An ultrafast, ultralarge change in reflectivity

Can be brought about with femtosecond lasers. In a recent experiment, 30-fs laser pulses impinging on an organic salt target produced a very rapid phase change in the material, taking it from an insulator to a semimetal. Reporting on his work at the September 2007 Frontiers in Optics meeting in San Jose, California, Jiro Itatani (Lawrence Berkeley National Laboratory and Japan Science and Technology Agency) said that the material’s reflectivity changed greatly along with its electronic properties. In fact, within 60 fs the reflectivity more than doubled, a far larger change than the few percent normally seen in photo-responsive materials. The laser pulse was not particularly intense; less than one photon was absorbed per molecule. Itatani thinks that the large, fast changes are driven by light-induced motions of the salt molecules that are strongly coupled to the electronic degrees of freedom. The dramatic reflectivity changes could come in handy for direct ultrafast optical switching.