-wave region. Conversely, when the CRLH TL is operating in the fast-wave region conformation will result in significant performance changes. These changes occur in terms of the leaky-wave radiation characteristics of the structure. In the later case, dispersion engineering is employed to modify the conformal structure such that it provides comparable performance to the planar version in terms of radiation characteristics.

Coupled Composite Right/Left-Handed Leaky-Wave Transmission Lines Based on Common/Differential-Mode Analysis

A systematic analysis for two types of symmetrical coupled composite right/left-handed (CRLH) leaky-wave (LW) transmission lines (TL) based on the common/differential-mode excitations is presented. The first proposed coupled CRLH LW-TL is a 4-port via-free coupled CRLH LW-TL that behaves as a conventional CRLH LW-TL under differential-mode excitation. But when common-mode excitation is applied, the structure has the ability to suppress the left-handed waves by operating in the cutoff mode, and only supporting right-handed guided as well as radiating waves. The second type of the proposed coupled CRLH LW-TL is a modification of the first type by inserting vias at the center of the left-handed stub of each unit-cell along the symmetry plane to provide a physical short-circuit to the ground. This structure behaves as a conventional CRLH LW-TL under both common- and differential-mode excitations but with orthogonal polarizations when operating in the fast-wave region. As a result, a dual polarized CRLH LW-TL is obtained.

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.

Dual-mode leaky-wave excitation in symmetric composite right/left-handed structure with center vias

In this paper a novel four port symmetric coupled composite right/left-handed leaky-wave (LW) transmission-line is introduced. Unlike the other coupled CRLH-TLs, this structure has the ability to support both right-handed as well as left-handed guided and leaky-wave propagations under both common-mode and differential-mode excitations. These unique properties are achievable through providing physical short circuit by means of ground vias at the center of each unit-cell along the symmetry plane of the structure. The advantage of this structure is that not only backward-wave is supported under both common- and differential-mode excitations but also when the TL operates in the fast-wave region, the radiated field under differential-mode excitation has an orthogonal polarization in comparison to that of the common-mode excitation resulting in a dual polarized four port coupled CRLH LW-TL antenna. The operation mechanism which is based on the direction of the current maximum will be discussed in this paper. For illustration purposes, a 25-cell four port coupled CRLH LW-TL is simulated, fabricated and measured and the obtained results verify the unique features of the structure mainly CRLH nature as well as its dual polarity under both common-mode and differential-mode excitation.

Dual-Band Composite Right/Left-Handed Metamaterial Concept

In this letter a concept of a simple dual-band (DB) composite right/left-handed (CRLH) metamaterial is presented and discussed. This DB-CRLH concept is based on the combination of a conventional CRLH unit-cell with two conventional right-handed transmission-line sections. The equations required for analysis of such structure is derived and presented. The effect of the equivalent circuit model parameters on the dispersion diagram of the DB-CRLH unit-cell is investigated.

Electronically controlled metamaterial-based leaky-wave transmission-line for conformal surface applications

In this paper a novel multifunctional electronically controlled conformal metamaterial-based transmission-line (TL) is presented. The proposed structure is a 25 unit-cell composite right/left-handed (CRLH) TL that is conformed on a cylindrical surface with a radius of 25 cm. This TL supports both guided-wave mode and radiation mode under its fundamental mode operation. Varactor diodes are introduced in the unit-cells to electronically control its guided and radiation characteristics. By applying different reverse-bias voltages the capacitance of the unit-cell is tuned. It will be shown that the proposed TL has the ability to operate partially in the slow-wave (SW) region and partially in the fast-wave (FW) region by applying proper biasing voltages to the different sections of the TL. Therefore, it has a unique radiation aperture selectivity function which results in controlling the radiation angle and beamwidth at a fixed frequency. In addition it will be shown that this electronically controlled conformal CRLH TL has the capability to compensate for the conformation effect by adjusting the biasing voltages properly. The proposed method allows the conformal structure to refocus its radiation main beam and have a comparable performance as the planar version.

Circularly polarized composite right/left-handed leaky-wave antenna

In this paper a circular polarized (CP) composite right/left-handed (CRLH) leaky-wave antenna (LWA) is presented. This CP CRLH LWA consists of two 25-cell conventional CRLH LWAs oriented orthogonal to each other which are connected to a quadrature hybrid coupler for 90° phase difference feeding purpose. This antenna maintains its CP characteristic while performing continuous frequency beam scanning from backward direction to forward direction including broadside radiation. To illustrate the scanning CP characteristics of this CRLH LWA, axial ratio and radiation patterns are obtained and shown for three frequencies of 3.4GHz (backward), 3.7GHz (broadside) and 4.3GHz (forward).