We are developing a technique for the stand-off detection of trace analytes and residues (explosives, hazardous chemicals, drugs, etc.) using photo-thermal infrared imaging spectroscopy (PT-IRIS). Herein, we refer to this technique as "RED" for "Remote Explosives Detection" or "Resonance Enhanced Detection". This approach leverages recent developments in critical enabling micro and nano-technology components. The first component, a compact IR quantum cascade laser (QCL), is tuned to fundamental absorption bands in the analytes and directed to illuminate a surface of interest. The second component, an IR focal plane array (FPA), is used to image the surface and detect any small increase in the thermal emission upon laser illumination. We have demonstrated the technique at up to 30 meters of stand-off distance indoors and in field tests, while operating the lasers below the eye-safe intensity limit (100 mW/cm2). In this manuscript we detail several recent improvements to the method and system, as well as some recent results for explosives on complex substrates such as car panels and fabrics. We also introduce a computational framework for modeling and simulating the optical and thermal phenomena associated with the photothermal process.