Bragg dielectric stack

Figure 4.14. Typically the mirrors are fabricated using a Bragg dielectric stack consisting of multilayers of dielectrics with different indices of refraction, (high) and 2 (low), stacked alternately with high and low indices of refraction for several cycles. The thickness of each layer is chosen so that a quarter-wavelength of the light that is to be filtered fits in the gap ...

Bragg mirrors on periodic stacks of layers Periodic stacks of metal nanoparticles or dielectric layers with alternating high and low refractive index produce a desired reflectance of the mirror that depends on the thickness and the refractive index of the layers in the stack 16,17... 

It is evident from physical considerations that the Bragg conditions for diffraction will be of exactly the same kind as for X-rays. However, quantitative results require the solution of the optical problem of diffraction by a finite stack of dielectric sheets having a periodic variation of dielectric constant. This problem is best handled by numerical means and the question is discussed by Hayter et al. [53J, who also give references to earlier papers in which the numerical methods are fully discussed. Similar methods have also been made use of by Nicklow et al. [541. Further studies were made by Highfield et al. [55]. 

Microcavity OLEDs fabricated on distributed Bragg reflectors (quarter wave stacks) [94]. Such OLEDs are fabricated on dielectric layers with significant dielectric contrast, so they narrow the emission spectrum by constructive interference. The narrow emission spectra also result in more efficient and more stable devices than regular OLEDs. The emission spectrum can be tailored to the specific sensor requirements (i.e., the absorption peak of the sensing element). 

Fig. 12.5 Comparison of the non-polarized light transmission by a stack of dielectric layers and a cholesteric liquid crystal (CLC). The two materials have the same Bragg reflection frequency (numerical calculations, for parameters see the text), (a) Transmission spectra on the frequency scale showing the absence of high harmonics in the case of CLC (b) blown transmission spectra at the wavelength scale showing the flat form of the CLC Bragg band and oscillations of transmission at the edges of the band...

Interference mirrors are dielectric thin film coatings where low- and high-refractive index layers alternate. The optical thickness of each of the layers is equal to quarter-wavelength QJAn). They are denoted as distributed Bragg mirrors or distributed Bragg reflectors (DBR), sometimes simply as Bragg mirrors. Other names include quarter-wave mirrors (QWM), quarterwave stacks (QWS) and highly reflective (HR) layers. 

Bragg mirrors are routinely fabricated by alternatively depositing quarter-wavelength stacks with high (m/,) and low (m/) refiactive index (approximately lossless dielectrics are assumed). Their principle of operation is similar to that of multilayer antirellection films, but the incident beam arrives at the layer with high-refractive index, and in this case the interference on the quarter-wavelength stacks is constructive. 

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. 

While not the optimal layer thickness for the Bragg stack, which would have dielectric layer thicknesses that are one-quarter of the wavelength of light that is to be filtered, it makes good use of the available layers in the... 

Lay out a tunable Fabry-Perot optical filter in a multiproject wafer process as shown in Figure 4.24. The top Bragg mirror should be composed of a stack of dielectric films and should be flat to within a fraction of a wavelength of light that will be filtered. 

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