IR electroabsorption (IREA) spectrometer

The IR electroabsorption (IREA) spectrometer enables to detect the molecular response in an external electric field system with high sensitivity. The detection limit of the ΔA signal is as small as 6 x 10^-8. The IREA spectrometer involves two devices; the sample cell for the electric field application and the signal amplification method for the small signal detection.

The sample cell The sample cell uses a doped silicon as a window and an electrode. The silicon plates flank a mylar film (typically 10 um), thereby enabling the application of the electric field up to 1 x 10⁷ Vm^-1. The application of the large electric field results in large ΔA signal and facilitates the detection of ΔA.

The small signal detection The IREA spectrometer includes a dispersive infrared spectrometer. The IR light intensity at certain wavenumber is recoreded step by step. The ΔI signal (i.e., the ΔA signal) is also measured at the fixed wavenumber.

In the IREA experiment, the electric field modulates the intensity of the transmitted IR light (that is, ΔI = I_on - I_off) because of responses of the molecules to the perturbation.

The ΔI can be measured with very high sensitivity by (i) eliminating I_off (the DC component) and amplifying largely (AC coupling method), and (ii) using the phase-sensitive detection with lock-in voltmeter. These devices shows the advantage of the dispersive IR spectrometer.