High-harmonic generation in metallic titanium nitride.

High-harmonic generation is a cornerstone of nonlinear optics. It has been demonstrated in dielectrics, semiconductors, semi-metals, plasmas, and gases, but, until now, not in metals. Here we report high harmonics of 800-nm-wavelength light irradiating metallic titanium nitride film. Titanium nitride is a refractory metal known for its high melting temperature and large laser damage threshold. We show that it can withstand few-cycle light pulses with peak intensities as high as 13 TW/cm2, enabling high-harmonics generation up to photon energies of 11 eV. We measure the emitted vacuum ultraviolet radiation as a function of the crystal orientation with respect to the laser polarization and show that it is consistent with the anisotropic conduction band structure of titanium nitride. The generation of high harmonics from metals opens a link between solid and plasma harmonics. In addition, titanium nitride is a promising material for refractory plasmonic devices and could enable compact vacuum ultraviolet frequency combs.

Negative Refraction in Time-Varying Strongly Coupled Plasmonic-Antenna-Epsilon-Near-Zero Systems.

Time-varying metasurfaces are emerging as a powerful instrument for the dynamical control of the electromagnetic properties of a propagating wave. Here we demonstrate an efficient time-varying metasurface based on plasmonic nano-antennas strongly coupled to an epsilon-near-zero (ENZ) deeply subwavelength film. The plasmonic resonance of the metal resonators strongly interacts with the optical ENZ modes, providing a Rabi level spitting of ∼30%. Optical pumping at frequency ω induces a nonlinear polarization oscillating at 2ω responsible for an efficient generation of a phase conjugate and a negative refracted beam with a conversion efficiency that is more than 4 orders of magnitude greater compared to the bare ENZ film. The introduction of a strongly coupled plasmonic system therefore provides a simple and effective route towards the implementation of ENZ physics at the nanoscale.

Synchrotron radiation from an accelerating light pulse.

Synchrotron radiation-namely, electromagnetic radiation produced by charges moving in a curved path-is regularly generated at large-scale facilities where giga-electron volt electrons move along kilometer-long circular paths. We use a metasurface to bend light and demonstrate synchrotron radiation produced by a subpicosecond pulse, which moves along a circular arc of radius 100 micrometers inside a nonlinear crystal. The emitted radiation, in the terahertz frequency range, results from the nonlinear polarization induced by the pulse. The generation of synchrotron radiation from a pulse revolving about a circular trajectory holds promise for the development of on-chip terahertz sources.

Patterned multilayer metamaterial for fast and efficient photon collection from dipolar emitters.

Solid-state quantum emitters are prime candidates for the realization of fast, on-demand single-photon sources. The improvement in photon emission rate and collection efficiency for point-like emitters can be achieved by using a near-field coupling to nanophotonic structures. Plasmonic metamaterials with hyperbolic dispersion have previously been demonstrated to significantly increase the fluorescence decay rates from dipolar emitters due to a large broadband density of plasmonic modes supported by such metamaterials. However, the emission coupled to the plasmonic modes must then be outcoupled into the far field before it succumbs to ohmic losses. We propose a nano-grooved hyperbolic metamaterial that improves the collection efficiency by several times compared to a conventional planar lamellar hyperbolic metamaterial. Our approach can be utilized to achieve broadband enhancement of emission for diverse types of quantum emitters.

Corrigendum: Controlling hybrid nonlinearities in transparent conducting oxides via two-colour excitation.

This corrects the article DOI: 10.1038/ncomms15829.

Controlling hybrid nonlinearities in transparent conducting oxides via two-colour excitation.

Nanophotonics and metamaterials have revolutionized the way we think about optical space (ɛ,µ), enabling us to engineer the refractive index almost at will, to confine light to the smallest of the volumes, and to manipulate optical signals with extremely small footprints and energy requirements. Significant efforts are now devoted to finding suitable materials and strategies for the dynamic control of the optical properties. Transparent conductive oxides exhibit large ultrafast nonlinearities under both interband and intraband excitations. Here we show that combining these two effects in aluminium-doped zinc oxide via a two-colour laser field discloses new material functionalities. Owing to the independence of the two nonlinearities, the ultrafast temporal dynamics of the material permittivity can be designed by acting on the amplitude and delay of the two fields. We demonstrate the potential applications of this novel degree of freedom by dynamically addressing the modulation bandwidth and optical spectral tuning of a probe optical pulse.

Enhanced Nonlinear Refractive Index in ε-Near-Zero Materials.

New propagation regimes for light arise from the ability to tune the dielectric permittivity to extremely low values. Here, we demonstrate a universal approach based on the low linear permittivity values attained in the ε-near-zero (ENZ) regime for enhancing the nonlinear refractive index, which enables remarkable light-induced changes of the material properties. Experiments performed on Al-doped ZnO (AZO) thin films show a sixfold increase of the Kerr nonlinear refractive index (n_{2}) at the ENZ wavelength, located in the 1300 nm region. This in turn leads to ultrafast light-induced refractive index changes of the order of unity, thus representing a new paradigm for nonlinear optics.

Experimental demonstration of titanium nitride plasmonic interconnects.

An insulator-metal-insulator plasmonic interconnect using TiN, a CMOS-compatible material, is proposed and investigated experimentally at the telecommunication wavelength of 1.55 µm. The TiN waveguide was shown to obtain propagation losses less than 0.8 dB/mm with a mode size of 9.8 µm on sapphire, which agree well with theoretical predictions. A theoretical analysis of a solid-state structure using Si(3)N(4) superstrates and ultra-thin metal strips shows that propagation losses less than 0.3 dB/mm with a mode size of 9 µm are attainable. This work illustrates the potential of TiN as a realistic plasmonic material for practical solid-state, integrated nano-optic and hybrid photonic devices.

Improving the radiative decay rate for dye molecules with hyperbolic metamaterials.

We directly demonstrate an improvement in the radiative decay rate of dye molecules near multilayer hyperbolic metamaterials (HMMs). Our comprehensive study shows a radiative decay rate for rhodamine 800 (Rh800) that is several times higher due to the use of HMM samples as compared to dielectric substrates. This is also the first experimental demonstration that multilayer hyperbolic metamaterials provide an increase in the radiative decay rate relative to those from either thin or thick gold films.

Efficient unidirectional ridge excitation of surface plasmons.

Using leakage-radiation microscopy, we characterize the efficiency of unidirectional surface-plasmon excitation with periodic (800 nm) arrays of 130-nm-high and 330-nm-wide gold ridges on a thin gold film illuminated with a focused (5-microm-wide) laser beam. We demonstrate that, at the resonant wavelength of 816 nm, the excitation efficiency of > 0.4 can be obtained with >or= 5 ridges by adjusting the beam position. Conducting numerical simulations, we account for the experimental results and calculate the electric-field enhancement achieved near the gold surface.

Thin film Ag superlens towards lab-on-a-chip integration.

A thin metal film near-field superlens, as originally suggested by Pendry and realized by Fang et al. and Melville et al., is investigated with emphasis on materials suitable for integration on a lab-on-a-chip platform. A chemically resistant cyclo-olefin copolymer (COC), mr-I-T85 from microresist technology, is applied as dielectric matrix/spacer for an Ag thin film superlens. The superlens successfully resolves 80 nm half-pitch gratings when illuminated with UV radiation at a free space wavelength of 365 nm. The superlens design, fabrication and characterization is discussed.

Long-range surface plasmon polariton nanowire waveguides for device applications.

We report an experimental study of long-range surface plasmon polaritons propagating along metallic wires of sub-micrometer rectangular cross-sections (nanowires) embedded in a dielectric. At telecom wavelengths, optical signals are shown to propagate up to several millimeters along such nanowires. As the wires approach a square cross-section, the guided mode becomes more symmetric and can, for example, be tuned to match closely the mode of a standard single-mode optical fiber. Furthermore, symmetric nanowires are shown to guide both TM and TE polarizations. In order to illustrate the applicability of plasmonic nanowire waveguides to optical circuits, we demonstrate a compact variable optical attenuator consisting of a single nanowire that simultaneously carries light and electrical current.

Experimental studies of surface plasmon polariton band gap effect.

Surface plasmon polaritons (SPPs) propagation at a gold film surface covered by periodic arrays of approximately 40-nm-high scatterers arranged in a triangular lattice of different periods containing straight line defects is studied using collection scanning near-field optical microscopy. The results reveal the dependence of the SPP band gap (SPPBG) effect manifested via the SPP reflection and guiding (along line defects) on the parameters of the surface structures (period, filling factor and lattice orientation). We find that the SPPBG effect is stronger along GammaK direction for all investigated periodic structures. Our results demonstrate that the SPPBG effect becomes less pronounced with a decrease of the filling factor and disappears for a filling factor close to 0.2. We also observe that the centre of the SPPBG shifts towards longer wavelengths with an increase of the lattice period.

Surface plasmon polariton waveguiding in random surface nanostructures.

In this study, guiding of surface plasmon polaritons excited at a gold film surface along corrugation-free channels in regions that are covered with randomly located surface scatterers, is considered using near-field microscopy for imaging of surface plasmon polariton intensity distributions at the surface. In the wavelength range 713-815 nm, we observed complete inhibition of the surface plasmon polariton propagation inside the random structures composed of individual ( approximately 70 nm high) gold bumps (and their clusters) placed on a 55 nm thick gold film with a bump density of 75 micro m-2. We demonstrate well-defined surface plasmon polariton guiding along corrugation-free 2 micro m wide channels in random structures and, in the wavelength range 738-774 nm, low-loss guiding around 20 degrees bends having a bend radius of approximately 15 micro m.