Chapter 10: Reflectors (click on figure for full-size image)

Fig. 10.1. Different types of reflectors including metallic reflector, distributed Bragg reflector (DBR), hybrid reflector, total internal reflector (TIR), and omni-directional reflector (ODR). Also given are angles of incidence for high reflectivity and typical reflectances and transmittances. Fig. 10.2. Measured reflectance of a silver/air reflector for normal incidence. The average reflectivity in the visible spectrum is 98.5 %.
Fig. 10.3. Attenuation of waveguide mode due to lossy reflector. Fig. 10.4. Reflected and refracted light ray at the boundary between two media with refractive indices n1 and n2, where n1 > n2.
Fig. 10.5. (a) Historical drawing and (b) schematic illustration of apparatus used in 1841 by Swiss engineer Daniel Colladon to demonstrate the guiding of light by total internal reflection in a jet of water. Fig. 10.6. Reflectance of a silver/air reflector and a 25-pair AlAs/GaAs distri- buted Bragg reflector (DBR).
Fig. 10.7. LED with a distributed Bragg reflector (DBR) located between the substrate and the lower confinement layer. Fig. 10.8. Reflectance of two distributed Bragg reflectors (DBRs) versus wavelength. (a) Four-pair Si/SiO2 reflector with high index contrast. (b) 25-pair AlAs/GaAs reflector. The high-index-contrast DBR only needs four pairs to attain high reflectivity.
Fig. 10.9. Illustration of the DBR penetration depth. (a) DBR consis-ting of two materials with thickness L1 and L2. (b) Ideal (metallic) re-flector displaced from the DBR sur-face by the penetration depth. Fig. 10.10. Calculated reflectivity (inside the cladding GaP) versus (a) wavelength and (b) polar angle of a transparent AlGaInP/AlInP DBR and an absorbing AlAs/GaAs DBR.
Fig. 10.11. (a) DBR stucture used in calculation. (b) Reflectivity versus angle of incidence and critical angle at which reflectivity decreases. (c) DBR reflectivity versus wavelength for two angles of incidence. Fig. 10.12. Structure of omnidirectional reflector consisting of semiconductor, low-refractive index dielectric layer, and metal layer. The dielectric is perforated by an array of microcontacts providing electrical conductivity (after Gessmann et al., 2003).
Fig. 10.13. (a) Calculated relflectivity at normal-incidence versus wavelength and (b) reflectivity versus angle of incidence for an omnidirectional reflcetor (ODR), a transparent AlGaInP/AlInP DBR, and an absorbing AlGaAs/GaAs DBR (after Gessmann et al., 2003). Fig. 10.14. Light-output power versus injection current of different types of LEDs. The ODR device has a higher output power than the DBR device (after Gessmann et al., 2003).
Fig. 10.15. Current-voltage and light-output-versus-current characteristic of a GaInN LED with a GaInN/RuO2/SiO2/Ag omnidirectional reflector (after Kim et al., 2004). Fig. 10.16. Schematic of a specular and diffuse (lambertian) reflector. The reflected power distribution of a lambertian reflector follows a cos theta dependence.
Fig. 10.17. Photograph showing that the sunís surface has a constant brightness independent of the viewing angle with respect to the sunís surface. It is a good example of a lambertian source. Fig. 10.18. (a) Optical mode guided by specular reflector at the epilayer/substrate interface and the epilayer/air interface. (b) Optical ray propagating in epilayer guided by lambertian reflector at the epilayer/substrate interface and the epilayer/air interface.
Fig. 10.19. Reflection contours of a roughened silica layer for different angles of incidence of the laser.