Thermodynamic limit to light trapping in thin planar structures

We calculate the degree to which absorption can be enhanced by light trapping in a material whose thickness is on the order of a wavelength of light. The calculation makes use of an extension of the radiance theorem in the wave domain and represents the ultimate upper limit as determined by the laws of thermodynamics. It is a general upper limit, based solely on considerations of the modal structure of the device and independent of the technique used to couple light into the material. We assume only that the coupling mechanism is isotropic. An important parameter in our result is the density of guided modes within the planar structure, which we calculate. We find that even for structures supporting only a small number of guided modes, significant enhancements in absorption are possible. (C) 1997 Optical Society of America.