Themistoklis P. H. Sidiropoulos

Multiresonant Broadband Optical Antennas As Efficient Tunable Nanosources of Second Harmonic Light

We report the experimental realization of efficient tunable nanosources of second harmonic light with individual multiresonant log-periodic optical antennas. By designing the nanoantenna with a bandwidth of several octaves, simultaneous enhancement of fundamental and harmonic fields is observed over a broad range of frequencies, leading to a high second harmonic conversion efficiency, together with an effective second order susceptibility within the range of values provided by widespread inorganic crystals. Moreover, the geometrical configuration of the nanoantenna makes the generated second harmonic signal independent from the polarization of the fundamental excitation. These results open new possibilities for the development of efficient integrated nonlinear nanodevices with high frequency tunability.

Adiabatic Nanofocusing in Hybrid Gap Plasmon Waveguides on the Silicon-on-Insulator Platform

We present an experimental demonstration of a new class of hybrid gap plasmon waveguides on the silicon-on-insulator (SOI) platform. Created by the hybridization of the plasmonic mode of a gap in a thin metal sheet and the transverse-electric (TE) photonic mode of an SOI slab, this waveguide is designed for efficient adiabatic nanofocusing simply by varying the gap width. For gap widths greater than 100 nm, the mode is primarily photonic in character and propagation lengths can be many tens of micrometers. For gap widths below 100 nm, the mode becomes plasmonic in character with field confinement predominantly within the gap region and with propagation lengths of a few microns. We estimate the electric field intensity enhancement in hybrid gap plasmon waveguide tapers at 1550 nm by three-photon absorption of selectively deposited CdSe/ZnS quantum dots within the gap. Here, we show electric field intensity enhancements of up to 167 ± 26 for a 24 nm gap, proving the viability of low loss adiabatic nanofocusing on a commercially relevant photonics platform.

Plasmon-Induced Optical Anisotropy in Hybrid Graphene–Metal Nanoparticle Systems

Hybrid plasmonic metal-graphene systems are emerging as a class of optical metamaterials that facilitate strong light-matter interactions and are of potential importance for hot carrier graphene-based light harvesting and active plasmonic applications. Here we use femtosecond pump-probe measurements to study the near-field interaction between graphene and plasmonic gold nanodisk resonators. By selectively probing the plasmon-induced hot carrier dynamics in samples with tailored graphene-gold interfaces, we show that plasmon-induced hot carrier generation in the graphene is dominated by direct photoexcitation with minimal contribution from charge transfer from the gold. The strong near-field interaction manifests as an unexpected and long-lived extrinsic optical anisotropy. The observations are explained by the action of highly localized plasmon-induced hot carriers in the graphene on the subresonant polarizability of the disk resonator. Because localized hot carrier generation in graphene can be exploited to drive electrical currents, plasmonic metal-graphene nanostructures present opportunities for novel hot carrier device concepts.

Ultrafast Dynamics of Lasing Semiconductor Nanowires

Semiconductor nanowire lasers operate at ultrafast timescales; here we report their temporal dynamics, including laser onset time and pulse width, using a double-pump approach. Wide bandgap gallium nitride (GaN), zinc oxide (ZnO), and cadmium sulfide (CdS) nanowires reveal laser onset times of a few picoseconds, driven by carrier thermalization within the optically excited semiconductor. Strong carrier-phonon coupling in ZnO leads to the fastest laser onset time of ∼1 ps in comparison to CdS and GaN exhibiting values of ∼2.5 and ∼3.5 ps, respectively. These values are constant between nanowires of different sizes implying independence from any optical influences. However, we demonstrate that the lasing onset times vary with excitation wavelength relative to the semiconductor band gap. Meanwhile, the laser pulse widths are dependent on the optical system. While the fastest ultrashort pulses are attained using the thinnest possible nanowires, a sudden change in pulse width from ∼5 to ∼15 ps occurs at a critical nanowire diameter. We attribute this to the transition from single to multimode waveguiding, as it is accompanied by a change in laser polarization.

Silicon-based metal-loaded plasmonic waveguides for low-loss nanofocusing

We introduce plasmonic waveguides based on metal loading of silicon-on-insulator (SOI) substrates. Here slab waveguide modes hybridize with the plasmonic modes of either a metal nanowire or a slot in a metal film. By tapering a single dimension of either structure, the resulting hybrid mode can be converted from photon-like to plasmon-like, allowing up to millimeter-range transport and rapid nanoscale focusing down to mode areas ∼λ2/400. Metal loading is achievable with a single lithography step directly on SOI without the need for etching and, thus, opens practical possibilities for silicon nanoplasmonics.

The Interplay of Symmetry and Scattering Phase in Second Harmonic Generation from Gold Nanoantennas

Nonlinear phenomena are central to modern photonics but, being inherently weak, typically require gradual accumulation over several millimeters. For example, second harmonic generation (SHG) is typically achieved in thick transparent nonlinear crystals by phase-matching energy exchange between light at initial, ω, and final, 2ω, frequencies. Recently, metamaterials imbued with artificial nonlinearity from their constituent nanoantennas have generated excitement by opening the possibility of wavelength-scale nonlinear optics. However, the selection rules of SHG typically prevent dipole emission from simple nanoantennas, which has led to much discussion concerning the best geometries, for example, those breaking centro-symmetry or incorporating resonances at multiple harmonics. In this work, we explore the use of both nanoantenna symmetry and multiple harmonics to control the strength, polarization and radiation pattern of SHG from a variety of antenna configurations incorporating simple resonant elements tuned to light at both ω and 2ω. We use a microscopic description of the scattering strength and phases of these constituent particles, determined by their relative positions, to accurately predict the SHG radiation observed in our experiments. We find that the 2ω particles radiate dipolar SHG by near-field coupling to the ω particle, which radiates SHG as a quadrupole. Consequently, strong linearly polarized dipolar SHG is only possible for noncentro-symmetric antennas that also minimize interference between their dipolar and quadrupolar responses. Metamaterials with such intra-antenna phase and polarization control could enable compact nonlinear photonic nanotechnologies.

Mode Switching and Filtering in Nanowire Lasers

Coherent light sources confining the light below the vacuum wavelength barrier will drive future concepts of nanosensing, nanospectroscopy, and photonic circuits. Here, we directly image the angular emission of such a light source based on single semiconductor nanowire lasers. It is confirmed that the lasing switches from the fundamental mode in a thin ZnO nanowire to an admixture of several transverse modes in thicker nanowires approximately at the multimode cutoff. The mode competition with higher order modes substantially slows down the laser dynamics. We show that efficient photonic mode filtering in tapered nanowires selects the desired fundamental mode for lasing with improved performance including power, efficiency, and directionality important for an optimal coupling between adjacent nanophotonic waveguides.

Efficient low dispersion compact plasmonic-photonic coupler

We report efficient low dispersion light coupling into a silicon waveguide using an antenna consisting of two metallic nanoparticles. We find that strong multiple scattering between the nanoparticles dictates the coupling efficiency. We also explore directional coupling, by using different particles with a relative scattering phase, but find that optimum directionality corresponds to minimum efficiency. A dipole model highlights a subtle interplay between multiple scattering and directionality leading to a compromise allowing up to 30% transmission into a single direction. With a 500 nm bandwidth near infrared telecoms bands, group delay dispersion is sufficiently low to faithfully couple pulses as short as 50 fs.

Generating intense optical fields with hybrid-gap plasmon lasers

Plasmonics is a potential route to optical devices that generate intense localized electric fields with unique capabilities in nonlinear optics. Many predict that sub-wavelength optical systems will be essential in the development of future optical integrated circuits, but realising this potential will be contingent on the ability to exploit plasmonic effects with semiconductor materials. Furthermore, the capability to focus light beyond the wavelength limit presents opportunities to explore intense optical field physics. To these ends we present two implementations of semiconductor hybrid gap plasmon devices that look promising for such applications. The first is a nanofocusing element [1] that boosts the electric fields of guided waves within a 24nm wide gap. The second device is a GaAs nano-plasmonic laser [2] with the potential to generate extremely intense optical field within nanoscale gaps [3].

Linearly polarized dipolar second harmonic generation from gold nano-antennas by controlling their radiation phase

A recurring theme in optics and photonics is the ability of metal nanostructures to imbue artificial materials with new functions. Metallic nano-antennas [1], so-called meta-atoms, are the building blocks of such metamaterials that boast unusual linear [2, 3] and nonlinear [4- 6] characteristics not observed in natural materials. Recently, nonlinear metamaterials have generated considerable excitement; while nonlinear effects in natural materials must gradually accumulate weak nonlinearity across macroscopic crystal dimensions, a small volume of metamaterial [7, 8], and even isolated antennas [9-11], can create a surprisingly strong effect. This capability stems from additional nanoscopic degrees of freedom that include couplings between the constituent nanoparticles within antennas or between antennas and material resonances [7, 9, 10]. Second harmonic generation (SHG) enables diversely coloured light sources through an energy exchange process between light at initial, ω, and final, 2ω, frequencies typically in optically thick non-linear crystals. Recently, metamaterials imbued with nonlinearity from their constituent nano-antennas have generated excitement by opening the possibility of wavelength-scale nonlinear optics. The nonlinear selection rules typically prevent dipole SHG from nano-antennas leading to much discussion concerning the best geometries. Following recent literature, we examine individual antennas that are designed to efficiently radiate SHG (ηSHG > 10-7W-1, corresponding to |χeff(2)| > 40pmV-1) by incorporating simple resonant elements tuned to light at both ω and 2ω. We show that antennas exhibiting both non-centro-symmetry and a mirror symmetry plane exhibit the strongest normal incidence SHG emission with a high degree of linear polarization. We confirm this by direct measurement of SHG in the back focal plane of isolated nano-antennas in a variety of configurations. We also show that antennas incorporating multiple 2ω-elements provide greater flexibility to ensure that both symmetries are met. By interpreting the SHG emission patterns using a multi-dipole model we also identify the phase and the orientation of each antenna element as key design parameters to control SHG emission pattern and polarisation. Metamaterials incorporating such antennas could enable compact nonlinear photonic nanotechnologies.