We present a fully automated laser system with low-intensity noise for coherent Raman scattering microscopy. The robust two-color system is pumped by a solid-state oscillator, which provides Stokes pulses fixed at 1043 nm. The tunable pump pulses of 750 to 950 nm are generated by a frequency-doubled fiber-feedback femtosecond optical parametric oscillator. The resulting pulse duration of 1.2 ps provides a viable compromise between optimal coherent Raman scattering signal and the necessary spectral resolution. Thus a spectral range of 1015 to 3695 cm − 1 with spectral resolution of <13 cm − 1 can be addressed.
We present an ultra‐low noise, near to mid infrared light source for a variety of multiphoton imaging and spectroscopy techniques. The system is based on an optical parametric oscillator (OPO) pumped by a femtosecond Ytterbium solid state oscillator with tens of megahertz repetition rate. This light source supplies three intrinsically synchronized light beams at wavelengths: 1040 nm, 1400-2000 nm (tunable) and 2200-4200 nm (tunable). Without active stabilization, the OPO preserves the shot-noise limited performance of the Yb-oscillator, along with a high long-term stability and a TEM00 beam profile. While this tuning range is already suitable for two- and three-photon microscopy, it now becomes possible to address vibrational modes and thus molecular specificity by employing further frequency conversion stages. Tailored frequency doubling provides either a narrow linewidth (0.5-1.2 nm) or a broadband (>40 nm) beam, tunable from 750-950 nm.
The fixed Stokes beam of the Yb-oscillator can directly be used as a pump source for coherent Raman scattering such as SRS or CARS spectroscopy/microscopy. To this end, we will demonstrate the capability of our system for both SRS imaging at video rate with a spectral precision of 13 cm-1 as well as SRS spectroscopy with more than 400 cm-1 bandwidth in a single shot. By further mixing the two output beams of the OPO, we are able to additionally produce mid-infrared light that is tunable from 4-16 µm. With the help of vibrational sum frequency generation, our system will allow us to cover a spectral range of 700-7000 cm-1.
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.