arXiv subjects

S. Anantha Ramakrishna

Publications and source records attributed to S. Anantha Ramakrishna.

At least 19 recordsLinked to original sources

arXiv2023

Direct Determination of Photonic Stopband Topological Character: A Framework based on Dispersion Measurements

Ascertainment of photonic stopband absolute topological character requires information regarding the Bloch eigenfunction spatial distribution. Consequently, the experimental investigations predominantly restrict themselves to the bulk-boundary correspondence principle and the ensuing emergence of topological surface state. Although capable of establishing the equivalence or inequivalence of bandgaps, the determination of their absolute topological identity remains out of its purview. The alternate method of reflection phase-based identification also provides only contentious improvements owing to the measurement complexities pertaining to the interferometric setups. To circumvent these limitations, we resort to the Kramers-Kronig amplitude-phase causality considerations and propose an experimentally conducive method for bandgap topological character determination directly from the parametric reflectance measurements. Particularly, it has been demonstrated that in case of one-dimensional photonic crystals, polarization-resolved dispersion measurements suffice in qualitatively determining bandgap absolute topological identities. By invoking the translational invariance of the investigated samples, we also define a parameter Differential Effective Mass that encapsulates bandgap topological identities and engenders an experimentally discernible bandgap classifier.

physics.optics↗

arXiv2022

Nanoporous alumina microtubes for metamaterial and plasmonic applications

Double anodization of aluminium microwires in an acid bath yields cylindrical nanoporous alumina with nonbranching radially emanating pores. The obtained nanoporous alumina is a cylindrically anisotropic as well as a radially inhomogeneous optical medium. Detailed structural characterization reveals that the nanopore diameter varies linearly with the radius of the aluminium microwire along the radial direction. Microcracks form on the alumina shell during the anodization when sufficient thickness is formed due to volume expansion and stress accumulation. The formation of the microcracks can be monitored by the anodization current which shows sudden jumps when the cracks are formed. After removing the remaining aluminium at the core of the anodised wire the anisotropic and inhomogeneous alumina microtube is obtained. Such nanoporous alumina microtubes form unique optical waveguides and are useful for microscale heat transfer applications.

physics.app-ph↗

arXiv2021

Clocking the Quantum Sojourn Time: Spurious Scatterings and Correction to the Larmor Clock

We revisit the notions of the quantum-mechanical sojourn time in the context of the quantum clocks to enquire whether the sojourn time be clocked without the clock affecting the dynamics of the wave motion. Upon recognizing that the positivity of conditional sojourn time is not ensured even in the case of physically co-evolving clock mechanisms, we trace its origins to the non-trivial inadvertent scattering arising from the disparity, however weak, engendered by the very clock potential. Specifically, our investigations focus on the Larmor spin rotation-based unitary clock where the alleviation of these unphysical contributions has been achieved by correcting the mathematical apparatus of extracting the sojourn times. The corrections have been obtained for both the spin precession-based and spin alignment-based scenarios. The sojourn times so obtained are found to have proper high- and low-energy limits and turn out to be positive definite for an arbitrary potential. The regimen provided here is general and appeals equivalently for unitary as well as non-unitary clocks where the clock-induced perturbations couple to the system Hamiltonian.

quant-ph↗

arXiv2019

A simple metamaterial absorber associated with Fano-like resonance

A simple metamaterial absorber suitable for fabrication over large areas associated with a Fano like resonance is proposed. The proposed designed of the metamaterial absorber consists of photoresist disk arrays on a silicon substrate followed by the deposition of a tri layer of Au, ZnS and Au on the top of the structure. Due to the tri layer, there is a formation of a cavity between the substrate and gold layer on the top of photoresist disk and a waveguide in the ZnS layer in between the gold layers on the substrate and the ZnS layer itself. The Fano like resonance arises due to the interference of the cavity mode and the guided-mode resonance as well as the Woods anomaly. The cavity mode resonance works as continuum mode whereas the guided mode resonance and Woods anomaly work as discrete modes. The spectral position of the resonance can be tuned by just controlling the thickness of the tri layer instead of the structural size and shape modification of the micro and nano-structures as usually done in conventional metamaterial absorbers. This design can be easily scaled for fabrication over large areas as it separates the structuring and deposition processes and makes them sequential thereby avoiding expensive and complex lift off or etching processes which are usually required for conventional metamaterial processing.

physics.optics↗

arXiv2017

Anisotropic Metamaterial Optical Fibers

Internal physical structure can drastically modify the properties of waveguides: photonic crystal fibers are able to confine light inside a hollow air core by Bragg scattering from a periodic array of holes, while metamaterial loaded waveguides for microwaves can support propagation at frequencies well below cutoff. Anisotropic metamaterials assembled into cylindrically symmetric geometries constitute light-guiding structures that support new kinds of exotic modes. A microtube of anodized nanoporous alumina, with nanopores radially emanating from the inner wall to the outer surface, is a manifestation of such an anisotropic metamaterial optical fiber. The nanopores, when filled with a plasmonic metal such as silver or gold, greatly increase the electromagnetic anisotropy. The modal solutions in anisotropic circular waveguides can be uncommon Bessel functions with imaginary orders.

physics.optics↗

arXiv2016

How long does a quantum particle or wave stay in given region of space?

The delay time associated with a scattering process is one of the most important dynamical aspects in quantum mechanics. A common measure of this is the Wigner delay time based on the group velocity description of a wave-packet, which my easily indicate super-luminal or even negative times of interaction that are unacceptable. Many other measures such as dwell times have been proposed, but also suffer from serious deficiencies, particularly for evanescent waves. One important way of realising a timescale that is causally connected to the spatial region of interest has been to utilize the dynamical evolution of extra degrees of freedom called quantum clocks, such as the spin of an electron in an applied magnetic field or coherent decay or growth of light in an absorptive or amplifying medium placed within the region of interest. Here we provide a review of the several approaches developed to answer the basic question - how much time does a quantum particle (or wave) spend in a specified region of space? While a unique answer still evades us, important progress has been made in understanding the timescales and obtaining positive definite times of interaction by noting that all such clocks are affected by spurious scattering concomitant with the very clock potentials, however, weak they be and by eliminating the spurious scattering.

quant-ph↗

arXiv2016

Resonant enhancement of Raman scattering in metamaterials with hybrid electromagnetic and plasmonic resonances

A tri-layer metamaterial perfect absorber of light, consisting of (Al/ZnS/Al) films with the top aluminium layer patterned as an array of circular disk nanoantennas, is investigated for resonantly enhancing Raman scattering from C-60 fullerene molecules deposited on the metamaterial. The metamaterial is designed to have resonant bands due to plasmonic and electromagnetic resonances at the Raman pump frequency (725 nm) as well as Stokes emission bands. The Raman scattering from C60 on the metamaterial with resonantly matched bands is measured to be enhanced by an order of magnitude more than from C60 on metamaterials with off-resonant absorption bands peaked at 1090 nm. The Raman pump is significantly enhanced due to the resonance with a propagating surface plasmon band, while the highly impedance matched electromagnetic resonance is expected to couple out the Raman emission efficiently. The nature and hybridization of the plasmonic and electromagnetic resonances to form compound resonances are investigated by numerical simulations.

physics.optics↗

arXiv2015

Thermally Induced Nonlinear Optical Absorption in Metamaterial Perfect Absorbers

A metamaterial perfect absorber consisting of a tri-layer (Al/ZnS/Al) metal-dielectric-metal system with top aluminium nano-disks is fabricated by laser-interference lithography and lift-off processing. The metamaterial absorber had peak resonant absorbance at 1090 nm and showed nonlinear absorption for 600ps laser pulses at 1064 nm wavelength. A nonlinear saturation of reflectance was measured to be dependent on the average laser power incident and not the peak laser intensity. The nonlinear behaviour is shown to arise from the heating due to the absorbed radiation and photo-thermal changes in the dielectric properties of aluminium. The metamaterial absorber is seen to be damage resistant at large laser intensities of 25 MW/cm2.

physics.optics↗

arXiv2014

Asymmetric coupling and dispersion of surface-plasmon-polariton waves on a periodically patterned anisotropic metal film

The morphology of a columnar thin film (CTF) of silver renders it an effectively biaxially anisotropic continuum. CTFs of silver deposited on one-dimensional gratings of photoresist showed strong blazing action and asymmetrically coupled optical radiation to surface plasmon-polariton (SPP) waves propagating only along one direction supported by either the CTF/photoresist or the CTF/air interfaces. Homogenization of the CTFs using the Bruggeman formalism revealed them to display hyperbolic dispersion, and the dispersion of SPP waves was adequately described thereby.

physics.optics↗

arXiv2012

Focussing light in a bi-anisotropic slab with negatively refracting materials

We investigate the electromagnetic response of a pair of complementary bi-anisotropic media, which consist of a medium with positive refractive index ($+\ep$, $+μ$, $+ξ$) and a medium with negative refractive index($-\ep$, $-μ$, $-ξ$). We show that this idealized system has peculiar imaging properties in that it reproduces images of a source, in principle, with unlimited resolution. We then consider an infinite array of line sources regularly spaced in a one-dimensional photonic crystal (PC) consisting of 2n-layers of bi-anisotropic complementary media. Using coordinate transformation, we map this system into 2D corner chiral lenses of 2n heterogeneous anisotropic complementary media sharing a vertex, within which light circles around closed trajectories. Alternatively, one can consider corner lenses with homogeneous isotropic media and map them onto one dimensional PCs with heterogeneous bi-anisotropic layers. Interestingly, such complementary media are described by scalar, or matrix valued, sign-shifting parameters, which satisfy a generalized lens theorem, which can be derived using Fourier series solutions of the Maxwell's equations (in the former case), or from space-time symmetry arguments (in the latter case). Also of interest are 2D periodic checkerboards alternating rectangular cells of complementary media which are such that one point source in one cell gives rise to an infinite set of images with an image in every other cell. Such checkerboards can be mapped into a class of three-dimensional corner lenses of complementary bi-anisotropic media.

physics.optics↗

arXiv2012

Periodically patterned columnar thin films as Blazed gratings

Periodically patterned columnar thin films (PP-CTFs) were made by evaporating CaF2 and directing the vapor flux obliquely towards lithographically fabricated micrometer/sub-micrometer gratings. The growth of the PP-CTFs was controlled by the deposition rate to form prismatic air cavities within them and they function like blazed diffraction gratings with asymmetric diffraction patterns and diffraction efficiencies upto 52% in transmission at visible wavelengths. Scalar diffraction theory qualitatively explained the measured diffraction efficiencies.

physics.optics↗

arXiv2012

Interface Induced Anisotropy and nematic glass/gel state in Jammed Aqueous Laponite Suspensions

Aqueous suspensions of Laponite, a system composed of disk-like nanoparticles, are found to develop optical birefringence over several days, well after the suspensions solidified due to jamming. The optical anisotropy is particularly enhanced near the air Laponite suspension interface over length-scales of several millimetres, which is beyond five orders of magnitude larger than the particle length scale, suggestive of large scale ordering influenced by the interface. The orientational order increases with time and is always greater for higher concentration of salt, higher concentration of Laponite and higher temperature of the suspension. While weakly birefringent, Laponite suspensions covered by paraffin oil do not show any enhancement in the optical anisotropy near the interface compared to that in the bulk. We suggest that the expedited structure formation near the air interface propagating progressively inside the sample is responsible for the observed behaviour. We discuss the observed nematic ordering in the context of glasslike and gellike microstructure associated with aqueous Laponite suspensions.

cond-mat.soft↗

arXiv2011

Plasmonic interaction of light with negative index and gold nano-checkerboards

Negative refractive index materials (NRIM) make possible unique effects such as a convergent at lens due to the reversed Snell-Descartes laws of refraction. NRIM are also known to be able to support a host of surface plasmon states for both polarizations of light which are responsible for the sub-wavelength image resolution achieved by a slab of NRIM. A generalized lens theorem provides us with a class of spatially varying slab lenses satisfying the prerequisite symmetries to fold the optical space onto itself. This theorem can be derived using powerful tools of transformational optics. A paradigm of such complementary media are checkerboards consisting of alternating cells of positive and negative refractive index that represent a very singular situation in which the local density of modes at the corners are enormously enhanced. We have considered several theoretical and numerical aspects of such structured films including a finite slabs of multi-scale checkerboards of NRIM satisfying the generalized lens theorem which are host of strongly enhanced electromagnetic feld. Such checkerboards can be mapped using transformational optics onto three-dimensional corner lenses consisting of semi-infinite heterogeneous anisotropic regions of space satisfying the generalized lens theorem. It is also possible to design three-dimensional checkerboards of complementary media, the only restriction being that corresponding unfolded structures in the plane are constrained by the four color theorem. Some of these meta-surfaces in the plane display thin bridges of complementary media, and this further enhances their plasmonic response. Since plasmonic metals mimic the behaviour of NRIM at small length scales, opaque gold films structured at sub-micron scales in a checkerboard fashion were fabricated using focussed-ion-beam technologies and their scattering spectra measured.

physics.optics↗

arXiv2010

Plasmonic space folding: focussing surface plasmons via negative refraction in complementary media

We extend designs of perfect lenses to the focussing of surface plasmon polaritons (SPPs) propagating at the interface between two anisotropic media of opposite permittivity sign. We identify the role played by the components of anisotropic and heterogeneous tensors of permittivity and permeability, deduced from a coordinate transformation, in the dispersion relation governing propagation of SPPs. We illustrate our theory with three-dimensional finite element computations for focussing of SPPs by perfect flat and cylindrical lenses.

physics.optics↗

arXiv2010

Controlling Metamaterial Resonances with Light

We investigate the use of coherent optical fields as a means of dynamically controlling the resonant behaviour of a variety of composite metamaterials, wherein the metamaterial structures are embedded in a dispersive dielectric medium. Control and switching is implemented by coherently driving the resonant permittivity of the embedding medium by applied optical radiation. The effect of embedding Split ring resonators (SRR) in a frequency- dispersive medium with Lorentz-like dispersion or with dispersion engineered by electromagnetic induced transparency (EIT), is manifested in the splitting of the negative permeability band, the modified (frequency-dependent) filling fractions and dissipation factors. The modified material parameters are strongly linked to the resonant frequencies of the medium, while for an embedding medium exhibiting EIT, also to the strength and detuning of the control field. The robustness of control against the deleterious influence of dissipation associated with the metallic structures as well as the inhomogeneous broadening due to structural imperfections is demonstrated. Studies on plasmonic metamaterials that consist of metallic nanorods arranged in loops and exhibit a collective magnetic response at optical frequencies are presented. Control and switching in this class of plasmonic nanorod metamaterials is shown to be possible, for example, by embedding these arrays in a Raman active liquid like CS$_2$ and utilizing the Inverse Raman Effect.

physics.optics↗

arXiv2009

Switching a plasma-like metamaterial via embedded resonant atoms exhibiting electromagnetically induced transparency

We theoretically demonstrate control of the plasma-like effective response of a metamaterial composed of aligned metallic nanorods when the electric field of the incident radiation is parallel to the nanorods. By embedding this metamaterial in a coherent atomic/molecular medium, for example silver nanorod arrays submerged in sodium vapor, we can make the metamaterial transmittive in the forbidden frequency region below its plasma frequency. This phenomenon is enabled by having Lorentz absorbers or other coherent processes like stimulated Raman absorption in the background medium which provide a large positive dielectric permittivity in the vicinity of the resonance, thereby rendering the effective permittivity positive. In particular, processes such as electromagnetically induced transparency are shown to provide additional control to switch and tune the new transmission bands.

physics.optics↗

arXiv2009

Spectral shifts in the properties of a periodic multilayered stack due to isotropic chiral layers

Investigating the canonical problem of a periodic multilayered stack containing isotropic chiral layers, we homogenized it as a uniaxial bianisotropic medium and derived its effective constitutive parameters. The stack shows a resonant behavior, when its unit cell consists of a metallic layer and an isotropic chiral layer. The presence of isotropic chirality can result in small shifts of the resonance frequency for reasonably large values of the chirality parameter, implying that the sign of an effective permittivity can be switched. Such spectral shifts in the dielectric properties can be potentially useful for spectroscopic purposes.

physics.optics↗

arXiv2008

Traversal of pulses through negative ($\varepsilon$, $μ$) materials

We study the traversal times of electromagnetic pulses across dispersive media with negative dielectric permittivity ($\varepsilon$) and magnetic permeability ($μ$) parameters. First we investigate the transport of optical pulses through an electrical plasma and a negative refractive index medium (NRM) of infinite and semi-infinite extents where no resonant effects come into play. The total delay time of the pulse constitutes of the group delay time and the reshaping delay time as analyzed by Peatross et al \cite{peatross}. For evanescent waves, even with broadband width, the total delay time is negative for an infinite medium whereas it is positive for the semi-infinite case. Evidence of the Hartman effect is seen for small propagation distance compared to the free space pulse length. The reshaping delay mostly dominates the total delay time in NRM whereas it vanishes when $\varepsilon(ω)=μ(ω)$. Next we present results on the propagation times through a dispersive slab. While both large bandwidth and large dissipation have similar effects in smoothening out the resonant features that appear due to Fabry-Pérot resonances, large dissipation can result in very small or even negative traversal times near the resonant frequencies. We investigate the traversal and the Wigner delay times for obliquely incident pulses. The coupling of evanescent waves to slab plasmon polariton modes results in large traversal times at the resonant conditions. We also find that the group velocity mainly contributes to the delay time for pulse propagating across a slab with refractive index (n) = -1. The traversal times are positive and subluminal for pulses with sufficiently large bandwidths.

cond-mat.mtrl-sci↗