arXiv subjects

Mohammad Neshat

Publications and source records attributed to Mohammad Neshat.

12 recordsLinked to original sources

arXiv2024

Evaluation of Analytical Models in Scattering Scanning Near-field Optical Microscopy for High Spatial Resolution Spectroscopy

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 interaction with a dielectric material. We evaluate the validity of these models by retrieving the permittivity spectrum of a sample material through an inverse method.

physics.optics↗

arXiv2024

Realization of Reconfigurable Intelligent Surfaces with Space-Time Coded Metasurfaces

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 proper binary codes manipulating the state of unit cells, the RIS can act as a digital data modulator with beam steering capability. We describe the experimental setup and computational tools, followed by validation through harmonic generation and investigation of beam steering and data modulation. Additionally, four digital modulation schemes are evaluated. By implementing customized binary codes, constellations under varying conditions are compared, showcasing the potential for real-world applications. This study offers new insights into the practical implementation of RIS for advanced wireless communication systems.

eess.SP↗

arXiv2024

Wireless Localization with Space-Time Coded Reconfigurable Intelligent Surfaces

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 introduce different scenarios that such technique can be used for localization. By incorporating RIS, a passive component, the method significantly reduces the complexity found in previous localization techniques. Additionally, the use of 1-bit codes minimizes hardware requirements, offering a reliable, low-cost solution for localization in advanced telecommunications networks.

eess.SY↗

arXiv2023

A Study on Angular Distribution of THz Radiation Driven by Two-Color Laser-induced Microplasma

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 current from which the far field THz radiation is computed. We have also studied the effect of optical polarization state and phase difference on the angular distribution of radiation. Our study shows that for two orthogonally-polarized colors, not only THz polarization state, but also the angular distribution of THz radiation is mostly influenced by the second optical harmonic. The possibility of rotating the angular radiation pattern around the axis of the laser propagation is illustrated for circularly-polarized laser drivers by changing the optical phase difference.

physics.optics↗

arXiv2023

Theory and Instrumentation of Fourier Transform Spectroscopy

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 components are identified. A simple theory is presented to show the Fourier transform relationship between the measured interferogarm signal at the detector and the spectral intensity of the source. Then, simulation results based on ray-tracing method are demonstrated to show the effect of coherent length of the source on the spectral intensity. Finally, various practical considerations to implement an FTS setup including noise/interference reduction are discussed, and the effect of alignment on the measurement accuracy is investigated. Since the design and implementation of an FTS measurement setup requires the knowledge from various parts of physics and engineering education, therefore, we believe that FTS can provide a very comprehensive educational tool for the last year undergraduate and graduate students.

physics.ins-det↗

arXiv2022

Quantum Modeling of Scanning Near-Field Optical photons Scattered by an Atomic-Force Microscope Tip for Quantum Metrology

Scattering scanning near-field optical microscopy (s-SNOM) is 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 dielectric sample that the incident beam lies within the near-field regime and displays nonlinear behaviour. We replace the incident EM field by photons generated by a single photon emitter, and propose a quantum model for the suggested system by employing electric-dipole approximation, image theory, and perturbation theory. Quantum state of scattered photons from the AFM tip is extracted from the proposed model, which contains information about electrical permittivity of the dielectric material beneath the tip. The permittivity of the sample can be extracted through spectroscopic setups. Our proposed scheme has potential applications for high-resolution quantum sensing and metrology, especially for quantum imaging and quantum spectroscopy.

quant-ph↗

arXiv2020

Digital Metasurface Based on Graphene: An Application to Beam Steering in Terahertz Plasmonic Antennas

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 coding metasurfaces have revolutionized the field by describing the device as a matrix of discrete building block states, thus drawing clear parallelisms with information theory and opening new ways to model, compose, and (re)program advanced metasurfaces. This paper joins the reconfigurable and digital approaches, and presents a metasurface that leverages the tunability of graphene to perform beam steering at terahertz frequencies. A comprehensive design methodology is presented encompassing technological, unit cell design, digital metamaterial synthesis, and programmability aspects. By setting up and dynamically adjusting a phase gradient along the metasurface plane, the resulting device achieves beam steering at all practical directions. The proposed design is studied through analytical models and validated numerically, showing beam widths and steering errors well below 10 degrees and 5% in most cases. Finally, design guidelines are extracted through a scalability analysis involving the metasurface size and number of unit cell states.

physics.app-ph↗

arXiv2019

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.

physics.optics↗

arXiv2018

MAC-Oriented Programmable Terahertz PHY via Graphene-based Yagi-Uda Antennas

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 reconfigurable antenna structures which, in turn, enable novel communication protocols at different levels of the stack. This paper explores both aspects by, first, presenting a terahertz Yagi-Uda-like antenna concept that achieves reconfiguration both in frequency and beam direction simultaneously. Then, a programmable antenna controller design is proposed to expose the reconfigurability to the PHY and MAC layers, and several examples of its applicability are given. The performance and cost of the proposed scheme is evaluated through full-wave simulations and comparative analysis, demonstrating reconfigurability at nanosecond granularity with overheads below 0.02 mm$^{2}$ and 0.2 mW.

cs.ET↗

arXiv2018

Terahertz Dielectric Resonator Antenna Coupled to Graphene Plasmonic Dipole

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 theory and the finite-element method. The coupling amount between the plasmonic graphene mode and the dielectric wave mode is explored based on different parameters. These results, together with DRA and plasmonic antenna theory, are then used to design a DRA antenna that supports the $TE_{y}^{112}$ mode at 2.4 THz and achieves a gain (IEEE) of up to 7 dBi and a radiation efficiency of up 70%. This gain is 6.5 dB higher than that of the graphene dipole alone and achieved with a moderate area overhead, demonstrating the value of the proposed structure.

cs.ET↗

arXiv2012

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 $15^\circ$--$85^\circ$ 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.

physics.optics↗

arXiv2010

Signal Recovery in Pulsed Terahertz Integrated Circuits

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 one single-port measurement. The method employs a wave-transfer model for analysis of the TIC, and the model parameters are obtained by a proposed blind estimation algorithm. A complete implementation of the method is demonstrated for a fabricated TIC, when used in an on-chip sensing application. The features of interest in the measured signal, such as absorption lines, can be masked or weakened by the distortion of the THz signal happening in a TIC. The proposed signal recovery approach improves the detection of those otherwise hidden features, and can significantly enhance the performance of existing TICs. To show the effectiveness of the proposed de-embedding method, numerical results are presented for simulated and measured signals. The method presented in this article is enabling for accurate TIC applications, and can be utilized to optimally design novel TIC structures for specific purposes.

physics.optics↗