Metal-dielectric mirrors

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. 

A very old and obvious solution is to deposit a dielectric layer onto metal, thus reducing the absorptance and increasing reflectance. The transmittance remains very low or zero in such structures, but this fact is of no consequence for light trapping structures for photodetectors. 

A generalization of this approach is to deposit a quarterwave dielectric mirror onto a metal reflector. In this way, very high reflectance is obtained for near-normal incidence. For more oblique incident angles the aU-dielectric part will be transmissive, but in that case the metal part will reflect these beams, albeit with a lower reflectance. In this way the benefits of both types of the used structures are simultaneously employed. In [251] the authors mention the use of hybrid reflectors consisting of a Bragg-type dielectric mirror with an additional thin metal reflector deposited on its backside. 

Another type of structures for metal-dielectric mirrors are metallodielectric multilayers. In this case alternating quarterwave or subwavelength stacks of metal and dielectric are deposited. Typical for such multilayers is alow reflection in visible, but large in infrared wavelength range. Thus they basically behave as low-pass optical filters. Such stmctures were denoted in literature as heat mirrors. First heat mirrors were fabricated as early as in 1950s [252]. The simplest heat mirrors consist of three layers only, dielectric-metal-dielectric or, alternatively, dielectric-transparent conductive oxide-dielectric [253]. Full multilayer metal-dielectric reflectors with binary but also ternary layers were also considered [254]. Because of their high reflectance in infrared, but also because of their plasmonic properties [255] metal-dielectric multilayer mirrors are of interest for cavity enhancement of infrared detectors. 

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Figure 143 illustrates the high spectral selectivity of the microcavity structure. The EL spectrum of the Eu complex-based conventional LED from Fig. 136a is compared with the EL spectrum of the same organic layers system placed in a microcavity formed by the MgAg metal/100% mirror (150 nm) and a dielectric half mirror (a quarter-wave stack... 

On the other hand, layer dependent screening contributions can be estimated for metal-dielectric interfaces applying a dielectric continuum model according to Aliis(d) A/ 7i(co) = -e2 /(1 (me(f dj [4, 8], where d is the distance from the mirror plane, AEB(d) and AEB(oo) is referred to the distance d and the infinitely thick film, respectively. Here, we assume e 3 and 0.34 nm for the molecule-molecule-distance. The distance of the first layer to the mirror plane of the metal di could be different on a microscopic scale. We apply the van der Waals radius of carbon in organic compounds (analogously to [8]) di = 0.17 nm and for comparison a distinct larger value (0.23 nm). The results are summarized in Table 1 ... 

Transmittance and reflectance of a transparent heat mirror using a thin metal-dielectric film combination [62]. 

When using a metal-dielectric interference filter, the uncemented mirror surface must always face the light source. Where filters are to be used above 40°C, the time... 

Various structures of microcavity OLEDs employing different combinations of mirrors using one metal mirror and one dielectric mirror (a and b), using two dielectric mirrors (c), and using two metal mirrors (d and e). (From Wu, C.-C. et al., /. Display Tedmol., 1,248,2005. With permission.)... 

The general complexity in the fabrication of dielectric mirrors and their strong wavelength-dependent reflection properties makes it very difficult for them to be implemented in real OLED displays. In view of these, microcavity OLEDs using metal mirrors (one reflective and one semi-transparent/reflective, Figure 9.10d and e) are more practical for display applications [3,10,27-29,33]. However, due to absorption (loss) in metals, it is not clear whether one can still obtain luminance enhancement as in microcavity OLEDs using lossless dielectric mirrors, and it is also not clear to what degree and under what conditions one obtains most luminance enhancement. 

The conventional solution to achieve specular reflectance is to use flat metal surfaces. Other solutions are interference-based multilayer dielectric reflectors (Bragg mirrors) and, as their generalization, photonic bandgap stmctures (photonic crystals) of all-dielectric and metal-dielectric type, etc. Nanoscale interferometric and diffractive stmctures offer extremely large values of reflection coefficient (in excess of 99.99 %). 

Surface plasmon waves are surface electromagnetic modes that travel along a metal-dielectric interface as bound nonradiative waves with their field amplitude decaying exponentially perpendicular to the interface (Raether 1988). Surface plasmons are usually excited by coupling them to an evanescent wave at a dielectric surface. A plasmon wave atom mirror can be formed on a glass surface with a thin deposited... 

Conventionally RAIRS has been used for both qualitative and quantitative characterization of adsorbed molecules or films on mirror-like (metallic) substrates [4.265]. In the last decade the applicability of RAIRS to the quantitative analysis of adsorbates on non-metallic surfaces (e.g. semiconductors, glasses [4.267], and water [4.273]) has also been proven. The classical three-phase model for a thin isotropic adsorbate layer on a metallic surface was developed by Greenler [4.265, 4.272]. Calculations for the model have been extended to include description of anisotropic layers on dielectric substrates [4.274-4.276]. 

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... 

Another interface that needs to be mentioned in the context of polarized interfaces is the interface between the insulator and the electrolyte. It has been proposed as a means for realization of adsorption-based potentiometric sensors using Teflon, polyethylene, and other hydrophobic polymers of low dielectric constant Z>2, which can serve as the substrates for immobilized charged biomolecules. This type of interface happens also to be the largest area interface on this planet the interface between air (insulator) and sea water (electrolyte). This interface behaves differently from the one found in a typical metal-electrolyte electrode. When an ion approaches such an interface from an aqueous solution (dielectric constant Di) an image charge is formed in the insulator. In other words, the interface acts as an electrostatic mirror. The two charges repel each other, due to the low dielectric constant (Williams, 1975). This repulsion is called the Born repulsion H, and it is given by (5.10). 

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