Quantum dot solutions fluorescing under different illumination methods. The cuvettes containing the solutions are placed on microscope stages. At left an epifluorescence microscope illuminates the sample with blue light at a wavelength of 488 nanometres (nm). The sample is excited throughout its thickness. At centre a confocal microscope also illuminates the sample at 488nm. The illumination is in a single plane, but the sample is still excited throughout its thickness. At right is 2-photon illumination at an infrared wavelength of 800nm. Only quantum dots in the focal plane are excited. Quantum dots are nanocrystals of semiconductor material that emit photons when excited. The size of the quantum dot can be tuned during manufacturing to produce a certain wavelength of photon emission. Quantum dots have a number of applications including use in light emitting diodes (LEDs), quantum computing, solar panels and labelling biological tissue.