Insulator to metal within a picosecond

A group from the University of California, San Diego, and the University of Quebec studied a 200-nm thick film of vanadium oxide (VO₂). They fired a 50-fs laser pulse at the sample, causing what they believe to be two phase transitions: a structural one (the unit cell size increases a bit), monitored with short x-ray pulses; and an electrical one (insulatorto-metal), monitored by short pulses of visible light. The simultaneous, ultrafast measurement of more than 1 degree of freedom showed that both transitions happened essentially all at once. Therefore, the experiment still did not settle an old question in condensed matter physics: Which comes first, the structural change in the sample or the electrical change? Because the crystalline reordering occurs in a few hundred femtoseconds and is reversible, and because x rays scatter differently from the two contrasting crystalline forms, it might be possible to use this whole process as an ultrafast “Bragg switch” to divert subpicosecond portions of a longer x-ray wavetrain. (A. Cavalleri et al., Phys. Rev. Lett., 87, 237401, 2001 http://dx.doi.org/10.1103/PhysRevLett.87.237401 .)