randomness: plasmons

plasmon imaging link

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plasmons

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Plasmons play a large role in the optical properties of metals. Light of frequency below the plasma frequency is reflected, because the electrons in the metal screen the electric field of the light. Light of frequency above the plasma frequency is transmitted, because the electrons cannot respond fast enough to screen it. In most metals, the plasma frequency is in the ultraviolet, making them shiny (reflective) in the visible range. On the other hand, some metals, such as copper, have a plasmon frequency in the visible range, yielding their distinct color. For other metals, such as gold, the plasma frequency lies deeply in the ultraviolet, but geometric factors come into play which reduce the plasmon frequency to the visible. In doped semiconductors, the plasma frequency is usually in the infrared.
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surface plasmon resonance

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Typical metals that support surface plasmons are silver and gold, but metals such as copper, titanium, or chromium can also support surface plasmon generation. Surface plasmons have been used to enhance the surface sensitivity of several spectroscopic measurements including fluorescence, Raman scattering, and second harmonic generation. However, in their simplest form, SPR reflectivity measurements can be used to detect DNA or proteins by the changes in the local index of refraction upon adsorption of the target molecule to the metal surface. If the surface is patterned with different biopolymers, the technique is denoted as Surface Plasmon Resonance Imaging (SPRI).
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