8 February 2012
Infrared spectroscopy provides detailed information about sample composition by probing its molecular structure. Most molecules have a unique ‘fingerprint’ in the mid-infrared (MIR) spectral region with wavelengths spanning 3–25μm. In addition, molecules absorb strongly in the MIR, making detection of trace quantities possible. Fourier transform infrared (FTIR) spectroscopy is routinely used for chemical identification of gases, liquids, and solids. However, for microscopy applications, FTIR spectroscopy is limited by low sample illumination brightness leading to long acquisition times. Synchrotron radiation sources provide a higher brightness MIR source, but are limited in availability.
Our approach to infrared microspectroscopy uses an external cavity QCL (ECQCL) to provide high brightness, narrow linewidth, and broadly tunable MIR illumination.4 Quantum cascade lasers are semiconductor laser sources emitting in the MIR spectral region that provide high power, small size, and robust operation at room temperature and above.5, 6 The wide tuning range of an ECQCL allows measurement of MIR spectra of solid-state materials, which typically exhibit broad spectral features. By combining an ECQCL with a transmission microscopy setup, we have achieved a system capable of rapidly imaging microscopic samples and measuring their MIR absorption spectrum, which can be used for chemical identification.
Figure 1 shows a schematic of our hyperspectral microscopy setup. The system uses a custom-built ECQCL7 to provide illumination of a sample at continuously tunable wavelengths spanning 9.13–10.53μm. (Other wavelength ranges can be accessed by changing the ECQCL.) The sample is imaged in transmission using high-magnification optics and detected using an uncooled microbolometer infrared camera. A sequence of infrared images is acquired while tuning the ECQCL illumination over a range of wavelengths, generating an image hypercube. By acquiring an entire image in one frame with the infrared camera, fast acquisition times of 40 milliseconds per wavelength band are achieved. For example, hyperspectral images containing 100 wavelength bands were acquired in four seconds, limited by the camera’s frame rate. In contrast, raster-scanning techniques build up an image point by point, increasing acquisition time.
We used the hyperspectral microscopy system to image small particles and residues of explosives. Figure 2 shows images of the mean infrared transmission through particles of common explosives known as PETN, RDX, and tetryl. The spatial resolution of the system was measured to be 13μm, limited by diffraction at the infrared wavelengths used. Each pixel in the hyperspectral images has an associated transmission spectrum. Figure 2shows the measured spectra taken from the different explosive particles. The characteristic peaks in the spectra are used for identification and characterization of the materials.
Individual particles with masses less than 1ng were imaged and could be identified based on their infrared absorption spectrum. Figure 3 shows an image of a tetryl particle with an estimated mass of 700pg. For reference, a single 1ng particle in the field of view of the microscope represents a surface concentration of 30ng/cm2. Additional optimization of the imaging optics and techniques to mitigate coherent illumination artifacts are expected to improve the image quality and detection limits.
In summary, we have demonstrated a technique to perform microscopy using broadly tunable ECQCL illumination, which measures the MIR transmission of a sample to provide chemical identification and mapping. The combination of a high-brightness, widely tunable MIR laser source with a low-cost infrared camera allows high-quality images to be acquired rapidly. The study of microscopic explosive particles will increase our understanding of the factors limiting the performance of hyperspectral imaging with MIR laser illumination and improve the performance of a system optimized for explosives detection at larger standoff distances. The microscopy technique could also have application for non-contact and non-destructive trace particle identification and characterization in forensics or analytical laboratory use. Future work will include adapting the microscopy technique to reflection and scattering geometries, and investigating additional materials.
The research described in this paper was conducted under the Laboratory Directed Research and Development Program at Pacific Northwest National Laboratory, a multiprogram national laboratory operated by Battelle for the US Department of Energy under contract DE-AC05-76RL01830.
Pacific Northwest National Laboratory
Mark C. Phillips is currently researching applications of mid-infrared technology and QCLs.