Radar return enhanced by a grating of species-selective multiphoton ionization as a probe for trace impurities in the atmosphere

Two-color time-ordered dyads of short laser pulses induce a spatially periodic modulation of the refractive index of the atmosphere through resonance-enhanced multiphoton ionization (REMPI), enhancing the back reflection of radio waves. The carrier frequencies and the timing of laser pulses in a dyad provide a unique code for selectively accessing the manifold of energy levels of impurity molecules, inducing REMPI gratings of the refractive index only in the presence of impurity molecules, thus suggesting an attractive strategy for the radarbased stand-off detection of trace impurities in the atmosphere.