Laser Ignition

Short answer: Light can set off a gas mixture in several ways. A 2013 review in Optics Express by Geoff Dearden and Tom Shenton lists four mechanisms.

In thermal initiation, the laser heats the gas or a surface. In non-resonant breakdown, a tightly focused pulse ionizes the gas into a tiny plasma spark. Resonant breakdown is a variant that relies on the gas absorbing specific wavelengths. In photochemical ignition, single photons split molecules, which usually requires ultraviolet light. Chlorine is a classic example of the photochemical route. Harvard's lecture-demonstration notes give the Cl–Cl bond energy as 243 kJ/mol, matching a photon of about 2.51 eV, or 492 nm. Photons at 405–450 nm carry about 2.8–3.1 eV, so they are energetic enough. Once a photon splits a chlorine molecule, a chain reaction (Cl + H₂ → HCl + H; H + Cl₂ → HCl + Cl) can turn explosive in a hydrogen–chlorine mixture. Energy alone isn't enough, though. Harvard's demo uses colored LEDs and only the UV one sets off the tube, and oxygen inhibits that chain. Engine laser ignition, reviewed by Dearden and Shenton and by Morsy (2012), works differently. Engine tests have typically used nanosecond pulses from near-infrared Nd:YAG lasers (1064 nm) focused into a plasma spark. They've been studied as a replacement for spark plugs in high-compression engines, and Wikipedia notes proposed uses in rocket engines. The footage shows a blue laser triggering a fireball and black soot in a flask, then liquids poured to make a yellow-green gas that appears to contain chlorine. A solid is added, and the laser fires again.