Switchable Optical Nonlinearity at the Metal to Insulator Transition in Magnesium Thin Films

Abstract

We report resonantly enhanced and switchable third order nonlinearity in magnesium thin films. Utilizing an optical parametric oscillator as a tunable broadband light source, we find a highly wavelength dependent third harmonic conversion efficiency, exhibiting a 40-to 50-fold signal increase over the investigated wavelength range between 1300 and 1800 nm. We relate this unusually strong increase of the intrinsic magnesium material nonlinearity to resonant sp-interband transitions around 0.7 eV or 1770 nm in the fundamental wavelength range, multiplying their impact on the conversion efficiencies. Moreover, we make use of the drastic change in the optical properties of magnesium at the phase transition between metallic magnesium and dielectric magnesium hydride by in situ hydrogenation. We are able to cyclically switch the nonlinearity and the efficiency of the third harmonic conversion process. The time-dependence of the nonlinear signal is complex and is a consequence of several contributions, including the hydrogen concentration-dependent change in the intrinsic nonlinear susceptibility, the change in the surface morphology during hydrogenation and dehydrogenation, as well as the volume change of the material. While shedding new light on the structural and morphological changes in such magnesium thin films, we envision that the switchable and large resonant third-order nonlinearity in magnesium will find applications in nanostructured systems, unlocking additional degrees of freedom in design and implementation, such as wavelength dependency and switchable material nonlinearities.

Spatially Resolved Nonlinear Plasmonics

“…In the work by Wolf et al, k-space imaging was utilized, which revealed the directionality of the emitted THG light. Kauranen and Zayats, Chen et al, Drechsler et al, Krauth et al, Metzger et al, and Wegener et al ,− have studied second-harmonic generation (SHG) and THG emission of plasmonic nanostructures depending on shape, material, spectral detuning, pulse duration, etc. However, in most of those cases, the local microscopic origin was also not spatially imaged but mostly derived by using linear or nonlinear optical simulations.…”

Liquid Hydrogenation of Plasmonic Nanoantennas via Alcohol Deprotonation

“…Especially plasmonic optical sensing platforms are an emerging field of interest as they allow, for example, to detect hydrogen (H 2 ) via an optical readout without further need of electronics in highly explosive environments. , Such sensors rely on the used plasmonic materials to be highly sensitive to a hydrogen exposure. Predestined and frequently used materials to show such unique properties are, for example, magnesium (Mg), − niobium (Nb), , yttrium (Y), , or palladium (Pd). − The latter is typically used as a catalyst to split hydrogen molecules at the Pd surface and to allow further diffusion of hydrogen atoms into the attached hydrogen-sensitive layers.…”