Broadband Surface-Enhanced Mid-Infrared Laser Spectroscopy With Adaptive Spectrum Compensation

External cavity-quantum cascade lasers (EC-QCLs) offer high spectral power density (SPD) over several hundred wavenumbers, making them ideal for broadband mid-infrared absorption spectroscopy. Surface-enhanced infrared absorption spectroscopy (SEIRA) leverages nanoantenna structure to enhance the signal in an infrared absorption spectrum. However, the highly uneven SPD of the EC-QCL and strong absorption of the SEIRA nanoantennas challenge the limited dynamic range of the detection system and thus introduce higher noise levels. Here, an adaptive spectrum compensation (ASC) method is proposed and applied in our home-built broadband surface-enhanced mid-infrared laser absorption spectrometer. The ASC method adaptively optimizes the laser beam and adjusts the SPD of the spectra reaching the detector with minimal loss. This reduces the dynamic range noise, detector noise and shot noise of the detection system, and allows quick adaptive switching between different background spectra. For our spectrometer, the ASC method has reduced the noise level of the absorption spectrum across 1660-975 cm−1 by more than 3 times. The proposed simple and low-cost spectrum compensation method could be applied to other tunable laser absorption spectroscopies with highly uneven SPD.