Near infrared phase-modulation

Another important method for photonic crystal fabrication employs colloidal particle self-assembly. A colloidal system consists of two separate phases a dispersed phase and a continuous phase (dispersion medium). The dispersed phase particles are small solid nanoparticles with a typical size of 1-1000 nanometers. Colloidal crystals are three-dimensional periodic lattices assembled from monodispersed spherical colloids. The opals are a natural example of colloidal photonic crystals that diffract light in the visible and near-infrared (IR) spectral regions due to periodic modulation of the refractive index between the ordered monodispersed silica spheres and the surrounding matrix. 

In contrast, the nonlinearities in bulk materials are due to the response of electrons not associated with individual sites, as it occurs in metals or semiconductors. In these materials, the nonlinear response is caused by effects of band structure or other mechanisms that are determined by the electronic response of the bulk medium. The first nonlinear materials that were applied successfully in the fabrication of passive and active photonic devices were in fact ferroelectric inorganic crystals, such as the potassium dihydrogen phosphate (KDP) crystal or the lithium niobate (LiNbO,) [20-22]. In the present, potassium dihydrogen phosphate crystal is broadly used as a laser frequency doubler, while the lithium niobate is the main material for optical electrooptic modulators that operate in the near-infrared spectral range. Another ferroelectric inorganic crystal, barium titanate (BaTiOj), is currently used in phase-conjugation applications [23]. 

K. Alford, Y. Wickramasinghe, Phase-amplitude crosstalk in intensity modulated near infrared spectroscopy. Rev. Sci. Instrum. 71, 2191-2195 (2000)... 

The polarization measurements in the mid-infrared (IR) and near-infrared (NIR) were performed on a Bruker IFS 88 FTIR/FTNIR spectrometer utilizing a wire-grid polarizer on KRS5-substrate which could be rotated pneumatically parallel or perpendicular to the selected reference direction. For the experiments in the step-scan mode a mercury-cadmium-telluride (MCT) detector with a DC-coupled preamplifier was used. This allowed an absolute intensity at each mirror position to be recorded and the use of phase-modulation-demodulation techniques, which are often applied for the step-scan mode [28-30] to be avoided. Further specific instrumental details of the individual applications are given in the corresponding sections. 

RAIRS is a non-destructive infrared technique with special versatility - it does not require the vacuum conditions essential for electron spectroscopic methods and is, therefore, in principle, applicable to the study of growth processes [4.270]. By use of a polarization modulation technique surfaces in a gas phase can be investigated. Higher surface sensitivity is achieved by modulation of the polarization between s and p. This method can also be used to discriminate between anisotropic near-sur-face absorption and isotropic absorption in the gas phase [4.271]. 

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