Ultrathin gold films are attractive for plasmonic and metamaterial devices, thanks to their useful optical and optoelectronic properties. However, deposition of ultrathin continuous Au films of a few nanometer thickness is challenging and generally requires wetting layers, resulting in increased optical losses and incompatibility with optoelectronic device requirements. We demonstrate wetting layer-free plasmonic gold films with thicknesses down to 3 nm obtained by deposition on substrates cooled to cryogenic temperatures. We systematically study the effect of substrate temperature on the properties of the deposited Au films and show that substrate cooling suppresses the Volmer−Weber growth mode of Au, promoting early stage formation of continuous Au films with improved surface morphology and enhanced optoelectronic properties. Our results pave the way for straightforward implementation of ultrathin Au-based optoelectronic and plasmonic devices, as well as metamaterials and metasurfaces.

We demonstrate that rapidly switched high-Q metasurfaces enable spectral regions of negative optical extinction.

Abstract

There is an active demand to develop efficient nanoscale nonlinear sources for applications in photonic circuitry, quantum optics and biosensing. To this end, plasmonic systems have been utilized to boost the nonlinear signal generation, however high efficiencies of frequency conversion for realistic applications remain a challenge. Metal-insulator-metal (MIM) nanocavities are good candidates for strongly concentrating the fields at the nanoscale to enhance the optical nonlinearities, however they typically suffer from the requirement to have a quadrupolar resonance at the emission wavelength. Here, we introduce nonplanar MIM nanocavities with a nonlinear spacer that can strongly enhance the second harmonic generation (SHG) despite of having fundamental and emission modes of the same parity. Our experimental and numerical results indicate that the enhancement is due to the non-planar design of the cavities and the bulk nonlinearity of the spacer layer.The emergence and the rapid advance of nanophotonic systems allowed an unprecedented control of the optical modes and interactions at the nanoscale [1]. This has been especially beneficial for nonlinear optical processes as the subwavelength nature of these platforms has opened up a range of novel possibilities [2, 3]. For example, at the nanoscale the strict phase matching conditions are relaxed, unlocking a host of strongly nonlinear materials that cannot be used in the bulk [4][5][6]. Furthermore, the field confinement afforded by nanoresonators concentrates and enhances the local fields leading to greater increase in the efficiency of the nonlinear light-matter interactions [2]. Finally, the ability to control the spatial distribution and symmetry of optical modes at the nanoscale is important for the engineering of strong nonlinearities [7,8].In this context, plasmonic systems have been used extensively to confine the optical fields into deep subwavelength volumes and to enhance the nonlinear optical phenomena at the nanoscale. Various nonlinear processes have been shown to benefit from plasmonic enhancement including SHG, third harmonic generation, four wave mixing, optical Kerr effect [9][10][11][12][13][14][15][16][17][18][19][20][21][22]. MIM nanocavities are of particular interest as they can reproducibly achieve strong field enhancement in the thin insulating layer [23,24]. Moreover, the plethora of modes supported by MIM nanoresonators allows to design multi-resonant structures enhancing the optical fields at all the frequencies involved in the nonlinear process [10].2nd order processes such as SHG require inversion symmetry breaking for electron motion. The pure plasmonic structures are made of noble metals which possess centrosymmetric crystal lattice and thus the symmetry breaking is achieved at the material interface. Thus, the effective nonlinear susceptibility tensor element ⊥⊥⊥ (2) is typically the dominant source for nonlinearity [25]. The major drawback of this approach is that it requires the optical mode at the SHG frequency to have different ...

Active nanostructured optical components show promise as potential building blocks for novel light‐based computing and data processing architectures. However, nanoscale all‐optical switches that have low activation powers and high‐contrast ultrafast switching have been elusive so far. Here, pump–probe measurements performed on amorphous‐Ge‐based micro‐resonator metasurfaces that exhibit strong resonant modes in the mid‐infrared are reported. Relative change is observed in transmittance of ΔT/T ≈ 1 with picosecond (down to τ ≈ 0.5 ps) free carrier relaxation rates, obtained with very low pump fluences of 50 μJ cm−2. These observations are attributed to efficient free carrier promotion, affecting light transmittance via high quality‐factor optical resonances, followed by an increased electron–phonon scattering of free carriers due to the amorphous crystal structure of Ge. Full‐wave simulations based on a permittivity model that describes free‐carrier damping through crystal structure disorder find excellent agreement with the experimental data. These findings offer an efficient and robust platform for all‐optical switching at the nanoscale.

Here, we report the comparative study of enhanced second harmonic generation using defect engineering in pyramid-like MoS2 (P-MoS2) flakes to vertically aligned MoS2 (VA-MoS2) flakes. P-MoS2 and VA-MoS2 is synthesized via the modified chemical vapor deposition technique. The second harmonic generation measurements on P-MoS2 and VA-MoS2 are performed by sweeping the excitation wavelength from 1200 nm to 1310 nm in identical conditions. The P-MoS2 flakes show a high SHG signal. The high SHG signal in pyramid-like MoS2 is attributed to the broken inversion symmetry and high thickness of grown MoS2 flakes. VA-MoS2 flakes under the identical conditions show a 34% enhanced SHG signal in comparison to P-MoS2. The midgap states generated due to defects in the form of S vacancies in VA-MoS2 are responsible for this enhancement. These midgap states confine the photons and result in enhanced SHG properties. Our study will pave a new path to understand the role of 2D material morphology in fabricating versatile optical and photonics devices.

The phase matching between the propagating fundamental and nonlinearly generated waves plays an important role in the efficiency of the nonlinear frequency conversion in macroscopic crystals. However, in nanoscale samples, such as nanoplasmonic structures, the phase-matching condition is often ignored due to the sub-wavelength nature of the materials. Here, we first show that the phase matching of the lattice plasmon modes at the fundamental and second-harmonic frequencies in a plasmonic nanoantenna array can effectively enhance the surface-enhanced second-harmonic generation. Additionally, a significant enhancement of the second-harmonic generation is demonstrated using stationary band-edge lattice plasmon modes with zero phase.

Microfocus Brillouin light scattering is a powerful technique for the spectroscopic and spatial characterization of elementary excitations in materials. However, the small momentum of light limits the accessible excitations to the center of the Brillouin zone. Here we utilize a metallic nanoantenna fabricated on the archetypal ferrimagnet yttrium iron garnet to demonstrate the possibility of Brillouin light scattering from large-wave-vector, highfrequency spin wave excitations that are inaccessible with free-space light. The antenna facilitates subdiffraction confinement of the electromagnetic field, which enhances the local field intensity and generates momentum components significantly larger than those of free-space light. Our approach provides access to high-frequency spin waves important for fast nanomagnetic devices, and can be generalized to other types of excitations and light-scattering techniques.