Lasers have long been used to cool atoms in traps. By using light slightly mistuned with the atom’s own internal quantum energy levels, the light can progressively slow the atoms almost to a halt. The same principles can be applied to larger objects made of trillions of atoms, such as a thin silicon cantilever.
Although light cooling of a cantilever-specifically the cantilever’s oscillatory motions---has been achieved before, scientists at the NIST lab in Boulder, Colorado are the first to do this using radio-frequency circuitry. In the NIST experiment, a micron-sized cantilever is chilled from room temperature down to 45 K in a process called capacitive cooling, in which the cantilever, pelted with radio waves, slows down (vibrates less) by transferring energy to the surrounding radio frequency resonant circuit. One of the NIST scientists, Kenton Brown (krbrown@boulder.nist.gov, 303-497-4364) says that the potential advantage here is that the cooling of the cantilever can be accomplished with standard radio-frequency technology instead of with precision optical elements or lasers, making it easier to put the whole setup on a chip and to immerse the chip in a cryogenic environment.
Why chill the cantilever (think of a tiny up-and-down vibrating diving board) in the first place? Because a cold enough cantilever could demonstrate quantum behavior in a macroscopic object. Besides the fundamental interest in such a feat, it might pave the way to very sensitive detectors. (Brown et al., Physical Review Letters, upcoming article)
An Ultrafast, Ultralarge Change In Reflectivity.
An ultrafast, ultralarge change in reflectivity can be brought about with femtosecond lasers. In a recent experiment short laser pulses, falling on an organic salt target, momentarily changed the material from an insulator (a bad reflector of light) to a semi-metal (good reflector of light). The change in reflectivity this large---more than 100%-has never been achieved before in a photonic material; photo-induced changes are usually more like a few percent. The laser pulse required doesn’t even have to be particularly intense to cause the change.
Thus gigantic photo-response work began as a Tokyo-Kyoto collaboration but now includes also LBL and Oxford. The new advance is that the change in reflectivity can be brought about in tens of femtoseconds rather than 150 ns. The new results are being reported this week at the Frontiers in Optics meeting in San Jose by Jiro Itatani, who has a joint appointment at LBL (jitatani@lbl.gov) and the Japan Science and Technology Agency. He says that dramatic reflectivity changes will be useful in bringing about direct ultrafast optical-to-optical switching. (Meeting website: http://www.osa.org/meetings/annual/default.aspx)
Explaining A Plasmon Version of Young's Experiment.
When light strikes a metallic array of subwavelength apertures surface plasmons may be created. An electromagnetic phenomenon like light itself, the plasmons propagate in the plane of the metal but with a wavelength smaller, sometimes appreciably smaller, than the illuminating light. Just as light can couple to surface plasmons, these plasmons propagating between apertures can also be reconstituted as light. The overall effect is that “large” light can pass through tiny holes.
If now the number of openings is limited to two, then one has the makings of a plasmonic version of the famous Young's experiment, the early nineteenth-century experiment in which light falling on two slits in a baffle produced an interference pattern---revealing the wave nature of light. A number of experiments have now been performed on exactly this version of Young's experiment. At the Frontiers in Optics meeting C.H. Gan of the University of North Carolina (Charlotte) reports on some new theoretical predictions relating to the coherence properties of light transmitted through the slits.
His detailed simulations, done with collaborators G. Gbur of UNC Charlotte and T.D. Visser of the Free University of Amsterdam, show how surface plasmons traveling between the apertures result in a correlation of the light fields emitted from the apertures. Gan (chgan@uncc.edu) shows how this effect can be tuned (such as by varying the size or spacing of the slits) to achieve varying degrees of spatial coherence (that is, the amount by which the waves are “in step”) of the emergent reconstituted light waves. This tunability in turn has the potential to be exploited in new forms of coherence-relating imaging, such as 'variable coherence scattering microscopy.
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