Omnidirectional reflectors

The concept of omnidirectional mirrors, also denoted as perfect mirrors, was introduced in 1998 [247, 248]. These are one-dimensional, all-dielectric mirrors (ID photonic crystals) that keep their reflective properties even for large oblique incident angles in a wide and fully tailorable range of frequencies. 

Basically, omnidirectional mirrors are dielectric quarterwave stacks in which TIR and Bragg mirroring wavelength ranges overlap, i.e., an overlapping band gap regime exists that extends above the light cone. 

For a normally incident beam the stop bands for transverse magnetic (TM) and transverse electric (TE) polarization is identical. For increasingly oblique incidence the TM gap decreases, and the TE gap increases. 

The width of the omnidirectional frequency range can be approximated for quarter-wave stacks as [249] 

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Skorobogatiy M (2005) Efficient anti-guiding of TE and TM polarizations in low index core waveguides without the need of omnidirectional reflector. Opt Lett 30 2991-2993... 

Fig. 1.17 Perspective view of an omnidirectional reflector (ODR). The ODR also serves as ohmic contact.

A very promising electrically conductive omnidirectional reflector suitable for use in LEDs is shown in Fig. 1.17 [62, 63], The reflector comprises the LED semiconductor material with a refractive index ns, a low-refractive index layer (nu), and a metal with a complex refractive index Nm = nm + i km, where km is the extinction coefficient. 

Fig. 1.19 Schematic of the omnidirectional reflector (ODR) LED. An array of microcontacts perforating the ODR serves as p-type ohmic contact to the epitaxial AIGalnP layers.

Kim J. K., Gessmann Th., Luo H., Schubert, E. F. GalnN light-emitting diodes with Ru02/Si02/Ag omnidirectional reflector Appl. Phys. Lett. 84, 4508 (2004). 

Chen C.H., Kelder E.M., Schoonman J. Unique porous LiCo02 thin layers prepared by electrostatic spray deposition. J. Mater. Sci. 1996 31 5437-5442 Chen C.H., Kelder E.M., Schoonman J. Electrostatic sol-spray deposition (ESSD) and characterisation ofnanostructured Ti02 thin films. Thin Solid Films 1999 342 35-41 Chen K.M., Sparks A.W., Luan H.C., Lim D.R., Wada K., Kimerling L.C. Si02/Ti02 omnidirectional reflector and microcavity resonator via the sol-gel method. Appl. Phys. Lett. 1999 75 3805-3807... 

Figure 13-9. (a) TEM images of a Ti02/Si02 omnidirectional reflector (top) and a microcavity resonator (bottom), both of which were fabricated by the sol-gel process, and (b) angle-dependent reflectance spectra of the omnidirectional reflectorfor both TE and TM modes (solid curves observed, dotted curves calculated) (Chen, 1999). 

Chen K.M., Sparks A.W., Luan H.C., Lim D.R., Wada K., Kimerling L.C. Si02/Ti02 omnidirectional reflector and irdcrocavity resonator via the sol-gel method. Appl. Phys. Lett. 1999 75 3805-3807... 

Fan S., Villeneuve P.R., Meade R.D., Joannopoulos J.D. Design of three-dimensional photonic crystals at submicron lengthscale. Appl. Phys. Lett. 1994 65 1466-1468 Fink Y., Winn J.N., Fan S., Chen C., Michel J., Joannopoulos J.D., Thomas E.L. A dielectric omnidirectional reflector. Science 1998 282 1679-1682 Fukuda K., Sun H., Matsuo S., Misawa H. Self-organizing three-dimensional colloidal photonic crystal structure with augmented dielectric contrast. Jpn. J. Appl. Phys. 1998 37 L508-L511... 

Ponnampalam N. and DeCorby R. G., Out-of-plane coupling at mode cutoff in tapered hollow waveguides with omnidirectional reflector claddings. Opt Express, 16,2894-2908 (2008). 

Pure metal mirrors are omnidirectional reflectors, but the price to pay are their non-negligible absorption losses. On the other hand, all-dielectric multilayers exhibit very high reflectivity, but their reflectivity drops for oblique incident angles. Thus the idea occurs to combine both into a single structure. 

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