Fundamentals of Photon Management in Photodetectors

The optical methods of optimization have a goal to maximize the quantum efficiency of a photodetector for a given geometry (thickness and active area) and a given spatial and frequency dependence of the absorption coefficient. They are described as photon management or light management methods. 

There are different methods of light management in a photodetector that can be divided into four groups. Fig. 2.1 optical concentration, use of antireflection stmctures, optical path increase, and light localization. 

Optical concentration methods actually collect incident radiation from a larger area (denoted as optical area) and concentrate it (focus) to the smaller active area of photodetector (the electrical area). The concentration efficiency can be then defined as the ratio between the optical and the electrical area, minus absorption and scattering losses. This method of light management is typically done by utilizing stmctures which are not themselves a part of the detector, but can be integrated with it. The simplest case of a concentrator would be an immersion lens, but there are a number of different other stmctures to serve the same purpose. Roughly, one could 

Jaksic, Micro and Nanophotonics for Semiconductor Infrared Detectors, 

After arriving to the detector surface, the incident beam should enter the photodetector area with losses as low as possible. Semiconductor materials used for photodetectors have large values of refractive index and thus large values of reflection coefflcient at the device surface. This reflection is minimized using antireflection structures. Basically, these stmctures serve as impedance matching media between the detector environment (most often, but not always free space) and the active region. 

---