Cooling — with a Light Touch

To make a mirror really cold — and possibly help reveal quantum effects in the macroscopic world — scientists are turning to the chilling effects of a photon breeze. In the Nov. 2 issue of Nature, three groups of researchers demonstrated that radiation pressure could induce the cooling of tiny mirrors to reach temperatures as low as 0.135 K.

An artist’s impression illustrates the principal working scheme of cooling a micromirror with radiation pressure. The moving, doubly clamped mirror is “stopped” by the impact of photons hitting it inside the cavity. The layer underneath shows a real CCD image acquired during experiments. Courtesy of Arkitek Studios Inc.

Cooling occurs when photons bounce off a springlike structure with a natural resonant frequency. If a laser is off that frequency, the light acts like an ultracold viscous fluid, damping movement and, therefore, cooling the structure. The key to achieving the lowest possible temperature is to have a low-mass, highly reflective mirror mounted on a springy structure of high mechanical quality.

One group of researchers — representing the University of Vienna, the Vienna-based Austrian Academy of Sciences, Johannes Kepler University in Linz, Austria, and the University of Maryland, College Park — built miniature mirrors that massed ∼400 ng and were more than 99.6 percent reflective. Using an Nd:YAG laser operating at 1064 nm, the investigators cooled their mirrors from 300 K to less than 10 K.

At the University of California, Santa Barbara, a second group used a diode laser at 780 nm to measure the location of a 30-μm-diameter mirror that was mounted on the end of a commercial microcantilever. They modulated a second diode laser at 980 nm, which they focused on the cantilever. This provided active feedback, with which they achieved cooling down to 0.135 K.

The third group, from the University of Pierre and Marie Curie and from l’École Normale Superieure (ENS), both in Paris, heated and cooled their micromirrors using a single-laser scheme, achieving temperatures as low as 10 K. As for how low such cooling can go, ENS assistant professor of physics Pierre-François Cohadon noted that there is a minimum. “The limit for such a technique basically should be only the quantum ground state of the resonator.