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.