SIMBAD references

We examine the wavelength dependence of flux ratios for six gravitationally lensed quasars using K and L' images obtained at the Gemini North 8 m telescope. We select lenses with source redshifts z_s< 2.8 so that K-band images probe rest-frame optical emission from accretion disks, while L'-band images probe rest-frame near-infrared flux emitted (in part) from the more extended surrounding torus. Since the observations correspond to different source sizes, the K and L' flux ratios are sensitive to structure on different scales and may be useful for studying small structure in the lens galaxies. Four of the six lenses show differences between K and L' flux ratios. In HE 0435-1223, SDSS 0246-0825, and HE 2149-2745, the differences may be attributable to known microlensing and/or intrinsic variability. In SDSS 0806+2006, the wavelength dependence is not easily attributed to known variations and may indicate the presence of substructure. By contrast, in Q0142-100 and SBS 0909+523 the K and L' flux ratios are consistent within the uncertainties. We discuss the utility of the current data for studying chromatic effects related to microlensing, dust extinction, and dark matter substructure.