Feedback as the source of imperfection in lossy perfect lenses

The major barrier to realizing a perfect lens with left-handed materials is perceived to be their intrinsic loss. Here we show that only specific designs of perfect lenses are limited by loss - those in which material loss is translated to internal feedback. The asymptotically uniform transmission required for perfect lensing is hindered by such feedback, which generates resonances that lead to a spatial cutoff in the lens transmission. Moreover, uniform transmission and its resonant deterioration stem from completely separate classes of modal excitations. A perfect lens made of lossy left-handed materials is therefore not forbidden in principle. Pursuing perfect lens designs that avoid internal feedback offers a path towards realization of practical perfect lenses.