The "rib waveguide photodetector" of the previous image provided a breakthrough in Si-based photodetector performance. However, it could not be incorporated into an integrated circuit. For integration, we needed a device that could be grown on top of the larger circuit, and illuminated with light from above. However, that would mean that the detector would be thin, too thin to absorb all of the light (and thus very insensitive). We worked with Prof. Joe Campbell to solve this problem. Using our techniques for growing GeSi/Si multilayers, we came up with the design (and patent) for a mirror that could be grown below the photodetector (the "DBR" = distributed Bragg reflector). Any light that was not absorbed in the detector would then bounce back up through it, giving us another chance to absorb and detect it. If the photodetector were just the right thickness, it would "resonate" with the light, bouncing it back and forth repeatedly. The net result was that our thin, integrated circuit compatible photodetector ended up acting like a much thicker, very efficient and fast photodetector. To make the photodetectors the percise "resonant" thickness, we developed the reflectometry techniques of the following images.