Extreme Sensitivity Charge Detection

After Theodore Maiman’s demonstration of the laser in 1960, researchers quickly discovered that tightly focused laser pulses generated a bright spark of ionized air. The initial reports caught the physics community off guard; in the words of an early researcher, C. Grey Morgan, a “flash of laser light can set the air on fire!” Because each laser photon didn’t have enough energy to knock an electron off an air molecule, it should have been impossible for the laser to ionize the air directly. Eventually, researchers realized that the extremely high electric fields at the laser’s focus were driving an electron avalanche breakdown, an already well-known process using high static fields and high-power microwaves. An initial population of free electrons gains energy by acceleration in the laser field, ionizing other molecules in a cascading, exponential process. The source of the initial population of electrons was a mystery, however, and it spurred pioneering theoretical work by Leonid Keldysh. In the quantum or multiphoton ionization (MPI) limit—at moderate intensity and short laser wavelength—of the theory, an electron is liberated when an atom absorbs many photons simultaneously. In the semiclassical limit (at high intensity and long wavelength), the laser’s large electric field pulls electrons out of atoms by tunneling ionization. With the basic process understood, researchers rushed to apply laser-driven avalanche breakdowns to such varied fields as breakdown spectroscopy, fast switching of high voltages, laser surgery, and laser machining. In this Quick Study, we recount the physics governing the laser-driven sparks and show how revisiting early experiments with new technology has uncovered the ability to pinpoint individual electrons in ambient gases.