Liquid Helium in a Janis bath cryostat. The temperature is T=4.22K, corresponding to -268.93°C. The boiling Helium is colorless, clear as water, and has a refractive index close to one. It is possible to see a mirror, which guides the light towards the microscope objective, which is upright in the cryostat. The system is designed to reach temperatures down to 1.4K. This is a typical cryostat for low temperature studies on single molecules.
An ongoing laser beam is recorded by a highly sensitive camera (Andor Ixon). In the optical path is a single molecule, embedded in a solid state matrix at T=1.4K. By scanning the laser frequency a few percent of the light is extinct on the optical path and the laser beam gets dimmer.
A single dipole is driven by an electric field (plain wave), emerging from the left. The dipole scatters and radiates itself. In this video the Poynting vector depicted with flow lines. Depending on the scattering ratio (beta) the ongoing beam is modified. This image from Paul and Fischer shows the extinction of light in a very intuitive way. When the scattering ratio is reduced (beta going to zero) the single dipole is getting more and more broad band and influences the flow of light less and less.
A single dipole, e.g. a single molecule, is undergoing Rabi-oscillations when driven by an electric field. If no spontaneous emission occurs, the dipole absorbs a photon and emits it stimulated directly out into the laser mode again. (Stimulated) absorption and stimulated emission alternate.
Same as before, but with a certain probability the system emits red-shifted photons. The time-averaged Bloch-vector has a slightly from zero shifted position.
If you supply short optical pulses to a single molecule and monitor the red-shifted fluorescence, it is possible to get an oscillatory behavior, depending on the intensity of the incident light.
A calculation after Bethe and Bouwkamp for a sub-wavelength aperture in a metal film. The displayed situation is for a 100nm hole. All three field components are calculated for different distances to the actual aperture.
Usually single photons are detected by avalanche photo diodes (APDs) - A photodiode is driven in strong inversion and when a photon arrives, the device shows a break-though, which can be associated to a single photon. If more photons are detected, the device cannot recapture normal operation, because of the intrinsic capacity. The video shows a number of photons arriving on the single photon detector in 2 us. If more and more photons arrive, the device gets "blind".