In plain words
The smallest possible amount of light – one indivisible packet of it. The packets of a given colour all carry the same energy, and blue ones carry more than red: that is why ultraviolet light gives you sunburn and red light, however bright, does not. A absorbs only photons with enough energy to cross its and gives light back at about the gap’s energy – which is why LEDs of different colours are made from different semiconductors.
Going deeper
Packets, not a stream
Around 1900 light was firmly established as a wave. Yet shining it on a metal produced a puzzle: brighter light knocked out more electrons but not faster ones, and below a certain frequency no electrons came out at all, however bright the light. In 1905 Albert Einstein explained this by proposing that light is absorbed in packets whose energy is fixed by its frequency – Planck’s constant times the frequency – and that each electron takes its energy from one packet. The name photon came two decades later.
Both pictures are right. Light spreads and interferes as a wave, but it is absorbed and detected in whole photons: a faint enough beam makes a sensitive detector click one photon at a time. A photon of green light, at 530 nanometres, carries about 2.3 electronvolts.
Why only some photons are absorbed
A photon is absorbed only if its energy fits a jump an electron can make. In a semiconductor the smallest jump is across the band gap, so photons with less energy pass straight through: silicon, with a gap of 1.1 eV, is transparent to much of the infrared, which is why infrared light is used to look through silicon . Above the gap a thin semiconductor absorbs strongly for its size – a of graphene takes 2.3 percent of visible light, and a monolayer several times as much at its peaks.
A photon carries plenty of energy but almost no momentum by the standards of an electron in a crystal. It can lift an electron straight up in energy but not sideways in momentum, which is why a direct band gap absorbs and emits light efficiently and an indirect one needs a vibration of the crystal to make up the difference.
One photon at a time
Ordinary light sources send out photons at random, sometimes two close together. A – a single defect or a strained spot in a of WSe2 or hexagonal boron nitride – gives out one photon and must be excited again before it can give out another. The proof is a photon-correlation measurement: split the light onto two detectors and count how often both click at once. For a single emitter such coincidences all but vanish, which no laser or lamp can manage.
Such sources matter for quantum communication, where a message carried by single photons cannot be copied without disturbing it. In a 2D host the emitter sits at the surface, so its light escapes easily, and it can be placed and tuned with or a gate.
For specialists
The quantum of the electromagnetic field, with energy E = hν and momentum h/λ but no mass or charge. Visible photons carry about 1.7–3.1 eV, comparable to the band gaps of semiconducting TMDCs, but a momentum far smaller than the crystal momenta across the Brillouin zone, so optical transitions are vertical and an indirect gap needs a to make up the difference. Single-photon emission is established by antibunching in photon-correlation measurements.