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Hamid Rajabalipanah

Publications and source records attributed to Hamid Rajabalipanah.

11 recordsLinked to original sources

LSTM-CNN Network for Audio Signature Analysis in Noisy Environments

There are multiple applications to automatically count people and specify their gender at work, exhibitions, malls, sales, and industrial usage. Although current speech detection methods are supposed to operate well, in most situations, in addition to genders, the number of current speakers is unknown and the classification methods are not suitable due to many possible classes. In this study, we focus on a long-short-term memory convolutional neural network (LSTM-CNN) to extract time and / or frequency-dependent features of the sound data to estimate the number / gender of simultaneous active speakers at each frame in noisy environments. Considering the maximum number of speakers as 10, we have utilized 19000 audio samples with diverse combinations of males, females, and background noise in public cities, industrial situations, malls, exhibitions, workplaces, and nature for learning purposes. This proof of concept shows promising performance with training/validation MSE values of about 0.019/0.017 in detecting count and gender.

cs.SD

A single metagrating metastructure for wave-based parallel analog computing

Wave-based signal processing has witnessed a significant expansion of interest in a variety of science and engineering disciplines, as it provides new opportunities for achieving high-speed and low-power operations. Although flat optics desires integrable components to perform multiple missions, yet, the current wave-based analog computers can engineer only the spatial content of the input signal where the processed signal obeys the traditional version of Snell's law. In this paper, we propose a multi-functional metagrating to modulate both spatial and angular properties of the input signal whereby both symmetric and asymmetric optical transfer functions are realized using high-order space harmonics. The performance of the designed compound metallic grating is validated through several investigations where closed-form expressions are suggested to extract the phase and amplitude information of the diffractive modes. Several illustrative examples are demonstrated to show that the proposed metagrating allows for simultaneous parallel analog computing tasks such as first- and second-order spatial differentiation through a single multi-channel structured surface. It is anticipated that the designed platform brings a new twist to the field of optical signal processing and opens up large perspectives for simple integrated image processing systems.

physics.optics

Reciprocal Metasurfaces for On-axis Reflective Optical Computing

Analog computing has emerged as a promising candidate for real-time and parallel continuous data processing. This paper presents a reciprocal way for realizing asymmetric optical transfer functions (OTFs) in the reflection side of the on-axis processing channels. It is rigorously demonstrated that the presence of Cross-polarization Exciting Normal Polarizabilities (CPENP) of a reciprocal metasurface circumvents the famous challenge of Green's function approach in implementation of on-axis reflective optical signal processing while providing dual computing channels under orthogonal polarizations. Following a comprehensive theoretical discussion and as a proof of concept, an all-dielectric optical metasurface is elaborately designed to exhibit the desired surface polarizabilities, thereby reflecting the first derivative and extracting the edges of images impinging from normal direction. The proposed study offers a flexible design method for on-axis metasurface-based optical signal processing and also, dramatically facilitates the experimental setup required for ultrafast analog computation and image processing.

physics.optics

Analytical Design for Full-space Spatial Power Dividers Using Metagratings

We present a rigorous theoretical framework for designing full-space spatial power dividers using metagratings. In our study, the current restrictions of spatial power dividing platforms such as reflection-only performance, operating at normal incidence, and small reflection/refraction angles have been totally relaxed. A modal expansion analysis based on Floquet-Bloch (FB) theorem is established so that a discrete set of spatial harmonics is considered in both reflection and transmission sides of a compound metallic grating in which the unknown coefficients are calculated by applying proper boundary conditions. By eliminating the unwanted scattering harmonics, the proposed metagrating has the ability to realize different functionalities from perfect anomalous refraction to reflection-transmission spatial power dividing, without resorting to full-wave numerical optimizations. The numerical simulations confirm well the theoretical predictions. Our findings not only offer possibilities to realize arbitrary spatial power dividers but also reveal a simple alternative for beamforming array antennas.

physics.app-ph

Time-modulated Measurements: Material Characterization Based on Scalar Reflection Data

This paper explores the time-modulated waveguide setups for unique and accurate permittivity and permeability extraction of lossy dispersive samples with phase-less measurements. We theoretically demonstrate that when the position of the short-circuit termination is dynamically modulated in a predefined way, the phase information of the reflection S-parameter manifests itself into the amplitude level of the emerging harmonics. Being insensitive to the calibration plane shifts and phase uncertainties in reflection measurements while bypassing a priori knowledge about the material under test (MUT) and also the transmission coefficient, can be enumerated as the main advantages of the proposed time-modulated retrieval scheme. Moreover, the presented reconstruction algorithm offers a simple post-processing step to facilitate fast computations of $\varepsilon_r$ and $μ_r$. Several illustrative examples at X-band frequencies have been presented to numerically verify the validity of the proposed approach for some applicable and practical types of homogeneous materials. Two possible realization and measurement configurations are suggested and discussed based on mechanical actuation and electrical phase control. We have also performed an uncertainty analysis to examine how the realization tolerances can affect the accuracy of results. By involving the temporal dimension, the proposed strategy takes a great step forwards in phase-less reconstruction of the electromagnetic parameters.

physics.app-ph

Electromechanically Programmable Space-Time-Coding Digital Acoustic Metasurfaces

Over the last couple of years, the digital coding acoustic metasurfaces have been developed rapidly as a highly active research area for their unique and flexible manipulation of acoustic wavefronts. Nevertheless, all recent attentions in the acoustic community have been mainly concentrated on space-encoded architectures, leaving the room free for benefiting from the unique features of spatiotemporally modulated metasurfaces. By entering the world of time, here, we propose a space-time-coding acoustic digital metasurface with exotic ability to dynamically transfer the energy of the carrier acoustic signal to a series of harmonic components, with equivalent magnitudes and phases that can be precisely and independently engineered. The contributing elements are composed of a straight pipe, and four shunted Helmholtz cavities (HCs) with transmission phases dynamically controlled by exploiting a high-speed electromechanical actuation system. Several illustrative examples have been presented to demonstrate that by distributing the coding sequences in both space and time dimensions, diverse scattering functionalities can be elaborately acquired for one or multiple harmonic frequencies in a programmable way. Numerical and theoretical results are in an excellent agreement, thereby elucidating that this next generation of programmable acoustic metasurfaces, without restoring to high-cost nonlinear components, opens up unprecedented potential for efficient harmonic control used in adaptive beamforming and acoustic imaging systems.

physics.app-ph

How Do Space-Time Digital Metasurfaces Serve to Perform Analog Signal Processing?

In the quest to realize analog signal processing using sub-wavelength metasurfaces, in this paper, we demonstrate the first experimental demonstration of programmable time-modulated metasurface processors based on the key properties of spatial Fourier transformation. Exploiting space-time coding strategy enables local, independent, and real-time engineering of not only amplitude but also phase profile of the contributing reflective digital meta-atoms at both central and harmonic frequencies. Several illustrative examples are demonstrated to show that the proposed multifunctional calculus metasurface is capable of implementing a large class of useful mathematical operators, including 1st- and 2nd-order spatial differentiation, 1st-order spatial integration, and integro-differential equation solving accompanied by frequency conversions. Unlike the recent proposals, the designed time-modulated signal processor effectively operates for input signals containing wide spatial frequency bandwidths with an acceptable gain level. Proof-of-principle simulations are also reported along with the successful realization of image processing functions like edge detection. This time-varying wave-based computing system can set the direction for future developments of programmable metasurfaces with highly promising applications in ultrafast equation solving, real-time and continuous signal processing, and imaging.

physics.app-ph

Real-time Terahertz Wave Channeling via Multifunctional Metagratings: A Sparse Array of All-Graphene Scatterers

Acquiring full control over a large number of diffraction orders can be strongly attractive in the case of realizing multifunctional devices such as multichannel reflectors. Recently, the concept of metagrating has been introduced which enables obtaining the desired diffraction pattern through a sparse periodic array of engineered scatterers. In this letter, for the first time, a tunable all-graphene multichannel meta-reflector is proposed for operating at terahertz (THz) frequencies. In the supercell level, the designed metagrating is composed of three graphene ribbons of different controllable chemical potentials which can be regarded as a five-channel THz meta-reflector. Several illustrative examples have been presented in which by choosing proper distribution of DC voltages feeding the ribbons, our design can realize different intriguing functionalities such as anomalous reflection, retro-reflection, and three-channel power splitting within a single shared aperture and with high efficiency. This work paves the way toward designing highly-efficient and tunable THz multichannel meta-reflectors with many potential applications in photonics and optoelectronics.

physics.app-ph

Self-biased Tri-state Power-Multiplexed Digital Metasurface Operating at Microwave Frequencies

Exploiting of nonlinearity has opened doors into undiscovered areas for achieving multiplexed performances in recent years. Although efforts have been made to obtain diverse nonlinear architectures at visible frequencies, the room is still free for incorporating non-linearity into the design of microwave metasurfaces. In this paper, a passive dual-band power intensity-dependent metasurface is presented. Here, power-multiplexing performance is achieved by embedding PIN-diodes in the coding particles. The proposed digital metasurface has three operational states: 1) it acts as a normal reflector at low power intensities whereas provides a dual-band nonlinear response upon illuminating by high-power incidences where 2) it perfectly absorbs the radiations at f1=6.7 GHz and 3) re-distributes the scattered beams by arranging the meta-atoms with a certain coding pattern at f2=9.37 GHz. The nonlinear performance of the designed tri-state coding elements has been characterized by using the scattering parameters captured in the full-wave simulations where the accurate models of the nonlinear diodes are involved. The emergence of microwave self-biased metasurfaces with smart re-actions upon illuminating by incidences of different power levels, revealing great opportunities for designing smart windows, smart camouflage coating surfaces and so on.

physics.app-ph

Addition Theorem Revisiting for Phase/Amplitude-Encoded Metasurfaces: Asymmetric Spatial Power Dividers

Recent years have witnessed an extraordinary spurt in attention toward the wave manipulating strategies revealed by coding metasurfaces as they build up a bridge between the physical and digital worlds. Newly, it has been shown that when two different coding patterns responsible for doing separate missions are added together based on the superposition theorem, the mixed coding pattern will perform both missions at the same time. In this paper, via a semi-analytical procedure, we demonstrate that such a theorem is not necessarily valid for all possible functionalities with considering phase-only coding distributions and ignoring the element pattern function. By revisiting the addition theorem, we introduce the concept of asymmetric spatial power divider (ASPD) with arbitrary power ratio levels in which modulating both amplitude and phase of the meta-atoms is inevitable to fully control the power intensity pattern of the metasurface. Numerical simulations illustrate that the proposed ASPD driven by proper coding sequences can directly generate a desired number of beams with pre-determined orientations and power budgets. The phase/amplitude-encoded Pancharatnam-Berry meta-atoms realize the required coding pattern in each case and good conformity between simulations and theoretical predictions verifies the presented formalism. This work exposes a new opportunity to implement spatial power dividers for various applications such as multiple-target radar systems, beamforming networks, and multiple-input multiple-output (MIMO) communication.

physics.app-ph

Generalized Optical Signal Processing Based on Multi-Operator Metasurfaces Synthesized by Susceptibility Tensors

This paper theoretically proposes a multichannel functional metasurface computer characterized by Generalized Sheet Transition Conditions (GSTCs) and surface susceptibility tensors. The study explores a polarization- and angle-multiplexed metasurfaces enabling multiple and independent parallel analog spatial computations when illuminated by differently polarized incident beams from different directions. The proposed synthesis overcomes substantial restrictions imposed by previous designs such as large architectures arising from the need of additional subblocks, slow responses, and most importantly, supporting only the even reflection symmetry operations for normal incidences, working for a certain incident angle or polarization, and executing only single mathematical operation. The versatility of the design is demonstrated in a way that an ultra-compact, integrable and planar metasurface-assisted platform can execute a variety of optical signal processing operations such as spatial differentiation and integration. It is demonstrated that a metasurface featuring non-reciprocal property can be thought of as a new paradigm to break the even symmetry of reflection and perform both even- and odd-symmetry mathematical operations at normal incidences. Numerical simulations also illustrate different aspects of multichannel edge detection scheme through projecting multiple images on the metasurface from different directions. Such appealing findings not only circumvent the major potential drawbacks of previous designs but also may offer an efficient, easy-to-fabricate, and flexible approach in wave-based signal processing, edge detection, image contrast enhancement, hidden object detection, and equation solving without any Fourier lens.

physics.optics