Session A12: Nanostructures and Metamaterials 1

Ultrafast and Nonlinear Nanoscopy

Combining plasmonic and optical antenna concepts with ultrafast and shaped laser pulses allows for the precise control of an optical excitation on femtosecond time scales, nanometer length scales, and into the strong coupling regime. I will discuss novel phenomena regarding the induced near-field light-matter interaction in terms of impedance matching to a quantum system for nano-spectroscopy and -imaging. This includes adiabatic nano-focusing on a tip for coherent spatio-temporal imaging with optical control with 10 nm spatial and 10 fs temporal resolution addressing competition and manipulation of optical coherences. I will extent into the behavior of ultra-small mode volume cavities and field localization giving rise to a new regime of nonlinear nano-optics with gradient-field induced phase matching for exceptionally efficient frequency conversion. Lastly I will present new insight into the enhanced nonlinear optical interaction on the nanoscale of highly efficient localized four-wave mixing in graphene.   

Photon emission through spontaneous parametric downconversion in plasmonic and metamaterial structures

Quantum nonlinear optics provides a robust platform for generating single and entangled photons – critical components of quantum information processing. Typically, the weak nonlinear interactions and high dispersion of homogeneous materials limit nonlinear optical processes for potential on-chip integration. In this work we study use of nanoplasmonic and metamaterial structures for enhancement and control of spontaneous quantum emission through nonlinear parametric downconversion. We develop a theoretical formalism based on eigenmode decomposition accounting for material dispersion, losses and anisotropy. We show that by engineering the optical dispersion in plasmonic waveguides and hyperbolic metamaterials, phase matching, efficient mode mixing, and large density of optical states may be achieved simultaneously to enable enhanced spontaneous quantum photon emission. We study several possible geometries using real material parameters and show that photon emission rates compact nanometer sized structures are comparable to those attainable in bulk crystals.
  

A Wave-based Approach to Solve Linear Integral Equations

Metamaterials have been explored as an interesting tool to transfer the well-known computational capabilities of electromagnetic waves into the realm of nano-photonics [Silva, Monticone, Castaldi, Galdi, Alu, and Engheta, Science 343, 6167 (2014)]. Along with performing the basic mathematical tasks such as differentiation and integration on the spatial profile of the wave, here we propose a more desired functionality for wave-based computation, namely solving integral equations. Our proposal is based on creating a feedback path for the wave, comprising the kernel integral operator. As the wave travels along this path, the solution to the integral equation is generated in the steady state. An implementable configuration is proposed including the mechanisms to excite and probe the system and the final output. The arbitrary kernel operator is designed in the form of a medium with inhomogeneous permittivity, and single-mode waveguides are employed as feedback lines. We will also discuss the potential advantages of this approach in terms of power consumption, capacity, and speed.   

Quantum Nonlocal Effects on the THz Absorption in Metallic Films

The quantum effects of spatial dispersion on the optical properties of a flat metallic film are studied within the Kubo formalism in the terahertz frequency range. As is shown, the results are qualitatively different from those obtained within either the model of the Boltzmann kinetic equation or the Drude-Lorentz model. Specifically, in sufficiently thin metallic slabs, the quantum resonances emerge and, as a result, significantly alter the absorption spectrum provided by the Landau damping. The latter is an inevitable property of the nonlocal electromagnetic response of conduction electrons. The absorption dependence on the electron relaxation rate, Fermi velocity, thickness of metallic slab and other parameters of the problem have also been analyzed. The predictions of the developed here quantum approach coincide with those of the Boltzmann kinetic equation when the thickness of the film is much greater than the metal skin depth. Within the classical local limit our results correspond to the Drude-Lorentz model.   

Visible to infrared tunable plasmonic nanoantennae via directed assembly

Radio- and microwave antennae are fundamental building blocks in modern technologies. As the miniaturization of devices continues down to nanometer length scales, the need to sense and signal using nanoantennae has become vital to enabling disruptive nanotechnologies. Yet, to-date, the widespread use of nanoantennae has been limited due in part to the inability to create these structures in large, uniform quantities with nanoscale precision. Here, we present a generalized approach to solve this longstanding issue by ‘gluing’ small aspect ratio gold nanorods together end to end with molecular linkers forming linear chains and ‘welding’ the chains together upon exposure to femtosecond laser pulses, producing trillions of infrared plasmonic nanoantennae per minute. We report experimental tuning ranges of over 2,000 nm from visible to infrared wavelengths. We also experimentally show that the nanoantenna absorption peak depends linearly on the chain length, in agreement with simulations, providing a straightforward means to understand and predict the infrared response of these materials.   

Controlling valley excitons in monolayer MoS2 with a metasurface

Excitons in monolayer transition metal dichalcogenides (TMDs) are formed at K and K’ points at the boundary of the Brillouin zone and inherent valley degree of freedoms. Using circularly polarized excitation lasers, excitons can be created in a specific valley. In MoS₂, it was found that the photoluminescence preserves the circular polarization at low temperature but not at room temperature, suggesting that the exciton valley depolarization occurs on longer or similar time scale in comparison to exciton recombination times at low temperature. Here, we demonstrate that by placing monolayer MoS₂ onto a metasurface, valley polarized exciton can couple to surface plasmon polaritons and be spatially separate even at room temperature. In addition, we found that emission from excitons in opposite valley can be separated in K space, i.e., excitons from opposite valleys emit photons to different directions.   

Topological photonic crystals in the visible: design and angle-resolved characterization of the bulk and edge states

We fabricated and characterized photonic crystals with non-trivial topological bulk states and pseudo-time-reversal-symmetry protected helical edge states in the visible regime. Hexagonal silicon photonic crystals with a lattice constant of 450nm were fabricated on 10nm Si₃N₄ membranes utilizing electron beam lithography and reactive ion etching. With a 30-keV electron beam, we excite coherent cathodoluminescence in the nanostructured material and derive photonic band structures based on angles as well as frequencies of the emitted light. Further investigation into polarization states of the emitted light with both plane wave excitation and electron beam excitation reveal edge states that are pseudo-spin orbit coupled.   

Hybrid plasmonic-photonic lasing from zinc doped GaAs nanowires

We investigated optically pumped lasing from zinc-doped GaAs nanowires (NWs) on glass and metal films as well as from gold coated NWs. NWs on glass with a diameter of 250 nm and an 8 nm Al₂O₃ coating showed photonic lasing at 80 K when the excitation fluence exceeded 53 μJ/cm². NWs with the same diameter on gold or silver exhibited enhanced lasing and a reduced lasing threshold which is attributed to the increased light confinement and gain feedback of hybrid plasmonic-photonic lasing modes. With increasing temperature, we observe a red-shift and weakening of the laser emission due to band-gap shrinkage and increasing non-radiative losses. While NWs on metal show lasing up to room temperature, NWs on glass stop emitting at ~200 K which is again attributed to the plasmon enhanced light confinement. NWs with diameters of 200 nm and 100 nm diminish the probability for photonic lasing due to increased losses of their sub-wavelength photonic cavity. When put on gold or silver these NWs showed lasing which is predominantly plasmonic. Gold coated NWs show weaker plasmonic lasing because of the reduced transmittance of the exciting laser and the laser emission through the gold film but, the lasing threshold was nearly not affected by the metal coating.   

Metasurfaces on Metamaterials: Surface Modes Induce Linear Dichroism in Gyroid Optical Metamaterials

Optical metamaterials offer the possibility of accessing extraordinary optical properties depending on the arrangement of sub-wavelength structural units. Gyroid-structured optical metamaterials possess a chiral, cubic, and triply-periodic bulk morphology, and exhibit both a circular and linear dichroism, the latter of which is not yet well understood. We have found a strong correlation between the symmetry of the gyroid surface and its polarization-dependent reflectivitiy. Depending on the cutting-plane, the surface terminations break the cubic symmetry of the gyroid, giving rise to the observed dichroism. Our results show that incident light couples into not only localized and propagating plasmonic modes but also nanocavities that form for specific cutting-planes, giving rise to the strong optical anisoptropy. This pronounced surface sensitivity of nanostructured metamaterials has significant consequences for both the design and application of optical metamaterials.   

Novel Superchiral Light Generation in Achiral Resonant Systems

In this work we introduce a novel superchiral light generation mechanism based on degenerate achiral systems. The theoretical model employs an electromagnetic rotating dipole to obtain a close form spatial chiral near-field distribution equation. This physical model predicts the solution of single-handed chiral near-field generation in the entire volume of interest with full handedness reversal based solely on the excitation condition. These physical predictions are further observed on a degenerate cavity-coupled achiral plasmonic substrate supporting a fundamental localized surface plasmon (LSP) displaying a dipolar resonance. FDTD simulations demonstrate the predicted superchiral near field generation mechanism with zero far-field circular dichroism signal on the same achiral substrate. Finally, surface enhanced vibrational circular dichroism is demonstrated on two chiral enantiomers on the same substrate showing up to four orders of magnitude enhancement over its volumetric counterpart. These results will promote further research in superchiral near-field generation and surface enhanced chiral light-matter interactions.   

Triply-Resonant Plasmonic Metasurfaces for Multimodal Enhancement

Multimodal plasmonic structures offer promise for applications such as fluorescence enhancement, wavelength multiplexing, and enhanced nonlinear generation. However, these structures may be leveraged by exploiting the high field enhancements present in film-coupled systems. Here, we present designs for triply-resonant film-coupled plasmonic metasurfaces. By exploiting asymmetry in several plasmonic unit cell geometries, we show that these geometries are capable of supporting three plasmonic modes that are independently tunable and polarization-dependent. Furthermore, we observe high field enhancements due to field confinement in the subwavelength gap between the nanostructure and film. By coupling absorption and spontaneous emission to the different plasmonic modes, and taking advantage of high field enhancement in the gap, one can potentially increase the efficiency of field-dependent processes, such as fluorescence, sum frequency generation (SFG) and difference frequency generation (DFG).   

Hybrid plasmonic-dielectric metamaterials for enhanced nonlinear response

Since its discovery in the 1960s, nonlinear optics has revolutionized optical technologies and laser industry. Development of efficient nanoscale nonlinear sources will pave the way for new applications in photonic circuitry, quantum optics and biosensing. However, nonlinear signal generation at dimensions smaller than the wavelength of light brings new challenges. These include the reduced light-matter interaction volume, mode overlap and increased losses. Here, we develop hybrid plasmonic – dielectric metamaterials that overcome these limitations and show a dramatic increase of the efficiency of nonlinear optical response at the nanoscale.