Learning objectives
Developing knowledge of the description of the microscopic mechanisms that determine the optical properties of matter, from the point of view of colour.
Prerequisites
Knowledge acquired in courses of mathematics, classical physics, and elements of quantum mechanics
Course unit content
1) The concept of colour. <br />
a) The role of the light source. The eye: photopic and scotopic vision. Vision in conditions of darkness: light intensifiers, IR visors. <br />
b) Geometrical optical phenomena and their role in colour. <br />
Reflection: mirror and diffuse reflection. The mechanisms responsible for colour in metals. Losses due to reflection. Anti-reflective films. Applications, remote localisation. <br />
Refraction and total internal reflection. Applications. Optic fibre. <br />
c) Physical optical phenomena and their role in colour. <br />
Diffusion. The colour of the sky, aerogels for aerospace applications, metallic nanoparticles in ancient glass. Surface plasmons. Light guides. Photochromic and photosensitive glass. Interference: examples, applications in thin films. Diffraction. Liquid crystals. From opals to photonic crystals. Applications. Grid structures and photonic crystals in nature. Polarisation. Pleochroism: cordierite. Induced birefringence for the analysis of deformation in man-made structures. <br />
d) Microscopic mechanisms and their role in colour. <br />
Vibrational absorption (e.g. water and ice). Transitions between molecular orbitals: organic compounds (chlorophyll, haemoglobin, opsin, natural and artificial colorants). Crystal field absorption and charge transfer: (Cr3+ in ruby and emerald, Co2+ e Ti3+ in different coordinated structures). Colour centres in ionic and mineral crystals, solarisation in optic fibre. Band-band transitions in semiconductors and insulators. Localised levels. Luminescence: photoluminescence deriving from impurities and nanocrystals. Cathodic luminescence. Chemical luminescence. Thermoluminescence. Electroluminescence. Applications. Exercises. <br />
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2) The propagation of electromagnetic waves in a medium. Complex refraction index. Energy absorption. Absorption coefficient. Coefficient of reflection at normal incidence. Losses in reflection. Optical properties due to free carriers. Notions on the free electron model in metals. Electrical conductance as a function of frequency. Plasma frequency. The transparency of metals to UV. Plasma oscillations. The Hagen-Rubens relation. <br />
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3) Optical constants of metals (n, κ, η, μ, R) due to free carriers: applications in the case of Au. Normal and anomalous skin effect. Experimental tests. The optical properties of semiconductors caused by free carriers. Plasma resonance. Examining experimental GaAs results. <br />
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4) The relationship between dielectric function and optical constants. Dielectric function and optical constants obtained from reflectance measurements by means of the Kramers-Kronig relations. Polarizability and dielectric function: the quantum approach. The dispersion function. Oscillator power for a transition. The sum rule. Determination of the dielectric function in the presence of extinction. Role of the internal field. Mechanisms of resonance and relaxation: examples. Reticular vibration and dielectric function. The Lyddane-Sachs-Teller relation. Optical constants of CdS. <br />
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5) Measurement of optical absorption spectra. Optical transparency and density. Spectrophotometers. Mode of operation. Elements of dispersion. Sources. Detectors. The effect of the atmosphere present in the spectrophotometer. Practical work using the UV, v.i.s. and n.i.r. dispersion spectrophotometer. Absorption spectra induced by Eu2+ and Eu3+ in ionic crystals. Resolution of spectral lines. Fourier transform spectrophotometers. Jacquinot and Felgett advantages. Resolution. Apodisation. Exercises: high-resolution CO2 and H2O spectra in the air. Analysis of car exhaust gases, with and without catalytic converter. Intrinsic GaAs vibrational absorption, of crystalline field (Fe2+ in InP and Er3+ and Ho3+ in crystalline matrices for laser, hyperfine structure) and localized vibrational absorption (H in InP and OH, and the respective isotopic replacements in crystalline and vitreous matrices). <br />
Bibliography
R. Capelletti: course notes <br />
R. Capelletti: Power Point lecture presentations <br />
Teaching methods
Oral lessons, laboratory demonstrations. Evaluation is performed by means of a written report on a subject chosen by the student on agreement with the professor, and by a subsequent oral presentation.
