We monitor the configuration of poly-L-lysine proteins using vibrational resonances at 6 µm (1667 cm^-1) by employing a broadband femtosecond solid-state laser for micro-FTIR spectroscopy. This laser system allows for detection of minute amounts of proteins due to a several orders of magnitude higher brilliance compared to standard FTIR light sources such as globars. Thus, absorption signals as small as 0.5% can be detected without averaging, compared to 6.4% using a globar, at a spatial resolution as small as 10x10 µm^2.
Our light source is based on a 98 fs, Yb-doped pump laser at 73 MHz repetition rate, providing 2.5 W average power. By pumping a fiber-feedback optical parametric oscillator (ffOPO) and a post-amplifier, signal and idler beams spanning from 1.33 – 2.0 and 2.1 – 4.6 µm are generated. The tuning range is extended to 8 µm by difference frequency generation between the signal and idler beams and can be further extended by using a pump laser with higher output power.
At 7 µm excellent long-term wavelength stability with fluctuations smaller than 0.1% rms measured over 9 hours is observed, without applying electronic stabilization. This is due to the combination of a ffOPO with a post-amplifier and is distinctly superior over other systems based on free-space OPOs.
Protein sensing is conducted by applying resonant surface-enhanced infrared absorption (SEIRA) spectroscopy, using a single gold nanoantenna. To the best of our knowledge, this is the first demonstration of resonant SEIRA spectroscopy using a single nanoantenna with a laser system as light source.
A key challenge for the development of active plasmonic nanodevices is the lack of materials with fully controllable
plasmonic properties. In this work, we demonstrate that a plasmonic resonance in top-down nanofabricated yttrium
antennas can be completely and reversibly turned on and off using hydrogen exposure. We fabricate arrays of yttrium
nanorods and optically observe in extinction spectra the hydrogen-induced phase transition between the metallic yttrium
dihydride and the insulating trihydride. Whereas the yttrium dihydride nanostructures exhibit a pronounced particle
plasmon resonance, the transition to yttrium trihydride leads to a complete vanishing of the resonant behavior. The
plasmonic resonance in the dihydride state can be tuned over a wide wavelength range by simply varying the size of the
nanostructures. Furthermore, we develop an analytical diffusion model to explain the temporal behaviour of the
hydrogen loading and unloading process observed in our experiments and gain information about the thermodynamics of
our device. Thus, our nanorod system serves as a versatile basic building block for active plasmonic devices ranging
from switchable perfect absorbers to active local heating control elements.
Metal nanowires with proper length give strong antenna-like plasmonic resonances in the infrared. Their resonance
spectrum is a sensitive measure not only of their geometry but also of their conductivity as we will show for lead