Titanium nitride (TiN) is a novel refractory plasmonic material which can sustain high temperatures and exhibits large optical nonlinearities, potentially opening the door for high-power nonlinear plasmonic applications. We fabricate TiN nanoantenna arrays with plasmonic resonances tunable in the range of about 950-1050 nm by changing the antenna length. We present second-harmonic (SH) spectroscopy of TiN nanoantenna arrays, which is analyzed using a nonlinear oscillator model with a wavelength-dependent second-order response from the material itself. Furthermore, characterization of the robustness upon strong laser illumination confirms that the TiN antennas are able to endure laser irradiation with high peak intensity up to 15 GW/cm(2) without changing their optical properties and their physical appearance. They outperform gold antennas by one order of magnitude regarding laser power sustainability. Thus, TiN nanoantennas could serve as promising candidates for high-power/high-temperature applications such as coherent nonlinear converters and local heat sources on the nanoscale.

Plasmonic nanoantennas provide unprecedented opportunities to concentrate light fields in subwavelength-sized volumes. By placing a nonlinear dielectric nanoparticle in such a hot spot, one can hope to take advantage of both the field enhancement provided by nanoantennas and the large, nonlinear optical susceptibility of dielectric nanoparticles. To test this concept, we combine gold gap nanoantennas with second-order, nonlinear zinc sulfide nanoparticles, and perform second harmonic generation (SHG) spectroscopy on the combined hybrid dielectric/plasmonic nanoantennas as well as on the individual constituents. We find that SHG from the bare gold nanoantennas, even though it should be forbidden due to symmetry reasons, is several orders of magnitude larger than that of the bare zinc sulfide nanoparticles. Even stronger second harmonic signals are generated by the hybrid dielectric/plasmonic nanoantennas. Control experiments with nanoantennas containing linear lanthanum fluoride nanoparticles reveal; however, that the increased SHG efficiency of the hybrid dielectric/plasmonic nanoantennas does not depend on the nonlinear optical susceptibility of the dielectric nanoparticles but is an effect of the modification of the dielectric environment. The combination of a hybrid dielectric/plasmonic nanoantenna, which is only resonant for the incoming pump light field, with a second nanoantenna, which is resonant for the generated second harmonic light, allows for a further increase in the efficiency of SHG. As the second nanoantenna mediates the coupling of the second harmonic light to the far field, this double-resonant approach also provides us with control over the polarization of the generated light.

Whispering-gallery resonators (WGR's), based on total internal reflection, possess high quality factors in a broad spectral range. Thus, nonlinear-optical processes in such cavities are ideally suited for the generation of broadband or tunable electromagnetic radiation. Experimentally and theoretically, we investigate the tunability of optical parametric oscillation in a radially structured WGR made of lithium niobate. With a 1.04  μm pump wave, the signal and idler waves are tuned from 1.78 to 2.5  μm--including the point of degeneracy--by varying the temperature between 20 and 62 °C. A weak off centering of the radial domain structure extends considerably the tuning capabilities. The oscillation threshold lies in the mW-power range.

Active plasmonics is enabling novel devices such as switchable metasurfaces for active beam steering or dynamic holography. Magnesium with its particle plasmon resonances in the visible spectral range is an ideal material for this technology. Upon hydrogenation, metallic magnesium switches reversibly into dielectric magnesium hydride (MgH), turning the plasmonic resonances off and on. However, up until now, it has been unknown how exactly the hydrogenation process progresses in the individual plasmonic nanoparticles. Here, we introduce a new method, namely nanoscale hydrogenography, that combines near-field scattering microscopy, atomic force microscopy, and single-particle far-field spectroscopy to visualize the hydrogen absorption process in single Mg nanodisks. Using this method, we reveal that hydrogen progresses along individual single-crystalline nanocrystallites within the nanostructure. We are able to monitor the spatially resolved forward and backward switching of the phase transitions of several individual nanoparticles, demonstrating differences and similarities of that process. Our method lays the foundations for gaining a better understanding of hydrogen diffusion in metal nanoparticles and for improving future active nano-optical switching devices.

Active plasmonic and nanophotonic systems require switchable materials with extreme material contrast, short switching times, and negligible degradation. On the quest for these supreme properties, an in-depth understanding of the nanoscopic processes is essential. Here, we unravel the nanoscopic details of the phase transition dynamics of metallic magnesium (Mg) to dielectric magnesium hydride (MgH2) using free-standing films for in situ nanoimaging. A characteristic MgH2 phonon resonance is used to achieve unprecedented chemical specificity between the material states. Our results reveal that the hydride phase nucleates at grain boundaries, from where the hydrogenation progresses into the adjoining nanocrystallites. We measure a much faster nanoscopic hydride phase propagation in comparison to the macroscopic propagation dynamics. Our innovative method offers an engineering strategy to overcome the hitherto limited diffusion coefficients and has substantial impact on the further design, development, and analysis of switchable phase transition as well as hydrogen storage and generation materials.

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Here we directly probe the electronic properties of bilayer graphene using s-SNOM measurements with a broadly tunable laser source over the energy range from 0.3 to 0.54 eV. We tune an OPO/OPA system around the interband resonance of Bernal stacked bilayer graphene (BLG) and extract amplitude and phase of the scattered light. This enables us to retrieve and reconstruct the complex optical conductivity resonance in BLG around 0.39 eV with nanoscale resolution. Our technique opens the door toward nanoscopic noncontact measurements of the electronic properties in complex hybrid 2D and van der Waals material systems, where scanning tunneling spectroscopy cannot access the decisive layers.

We demonstrate a highly efficient double pass optical parametric generator based on periodically poled MgO-doped congruent LiNbO3. More than two watts of tunable near-IR radiation (1370-1650 nm) are generated by directly pumping the system with 550 fs pulses from a 42 MHz repetition rate passively mode-locked Yb:KGW oscillator. Pulse durations below 200 fs were achieved without further compression techniques. The system is extremely efficient, compact, cost effective, easy to align and easy to operate, which makes it an interesting alternative to more complex optical parametric oscillators or optical parametric amplifiers.