Reflective index modulation

As shown in previous sections of this chapter, when an external perturbation is applied to the polymer film (such as irradiation), the ATR guided modes shift their angular positions and the reflectivity is modulated (Fig. 31b). These angular shifts are very small in the case of electrooptic experiments they correspond to refractive index variations of the order of 10 . One has then to modulate the measuring electric field at a low frequency Q( = cos fit) and to detect the modulated signal with lock-in amplifiers. The lock-in signals detected at the modulation frequency and its second harmonic give, respectively, the linear (or Pockels) and the quadratic (or Kerr) electrooptic effects. The amplitude of the modulation of the thickness and the refractive indices is evaluated by a computer fit, and allows the determination of Pockels (r) and Kerr (s) coefficients (Eqs. 28) ... 

The reflectivity and width of the band gaps increase as the refractive index modulation increases. The basic principle for optical limiting by a ID nonlinear photonic crystal material can be understood from Figure 2. Initially, (A), the refractive indices of the different layers are matched, and the sample... 

We described a facile multilayer extrusion technique for fabricating these novel polymer film materials. The presence of a discrete nanolayered structure was verified by an AFM study. The AFM study also demonstrated the presence a distribution of layer thickness within the polymer film. The distribution is essential to controlling the bandwidth of the induced reflectivity. A broad band reflectivity was demonstrated in a layered material with a permanent index modulation of 0.0045. This wavelength dependence of the reflectivity showed a fine structure characteristic of dielectric stack with a distribution of layer thicknesses. 

The laser interferometer consists of two coupled resonators, one containing the laser, the other the plasma under investigation (Fig. 10). The laser radiation, reflected back from mirror A/s, which contains phase information about the refractive index of the plasma, interferes with the laser wave in cavity A, resulting in an amplitude modulation of the laser output 267). This modulation can be related to the refractive index and therefore to the plasma frequency and electron density. With a curved rather than a planar mirror, the sensitivity can be increased by utilizing transverse cavity modes 268). 

Current production of solar cells is dominated by crystalline silicon modules however, due to the high refractive index of silicon, more than 30 percent of incident light is reflected back, which greatly reduces the conversion effi-... 

As the refractive index differs only slightly from unity, relative variations in refractive index in the interfaciEd region are also very small and the reflectivity may be treated as the Fresnel reflectivity between two bulk phases modulated by small interference effects from the interfacial region. Neglecting refraction, which is Justifiable for the usual situation that q = q at the critical angle), and absorption this leads to the kinematic or Bom approximation ... 

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