arXiv subjects

Nima Chamanara

Publications and source records attributed to Nima Chamanara.

At least 19 recordsLinked to original sources

arXiv2021

Front-induced transitions control THz waves

Relativistically moving dielectric perturbations can be used to manipulate light in new and exciting ways beyond the capabilities of traditional nonlinear optics. Adiabatic interaction with the moving front modulates the wave simultaneously in both space and time, and manifests a front-induced transition in both wave vector and frequency yielding exotic effects including non-reciprocity and time-reversal. Here, we introduce a technique called SLIPSTREAM, Spacetime Light-Induced Photonic STRucturEs for Advanced Manipulation. The technique is based on the creation of relativistic fronts in a semiconductor-filled planar waveguide by photoexcitation of mobile charge carriers. Here we demonstrate the capabilities of SLIPSTREAM for novel manipulation of THz light pulses through relativistic front-induced transitions. In the sub-luminal front velocity regime, we generate temporally stretched THz waveforms, with a quasi-static field lasting for several picoseconds tunable with the front interaction distance. In the super-luminal regime, the carrier front outpaces the THz pulse and a time-reversal operation is performed via a front-induced intra-band transition. We anticipate our platform will be a versatile tool for future applications in the THz spectral band requiring direct and advanced control of light at the sub-cycle level.

physics.optics↗

arXiv2020

Extreme Lightwave Electron Field Emission from a Nanotip

We report on sub-cycle terahertz light-field emission of electrons from tungsten nanotips under extreme conditions corresponding to a Keldysh parameter $γ_K\approx10^{-4}$. Local peak THz fields up to 40~GV/m are achieved at the apex of an illuminated nanotip, causing sub-cycle cold-field electron emission and acceleration in the quasi-static field. By simultaneous measurement of the electron bunch charge and energy distribution, we perform a quantitative test of quasi-static Fowler-Nordheim tunnelling theory under field conditions that completely suppress the tunnel barrier. Very high bunch charges of $\sim10^6$ electrons/pulse are observed, reaching maximum energies of 3.5~keV after acceleration in the local field. The energy distribution and emission current show good agreement with Fowler-Nordheim theory even in this extreme field regime. Extending this model to the single-shot regime under these conditions predicts peak electron distributions with a spectral purity of $10^{-4}$. THz field-induced reshaping and sharpening of the nanotip is observed, reducing the tip radius from 120~nm to 35~nm over roughly $10^9$ THz shots. These results indicate THz-driven nanotips in the extreme field limit are promising electron sources for ultrafast electron diffraction and microscopy.

physics.optics↗

arXiv2019

Uniform-Velocity Spacetime Crystals

We perform a comprehensive analysis of uniform-velocity bilayer spacetime crystals, combining concepts of conventional photonic crystallography and special relativity. Given that a spacetime crystal consists of a sequence of spacetime discontinuities, we do this by solving the following sequence of problems: 1) the spacetime interface, 2) the double spacetime interface, or spacetime slab, 3) the unbounded crystal, and 4) the truncated crystal. For these problems, we present the following respective new results: 1) an extension of the Stokes principle to spacetime interfaces, 2) an interference-based analysis of the interference phenomenology, 3) a quick linear approximation of the dispersion diagrams, a description of simultaneous wavenumber and frequency bandgaps, and 4) the explanation of the effects of different types of spacetime crystal truncations, and the corresponding scattering coefficients. This work may constitute the foundation for a virtually unlimited number of novel canonical spacetime media and metamaterial problems.

physics.optics↗

arXiv2019

Simultaneous Control of the Spatial and Temporal Spectra of Light with Space-Time Varying Metasurfaces

This paper presents space-time varying (STV) metasurfaces for simultaneously controlling the spatial and temporal spectra of electromagnetic waves. These metasurfaces transform incident electromagnetic waves into specified reflected and transmitted waves, with arbitrary temporal and spatial frequencies. They are synthesized in terms of time-domain generalized sheet transition conditions (GSTCs). Moreover, they are characterized using an analytical method and the unstaggered finite-difference time-domain (FDTD) technique adapted to space-time metasurfaces. STV metasurfaces performing pulse shaping, time reversal and differentiation are demonstrated as examples.

physics.optics↗

arXiv2018

Linear Pulse Compansion using Co-propagating Space-Time Modulation

This paper presents a pulse compansion, i.e. compression or expansion, technique based on co-propagating space-time modulation. An engineered asymmetric space-time modulated medium, co-propagating with a pulse compands the pulse continuously and at a constant rate. The space-time medium locally modifies the velocity of different sections of the pulse in order to shape the pulse as it propagates. There is no theoretical limit on the compansion factor with the proposed system. Moreover, it can be designed to transform the pulse shape and its modulation linearly, without any distortion. Therefore the proposed technique can be used for up or down-conversion of modulated pulses, with extreme conversion ratios. The presented compansion technique is linear with respect to the input wave and therefore can be used to perform compansion or frequency conversion on multiple pulses simultaneously.

physics.optics↗

arXiv2018

Wave Scattering by a Cylindrical Metasurface Cavity of Arbitrary Cross-Section: Theory and Applications

This paper presents a technique, combining the integral equations (IE) and the Generalized Sheet Transition Conditions (GSTCs) with bianisotropic susceptibility tensors, to compute electromagnetic wave scattering by cylindrical metasurfaces -- forming two-dimensional porous cavities -- of arbitrary cross sections. Moreover, it applies this technique to two problems -- cloaking with circular and rhombic shapes and illusion optics with an elliptic shape -- that both validate it, from comparison with specifications used in an exact synthesis of the metasurfaces, and reveal interesting capabilities of such metasurface structures. Particularly, active cylindrical metasurfaces can perfectly cloak and hence eliminate the extinction cross section of various cylindrical shapes, and simple purely passive versions of them, practically more accessible, still perform quite good cloaking and provide remakable extinction cross section reduction.

physics.comp-ph↗

arXiv2018

Efficient GSTC-FDTD Simulation of Dispersive Bianisotropic Metasurface

We present a simple and efficient Finite-Difference Time-Domain (FDFD) scheme for simulating dispersive (Lorentz-Debye) bianisotropic metasurfaces. This scheme replaces the conventional FDTD update equations by augmented update equations where the effect of the metasurface, positioned at a virtual node (or node plane) in the Yee grid, is accounted for by judiciously selected auxiliary polarization functions, based on the Generalized Sheet Transition Conditions (GSTCs). This scheme is computationally -- time- and memory-wise -- more efficient and easier to implement than a previously reported scheme for dispersive metasurfaces. It is validated in three illustrative examples.

physics.app-ph↗

arXiv2018

Metasurface Particle with Independent Transmission and Reflection Full Phase Coverage

A metasurface particle with independent transmission and reflection full phase coverage for circularly polarized waves is introduced. This particle is constituted of two parts, one controlling the power splitting and the reflection phase, and the other one controlling the transmission phase, both leveraging the Pancharatnam-Berry phase principle. Given its unique flexibility, this particle may find various applications in metasurface technology.

physics.app-ph↗

arXiv2018

Nonreciprocal Electromagnetic Scattering from a Periodically Space-Time Modulated Slab and Application to a Quasisonic Isolator

Scattering of obliquely incident electromagnetic waves from periodically space-time modulated slabs is investigated. It is shown that such structures operate as nonreciprocal harmonic generators and spatial-frequency filters. For oblique incidences, low-frequency harmonics are filtered out in the form of surface waves, while high-frequency harmonics are transmitted as space waves. In the quasisonic regime, where the velocity of the space-time modulation is close to the velocity of the electromagnetic waves in the background medium, the incident wave is strongly coupled to space-time harmonics in the forward direction, while in the backward direction it exhibits low coupling to other harmonics. This nonreciprocity is leveraged for the realization of an electromagnetic isolator in the quasisonic regime and is experimentally demonstrated at microwave frequencies.

physics.class-ph↗

arXiv2017

Inverse Prism based on Temporal Discontinuity and Spatial Dispersion

We introduce the concept of the inverse prism as the dual of the conventional prism and deduce from this duality an implementation of it based on temporal discontinuity and spatial dispersion provided by anisotropy. Moreover, we show that this inverse prism exhibits the following three unique properties: chromatic refraction birefringence, ordinary-monochromatic and extraordinary- polychromatic temporal refraction, and linear-to-Lissajous polarization transformation.

physics.optics↗

arXiv2017

Computational Analysis of Metasurfaces

Metasurfaces represent one of the most vibrant fields of modern science and technology. A metasurface is a complex electromagnetic structure, that is typically deeply subwavelength in thickness, electrically large in transverse size and composed of subwavelength scattering particles with extremely small features; it may generally be bianisotropic, spacevarying and time-varying, nonlinear, curved and multiphysics. With such complexity, the design of a metasurface requires a holistic approach, involving synergistic synthesis and analysis operations, based on a solid model. The Generalized Sheet Transition Conditions (GSTCs), combined with bianisotropic surface susceptibility functions, provide such a model, and allow now for the design of sophisticated metasurfaces, which still represented a major challenge a couple of years ago. This paper presents this problematic, focusing on the computational analysis of metasurfaces via the GSTC-susceptibility approach. It shows that this analysis plays a crucial role in the holistic design of metasurfaces, and overviews recently reported related frequency-domain (FDFD, SD-IE, FEM) and time-domain (FDTD) computational techniques.

physics.optics↗

arXiv2017

New Electromagnetic Modes in Space-Time Modulated Dispersion-Engineered Media

We report on new electromagnetic modes in space-time modulated dispersion-engineered media. These modes exhibit unusual dispersion relation, field profile and scattering properties. They are generated by coupled codirectional space-time harmonic pairs, and occur in space-time periodic media whose constituent materials exhibit specific dispersion. Excitation of a slab of such a medium with subluminal modulation results in periodic transfer of energy between the incident frequency and a frequency shifted by a multiple of the modulation frequency, whereas superluminal modulation generates exponentially growing frequencies. These modes may find applications in optical mixers, terahertz sources and other optical devices.

physics.optics↗

arXiv2017

Frequency Vectoralization and Frequency Birefringence

In view of momentum continuity at a temporal slab, it is shown that instantaneous switching of an isotropic medium to an anisotropic medium offers the incident frequency a directional property- a counterintuitive process which is called frequency vectoralization. By expressing the dispersion diagrams before and after the temporal transition, a general analogy between spatial and temporal interfaces is given and the concept of frequency birefringence, i.e. double frequency jump, will be explained. Furthermore, it will be shown that an anisotropic temporal interface diffracts a monochromatic beam in both the spatial and spectral domains.

physics.optics↗

arXiv2017

Generalized Sheet Transition Condition FDTD Simulation of Metasurface

We propose an FDTD scheme based on Generalized Sheet Transition Conditions (GSTCs) for the simulation of polychromatic, nonlinear and space-time varying metasurfaces. This scheme consists in placing the metasurface at virtual nodal plane introduced between regular nodes of the staggered Yee grid and inserting fields determined by GSTCs in this plane in the standard FDTD algorithm. The resulting update equations are an elegant generalization of the standard FDTD equations. Indeed, in the limiting case of a null surface susceptibility ($χ_\text{surf}=0$), they reduce to the latter, while in the next limiting case of a time-invariant metasurface $[χ_\text{surf}\neqχ_\text{surf}(t)]$, they split in two terms, one corresponding to the standard equations for a one-cell ($Δx$) thick slab with volume susceptibility ($χ$), corresponding to a diluted approximation ($χ=χ_\text{surf}/(2Δx)$) of the zero-thickness target metasurface, and the other transforming this slab in a real (zero-thickness) metasurface. The proposed scheme is fully numerical and very easy to implement. Although it is explicitly derived for a monoisotropic metasurface, it may be straightforwardly extended to the bianisotropic case. Except for some particular case, it is not applicable to dispersive metasurfaces, for which an efficient Auxiliary Different Equation (ADE) extension of the scheme is currently being developed by the authors. The scheme is validated and illustrated by five representative examples.

physics.class-ph↗

arXiv2017

Efficient Analysis of Metasurfaces in Terms of Spectral-Domain GSTC Integral Equations

We present a spectral-domain (SD) technique for the efficient analysis of metasurfaces. The metasurface is modeled by generalized sheet transition conditions (GSTCs) as a zero-thickness sheet creating a discontinuity in the electromagnetic field. The SD expression of these GSTCs for a specified incident field leads to a system of four surface integral equations for the reflected and transmitted fields, which are solved using the method of moments in the spectral domain. Compared to the finite-difference and finite-element techniques that require meshing the entire computational domain, the proposed technique reduces the problem to the surface of the metasurface, hence eliminating one dimension and providing substantial benefits in terms of memory and speed. A monochromatic generalized-refractive metasurface and a polychromatic focusing metasurface are presented as illustrative examples.

physics.optics↗

arXiv2017

Optical Isolation based on Space-time Engineered Asymmetric Photonic Bandgaps

Nonreciprocal electromagnetic devices play an important role in modern optical and microwave technologies. Conventional methods for realizing such systems are incompatible with integrated circuits. With recent advances in integrated photonics, the need for efficient on-chip magnetless nonreciprocal devices is more urgent than ever. This paper leverages space-time engineered asymmetric photonic bandgaps to generate optical isolation. It is shown that a properly designed space-time modulated slab is highly reflective/transparent for opposite directions of propagation. The proposed method requires a low modulation frequency, is magnetless and can achieve very high isolation levels. Experimental proof of concept at microwave frequencies is provided.

physics.optics↗

arXiv2016

Graphene Transverse Electric Surface Plasmon Detection using Nonreciprocity Modal Discrimination

We present a magnetically biased graphene-ferrite structure discriminating the TE and TM plasmonic modes of graphene. In this structure, the graphene TM plasmons interact reciprocally with the structure. In contrast, the graphene TE plasmons exhibit nonreciprocity. This nonreciprocity is manifested in unidirectional TE propagation in a frequency band close to the interband threshold frequency. The proposed structure provides a unique platform for the experimental demonstration of the unusual existence of the TE plasmonic mode in graphene.

cond-mat.mes-hall↗

arXiv2016

PML Inspired Transparent Metamaterials

Perfectly transparent metamaterial structures of arbitrary shapes, constructed from coordinate stretching and contractions, are presented. Coordinate stretching has been used for 2 decades in perfectly matched layers (PMLs) to electromagnetically simulate infinite domains in numerical techniques, but this concept is applied here for the first time to realize a physical transmission medium. The transparent medium does not scatter electromagnetic waves, i.e. it is reflection-less for all incidence angles and all excitation frequencies. It may be implemented in the form of metasurfaces and will clearly find a myriad of applications if it can be efficiently manufactured.

physics.optics↗