Dielectric, mirror coating

Figure 6.12 shows the schematic constraction of a typical optically addressed liquid crystal spatial light modulator (OALCSLM) operating in the reflective mode. It consists of an aligned liquid crystal layer sandwiched between transparent electrodes and adjacent to a photoconducting semiconductor layer (a-Si H) sensitive to the writing beams and a dielectric mirror coated to block the reading beam. 

Commercially available are various types of aluminium front surface mirrors to suit different requirements. For the visible spectral range, there are standard mirrors such as Alflex A . If improved reflection is required, a multiple film mirror Alflex B can be used. Both types of mirrors are provided with a hard and resistant dielectric protection coating. Such mirrors were first made by Hass et al. [73, 74]. The aluminium film on the surface mirror Alflex is even protected by an interference film system, which also enhances the reflectance for the visible range. In the visible and infrared, the spectral curve of the reflectance is approximately the same for Alflex A as that of an unprotected aluminium surface. With a mirror type Alflex B. the increase in reflection in the visible, with a maximum at 550 nm, can be clearly seen in Fig. 12. If required, this maximum can also be shifted to other wavelengths in the visible spectrum. 

Vital to the operation of an interference filter is a very high reflectance of the mirror coatings adjacent to the spacer layer. The absorption of the metal mirrors can be reduced and thus the maximum transmittance of the filter increased if both metal layers are increased in reflectance by additionally deposited high reflecting dielectric multilayers [124]. In this way, with a first-order filter, a half width of 2 nm and a transmittance maximum of 41% can be obtained [124]. If, however, the metal mirrors are completely replaced by absorption-free all-dielectric high reflecting multi-... 

The general approach used to make a dielectric mirror is to lay down a stack of thin films that have alternately higher and lower refractive indices. Manipulation of the thickness and the refractive index of each layer in the stack allows the optical properties to be modified at will to produce virtually perfect mirrors and virtually perfect antireflection coatings - both of which can be mned to respond to very specific wavelengths - as well as a variety of optical filters. The fabrication of such devices falls... 

The spectral range of thin metal mirrors is typically very wide. Also, they are relatively insensitive to the changes of the incident angle. Because of their low cost and relatively high-reflection coefficient they are often used for the deposition to the backside of the detector. Since thin metal mirrors quickly become covered by an oxide layer spoiling their reflection, in some applications additional dielectric protective coatings are deposited on them, for instance magnesium fluoride or silicon monoxide. This is usually not necessary when they are used for the improvement of photodetector quantum efliciency. 

Heat- and light-separation coatings (also known as hot and cold mirrors) are important applications that separate the hot (infrared) from the cold (visible) radiation. The principle is shown schematically inFig. 16.2. The transmittance of a cold mirror is shown in Fig. 16.3a. This mirror is coated with a dielectric film reflecting more than 90% of the... 

From a theoretical point-of view, significantly higher current densities are feasible, but require further improved front TCO films and perfect mirrors as back reflectors. This is illustrated by the dotted curve in Fig. 8.28, which shows simulations of quantum efficiency for a 1 pm thick pc-Si H solar cell. These simulations reveal a current potential of 29.2 mA cm-2 by improved optical components like reduced parasitic absorption in the front TCO, ideal Lambertian light scattering, dielectric back reflectors, and antireflection coatings on the front side [147]. However, this still has to be achieved experimentally. 

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