Plasmons

Plasmons are oscillating electrons that are common in plasma (including fire) which can vibrate at the frequency of visible light (~400-700 trillion hertz) although large scale alternating currents in copper wires are at most few hundred thousand hertz. Plasmons can capture photons forming polariton (generic name to photon together with material that absorbed it).
Plasmon frequency also determines if material reflects photons or transmits them. If plasmon has higher frequency than photon then material reflects photon but if photon has higher frequency than plasmon then photon gets transmitted through material. In case of metal high frequency x-rays and gamma rays get transmitted through metals with increased frequency increasing transmittance distance but visible light and every EM radiation with slower frequency gets reflected. Plasmons in metals and semiconductors have plasmon frequency in UV light range. Certain narrow wavelengths in copper and gold transmit some visible colors as different electrons around atoms have bit different frequencies. It seems to happen because if plasmon has faster frequency than photon then it can respond to photons electric field but if it is slower than photon then it can't disrupt it well. Also in simplified terms plasmon energy is its frequency times Planck constant which is same formula as energy for photon energy. Predictably whichever particle has higher energy is going to somehow dominate over lower energy particle.  

Surface plasmons exist mainly on surfaces of metals that are exposed to dielectric (nonconducting) environment like air or vacuum. They weaken exponentially with increased distance from this surface area. Electron and photons can both create plasmons which fade fast by being absorbed through material or by getting emitted as photons.

Surface plasmon polaritons are infrared or visible light waves trapped on surface of metal where they have smaller wavelength than in form of photons. Surface plasmons move parallel to surface but with much more compact wave sizes.
Plasmons can be limited to area with size of about 0,007-0,02 cubic wavelengths.

Surface plasmon resonance (SPR) refers to plasmon oscillation in response to light. SPR is often considered in certain detectors like in above one. It need consistent laser (same distance and angle), metal for plasmon waves and prism to make different wavelengths pass prism in bit different directions so certain part of detector gets light only from certain wavelength making it easier for detector to see which wavelength did or didn't reflect back. Reflectivity depends on bumps on surface in addition to electric fields.
To create resonance light should reflect from given metal and it needs to be polarized parallel to surface so electrons would be pushed parallel to surface. If polarization is perpendicular to surface then plasmons can't build up resonance energy.

SPR can be used in molecular biology to test presence of certain antigens like organic toxins or any molecule that binds with antibodies. If antibody binds with antigen it can increase resonance unit (RU) signal with 1000 RU causing ~0,1 degree difference in angle of reflection and further binding with antibody receptor can increase it further. 

Above images (source 1, 2) illustrate plasmonic laser from 2009 that creates light in 5 nm insulating solid transparent gap between nanowire and silver. As one use this could allow use of visible light in much smaller scales than their wavelength.
Even blue light has wavelength that is at least 16 times larger than processors with present time 22 nm architecture but since 2009 it seems light can be compacted to within 5 nm gaps.

Above nanolaser was created by 2012. These gold using "bowtie" laser are about as large as a virus with size of ~150 nm. Laser light is produced within 30 nm region and plasmon electrons can have almost any frequency without needing to have wavelength that photons have with same frequency.