2.5. Frequency measurements and IR spectroscopy
The sensor chip was mounted in an aluminium vacuum chamber providing electrical connection, a glass window for optical readout and a zinc selenide window for IR irradiation (Fig. 3 (2)). The resonators were actuated with an external piezoelectric element and read out with a laser-Doppler vibrometer (MSA-500, Polytec GmbH) mounted with a 632.8 nm helium neon readout laser. The nanomechanicalresonance frequency was monitored by means of a phase-locked loop (HF2LI-PLL, Zurich Instruments). For NAM-IR measurements, monochromatic IR light was produced by a tunable quantum cascade laser (QCL). The IR reference spectrum was recorded with an ATR-FTIR spectrometer (Spectrum 100, Perkin-Elmer).