Magnesium (Mg) has been widely investigated for solid-state hydrogen storage. It is able to absorb up to 7.6 wt % of hydrogen gas, making it one of the most promising candidates for hydrogen storage and also a model system for other energy storage materials. Upon hydrogenation the metallic Mg forms non-metallic magnesium hydride (MgH2), thus its electronic and thereby optical properties are drastically altered. This allows for monitoring the kinetics of the hydrogenation process by easily accessible optical measurements. This technique, termed hydrogenography, has been used to observe the nucleation and growth of MgH2 domains in Mg films. Like all far-field optical methods, however, these investigations suffer from the diffraction limit and thus possess only limited spatial resolution, therefore preventing the direct observation of the hydrogenation process on the nanoscale. Here, we overcome this limitation by employing scattering-type scanning near-field optical microscopy and atomic force microscopy. This technique enables us to map the local dielectric properties at different stages of hydrogenation and dehydrogenation, probing thus the electronic properties and hence the local material composition with a spatial resolution on the order of 10 nm. Upon monitoring the kinetics of hydrogen absorption and desorption in such polycrystalline nanoparticles we reveal that the nucleation of this process progresses within individual crystallites. Our combined measurement techniques additionally corroborate a correlation between structural and electronic properties during this dynamic process. To validate this novel technique we additionally monitor in parallel the far-field scattering spectra of individual nanoparticles, which exhibit plasmonic resonances in the visible spectral range.
Providing optical feedback by a resonator enhances the efficiency of nonlinear optical effects, e.g. frequency
conversion. The bow-tie cavity is known to be a very successful scheme and it has made its way into the
commercial world of second harmonic generation and parametric oscillation. We demonstrate a continuouswave
optical parametric oscillator based on a bow-tie cavity converting monochromatic pump light at 1.03 μm
wavelength to signal light being tunable from 1.25 to 1.85 μm and to corresponding idler light from 2.3 to 5.3 μm.
We observe a signal power of up to 7 W, an idler power up to 3 W, and a mode-hop free operation over 10 h
without any active stabilization. Furthermore, we have extended the tuning range of the parametric oscillator to
the terahertz region: Our system converts near-infrared pump light to a monochromatic wave with a frequency of
1.35 THz and a power of 2 μW. Now, the straightforward next development step is to reduce the footprint of such
devices. For this purpose another type of ring cavity is very promising: the whispering gallery resonator. This
system offers unequaled opportunities because of its low loss leading to a high finesse. We discuss the challenges
for transferring the parametric oscillation scheme to whispering gallery resonators, addressing the preparation
of suitable resonators with a quality factor of 107 and a finesse of 500 and locking of the pump laser to a cavity
mode for 3 hours.