Fig. 2.
Laser-based confocal IR microscopy. (A) The microscope consists of a quantum cascade laser (QCL) source, tunable for narrow-band emission across the mid-IR fingerprint region. A high-speed stage and focus unit raster scans the sample, while the detection is locked into the laser’s pulse rate and each pixel is triggered by the stage encoder counter. (B) Images of USAF 1951 resolution test targets show diffraction-limited performance for absorbance at 1,658 cm−1 with both the 0.56 N.A. objective (Top) at 2-μm pixel size and the 0.85 N.A. objective (Bottom) at 1-μm pixel size. (C) The optical contrast of each set of bars is plotted as a function of spatial frequency. The bars are no longer resolvable if the contrast drops below 26%, which corresponds to the Rayleigh criterion separation distance. The arrow indicates the deconvolution of the raw data (solid line) with the simulated PSF at the specified wavenumber to achieve a substantial resolution enhancement (dotted line). These results are compared with the simulated performance (dashed line) of a FT-IR instrument with optimized Schwarzschild objectives used in FT-IR imaging. (D) The unprocessed spectrum of a 5-μm layer of SU-8 epoxy acquired by our QCL instrument at 1 cm−1 resolution shows accurate spectral features compared with a reference FT-IR spectrometer. The 100% spectral profile lines show absorbance noise of ∼10−4 and 10−3 for the two objectives over most the fingerprint region.