Optical constants isotropic film

Infrared ellipsometry is typically performed in the mid-infrared range of 400 to 5000 cm , but also in the near- and far-infrared. The resonances of molecular vibrations or phonons in the solid state generate typical features in the tanT and A spectra in the form of relative minima or maxima and dispersion-like structures. For the isotropic bulk calculation of optical constants - refractive index n and extinction coefficient k - is straightforward. For all other applications (thin films and anisotropic materials) iteration procedures are used. In ellipsometry only angles are measured. The results are also absolute values, obtained without the use of a standard. 

The vibrational spectrum of benzene around 1000 cnf has also been measured. IQ. Benzene was physisorbed on a cooled copper substrate in the vacuum chamber. Figure 19 shows the transmission for several thicknesses of benzene and a prism separation of 3 cm. The thickness was determined from the measured transmission in transparent regions using Eg. (7). The solid curves were calculated from Eqs. (5) and (6) using optical constants for benzene obtained from an ordinary transmission experiment.il The benzene film was assumed to be isotropic. Of the two absorption lines seen, one belongs to an in-plane vibrational mode, and one to an out-of-plane vibration. Since the electric field of the SEW is primarily perpendicular to the surface, the benzene molecules are clearly not all parallel or all perpendicular to the copper surface. Also it should be noted that the frequencies are the same (within the experimental resolution) as those of solid benzene22 and of nearly the same width. These features indicate that the benzene interacts only weakly with the copper surface, as would be expected for physisorbed molecules. 

The purpose of obtaining spectral data on the complex refractive Index of polycarbonate polymer was to permit detailed Interpretations to be made of the IR-RA spectra collected In situ on metal-backed films of this material. Several of the principal methods for obtaining the optical constants n and k of an Isotropic medium have been reviewed by Humphreys-Owen [18]. All of the methods outlined are Insensitive to k when k Is close to zero, which Is the case for frequencies between absorbance bands. For this study, a polarlmetrlc technique (method D In Ref. 18) was chosen to obtain the optical constants of BPA-PC. To apply this method, the ratio of surface reflectances Rp/Rg at two large, but well-separated, angles of Incidence (9 ) was obtained for BPA-PC In the IR. Rp Is defined as the reflectance measured for a sample using radiation polarized parallel to the Incidence plane and R... 

Ellipsometry at noble metal electrode/solution interfaces has been used to test theoretically predicted microscopic parameters of the interface [937]. Investigated systems include numerous oxide layer systems [934-943], metal deposition processes [934], adsorption processes [934, 944] and polymer films on electrodes [945-947]. Submonolayer sensitivity has been claimed. Expansion and contraction of polyaniline films was monitored with ellipsometry by Kim et al. [948]. Film thickness as a function of the state of oxidation of redox active polyelectrolyte layers has been measured with ellipsometry [949]. The deposition and electroreduction of Mn02 films has been studied [950] below a thickness of 150 nm, the anodically formed film behaved like an isotropic single layer with optical constants independent of thickness. Beyond this limit, anisotropic film properties had to be assumed. Reduction was accompanied by an increase in thickness, which started at the ox-ide/solution interface. 

One needs to know optical constants to calculate IRRAS spectra of molecules either adsorbed at the electrode surface or resident inside the thin-layer cavity. The isotropic optical constants of a given compound are usually determined from transmittance spectra. A pressed peUet, prepared by grinding the dispersion of the compound with a KBr or KCl powder, is typically used as a sample. Recently, Arnold et al. [41] have demonstrated that this method can yield non-reproducible results due to different histories of the sample preparation. In addition, the optical constants determined using the powder method can be quite different from those of the film at the metal/electrolyte interface because of the difference in the environment. 

Figure 1.13. Simulated reflection spectra of isotropic polyethylene layer 100-nm thick on Al substrate in range of stretching vibrations of CH groups for angles of incidence (pt = 45° (short-dashed line) and

Successful IR spectroscopy of ultrathin films is very sensitive to the choice of the method and the optical geometry of the experimental set-up, maximizing spectral contrast and the amount of information obtained about the film. These choices should be made on the basis of a comparison of band intensities in film spectra calculated for different experimental conditions. In this section, this approach will be demonstrated using a 1-nm weakly absorbing hypothetical layer that models an isotropic organic monolayer with optical constants 2 = 1-3 and 2 = 0.1 in the region of the vCH vibrations (v = 2800 cm ). The layer is assumed to be located on a Ge or A1 substrate. The spectra were calculated for /7-polarized reflection IRRAS and ATR and single transmission. 

DETERMINATION OF OPTICAL CONSTANTS OF ISOTROPIC ULTRATHIN FILMS... 

In this section, only the optical constants of isotropic films determined by the multiwavelength approach in IRRAS will be discussed. The optical constants are assumed to be independent of the film thickness, and any gradient in the optical properties of the substrate (Section 3.5) is ignored. This undoubtedly lowers accuracy of the results. Anisotropic optical constants of a film are more closely related to real-world ultrathin films. At this point, it is worth noting that approaches to measuring isotropic and anisotropic optical constants are conceptually identical An anisotropic material shows a completely identical metallic IRRAS spectrum to the isotropic one if the complex refractive index along the z-direction for the anisotropic material is equal to that for the isotropic one [44]. However, to... 

Ellipsometry is a powerful tool to gain the optical properties of materials though measuring the change of polarization state of the probe light after interaction with the sample. It offers a sensitive, nondestructive and comprehensive way to accurately determine film thickness and optical constants of extensive materials, such as metals, ceramics, glasses, semiconductors, and its compounds and composites. These materials can be liquid phase or even gaseous phase, can be isotropic or anisotropic, and can be bulk materials or multi-layer thin films. 

The big advantages of the KKA, besides the relatively small experimental work required for obtaining the necessary data, are twofold. Firstly, it is possible to evaluate the two optical constants of a (assumed isotropic) thin film by measurements with s-polarized light only. Hence, problems arising from nonlocal effects (Section 2.4) are avoided. Secondly, if we are forced to assume an anisotropic film dielectric function, S2t and B2 can, in principle, be obtained from a KKA of the AR/R)j and (A/ // )n spectra. ... 

Since in the case of a uniaxial absorbing film, four unknown parameters instead of two have to be determined, the evaluation procedure for the film optical constants becomes quite complicated. Besides, it may not be obvious in the first place whether the more simple isotropic case is sufficient or not. It has been pointed out that, for example, a thin, transparent but anisotropic film on a metallic substrate can yield R/R values which, when used to calculate the optical constants for an isotropic film, may falsely indicate absorbing properties. " ... 

Electro-optic effects refer to the changes in the refractive index of a material induced by the application of an external electric field, which modulates their optical properties [61, 62], Application of an applied external field induces in an optically isotropic material, like liquids, isotropic thin films, an optical birefringence. The size of this effect is represented by a coefficient B, called Kerr constant. The electric field induced refractive index difference is given by... 

To obtain the values of magi ii and 2, several further assumptions are necessary. We take an approximation that the density and local angular distribution of the 8CB molecules in the adsorbed layers are the same as in the bulk smectic A phase, i.e. bulk values of optical dielectric constants are used in the calculation of Cxi and ezi- To account for the molecular tilting, the films are regarded to form a multi-domain structure with the molecular directors being azimuthally isotropically distributed on a cone with the tilt angle 0, . These assumptions result in... 

Maximum ON-state transmittance occurs when the refractive index of polymer ( p) matches with the ordinary refractive index ( ) of LC. During the film formation, it is possible that some fraction of LC dissolved in polymer matrix can have a profound effect mi the PDLC film properties. Therefore, determination of partition of LC between the polymer and LC phases is an important factor in evaluating the performance of PDLC films. The reorientation of LC portion in a PDLC composite film is responsible for the optical non-linearity and electro-optical properties of the device. The absorption of LC into an isotropic polymer results in the LC becoming a part of the polymer phase. In this state, reorientation of LC does not happen with an applied electric field, leaving less amount of LC behind for scattering of light. Therefore, selection of suitable concentration of LC in PDLC films is crucial in optimizing film properties. LC dissolved in polymer matrix alters refractive index, dielectric constant, viscosity etc. of the host polymer. As explained earlier, for best electro-optical responses, a polymer and LC material are chosen on the basis of... 

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