Most materials absorb infrared (IR) light at specific patterns of frequencies directly related to their structure. An IR spectrum can therefore be used to identify and quantify the constituents of an unknown sample, making detection of chemicals by their absorption of IR light an important analytical tool. Laser-based methods of IR detection offer the special attributes of very high sensitivity and long-range detection.
We at Sandia have developed a number of laser-based IR chemical measurements that use quasi-phase-matching (QPM) to generate the light. QPM is a relatively new technology that allows efficient nonlinear conversion of light of a particular color (wavelength) from an existing laser into new colors. We have successfully used QPM-based nonlinear mixing to convert light from available near-IR lasers (e.g., a Nd:YAG) into mid- (wavelength = 2-4 mm) or long-wave-IR (wavelength = 8-12 mm) light that can then be used for unique measurements.
Existing (traditional) lasers often cannot produce infrared light of a particular format (e.g., pulse length or wavelength). Nonlinear mixing, however, converts light of one color into other colors. With a typical nonlinear light source, photons from an existing laser or lasers are passed through a nonlinear material where new photons are produced. The new photons are produced by the injected laser beams that cause the material to oscillate at a frequency other than that of the driving laser field. This resonance oscillation is similar to the mechanical nonlinearity oscillation of a guitar string—when plucked, a guitar string emits its fundamental frequency but also may produce overtones, at twice the frequency. By producing an optical-frequency oscillation that moves charges around in the nonlinear medium, the nonlinear process allows light to be generated at the new frequency.