Laser refrigerator cools to 4 degrees above absolute zero

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Koji Usami holding the semiconductor nanomembrane inside its holder

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Researchers at the Niels Bohr Institute have succeeded in using a new form of laser cooling method to cool a two and a half square millimeter semiconducting gallium arsenide (GaAs) membrane with a thickness of 160 nm from room temperature to four degrees above absolute zero - the temperature of liquid helium.

In conventional laser cooling, an illuminated object is cooled when its motion drives a Doppler shift in light which reflects from the object. This would happen when an incoming photon is reflected along the axis of motion of the object. The transfer of energy from the object to the reflected light cools the object.

What happens if the incoming light is strong enough to move the object by radiation pressure (or some other influence)? In that case, not only is the Doppler effect still in evidence, but the light-driven motion of the object changes the size of the Doppler effect depending on those motions. Now, if the object is a mirror forming one end of an optical cavity, the motion will also change the resonant frequency of the cavity. This is called optical cavity back reaction, and can result in highly complex nonlinearly coupled vibrational behavior.

In this study, the Niels Bohr Institute group reports the first experimental realization of cavity cooling of mechanical modes in a semiconductor nanomembrane with the cooling mechanism involving both the internal electronic degrees of freedom and the externally tunable cavity resonance. Thermal stress due to non-radiative relaxation of electron-hole pairs optically generated in the semiconductor material of the membrane is the primary interaction which leads to membrane cooling.

Changing the distance between the membrane and the mirror can be accomplished through numerous pathways - moving the mirror, changing the laser intensity and/or wavelength, generating thermal vibrations in the membrane, as well as many higher-order effects.

The experimental group found in this study a complex interplay between all of these factors which can be controlled so as to cool the temperature of the membrane fluctuations. The dependence of the membrane dynamics on the frequency of the laser light suggests that the excitation of electron-hole pairs by the laser light passing through the membrane is responsible for the optomechanical force.

This is a new optomechanical mechanism, which is central to the new discovery. The paradox is that even though the membrane as a whole is gaining energy, specific vibrational modes of the membrane are losing energy faster than it is being added. Future development may extend this new mode of laser cooling to allow access of the quantum regime for macroscopic objects.

Quantum physics is the wheat to the bread of our live-a-day world. In recent years, the flour particles have been getting larger, as we grow more able to study single-quantum macroscopic quantum phenomena. Accessing the quantum regime is quite difficult for large objects, and hence new cooling methods are of continuing interest. A new form of laser cooling offers great promise for studying the macroscopic quantum regime.

For most of the past 80 years, quantum mechanical effects appeared unimportant to daily living, with the one exception of electronics, which most people realized has something to do with subatomic physics in several guises. We now realize that quantum physics is the wheat to the bread of our live-a-day world, but the notion that a pile of wheat can alone make a loaf of bread seems as unlikely as an ordinary beam of light suddenly revealing to our unaided eyes that it is composed of photons. Generally the importance of individual quanta does not appear to reach into our macroscopic world.

There have, however, been more and more demonstration of macroscopic quantum phenomena in recent years. Physicists have demonstrated an increasing ability to maintain entanglement, perhaps the most spooky of quantum effects, of large numbers of quanta as well as entangling macroscopic objects. They have shown us macroscopic mechanical Schrodinger's cats, and have produced light by shaking a mirror, thereby promoting photons from the quantum vacuum. We have learned to get a sharp view of quantum motion by not observing a mechanical oscillator until ready to record a picture. (Continuous observation keeps collapsing the quantum state, thereby removing many interesting quantum phenomena.)

Admittedly, most of the macroscopic quantum demonstration experiments have taken place on scales of tens of microns, but such objects still possess billions or trillions of atoms. But several recent experiments have demonstrated quantum behavior in millimeter sized objects.

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