Abstract
LASER breakdown, in which a substance is transformed explosively to a plasma by a focused pulsed laser beam, has been much studied1–8. Light emission from the plasma formed by breakdown of liquid media or suspended microparticles is generally observed lateral to the laser beam, and can be used for spectrochemical analysis4,8,9. Here we report the observation of highly anisotropic emission of hydrogen Balmer-α (Hα) radiation from the laser breakdown of polystyrene particles in water. The emission was strong and spectrally narrow in the forward and backward directions (with respect to the beam), and weak and broad in others. The incorporation of a single-mirror cavity in the optical arrangement and the observed nonlinear dependence of intensity on the focal length of the focusing lens suggest that the plasma may induce gain in the emitted radiation. The exact mechanism of the anisotropic emission is not yet known.
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