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Lasers that switch on when the temperature drops below a set point could be
used to make temperature-sensitive displays and temperature sensors. Diederik
Wiersma at the European laboratory for nonlinear spectroscopy in Florence mixed
dye-soaked liquid crystals into glass. Shining green laser light onto the
material made it emit laser light—but only when cooled below 42.5 °C
Nature (vol 414, p 708). Wiersma says that as the material gets colder, the
liquid crystals align more strongly and so scatter light more. This keeps
photons trapped inside the material for longer, giving them more opportunity to
excite electrons in the dye molecules. As the excited electrons lose that
energy, they produce light.