Spectroscopic investigations of topologically distinct photonic stopbands:
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Abstract: Topological band theory provides a framework to establish the equivalence/inequivalence of bandgaps in photonic topological insulators. However, experimental discernment of bandgap topological characteristics encounters inherent measurement complexities, particularly beyond the terahertz frequencies. To surmount this difficulty, we resort to the prolific optical technique of spectroscopic ellipsometry and carry out detailed experimental examination of attributes of one-dimensional photonic crystal stopbands and, in consequence, identify an appropriate classifier of the implicit topological characteristics. It is found that governed by the bulk topology, the band edge locations in the dispersion diagram provide a conditional site for the appearance of zeros of a complex reflection ratio. This leads to a selective appearance of topologically robust phase singularities with integer (unity positive) topological charge. We demonstrate that the presence of these phase singularities on either the blue or the red band edges of the stopbands provides us with an experimental marker of their distinctive topological characteristics.
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Switchable thermal emission from electro-optically induced topological phase transitions:
Abstract: Explorations into the photonic analogs of topological materials have garnered significant research interest due to their application potential. Particularly in planar systems, the prospects of engendering extinguishable topological states can have wide-ranging implications. With an objective of employing these concepts for thermal emission engineering, here, we design and numerically investigate a quasi-monochromatic highly directional mid-infrared source elicited from inversion symmetry-protected topological interface states. Notably, by relying on the architecture of electro-optic effect-induced topological phase transitions, we introduce the possibility of ultrafast switching of thermal radiation. These reversible phase transitions, being free from carrier transport are inherently fast and evoke thermal emission modulation with a modulation depth upto 0.99. Specifically, our platform exhibits a near-perfect extinguishable spectral emission peak at 4 μm with a quality factor of over 18500, displaying negligible parasitic emissions. Furthermore, the optimized interface state manifests itself for only one of the polarization modes, resulting in polarized emission under resonance conditions. To establish a methodical approach to parameter optimization, we also model our platform as a leaky mode resonator using the framework of temporal coupled-mode theory. We believe, our findings can provide a way forward in establishing complete control over the optical characteristics of the infrared thermal emitters.
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Optical biosensing with strongly coupled modes of a plasmonic-photonic trimer :
Abstract: A lithography-free plasmonic–photonic hybrid nanostructure exhibiting an interesting phenomenon of cavity-mediated normal-mode splitting among doubly-degenerate Tamm plasmon polariton modes has been designed and optimized to manifest three strongly coupled modes. The exotic dispersion of these supermodes is used to design a self-referenced spectroscopic refractive index sensor at optical frequencies with a substantial sensitivity value of 1410 nm RIU−1. The same structure is also shown to function as a singular-phase-based refractometric biosensing platform with multiple near-singular points, exhibiting a maximum sensitivity of around 27 000∘ RIU−1 with a sufficiently broad dynamic range of operation. Furthermore, the presence of three near-singular points provides the necessary flexibility in striking an appropriate balance between sensitivity and dynamic range of operation. The concomitant existence of the mentioned functionalities is an outcome of the strong coupling between the modes, which enables us to exhibit exquisite control over the dispersion of the supermodes. These distinctions enable our proposal to be of direct utility in highly demanding point-of-care biosensing applications.
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Surface plasmon coupling for selectively enhanced random lasing:
Abstract: Periodically patterned sculptured plasmonic thin films, consisting of forests of nanocolumns of metals like silver on a periodic grating, offer a very rich landscape for light–matter interactions. Multiple light scattering, plasmonic resonances, anisotropy, hyperbolic dispersions, and Bragg scattering: a plethora of effects come together in these systems to offer various possibilities. We realize an efficient random laser by infiltrating a laser dye into such a grating of silver nanocolumns and optically pumping the system. The densely packed plasmonic nanocolumns provide feedback through efficient scattering, while the optically pumped dye solution in the voids provides for amplification. The periodicity and anisotropy provide for an angle and polarization selective enhanced coupling of the pump laser via the propagating surface plasmon resonances in the system.
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Switchable thermal emission from electro-optically induced topological phase transitions:
Abstract: Super-resolution microscopy refers to a powerful set of imaging techniques that overcome the diffraction limit. Some of these techniques, the importance of which was recognized by the 2014 Nobel Prize for chemistry, are based on the concept of image reconstruction by spatially sparse sampling. Here, we introduce the concept of super-resolution spectroscopy based on sparse sampling in the frequency domain, and show that this can be naturally achieved using a random laser source. In its chaotic regime, the emission spectrum of a random laser features sharp spikes at uncorrelated frequencies that are sparsely distributed over the emission bandwidth. These narrow lasing modes probe stochastically the spectral response of a sample, allowing it to be reconstructed with a resolution exceeding that of the spectrometer. We envision that the proposed technique will inspire a new generation of simple, cheap, high-resolution spectroscopy tools with a reduced footprint.
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Sensor based on gold-coated nanoporous anodic alumina membrane:
Abstract: We report a refractive index sensor consisting of a gold-coated nanoporous anodic alumina membrane on aluminium substrate that can distinguish between different kinds of alcohols such as methanol and ethanol due to their different refractive indices. The nanoporous volume allows the loading of liquids with low surface energy into its nanopores. Upon dipping one end of the membrane into the alcohol, the entire nanoporous surface experiences wetting. The wavelength shift of the Fabry–Perot resonating modes formed between the gold-coated nanoporous alumina surface and aluminium on the other side due to the changed effective refractive index form the basis of the sensor. The sensitivity of the nanosensor to the refractive index of the loaded liquid is sufficient to distinguish between different alcohols such as methanol, ethanol and isopropanol, and to detect about 5–10% of methanol in a methanol–ethanol mixture.
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Remote control of liquid crystal elastomer random laser:
Abstract: We present a distinct design for a random laser based on a composite material consisting of an elastomeric liquid crystal with embedded TiO2 nanoparticles. Random lasing action can be controlled by an external, non-contact light stimulus; this induces a rearrangement of the elastomeric liquid crystals which moves the laser body in and out of the focal plane of a pump laser, pushing its emission above or below the lasing threshold.
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