Dielectric Interference Mirrors

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. 

After a light beam enters the stack, each interface between two adjacent layers will reflect a part of the incident radiation. All the emerging beams will be in phase and thus their intensities will add. In that way constmctive interference will greatly enhance the reflection of the whole stack. 

An approach that can be generally applied to various multilayer structures is the well-known transfer matrix technique [87, 180]. It can be used regardless of the geometry of the layers or the intended application of the multilayer. Besides being usable for the calculation of quarterwave Bragg mirrors, it can be used without modification to accurately determine the properties of antireflection coatings, step-down structures, various quasiperiodic, aperiodic and random stractures, but also 2D and 3D photonic crystals as well [241] 

In comparison to an arbitrary multilayer coating, a quaiterwave ID dielectric stack has additional symmetry which allows one to determine its transfer matrix analytically, thus significantly simplifying the procedure of calculation of its properties and optimization of the final structures. 

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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... 

In Fabry-Perot etalons, the cavity encloses air, a gas, or vacuum, while for interference filters transparent dielectric layers are used. The real length of the interfering beam is N times the length of the cavity, due to multiple reflection. /V, the so-called finesse, is determined by the reflectivity p of the mirrors N = K , /pl( - p). Therefore, the resolving power, as above, equals the length of the interfering beam in units of the wavelength ... 

A simple example of this is the case of a molecule (modeled as an oscillating dipole) close to a perfect mirror. If the dipole is parallel to the mirror, destructive interference between directly emitted light and reflected light causes a reduction in the radiative rate. In the presence of competing nonradiative decay processes, this leads to a reduction in the efficiency of emission. The variation of radiative rate with position and orientation for a molecule within an arbitrary planar dielectric structure has been modeled by Crawford.81 This model has been applied to polymer LEDs by Burns et al.,82 and Becker et al.,83 who predict significant variations in the efficiency of radiative decay in polymer LEDs depending on the distribution of exciton generation within the device. 

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. 

There are basically two ways to achieve high visual transmittance simultaneously with high infrared reflectance. One is the use of the interference effect in all-dielectric multilayers, the other is the use of intrinsic optical properties of electrically conducting films such as Au, Ag, and others which have high infrared reflection with relatively low visual absorption. Their suitability as transparent heat mirror can be improved by antireflection coating for the visible. Figure 25 shows an example for such types of heat mirrors, according to Fan et al. [102]. 

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-... 

Laser Fluorescence Detector. A helium-cadmium laser (Model 4240B, Llconlx, Sunnyvale, CA) was (diosen as the excitation source because of Its stability and convenient wavelengths (325 and 442 nm). The UV laser radiation (325 nm, 5-10 mH cw) was Isolated with a dielectric mirror and was focused on the miniaturized flowcell with a quartz lens. Sample fluorescence, collected perpendicular to and coplanar with the excitation beam, was spectrally Isolated by appropriate Interference filters and then focused on a photomultiplier tube (Centronlc Model Q 4249 B, Bailey Instruments Co., Inc., Saddle Brook, NJ). The resulting photocurrent was amplified with a plcoammeter (Model 480, Kelthley Instruments,... 

Examples of devices in which only two partial beams interfere are the Michelson interferometer and the Mach-Zehnder interferometer. Multiple-beam interference is used, for instance, in the grating spectrometer, the Fabry-Perot interferometer, and in multilayer dielectric coatings of highly reflecting mirrors. 

The constructive interference found for the reflection of light from plane-parallel interfaces between two regions with different refractive indices can be utilized to produce highly reflecting, essentially absorption-free mirrors. The improved technology of such dielectric mirrors has greatly supported the development of visible and ultraviolet laser systems. 

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 %). 

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