Metals reflection coefficient

The presence of adsorbed layers also affects the other parameters of the interaction between metastable atoms and a metal surface. Titley et al. [136] have shown that the presence of an adsorbed layer of oxygen on a W( 110) surface increases the reflection coefficient of helium metastable atoms. The reflection is of irregular nature and grows higher when the incidence angle of the initial beam increases. A series of publications [132, 136, 137] indicate that the presence of adsorbed layers causes an increase in the quantum yield of electron emission from a metal under the action of rare gas metastable atoms. 

It is well known that the SPR may be registered as the sharp minimum of the reflection coefficient for the plane-parallel light which depends on the incidence angle. The position of the resonance angle and the minimum depth of the incidence are determined by the parameters of the metal layer, and the optical constants of the external medium. As molecules adsorb and interact at the gold surface, the dielectric properties of the formed layer change, which leads to the transformation of the resonance curve and to the displacement of the resonance angle [7, 9, 15]. 

Let us next consider a metal hoUow-optical fiber with dielectric inner coating. When the coating thickness is properly designed, the dielectric layer enhances reflection in a specific wavelength range owing to the interference effect. Let us consider a metal surface coated with a dielectric film of thickness d as shown in Fig. 2. From Fresnel s formula, the reflection coefficient of the electric field on boundary I is expressed as... 

Using ri and r2 one can evaluate the reflection coefficient r of the film-coated metal surface for p- or -polarized light as follows [2]... 

In hollow-core metal waveguides, when excited with linearly polarized light, TE modes are dominant because reflection coefficients of metals are much higher for the i-polarization state. To reduce the loss for a randomly polarized incoherent light... 

Fig. 5. Schematic view of a metal layer, surrounded by arbitrary materials (pi2 and p2s can represent the reflection coefficient of multilayers, media 1 and 3 can be different).

Concerning the absorption of evanescent waves, a 50-mm circular parallel-plate metallic waveguide whose TMi mode has a cutoff at 3 GHz is explored. For a wideband attenuation, the PML of [26] is selected. All implementations are performed with operators (3.42) optimized for k = 1.437. Various setups are constructed and their reflection coefficient is shown in Figure 7.6. Observe that all absorbers display a promising annihilation of evanescent waves below the cutoff frequency, while HO counterparts attain a considerable decrease of PML depths for specific reflection coefficient values. 

Screening of interparticle interactions using LbL assembly has also been demonstrated by depositing inert monolayers between each two consecutive metal [11] or magnetic [15] nanoparticle layers. In such methods, only interlayer interactions are screened, so that spatially modulated coupling can be achieved and polarization effects on the reflection coefficient can be obtained. 

The understanding of such a problem is obvious from the example of a metallic plate coated with an homogeneous material receiving a plane wave at normal incidence. The calculation of the reflection coefficient is then simple. The surface impedance is defined by ... 

The first objective is the measurement with good precision of cither the RCS of a complex shape or the reflection coefficient of a metallic plate covered with an absorbing material. The basic principle consists in lighting the object and measuring the electromagnetic energy backscattered from it. For that purpose, the measurement is made in a room named anechoi c chamber since all the walls are covered with pyramidal foam absorbers suppressing all parasitic reflections. 

At high temperature thermolysis (1270K) of cobalt acetate with PS, PAA, and PMVK present, the metal clusters that are catalyzing an oxygen electrolytic reduction had formed [98]. As the thermal decomposition of silver trifluoro-acetyl-acetonate at 613 K occurs in polyimide film [99-101], the film becomes metallized (a structure called film on film ) and a nanocomposite is formed with high surface conductivity and light reflection coefficient (above 80%). 

Regarding materials, the structures of microreflectors and their arrays are usually made of metal with good reflection coefficient. There are a wide choice of metals and alloys convenient for the fabrication of reflector-type concentrators. 

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 %). 

Fig. 2.35 Spectral dependence of reflection coefficient of some metals used for reflective layers in IR detectors...

The spectral range of thin metal mirrors is typically very wide. Also, they are relatively insensitive to the changes of the incident angle. Because of their low cost and relatively high-reflection coefficient they are often used for the deposition to the backside of the detector. Since thin metal mirrors quickly become covered by an oxide layer spoiling their reflection, in some applications additional dielectric protective coatings are deposited on them, for instance magnesium fluoride or silicon monoxide. This is usually not necessary when they are used for the improvement of photodetector quantum efliciency. 

The most often used materials for the infrared range are aluminum and gold [5]. There is a wealth of literature data on these materials in flie IR range. As an example. Fig. 2.35 shows spectral characteristics of the reflection coefficient of some polished metals in the (3-14) pm range, according to [1, 238-240]. All dependences are valid for normal incidence and at room temperature. 

Chance, Frock and Silbey [28] computed explicit expressions for the normalized decay rates near a metal surface, expanding the dipolar field in plane-waves and considering for each of them the reflection coefficients from the metal substrate ... 

These rates are named "apparent" because correspond only to what can be measured, collecting the light coming out above the metal substrate. Without the metal substrate all the reflection coefficients vanish and we have ... 

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