Mohammad Neshat

• PhD
• Associate Professor at Queen's University Belfast

To enable various important applications of terahertz (THz) technology, sources that are compact, low-cost, highly efficient, and tunable at room temperature are needed. THz-guided sources can provide an optimal platform for such features along with on-chip integrability. We analytically investigate the THz difference frequency generation in a long...

Subwavelength plasmonic metasurfaces combined with multiple quantum wells (MQWs) heterostructures have recently demonstrated highly efficient nonlinear wave generation under low power input intensities. They can pave the way for developing highly efficient, compact, tunable and room temperature terahertz (THz) wave sources through the mixing and do...

Scattering scanning near-field optical microscopy (s-SNOM) is a technique to enhance the spatial resolution, and when combined by Fourier transform spectroscopy it can provide spectroscopic information with high spatial resolution. This paper studies two analytical models for the s-SNOM probe using atomic force microscopy (AFM) tip and its interact...

This paper presents experimental realization of a reconfigurable intelligent surface (RIS) using space-time coding metasurfaces to enable concurrent beam steering and data modulation. The proposed approach harnesses the capabilities of metasurfaces, allowing precise temporal control over individual unit cells of the RIS. We show that by employing p...

In this paper, a novel approach for wireless localization is proposed and experimentally validated that leverages space-time coded reconfigurable intelligent surfaces (RIS). It is demonstrated that applying proper single-bit codes to each RIS element, enables accurate determination of the direction of arrival (AOA) at the receiver. Moreover, we int...

We propose a versatile platform to design tunable metasurface devices based on Au/n-Si Schottky diodes embedded in a split-ring resonator (SRR) devised on a Si-on-insulator (SOI) wafer. The horizontally formed diodes are connected in the SRR radial direction, reducing the overall junction capacitance of the metasurface array compared to its counter...

We present an analytical method to study the angular distribution of THz radiation driven by two-color laser-induced microplasma in nitrogen gas. Directionally dependent temporal waveforms of the radiated THz pulse is calculated. To do so, we have extended the photo-current model to calculate three-dimensional distribution of the photo-induced curr...

Fourier transform spectroscopy (FTS) has been widely used as an analytical tool for many applications in science and engineering. In this paper, we describe the operation principle and practical implementation of an FTS prototype. First, the structure of an FTS setup based on optical interferometers is introduced, and its optical and electronic com...

A plasmonic-type nonlinear metasurface loaded with Multiple Quantum Well (MQW) is studied for terahertz beam generation through the difference frequency generation (DFG) phenomenon. We calculate the second-order nonlinear susceptibility coefficient of MQW. Furthermore, we investigate the effect of applying a bias voltage across the MQW on the magni...

Scattering scanning near-field optical microscopy (s-SNOM) is known as a promising technique for overcoming Abbe diffraction limit and substantially enhancing the spatial resolution in spectroscopic imaging. The s-SNOM works by exposing an atomic force microscope (AFM) tip to an optical electromagnetic (EM) field, while the tip is so close to a sam...

Supplementary: 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 mod...

We propose an RF front-end architecture with simultaneous direct antenna modulation and beam steering capability based on space-time-coding arrays. In a space-time-coding array, digital codes control the state of each array element. The element state determines the amplitude and phase of the radiated wave, and in a time scale over which the state o...

In this article, we propose a reconfigurable metasurface emitter (MSE) with beam shaping capability without using any phase shifter or a superstrate. MSEs consist of repeating radiative inclusions (unit cells) distributed on a surface. The type, shape, geometry, and the arrangement of the radiative inclusions provide great flexibility to design hig...

In this paper, we show that the digital code bits used in the coded time-modulated arrays (TMA) can be interpreted as vector elements, and with this new interpretation, a better understanding of the relationship between the codes and the amplitude/phase of the generated harmonic signals is obtained. Using such interpretation, we first select the pr...

We propose a hybrid guided structure consisting of a rectangular waveguide loaded with channel-shaped and its complementary wedge-shaped metallic pillars that supports spoof plasmonic guided modes. The complementary structure is used in order to gain a better control over tuning of the asymptotic frequency in the spoof plasmonic waveguide. As a pro...

Metasurfaces, the two-dimensional counterpart of metamaterials, have caught great attention thanks to their powerful capabilities on manipulation of electromagnetic waves. Recent times have seen the emergence of a variety of metasurfaces exhibiting not only countless functionalities, but also a reconfigurable response. Additionally, digital or codi...

In this paper, we present a rectification process through electron emission from sharp tips under a high electric field of 10⁹–10¹⁰ V/m based on the Fowler–Nordheim theory. A large planar array of 10⁴–10⁵ nanoantennas is proposed to achieve the required electric field for electron emission in the nanoantenna gap. Spatial coherence of the incident w...

A rectification concept based on field electron emission in a nanoantenna is proposed. The device consists of three parts, i.e., an aspherical micro-lens, a nanoantenna, and an electron collector electrode with proper geometry design to ensure rectified field electron emission. The incident radiation is enhanced in two steps, firstly by micro-lens...

A plasmonic THz PCA consisting Ti/Au nanodisks in the antenna gap is presented. • All structural parameters were optimized to reach highest electron-hole generation. • The PCA benefits from enhanced quantum efficiency enabled by plasmon excitation. • The finalized PCA resulted in more than 5.6 times THz electric field radiation. • This THz field en...

Metasurfaces, the two-dimensional counterpart of metamaterials, have caught great attention thanks to their powerful capabilities on manipulation of electromagnetic waves. Recent times have seen the emergence of a variety of metasurfaces exhibiting not only countless functionalities, but also a reconfigurable response. Additionally, digital or codi...

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 reco...

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 reco...

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 (...

Unlike the solar energy, infrared radiation from black bodies around us is always available. In this paper, a novel energy harvesting device is proposed for absorption of the infrared radiation from hot objects around us at temperatures beyond 300 K, in the vicinity of 10 μm wavelength, or from the Earth itself. Electron field emission from sharp e...

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 gra...

Photoconductive antennas (PCA) have attracted lots of attention for terahertz application, due to their room temperature operation and compact design. On the other hand, their main problem is acquiring low output power. Lately, plasmonic structures of different geometries have been proposed to overcome the mentioned issue. Here, we have proposed an...

This paper presents an efficient approach for exciting a dielectric resonator antenna (DRA) in the terahertz frequencies by means of a graphene plasmonic dipole. Design and analysis are performed in two steps. First, the propagation properties of hybrid plasmonic onedimensional and two-dimensional structures are obtained by using transfer matrix th...

Graphene is enabling a plethora of applications in a wide range of fields due to its unique electrical, mechanical, and optical properties. In the realm of wireless communications, graphene shows great promise for the implementation of miniaturized and tunable antennas in the terahertz band. These unique advantages open the door to new reconfigurab...

This paper presents an efficient approach for exciting a dielectric resonator antenna (DRA) in the terahertz frequencies by means of a graphene plasmonic dipole. Design and analysis are performed in two steps. First, the propagation properties of hybrid plasmonic onedimensional and two-dimensional structures are obtained by using transfer matrix th...

A photoconductive terahertz antenna based on a distributed Bragg reflector, recessed nanoplasmonic grating and recessed electrodes is proposed in this paper. By use of the finite element method and full wave simulation, the effect of geometrical parameters on the transient photocurrent of a proposed photoconductive antenna is investigated. The rece...

We present a new bias-free antenna-coupled CW terahertz photomixer with interdigitated electrodes. Each finger pair is made of metal/semiconductor/metal (MSM) electrodes with dissimilar Schottky barriers. The two dissimilar metals in each MSM have a barrier height of difference ( ) and a finite lateral spacing ( ). In the proposed teraheretz emitte...

We propose an array of unbiased antennaless continuous wave (CW) THz photomixers made of subwavelength buried metal–semiconductor–metal (MSM) gratings with dissimilar Schottky barriers. These dissimilar metals in each unit cell are in intimate contact, and yet the maximum electric field appearing in the semiconducting material, unlike that in the p...

In this article, a multiband, planar, direct energy rectification, and dual polarized infrared plasmonic nanoantenna is presented and numerically simulated using finite element method (FEM). Broadband characteristics are highly suitable for energy harvesting applications. The proposed nanoantenna structure comprises cross bowtie aluminum structures...

In this paper, we study the effect of different metallic nanoparticles loaded in the gap of a silicon-on-sapphire (SOS) photoconductive antenna. Three different metals (Au, Ag, and Al) are considered as the material of the nanostructures which were loaded in the gap of a silicon-on-sapphire (SOS) photoconductive antenna as a THz source. The whole s...

A novel structure for THz photoconductive antennas (PCA) has been proposed to enhance the optical absorption, and consequently the power conversion efficiency. In the proposed structure, the surface of the low-temperature-grown GaAs (LTG-GaAs) substrate is textured to create an array of nanodisks where an array of silver nanodisks is placed 300 nm...

An array of unbiased CW terahertz photomixer emitters based on excitation of different waveguide modes in the bimetallic contact grating is introduced. Unlike previous structures, this emitter does not require any external bias voltage. For photocarrier acceleration it rather makes use of the built-in electrostatic field formed in the vicinity of t...

In this paper, five different nanoantenna structures, i.e. dipole, bowtie, rounded bowtie, rounded cross bowtie and cross bowtie nanoantennas are investigated for solar energy harvesting. Electrical field enhancement by each structure is investigated when different metals (silver, aluminum, gold, and copper) are used to achieve the best performance...

A silicon-on sapphire photoconductive antenna (PCA) as a terahertz source has been simulated by COMSOL Multiphysics® using wave optics and semiconductor modules. A periodic nanoplasmonic structure has been incorporated to improve terahertz radiation.

Photoconductive antenna on LTG-GaAs has been simulated and fabricated. Reducing gap size, increasing bias voltage and increasing laser pulse power will increase terahertz field. Narrower laser pulse width results in wider terahertz bandwidth.

In the last decade, terahertz technology has attracted attention of researchers and scientists because of its unique properties. Developing reliable sources and detectors are the most prominent part of terahertz technology. Photoconductive antennas are the most common devices for terahertz wave generation and detection due to its compact structure,...

A comprehensive theoretical study is presented on the polarization state of THz wave radiation from a two-colour laser-induced plasma, and its dependency on optical pulse polarizations and phase difference. We extend the so-called photocurrent model to calculate three-dimensional distribution of photocurrent from which the polarization state of the...

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...

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 (ΔΦB) and a finite lateral spacing (s). Simulations s...

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 dissimilar 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 s...

Plasmonic mode propagation properties of a graphene strip placed on a substrate are studied in the THz range. Based on propagation properties (phase constant), a design guide for a dipole-like antenna made of graphene strips is presented. The input impedance and the radiation properties of such graphene-based antenna are investigated through full-w...

A new unbiased terahertz photoconductive emitter is introduced. Unlike previous structures, this emitter does not require any external bias voltage. For photocarrier acceleration it rather makes use of the built-in electrostatic Schottky contact field formed at the electrode-photoconductor interface. A hybrid numerical simulation method is used to...

In this paper, a dynamic nonlinear transmission-line model for a field-effect transistor (FET) is proposed. The model can be used for a field-effect transistor operating as a detector of terahertz waves. Plasma wave excitation and rectification in the channel of a zero drain-source biased FET enables terahertz-wave detection. The terahertz response...

Terahertz radiation mechanism from laser-induced plasma in air is studied through photo-current model. The effect of intensity and pulse duration of the femtosecond laser field on the spatio-temporal behavior of the photo-current is presented. The radiation pattern of such laser-induced plasma in air is calculated at THz range. It is shown that fro...

We study the existence and guidance feasibility of spoof surface plasmon polaritons on a corrugated conical copper wire at terahertz frequencies. Then, we propose a microprobe so it can localize the field with high concentration at the apex and interacts strongly with samples, here, gold strips located nearby. Finally, we examine imaging functional...

A new unbiased terahertz photoconductive emitter is introduced. Unlike previous structures, this emitter does not require any external bias voltage. For photocarrier acceleration it rather makes use of the built-in electrostatic Schottky contact field formed at the electrode-photoconductor interface. A hybrid numerical simulation method is used to...

A dipole-like antenna made of graphene strips and integrated with a THz photonic mixer is studied numerically. The input impedance and the radiation properties of such graphene-based antenna are investigated through full-wave numerical simulations. Full-wave simulations show that the graphene-based antennas can provide higher input impedance along...

Plasmonic mode propagation properties of a graphene strip placed on a substrate is studied in THz range. Based on propagation properties (phase constant), a design guide for a dipole-like antenna made of graphene strips is presented. The input impedance and the radiation properties of such graphene-based antenna are investigated through full-wave n...

Plasmonic mode propagation properties of a graphene strip placed on a substrate is studied in THz range. Based on propagation properties (phase constant), a design guide for a dipole-like antenna made of graphene strips is presented. The input impedance and the radiation properties of such graphene-based antenna are investigated through full-wave n...

Plasmonic mode propagation properties of a graphene strip placed on a substrate is studied in THz range. Based on propagation properties (phase constant), a design guide for a dipole-like antenna made of graphene strips is presented. The input impedance and the radiation properties of such graphene-based antenna are investigated through full-wave n...

A new plasmonic-like waveguide structure is proposed for integrated terahertz refractive index sensing. The surface plasmon-like wave is an effective means for high sensitivity sensing in the terahertz range, but most of the reported configurations are based on free space excitation using a prism, that is not convenient for integration. In the prop...

Ellipsometry is a technique whereby the measurement of the two orthogonal polarization components of light reflected at glancing incidence allows a characterization of the optical properties of a material at a particular frequency. Importantly, it obviates the need for measurement against a standard reference sample, and so can provide reliable spe...

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 p...

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...

A fast analysis method for a new terahertz waveguide for photo-mixing is proposed. The wave-guiding mixer structure is a modified parallel plate waveguide (PPWG) in which the top plate is replaced by a periodic array of sub-wavelength nano-slits. The substrate of the PPWG is made of a fast photoconductive material in which laser photomixing/absorpt...

A hybrid numerical simulation method is presented to model and analyze integrated terahertz (THz) photomixer antennas. The proposed computational method combines an optoelectronic solver and a full-wave electromagnetic solver to rigorously model continuous wave (CW) THz photomixer sources. In this hybrid computational approach, the photomixer sourc...

A dielectric waveguide-based structure coupled to a whispering gallery mode (WGM) disc resonator is introduced as a low-cost integrable millimeter-wave (mm-wave) bio-sensor. An efficient variational analysis method is developed and applied to the WGM. Three sets of sensors, operating in different ranges of frequency from 85 to 220 GHz, are fabricat...

A calibration scheme is presented for improved polarization state measurement of terahertz pulses. In this scheme the polarization response of a two-contact terahertz photoconductive detector is accurately measured, and is used to correct for the impact of the nonidealities of the detector. Experimental results show excellent subdegree angular accu...

A method for gain measurement of embedded on-chip antennas in mmW/THz range is presented. In this method, the radiation pattern is first measured in a quasi-optical configuration using a power detector. Subsequently, the radiated power is estimated from the integration over the radiation pattern. Finally, the antenna gain is obtained from measureme...

A calibration scheme is presented for improved polarization state measurement of terahertz pulses. In this scheme the polarization response of a two-contact terahertz photoconductive detector is accurately measured, and is used to correct for the impact of the non-idealities of the detector. Experimental results show excellent sub-degree angular ac...

A method for polarization state measurements of THz pulses is presented. A calibration scheme is applied to correct for the impact of the non-idealities of the detector. Experimental results show excellent sub-degree angular accuracy with this method.

In this paper, we present a computationally efficient hybrid spectral ray tracing (HSRT) method that requires only one spectral domain integration step for each observation point. The HSRT method is capable of modeling arbitrary three-dimensional dielectric and metallic structures. We compare and validate various versions of the HSRT method includi...

We present a computationally efficient general Hybrid Spectral Ray Tracing (HSRT) method that requires one spectral domain integration step for each observation point. The HSRT method is capable of modeling arbitrary three-dimensional dielectric and metallic structures. As an example, the HSRT method is applied to on-chip dipole antenna backed by S...

The experimental and theoretical study of a low cost, and high sensitivity biosensor based on the Whispering Gallery Mode (WGM) resonance at D-band is reported. The sensitivity, selectivity, and repeatability of the proposed sensor are examined and experimentally verified for different concentrations of glucose in water solution at 124.7 GHz.

In this paper, a monolithic fabrication process for implementing a sensor device consisting of a ring resonator and dielectric image-waveguide on Silicon-on-Insulator (SOI) is presented for sensing applications. The device is implemented on high resistive silicon with the resistivity greater than 1000 il-cm to minimize the dielectric loss. The set...

The concept of equivalent surface impedance is applied for electromagnetic modeling of thin-film samples, e.g., nano-layers of deoxyribonucleic acid, placed on a metallic surface. For the first time, it is shown that by using such a concept, the performance of resonance-based terahertz bio-chips can be easily evaluated in terms of analytic sensitiv...

Three approaches will be presented for low-cost sub-millimeter/THz integrated circuits and systems. Multi-layer planar line monolithic integration, dielectric waveguide hybrid technology, and SOI-based photonic-crystal technique will be described and recent progresses and typical developed integrated devices will be discussed.

The paper proposes to stabilize the oscillation frequency through direct interaction of the Gunn diode with a whispering gallery mode (WGM) dielectric resonator (DR). The dimensions of a WGM resonator can be much larger than the operating wavelength, and therefore less sensitive to fabrication tolerances. In order to transmit the generated mm-wave...

In this paper, a global and geometry-independent approach is proposed for accurate analysis of edge-coupled continuous wave (CW) traveling-wave terahertz photomixer sources. All major physical phenomena involved in the operation of such devices are included in three interconnected solvers, which are combined as a unified analysis tool. A photonic s...

We propose, for the first time, the application of whispering gallery mode (WGM) perturbation technique in dielectric analysis of disk shape pharmaceutical tablets. Based on WGM resonance, a low-cost high sensitivity sensor in milllimeter-wave frequency range is presented. A comprehensive sensitivity analysis was performed to show that a change in...

In this paper, we propose a hybrid method that uses commercially available Finite Element Method (FEM) software and our in-house developed Spectral Ray Tracing (SRT) algorithm. The method is used for calculating the radiation patterns of complex lens antenna such as a bow-tie antenna backed by silicon lens. A traveling-wave terahertz photomixer int...

In this article, a time-domain calibration procedure is proposed for pulsed Terahertz Integrated Circuits (TIC) used in on-chip applications, where the conventional calibration methods are not applicable. The proposed post-detection method removes the unwanted linear distortions, such as interfering echoes and frequency dispersion, by using only on...

In this paper, a low-cost planar antenna array structure made of tapered dielectric image-line (DIL) radiators with a T-junction type feeding configuration is presented for Ka band frequency range. The radiation beam can be steered by changing the operation frequency. A beam deflection of 12deg in horizontal plane is achieved by 2 GHz change in the...

A signal processing algorithm based on the propagation and reflection mechanisms is proposed for echo cancellation in pulsed terahertz integrated circuits.

A semi-analytical method based on distributed source transmission line model is proposed to analyze a traveling-wave terahertz photomixer integrated with a coplanar stripline waveguide. Multilayer spectral domain method along with complex image technique have been applied to calculate the distributed voltage source element in the transmission line...

A dielectric measurement method based on using dielectric image guide (DIG) is presented. This method is ideal for characterization of very small liquid samples, e.g. in the range of mul or less, and has the advantage of broadband frequency measurement. The open structure of DIG makes it very convenient to place the liquid droplet at a predetermine...

In this paper, a high-gain tapered dielectric image-line (DIL) antenna array is presented for 30 GHz frequency range. Each element of the array is designed and optimized for maximum gain and minimum side-lobes. A novel array structure is proposed to increase the gain and suppress the side-lobs. An estimated 21.5 dB gain has been achieved with a 3-e...