Amir Nader Askarpour

Chirality detection of enantiomers using twisted optical metamaterials

Many naturally occurring biomolecules, such as amino acids, sugars and nucleotides, are inherently chiral. Enantiomers, a pair of chiral isomers with opposite handedness, often exhibit similar physical and chemical properties due to their identical functional groups and composition, yet show different toxicity to cells. Detecting enantiomers in small quantities has an essential role in drug development to eliminate their unwanted side effects. Here we exploit strong chiral interactions with plasmonic metamaterials with specifically designed optical response to sense chiral molecules down to zeptomole levels, several orders of magnitude smaller than what is typically detectable with conventional circular dichroism spectroscopy. In particular, the measured spectra reveal opposite signs in the spectral regime directly associated with different chiral responses, providing a way to univocally assess molecular chirality. Our work introduces an ultrathin, planarized nanophotonic interface to sense chiral molecules with inherently weak circular dichroism at visible and near-infrared frequencies.

Infrared beam-steering using acoustically modulated surface plasmons over a graphene monolayer

We model and design a graphene-based infrared beamformer based on the concept of leaky-wave (fast traveling wave) antennas. The excitation of infrared surface plasmon polaritons (SPPs) over a ‘one-atom-thick’ graphene monolayer is typically associated with intrinsically ‘slow light’. By modulating the graphene with elastic vibrations based on flexural waves, a dynamic diffraction grating can be formed on the graphene surface, converting propagating SPPs into fast surface waves, able to radiate directive infrared beams into the background medium. This scheme allows fast on–off switching of infrared emission and dynamic tuning of its radiation pattern, beam angle and frequency of operation, by simply varying the acoustic frequency that controls the effective grating period. We envision that this graphene beamformer may be integrated into reconfigurable transmitter/receiver modules, switches and detectors for THz and infrared wireless communication, sensing, imaging and actuation systems.

The Feasibility of Communication Among Pumps in a District Heating System

The problem tackled is selecting a viable method for communication among pumps in a district heating system. This system is viewed as a metropolitan wireless sensor network, the nodes of which are confined underground and physically connected by pipes. In a further horizon, providing sophisticated control systems for similar urban utilities motivates this research problem. Here we have reported the results of investigating several potential methods for realizing the idea of “the talking pumps” in a district heating system. This includes a diverse list of key references, simulations for some methods and experimental results for some others, followed by selection of the most appropriate option. The methods considered used (1) acoustic waves through water and pipelines; (2) power-line communications; (3) the electrical conductivity of pipes; (4) cell-phone infrastructure; (5) free and guided radio-frequency (RF) electromagnetic (EM) waves; and (5) free and guided very low frequency (VLF) electric and magnetic fields, also known as magnetic induction. The viability of the latter method was verified by simulations and primitive experimental results.

Transmission-Line Model and Propagation in a Negative-Index, Parallel-Plate Metamaterial to Boost Electron-Beam Interaction

A negative-index metawaveguide (NIMW) comprised of stacks of planar metal sheets loaded with periodic split-ring resonator apertures is modeled using a transmission-line (TL) analysis, and analyzed in detail. The proposed model provides a useful tool to extract the wave impedance and effective homogenized parameters of the metamaterial, ideal for design and optimization purposes. The dominant mode of the NIMW is transverse-magnetic, with a large longitudinal component of the electric field that enables its strong interaction with an electron beam and opens new opportunities for microwave generation and radiation detection. The inherent bi-anisotropy of its loaded inclusions is also examined within our TL theoretical framework, and a solution design to suppress its effect is presented. Finally, since the proposed geometry is shown to support an additional transverse-electromagnetic mode due to strong spatial dispersion, proper excitation of the NIMW is discussed and simulated.

Experimental Demonstration of Negative-Index Propagation in a Rectangular Waveguide Loaded With Complementary Split-Ring Resonators

In a recent paper, we studied a metamaterial consisting of stacked metal plates loaded by embedded complementary split-ring resonators (C-SRRs), ideal to support negative-index propagation and reversed Cherenkov radiation. Here, we experimentally verify negative-index propagation in a related structure, a conventional X-band waveguide loaded with a single column of C-SRRs, emulating a unit cell of an effective bulk negative-index metamaterial. Measurements and supporting simulations reveal a passband between 4.2 and 4.6 GHz, with a phase response typical of negative-index propagation.

High-sensitivity chiral molecular sensing with optical metasurfaces

Determination of the absolute handedness of single enantiomer at small quantities is essential in chiral drug developments. Here we provide theoretical insights, numerical analysis and experimental demonstrations of high sensitivity to zepto moles of chiral molecules using optical metasurfaces. We introduce a figure of merit (FoM) for metasurfaces that can be extracted from simple far-field scattering measurements at visible to near-infrared frequencies, establishing the optimal performance to detect molecular handedness. Our results experimentally show opposite signs of FoM for different molecular chirality. The properly designed metamaterial serves as an ultra-sensitive probe to enhance circular dichroism measurements at frequencies for which these signals would be otherwise undetectable.

Electromagnetic wave interaction with 3D arrays of quadrupolar inclusions

In the homogenization of metamaterials, one of the important challenges yet to be fully resolved resides in the proper modeling of the influence of higher-order multipolar terms describing the basic constituent inclusions. As an important step in this direction, we consider here a metamaterial composed of a three-dimensional cubic array of purely quadrupolar inclusions, with zero electric and magnetic dipole moments, so that the leading term in their multipole expansion is the electric quadrupole moment. Applying a first-principle homogenization approach similar to (A. Alù, Phys. Rev. B, 84, 075153, 2011) the spatially dispersive permittivity of such an array in the long-wavelength limit is derived. This tensorial permittivity is a quadratic function of the propagation constant in the medium and it is shown to depend on the propagation direction, as a symptom of strong spatial dispersion.