L13-Optical_Characterization

L13-Optical_Characterization - Optical Analytical...

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Optical Analytical Techniques for Thin Film Characterization
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Nature of Light Light as wave The propagation of light can best be described as a travelling wave. It really is a coupled electric and magnetic field that oscillate together in time and space (electromagnetic wave). The speed of the wave is constant in free space (c = 3 x 10 8 m/s). Light as particle The interaction of light with matter is best described as a massless particle called a photon. The energy of the photon determines its frequency. The number of photons in a beam determines the total energy in that beam.
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Light as a Wave ± ² t kx A y Z ³ cos sm c / 10 3 8 u Speed of Light O S 2 k Wavenumber f 2 Angular Frequency fc T Period (s) Wavelength (m) Frequency (Hz) T f 1 k c
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Electromagnetic Spectrum
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Wave Properties of Light Polarization Direction of oscillation Interference Interaction of two coherent waves Diffraction Interaction of a wave with itself
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Polarization We said that light is a wave with an electric and a magnetic field. These fields are vector quantities, which means they have a direction as well as an amplitude. The direction of the electric field associated with the EM wave is called its polarization . Light can be linearly (horizontal or vertical) or circularly (clockwise or counter cw) polarized or unpolarized (random polarization). Laser light is generally polarized while sunlight and light from light bulbs are unpolarized. Unpolarized light can be turned in to polarized light by a polarizer. A polarizer can work by; selective absorption (polaroid films, sunglasses) selective scattering (blue skies, red sunsets) reflection (Brewster’s angle, ellipsometry) double refraction (calcite crystals) Selective Absorption Reflection
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Interference If the waves are emitted from a single source, we can adjust the synchronicity of the waves by adjusting their arrival times. If one wave arrives half a period of oscillation after the other, then it will be completely out of sync with the earlier one and they will cancel each other out – destructive interference . The same is true for any wave that arrives 1.5 periods later and 2.5 periods later, etc. If one wave arrives exactly a period later then it will be again completely in sync with the earlier one and they will add to each other constructive interference . Same is true for all waves arriving 2 periods, 3 periods, etc. later.
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Thin Film Interference Consider a thin film that is at least partially transparent to light. If light is incident from the top, some of the light will be reflected and some will be transmitted in to the film. Similarly, as the transmitted light tries goes on to exit the film at the bottom, there will be reflection and transmission.
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L13-Optical_Characterization - Optical Analytical...

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