Tunable s-SNOM for Nanoscale Infrared Optical Measurement of Electronic Properties of Bilayer Graphene.

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.

Compact harmonic cavity optical parametric oscillator for optical parametric amplifier seeding.

We present a broadly tunable highly efficient frequency conversion scheme, based on a low-threshold harmonic cavity optical parametric oscillator (OPO) followed by an idler-seeded power amplifier. By choosing the cavity length of the OPO equal to the 10th harmonic of its 41 MHz Yb:KGW solid-state pump laser, a very compact optical setup is achieved. A singly-resonant cavity without output coupler results in a low oscillation threshold of only 28-100 mW in the entire signal tuning range of 1.37-1.8 µm. The 2.4-4.15 µm idler radiation is coupled out at the 41 MHz pump frequency and employed to seed a post amplifier with nearly Watt-level output power. In addition, the seeder plus power amplifier concept results in clean signal and idler pulses at the fundamental repetition rate of 41 MHz with a time-bandwidth product below 0.4 and a relative intensity noise 10 dB lower compared to the solid-state pump laser.

Watching in situ the hydrogen diffusion dynamics in magnesium on the nanoscale.

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.

Alignment-free difference frequency light source tunable from 5 to 20†µm by mixing two independently tunable OPOs.

Tunable mid-infrared ultrashort lasers have become an essential tool in vibrational spectroscopy in recent years. They enabled and pushed a variety of spectroscopic applications due to their high brilliance, beam quality, low noise, and accessible wavelength range up to 20 µm. Many state-of-the-art devices apply difference frequency generation (DFG) to reach the mid-infrared spectral region. Here, birefringent phase-matching is typically employed, resulting in a significant crystal rotation during wavelength tuning. This causes a beam offset, which needs to be compensated to maintain stable beam pointing. This is crucial for any application. In this work, we present a DFG concept, which avoids crystal rotation and eliminates beam pointing variations over a broad wavelength range. It is based on two independently tunable input beams, provided by synchronously pumped parametric seeding units. We compare our concept to the more common DFG approach of mixing the signal and idler beams from a single optical parametric amplifier (OPA) or oscillator (OPO). In comparison, our concept enhances the photon efficiency of wavelengths exceeding 11 µm more than a factor of 10 and we still achieve milliwatts of output power up to 20 µm. This concept enhances DFG setups for beam-pointing-sensitive spectroscopic applications and can enable research at the border between the mid- and far-IR range due to its highly efficient performance.

Coherently broadened, high-repetition-rate laser for stimulated Raman scattering-spectroscopic optical coherence tomography.

We present a novel light source specifically tailored for stimulated Raman scattering-spectroscopic optical coherence tomography (SRS-SOCT), which is, to the best of our knowledge, a novel molecular imaging method that combines the molecular sensitivity of SRS with the spatial and spectral multiplexing capabilities of SOCT. The novel laser consists of an 8 W, 450 fs Yb:KGW oscillator, with a repetition rate of 40 MHz, which delivers the Stokes beam for SRS-SOCT and also pumps and amplifies an optical parametric oscillator (OPO). The output of the amplified OPO is then frequency doubled and coherently broadened using a custom-made tapered fiber that generates bandwidth pulses >40 nm, compressible to <50 fs, with the average power over 150 mW, near the shot-noise limit above 250 kHz. The broadened and compressed pulse simultaneously serves as the pump beam and SOCT light source for SRS-SOCT. This light source is assessed for SRS-SOCT, and its implications for other imaging methods are discussed.

Nanoscale Hydrogenography on Single Magnesium Nanoparticles.

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 (MgH2), 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.

Low drift cw-seeded high-repetition-rate optical parametric amplifier for fingerprint coherent Raman spectroscopy.

We introduce a broadly tunable robust source for fingerprint (170 - 1620 cm-1) Raman spectroscopy. A cw thulium-doped fiber laser seeds an optical parametric amplifier, which is pumped by a 7-W, 450-fs Yb:KGW bulk mode-locked oscillator with 41 MHz repetition rate. The output radiation is frequency doubled in a MgO:PPLN crystal and generates 0.7 - 1.3-ps-long narrowband pump pulses that are tunable between 885 and 1015 nm with >80 mW average power. The Stokes beam is delivered by a part of the oscillator output, which is sent through an etalon to create pulses with 1.7 ps duration. We demonstrate a stimulated Raman gain measurement of toluene in the fingerprint spectral range. The cw seeding intrinsically ensures low spectral drift.

Narrowband cw injection seeded high power femtosecond double-pass optical parametric generator at 43 MHz: Gain and noise dynamics.

We demonstrate narrowband cw injection seeding of a femtosecond double-pass optical parametric generator at 43 MHz repetition rate with a simple, low power external cavity diode laser. Up to 2.5 W of near-IR radiation (1.5 - 1.66 µm) as well as 800 mW of tunable mid-IR radiation (2.75 - 3.15 µm) with pulse durations below 300 fs are generated with a remarkable pulse-to-pulse and long term power stability. Compared to conventional, vacuum noise seeded optical parametric generators, the presented frequency conversion scheme does not only exhibit superior gain and noise dynamics, but also a high degree of flexibility upon control parameters such as pump power, seed power, or spectral position of the seed.

Nonlinear Refractory Plasmonics with Titanium Nitride Nanoantennas.

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.

Second harmonic generation spectroscopy on hybrid plasmonic/dielectric nanoantennas.

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.

High repetition rate femtosecond double pass optical parametric generator with more than 2 W tunable output in the NIR.

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.