Session L48: Artificially Structured Materials II: Optical Phenomena and Plasmons

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The interaction between the magnetization vector and the optical properties is described by the off-diagonal components of the dielectric tensor, which depends on the magnetization orientation. Here, we present our study of magneto-optical anisotropy (MOA) in Fe_x/Cu_x superlattice, where x is the number of atomic-layers, based on first-principle study. Calculations were carried out by using a full-potential linearized augmented plane wave (FLAPW) method with the generalized gradient approximation.[1] The optical conductivities tensor (or dielectric tensor) were estimated by the Kubo formula. The anisotropy of the absorptive parts of the off-diagonal optical conductivity tensor is determined in the photon energy range 0-8 eV. As a result, we observed that MOA was highest in Fe₂Cu₂ due to the orbital character of the partial density of d state significantly different by the change in the magnetization direction. The band-by-band decomposition of the MOA in Fe₁Cu₁, where the interband transitions are responsible for the MOA spectra around 1 eV, 2 eV, 3.5 eV, and 6 eV, is discussed. On the other hand, at 1.96 eV, magneto-optical constant Q increases in proportion to the value of x in transversal Kerr geometry.
Keywords: magneto-optical anisotropy, first-principles study, superlattice   

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The carrier recombination and localization in near strain-balanced InGaN/AlGaN superlattices grown by plasma-assisted molecular-beam epitaxy was investigated by photoluminescence (PL) spectroscopy. Time-resolved PL shows a recombination lifetime of ~0.3ns, much shorter than that in conventional polar c-plane InGaN/(Al)GaN quantum wells. The temperature dependence of PL energies is not monotonic and experimental PL peak energies are lower than inter-band transition energies calculated using structural parameters obtained from high-resolution x-ray diffraction. This indicates the existence of In composition fluctuation. Sites with high In composition act as localization centers. Time-resolved PL in magnetic field indicates that radiative recombination mainly takes place in localization centers. The localization depth is found to be ~13meV, which is much lower than previously reported for m-plane InGaN/GaN quantum wells. Shallow localization potential contributes to narrower transition peak, but reduces the isolation from non-radiative recombination centers. Rapid drop of PL intensity as temperature increases indicates strong non-radiative recombination at room temperature.   

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Recently, the bound states in the continuum attracted a lot of interest due to potential applications in modern laser, quantum, and nanoscale technologies, including quantum computing and information processing. A few examples of such extremely long-lived bound states have been found and studied, mostly via numerical methods, in optoelectronics, condensed matter physics, and material science. Many efforts have been devoted to new design-optimization numerical schemes. Remarkably, the optimized structures look somewhat disordered and could possibly never have been guessed on the basis of the existing analytic results and intuition. Here, we apply a powerful analytic theory of mode coupling in optical gratings, that we proposed in Phys. Rev. A 100, 053854 (2019), to reveal the nature and properties of such bound states in photonic crystals. The results based on such an analytic, as opposed to purely numeric, approach could greatly facilitate finding and designing the optimal structures for bound states.   

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Resonant dipole-dipole interactions (RDDI) govern phenomena such as Van der Waals forces, Casimir-Polder forces, and Forster resonant energy transfer. In the limit of many dipoles, it is known that the fluorescent decay profile of donor molecules changes from a single-exponential to a stretched-exponential decay trace due to resonant dipole-dipole interactions with acceptor molecules. While the stretched exponential profile, I(t) = I₀exp(-γ₀t-αt^1/2), is well-known to 3D geometries, the fluorescence decay dynamics in more complicated nanophotonic environments remain largely unexplored. To this end, we present a systematic study of the fluorescent decay profile of donor (Alq3) and acceptor (R6G) molecules spin-coated on a plasmonic metasurface consisting of 2D silver nanoparticle lattice. We observe modified fluorescence decay traces on the plasmonic meta-surface compared to conventional control samples such as glass or silver substrates. Our work sheds light on the origin of fluorescent decay dynamics and shows that they provide a way of uncovering the dimensionality of dipole-dipole interactions modified by a nanophotonic environment.   

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Nanolayered quantum wells (QWs) composed of III-V semiconductors present unexplored opportunities to engineer χ⁽²⁾ (e.g. for electro-optic and quantum information applications) by optimizing thickness, separation, and shape of individual QW layers, and the number N of repeated layers. A digital alloys growth technique was used, which avoids phase segregation that often plagues III-V alloy growth, and allows nano-structuring to enhance χ⁽²⁾. Second harmonic generation (SHG) was used to probe the total nonlinear χ⁽²⁾ response of a series of N multiple-QW (MQW) layers made up of InAs QWs and AlSb barriers, sandwiched in between a GaSb oxidation cap and GaSb buffer layer, all based on a GaSb substrate. Spectroscopic ellipsometry was used to model an effective medium describing the MQWs and extract optical constants and thicknesses. Then, these supplied the nonlinear transfer matrix formalism employed to model the SHG signal as a coherent superposition of a variable-N MQW layer with fixed substrate and cap layer SH polarizations. Experimental results reveal up to 25x stronger SHG ∝ (χ⁽²⁾)² from MQW structures compared to a GaSb substrate. The model attributes this enhancement to geometric and interference effects in the MQW structures.   

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Erbium arsenide (ErAs) is a semi-metallic compound that can be epitaxially grown and integrated to III-V semiconductors such as GaAs and InGaAs, etc., to generate tunable optoelectronic properties, for example, the ultrafast photo-carrier lifetimes. [1] However, the wider bandgap of GaAs limits the application of ErAs/GaAs in 1550 nm. In this study, we present different ways to integrate ErAs and GaAs and utilize the absorption within the GaAs bandgap to realize ultrafast photo-response at 1550 nm. The infrared absorption and 1550 nm ultrafast photo-response in epitaxial ErAs/GaAs systems are studied by Fourier transform infrared (FTIR) spectroscopy and time-resolved pump-probe technique. The sub-bandgap absorption in ErAs/GaAs has been observed in all the samples and can be tuned by the ErAs incorporation. The ultrafast photo-induced carrier lifetime at 1550 nm is measured to be as low as 200 fs, and the relaxation mechanism is discussed in detail. This material system is promising for 1550-nm-technology-compatible THz sources with high breakdown-voltages.
[1] M. Griebel, J. H. Smet, D. C. Driscoll, J. Kuhl, C. A. Diez, N. Freytag, C. Kadow, A. C. Gossard, K. Von Klitzing, Nat. Mater. 2003, 2, 122.   

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We demonstrate how various fabrication methods of amorphous silicon can drastically modify the ultrafast optical properties of dielectric metasurfaces. Silicon thin films were made using either electron beam evaporation or plasma-enhanced chemical vapor deposition. The fabricated samples were fully characterized optically and structurally via ellipsometry and X-ray diffraction respectively. Moreover, we use linear and ultrafast nonlinear spectroscopy to understand the effect of two-photon absorption, free carrier relaxation, and lattice heating on the overall optical response of our various samples. To confirm results of our ultrafast optical experiments, we modelled the dynamics using a differential model coupled to a full-field finite-difference time-domain electromagnetic solver that accurately reproduces our experimental observations.   

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Broadband cloaking can be achieved via scattering suppression in the visible (VIS) and near infra-red (NIR) regions of the electromagnetic spectrum. Scattering suppression is possible by leveraging the plasmonic properties of the metallic nanoparticles (NPs). We have developed a model¹ to calculate the efficiency of scattered light when a spherical homogenous bare core is covered by a shell comprising a randomly-ordered collection of metallic NPs. Multiple scattering theory of Foldy and Lax was applied to calculate the interaction of light with the metallic NPs, in combination with the method of fundamental solutions. Numerical results demonstrated significant scattering suppression of up to 50% from the core-shell composite over a broad visible spectral range (400 nm-600 nm). The core sizes for the numerical experiments were varied between 400 nm to 900 nm along with an appropriate combination of NP sizes and NP filling fractions in the shell.   

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Crossing a percolation threshold radically changes the response of a structure, including optical spectra such as transmittance. Understanding and, in some cases, predicting the critical nature of the threshold point provides novel insights into the capabilities and applications of engineering particular structures. Here, we investigate the self-complimentary, plasmonic checkerboard structure and the relation to its percolation effects. As seen through finite element simulated transmittance and electromagnetic fields, direct control of extraordinary, resonant transmittance is attained by tuning of simple geometrical structure parameters, while also exploiting extreme sensitivity due to the proximity of the percolation threshold. We supply evidence that devices fabricated from these structures yield potential possibilities as parametrically tuned sensors.   

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Magneto-optical coupling incorporates photon-induced change of magnetic polarization that can be adopted in ultrafast switching, optical isolators, mode convertors, and optical data storage for advanced optical integrated circuits. But integration of plasmonic, magnetic and dielectric properties in one single material system is challenging. We use a bottom-up self-assembling synthesis method to integrate multifunctional phases as a nanopillar-in-matrix thin film heterostructure that realizes epitaxial quality, sharp atomic interface and large throughput. Using titanium nitride (TiN) as a durable plasmonic matrix, a metal-free metamaterial platform with embedded nickel oxide (NiO) vertical nanorods that function as tunable ferromagnetic nanodomains has been demonstrated. Such a dissimilar ceramic-ceramic combination enables a strong hyperbolic dispersion in the visible and near infrared frequencies. More interestingly, when Au is introduced in the TiN-NiO heterostructure, a hybrid core-shell nanopillar array is formed where the two-monolayer Au shell serves to release the strain energy at the TiN/NiO interface. We demonstrate a significantly enhanced long-range ordering of the core-shell nanopillars which enables a stronger Kerr anisotropy.   

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Hybrid organic-inorganic perovskite (HOIP) becomes attractive due to its high refractive index and excellent electrical properties. Recent demands of ultrathin materials for photoelectric battery and skin-like sensors especially require high optical absorption, which is difficult for conventional homogeneous thin films. In this work, by introducing nanohole arrays on HOIP ultrathin film, we achieve all-dielectric HOIP metasurfaces with broadband optical absorption enhancement. Based on the finite-difference time-domain method, we show that Mie resonances occur in the HOIP metasurfaces, which depend on the surface structure, including the nanohole geometry and the spatial periodicity. Consequently, the absorption can be tuned by the nanohole arrays on the HOIP film. Compared with the flat HOIP film, significant absorption occurs in the HOIP metasurface over entire visible regime. An assembly of nanoholes are designed and we achieve broadband absorption from 51% to 87% in the wavelength range of 400-770nm, and the external quantum efficiency of the solar cell with this HOIP metasurface can be 44% higher than that of a flat HOIP film. We suggest that these results are inspiring for effective light trapping and efficient photoelectric conversion with ultrathin HOIP metasurfaces.   

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Surface plasmon polaritons (SPPs) are hybrid modes of light and electrons on conducting surfaces. Analogous to Bloch electrons in crystals, SPPs traveling in a medium with spatially varied properties experience scattering and interference, which transforms SPPs into Bloch polaritons. Bloch polaritons possess the universal attributes of Bloch quasiparticles whose energy-momentum dispersion is dictated by the underlying band structure. It is therefore feasible to reconstruct the polariton bands structure by exploring the real space motion of ballistic Bloch wavepackets. We visualized the band structure of Bloch polaritons in a graphene-based polariton crystal by utilizing near-field imaging techniques to record the propagation of Bloch polaritons launched by nano-structured antennas. Bloch polaritons emanating from a point antenna reveal star-shaped patterns which encode information for the underlying polariton band structure. Fourier analysis of the near-field images reveals the isofrequency contours of the polariton bands which can subsequently be stacked to form a complete band structure. As the energy bands in graphene polariton crystals are tunable by gate voltage, we were able to switch on demand the propagation direction of Bloch polaritons and make a polariton switch.   

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We report on the surface plasmon excitations in the multicoaxial, negative-index metamaterials (NIM) cylindrical canles -- using an elegant response function theory, which does not require matching of the messy boundary conditions. The multicoaxial cables with {\em dispersive} metamaterial components exhibit rather richer (and complex) plasmon spectrum with each interface supporting two modes: one TM and the other TE for (the integer order of the Bessel function) $m \ne 0$. The cables with {\em nondispersive} metamaterial components bear a different tale: they do not support simultaneously both TM and TE modes over the whole range of propagation vector. The computed local
and total density of states enable us to substantiate spatial positions of the modes in the spectrum. Such quasi-one dimensional systems as studied here should prove to be the milestones of the emerging optoelectronics and telecommunications systems. {see, for instances,
J. Opt. Soc. Am. B {\bf 27}, 148 (2010); {\bf 27}, 605 (2010). }