The R1 research group is responsible for the study of the fundamental physical mechanisms involved in subsurface imaging. Two approaches are particularly emphasized: nonlinear probes and dual- or multiple-wave probes. CenSSIS is investigating nonlinear acoustic and optical probes, as well as probes exhibiting quantum effects at the single-photon level. These techniques offer numerous benefits not available in conventional imaging. The fundamental barrier facing nonlinear modalities is the incomplete understanding of the physical mechanisms that produce the results. In dual-wave probes, two fields probe the target. CenSSIS is investigating ultrasound-assisted diffuse optical tomography and laser-induced non-contact acoustic imaging. Multi-wave imaging is also being developed using spectral-domain interferometry, with applications in nano-scale microscopy. As in nonlinear probes, the underlying physics of dual-wave mechanisms is not well enough understood to predict the fundamental limits on imaging quality.