Sebastian Beer

A systematic exploration of amorphous ABC heterostructures revealed that nanoscale morphological modifications markedly improved their artificial bulk second-order susceptibility. These amorphous birefringent heterostructures were fabricated using plasma-enhanced atomic layer deposition of three oxides, thereby breaking centrosymmetry at the nanoscale. We observe that the optical nonlinearity depends on the thickness variation of the three constituent materials, SiO ₂ , TiO ₂ , and Al ₂ O ₃ , ranging from tens of nanometers to the atomic scale, and where the thin films exhibit second-order susceptibility at their interfaces. Our findings reveal that the enhancement of nonlinear optical properties is strongly correlated with a high interface density and superior interface quality, where the interface second-order nonlinearity transitions to bulk-like second-harmonic generation. An effective bulk second-order susceptibility of χ _zzz = 2.0 ± 0.2 pm/V at 1032 nm is achieved, comparable to some conventional monocrystalline nonlinear materials.

We investigate second harmonic generation from vertically stacked metal–insulator–metal nanobars arranged in a two-dimensional rectangular array. At specific angles of incidence, the nonlinear emission is maximized due to the near-field enhancement associated with the surface lattice resonance from the periodic arrangement, regardless of the number of insulator layers embedded in each nanoantenna. Intuitively, stacking should increase second harmonic generation with the number of metal layers. In fact, the maximum emission was reached at the highest stacking, but experimental results also indicate a far more complex relation between the individual layers. Due to the small vertical distance between the metallic layers, the fields in each nanogap are strongly coupled, leading to the generation of gap-plasmons and magnetic modes.

In this work, we demonstrate the potential of iridium (Ir) and iridium/alumina (Ir/Al₂O₃) based atomic-scale heterostructures as promising non-linear optical materials. We investigated the surface second harmonic generation (SHG) response from Ir nanoparticles and their heterostructures. The SHG signal from Ir increases with the surface coverage of nanoparticles but is constrained by self-absorption beyond a certain thickness. Notably, the Ir/Al₂O₃ heterostructures demonstrate up to two orders of magnitude higher SH response than bare Ir, attributed to improved optical properties by incorporating the dielectric material. This study highlights the potential of precisely grown Ir/Al₂O₃ metal-dielectric heterostructures based on atomically thin layers for advanced optical applications.

In the rapidly evolving field of plasmonic metasurfaces, achieving homogeneous, reliable, and reproducible fabrication of sub‐5 nm dielectric nanogaps is a significant challenge. This article presents an advanced fabrication technology that addresses this issue, capable of realizing uniform and reliable vertical nanogap metasurfaces on a whole wafer of 100 mm diameter. By leveraging fast patterning techniques, such as variable‐shaped and character projection electron beam lithography (EBL), along with atomic layer deposition (ALD) for defining a few nanometer gaps with sub‐nanometer precision, we have developed a flexible nanofabrication technology to achieve gaps as narrow as 2 nm in plasmonic nanoantennas. The quality of our structures is experimentally demonstrated by the observation of resonant localized and collective modes corresponding to the lattice, with Q‐factors reaching up to 165. Our technological process opens up new and exciting opportunities to fabricate macroscopic devices harnessing the strong enhancement of light–matter interaction at the single nanometer scale.

Second-harmonic generation (SHG) is a second-order nonlinear optical process that is not allowed in media with inversion symmetry. However, due to the broken symmetry at the surface, surface SHG still occurs, but is generally weak. We experimentally investigate the surface SHG in periodic stacks of alternating, subwavelength dielectric layers, which have a large number of surfaces, thus enhancing surface SHG considerably. To this end, multilayer stacks of SiO₂/TiO₂ were grown by Plasma Enhanced Atomic Layer Deposition (PEALD) on fused silica substrates. With this technique, individual layers of a thickness of less than 2 nm can be fabricated. We experimentally show that under large angles of incidence (> 20 degrees) there is substantial SHG, well beyond the level, which can be observed from simple interfaces. We perform this experiment for samples with different periods and thicknesses of SiO₂/TiO₂ and our results are in agreement with theoretical calculations.

We investigate experimentally third harmonic generation (THG) from plasmonic metasurfaces consisting of two-dimensional rectangular lattices of centrosymmetric gold nanobars. By varying the incidence angle and the lattice period, we show how surface lattice resonances (SLRs) at the involved wavelengths are the major contributors in determining the magnitude of the nonlinear effects. A further boost on THG is observed when we excite together more than one SLR, either at the same or at different frequency. When such multiple resonances take place, interesting phenomena are observed, such as maximum THG enhancement for counter-propagating surface waves along the metasurface, and cascading effect emulating a third-order nonlinearity.

Second harmonic generation is enhanced at the surface lattice resonance in plasmonic nanoparticle arrays. We carried out a parametric investigation on two-dimensional lattices composed of gold nanobars where the centrosymmetry is broken at oblique incidence. We study the influence of the periodicity, the incidence angle and the direction of the linear input polarization on the second harmonic generation. Excitation of the surface lattice resonance either at the fundamental or second harmonic wavelength, achieved by varying the incidence angle, enhance the conversion efficiency. As a special case, we demonstrate that both the wavelengths can be simultaneously in resonance for a specific period of the lattice. In this double resonant case, maximum second harmonic power is achieved.