arXiv subjects

Safieddin Safavi-Naeini

Publications and source records attributed to Safieddin Safavi-Naeini.

14 recordsLinked to original sources

arXiv2024

Millimeter-Wave Circular Synthetic Aperture Radar Imaging

In this paper, we present a high resolution microwave imaging technique using a compact and low cost single channel Frequency Modulated Continuous Wave (FMCW) radar based on Circular Synthetic Aperture Radar (CSAR) technique. We develop an algorithm to reconstruct the image from the raw data and analyse different aspects of the system analytically. Furthermore, we discuss the differences between the proposed systems in the literature and the one presented in this work. Finally, we apply the proposed approach to the experimental data collected from a single channel FMCW radar operating at $\rm 79 \;GHz$ and present the results.

eess.SP↗

arXiv2024

CDM Based Virtual FMCW MIMO Radar Imaging at 79GHz

Multiple Input Multiple Output (MIMO) Frequency Modulated Continuous Wave (FMCW) radars operating at 79GHz are compact, light and cost effective devices with low peak-to-average power ratio that have applications in different areas such as automotive industry and Unmanned Arial Vehicle (UAV) based radar imaging. In order to keep the structure small and simple, these radars come with small number of transmitters and receivers. The number of elements can be virtually increased using techniques such as Time Division Multiplexing (TDM), Frequency Division Multiplexing (FDM) or Code Division Multiplexing (CDM) and as a result higher angular resolution can be achieved. Both TDM and FDM based virtual FMCW MIMO radar imaging process have been reported in literature. However, to the best of our knowledge CDM based virtual FMCW MIMO radar has not received any attention. In this paper we will be using an 79GHz FMCW MIMO radar and apply the idea of the CDM method to increase the number of elements virtually which in turn enhances the angular resolution.

eess.SP↗

arXiv2021

Single Channel MMWave FMCW Radar for 2D Target Localization

In this paper, we present a 2D target localization method using two low cost and compact Mellimeter Wave Frequency Modulated Continuous Wave (MMW-FMCW) radars. To create a 2D map we exploit the bilateration method followed by a multi-target tracking block to remove the ghost targets. Finally, we present experimental results based on the data gathered from two FMCW radars operating at $\rm 79 \;GHz$.

eess.SP↗

arXiv2021

3D Near-Field Millimeter-Wave Synthetic Aperture Radar Imaging

In this paper, we present 3D high resolution radar imaging process at millimeter-Wave (mmWave) frequencies by creating the effect of a large aperture synthetically. We use a low cost fully integrated Frequency Modulated Continuous Wave (FMCW) radar operating at $\rm 79\;GHz$ and then perform Synthetic Aperture Radar (SAR) imaging in the near-filed zone. At the end, we conduct a real experiment and present the reconstructed image.

eess.SP↗

arXiv2021

Interaction between a Waveguide Mode and an Absorptive Medium

Absorption spectroscopy studies the absorption of electromagnetic wave in a material sample under test. The interacting electromagnetic wave can be propagating in free space or in a waveguide. The waveguide-based absorption spectroscopy method has several advantages compared to the free space setup. The effect of the waveguide cross section on the interaction between the waveguide mode and the sample can be expressed by a factor, called interaction factor. In this article, a new formulation for the interaction factor is derived. It is shown that this factor is inversely proportional to the energy velocity of the waveguide mode.

physics.optics↗

arXiv2020

Learning in the Sky: An Efficient 3D Placement of UAVs

Deployment of unmanned aerial vehicles (UAVs) as aerial base stations can deliver a fast and flexible solution for serving varying traffic demand. In order to adequately benefit of UAVs deployment, their efficient placement is of utmost importance, and requires to intelligently adapt to the environment changes. In this paper, we propose a learning-based mechanism for the three-dimensional deployment of UAVs assisting terrestrial cellular networks in the downlink. The problem is modeled as a non-cooperative game among UAVs in satisfaction form. To solve the game, we utilize a low complexity algorithm, in which unsatisfied UAVs update their locations based on a learning algorithm. Simulation results reveal that the proposed UAV placement algorithm yields significant performance gains up to about 52% and 74% in terms of throughput and the number of dropped users, respectively, compared to an optimized baseline algorithm.

eess.SP↗

arXiv2019

Triple-Poles Complementary Split Ring Resonator for Sensing Diabetics Glucose Levels at cm-Band

Microwave sensors are very promising for sensing the blood glucose levels non-invasively for their non-ionizing nature, miniaturized sizing, and low health risks for diabetics. All these features offer the possibility for realizing a portable non-invasive glucose sensor for monitoring glucose levels in real time. In this article, we propose a triple poles complementary split ring resonator (CSRR) produced on a FR4 substrate in microstrip technology in the cm-wave band (1-6 GHz). The proposed bio-sensor can detect the small variations in the dielectric properties (relative permittivity and dielectric losses) of glucose in the blood mimicking aqueous solutions due their intense interaction with the electromagnetic field at harmonic resonances. The resonator exhibits higher sensitivity performance at the different resonances compared to the single and double-poles counterparts as demonstrated by simulations in a 3D full-wave EM solver.

physics.med-ph↗

arXiv2019

Glucose Levels Sensing using Whispering Gallery Modes at mm-Wave Band

In this study, an integrated low-cost and complexity mm-wave structure of Whispering Gallery Mode (WGM) is proposed for sensing the glucose levels in mimicking aquatic solutions of concentrations similar to type 2 diabetics. The bio-sensor is composed of a curved dielectric waveguide coupled to a dielectric disc resonator that is loaded with the glucose sample under test. The intense WGM field induced towards the boundary of the DDR is exploited to detect the slight variations in the dielectric properties of different glucose levels via tracing the variations in the magnitude of the transmission coefficient S_{21} in the mm-wave frequency band (49-70 GHz). The WGM resonator shows a high sensitivity performance (0.025-0.077 dB/(mg/dL)) at the lower-order modes WGH_{600} and WGH_{700} as demonstrated by simulations in a 3D full-wave EM solver (Ansys HFSS).

physics.app-ph↗

arXiv2019

Anomalous optical saturation of low-energy Dirac states in graphene and its implication for nonlinear optics

We reveal that optical saturation of the low-energy states takes place in graphene for arbitrarily weak electromagnetic fields. This effect originates from the diverging field-induced interband coupling at the Dirac point. Using semiconductor Bloch equations to model the electronic dynamics of graphene, we argue that the charge carriers undergo ultrafast Rabi oscillations leading to the anomalous saturation effect. The theory is complemented by a many-body study of the carrier relaxations dynamics in graphene. It will be demonstrated that the carrier relaxation dynamics is slow around the Dirac point, which in turn leads to a more pronounced saturation. The implications of this effect to the nonlinear optics of graphene is then discussed. Our analysis show that the conventional perturbative treatment of the nonlinear optics, i.e., expanding the polarization field in a Taylor series of the electric field, is problematic for graphene, in particular at small Fermi levels and large field amplitudes.

cond-mat.mes-hall↗

arXiv2019

Experimental Characterization of Ultrafast, Tunable and Broadband Optical Kerr Nonlinearity in Graphene

In this study we systematically measure the near-infrared spectral dependence, the sub-picosecond temporal evolution and pulse-width dependence of the effective Kerr coefficient ($n_{2,eff}$) of graphene in hundreds of femtosecond regime. The spectral dependence measured using the Z-scan technique is corroborated by quantum theory to extract a $n_{2,eff} \propto λ^2$ dependence. The temporal evolution extracted using the time-resolved Z-scan measurement shows the nonlinear response peaking at zero delay time and relaxing on a time scale of carrier relaxation. Since the Kerr-type response originated from the optically induced carrier population difference, the time-scale of the evolution of the nonlinear response is apt. The $n_{2,eff}$ shows a dependence on the pulse-width attributed to the relative heating and cooling times of the carriers. This dependence is strong when the pulse-duration is on the same time-scale as the decay constant. Throughout our study the $n_{2,eff}$ remains positive.

physics.optics↗

arXiv2016

Bistable Circuit Behaviour as a 2 level (stable/metastable) potential energy system

A novel model of analyzing Schmitt trigger as 2 level (metastable/stable) energy system is presented. The energy of the 2 level system is identified. In an MOS implementation, this arises from the electrostatic potential energy of the electrons on the gate to source capacitance of the cross coupled MOS transistors. The electrostatic energy, upon co-ordinate transformation, (incorporating the nonlinear MOS device behaviour and cross coupling), yields an expression for the 2 level system energy. The 2 level system, described under the transformed co-ordinate, is simulated. Simulation on the dynamics of the system shows a metastsable state and two stable states, corresponding to the a maximum(metastable) energy and a minimum(stable) energy, as predicted by the energy expression in the model. This result agrees with earlier attempt based on nonlinear dynamic system approach, whereby the potential is interpreted from the point of view of the gradient of vector field. In addition, the model gives physical design insight on how the change in circuit parameters (such as gm, R), affects the energy characteristics, hence leads to effective design of such 2 level system.

physics.class-ph↗

arXiv2015

Nonlinear quantum optical properties of graphene: the role of chirality and symmetry

We present a semiclassical theory of linear and nonlinear optical response of graphene. The emphasis is placed on the nonlinear optical response of graphene from the standpoint of the underlying chiral symmetry. The Bloch quasiparticles in low energy limit, around the degeneracy points are dominantly chiral. It is shown for the first time that this chiral behavior in conjunction with scale invariance in graphene around the Dirac points results in the strong nonlinear optical response. Explicit expressions for the linear and nonlinear conductivity tensors are derived based on Semiconductor Bloch Equations (SBEs). The linear terms agree with the result of Kubo formulation. The three main additive mechanisms contribute in the nonlinear optical response of graphene: pure intraband, pure interband and the interplay between them. For each contribution, an explicit response function is derived. The Kerr-type nonlinearity of graphene is then studied and it is demonstrated that its Kerr nonlinear coefficient is several orders of magnitude higher than that of many other known semiconductors. In addition, the nonlinear refractive index of graphene can also be tuned and enhanced by applying a gate voltage.

cond-mat.mes-hall↗

arXiv2014

A Fundamental Limit on Propagation Loss of Waveguides with Subwavelength Mode Size

A fundamental trade-off relation between the cross sectional confinement and propagation length of an arbitrary mode of a general waveguide is presented. This limit is a generalization of the well-known diffraction limit for guided modes. The results provide a lower bound on propagation loss of plasmonic waveguides which are attractive for their deep subwavelength mode dimensions. We also introduce a material loss merit factor that sets a criterion for comparing different plasmonic materials for achieving the best trade-off between confinement and loss.

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↗