Particle Detectors: (4.24) Resonant-Cavity-Enhanced Photo Detector: (2) Theory of RCE Photo Detectors: The RCE photo detectors can provide: (a) Higher quantum efficiency, (b) Higher detection speed, (c) Wavelength selective detection, than compare to a conventional photodiode. (2. 1) Quantum efficiency of RCE photo etectors: The RCE photodetectors are expected to have higher quantum efficiency η than compare to conventionalphotodiodes. The formulation of η for RCE devices gives insight to the design criteria. A generalized RCE photodetector schematic as given in Figure 1 can give the required theoretical model ofphotodetection. A thin absorption region of thickness d is sandwiched between two relatively less absorbing region, substrate, of thickness L1 and L2. The optical cavity is formed by a period of λ/4 distributed Bragg reflector (DBR), made of non-absorbing larger bandgap materials, at the both end of the substrate. The front mirror has atransmittance of t1 and generally has lower reflectivity than compare to the mirror at back (R1 < R2). Transmittance t1 allows light to enter into the cavity, and reflectivity R1 (=r12) and R2 (=r22) provides the optical confinement in the cavity. The active region and the substrate region have absorption coefficient α and αex respectively. The field reflection coefficients of the front and the back mirrors are   and   respectively, where ф1 and ф2 are the phase shifts due to the light penetration (See: Penetration Depth) into the mirrors. The optical microcavity allows building up an optical field inside the optical cavity. In compare to conventional detector, where light is absorbed in a single pass through the absorption region, for RCE detectors trapped light is absorbed each time it traverses through the absorption region. The Quantum efficiency   for a RCE detector is given by: Here αc = (αexL1 + αexL2 + αd) /L. In practical detector design αex << α, so αex can be neglected and   can be given as: The term inside the represents the cavity enhancement effect. This is a periodic function of 2βL+ ф1 + ф2, which has minima at 2βL+ ф1 + ф2 = 2mп. And η enhanced periodically at resonance wavelength that meets this condition