Gapsy

Meet Gapsy, a semiconductor with a gap she won't let small photons cross.

In a solid, electrons can only have energies inside allowed bands. In a semiconductor the valence band is nearly full and the conduction band nearly empty, and between them lies a forbidden band gap of energy E_g. To free an electron so it can carry current, something must lift it across that gap. A single photon can do it only if its energy hc/λ is at least E_g, so there is a longest useful wavelength, λ_max = hc/E_g.

For silicon, E_g is about 1.12 electronvolts at room temperature, which gives λ_max of about 1.11 micrometres, in the near infrared. Light at longer wavelengths passes through silicon without being absorbed this way.

This one number shapes a lot of technology: it sets which colours a silicon solar cell or camera sensor can use, and why LEDs made of different semiconductors glow in different colours.

Silicon's gap is indirect, which makes absorption near the edge weak, and the gap shifts slightly with temperature, so the cutoff is softer than a single number suggests.