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  • Unique Cavity Demonstrates Highest Optomechanical Coupling

Photonics Spectra
May 2008
Silicon nitride membrane may lead to better quantum mechanical studies.

Richard Gaughan

Photons are nearly ideal for investigating quantum phenomena: They are easy to generate and have well-defined states and straightforward interactions with matter. Quantum phenomena in macroscopic mechanical systems, on the contrary, are difficult to stimulate and to measure.

Now scientists at Yale University in New Haven, Conn., and at Ludwig Maximilians University in Munich, Germany, have developed an optical cavity design that demonstrates unprecedented ability to control the coupling between photons and a micromechanical structure.


Quantum states of mechanical structures are difficult to stimulate and observe. By separating the design of a high-finesse cavity from a mechanical membrane with a high quality factor, a new device is getting close to observing quantum transitions in the membrane. The device is fabricated by depositing SiN on a silicon wafer and using a selective etch to remove part of the wafer, leaving a free-standing 50-nm-thick membrane supported on a silicon frame. Photo courtesy of Bernie Staggers, Yale University.

Previous attempts to couple optical cavity modes to mechanical motion have been hampered by opposing technical needs. A high-quality cavity demands fixed and stable end mirrors, but mechanical coupling requires a flexible, movable element. These competing requirements have made it impossible to push the sensitivity of optomechanical coupling to the quantum level. The research team sidestepped the problem by adding a third element to the system: a mechanical membrane that can be positioned within the cavity.

Two commercial dielectric mirrors on a stable mount made of the iron-nickel alloy Invar form the high-finesse optical cavity. “High finesse” means that photons remain within the cavity for many transits. The mechanical membrane — a 1 × 1-mm, 50-nm-thick SiN dielectric carried by a silicon frame — is held on a piezoelectric stage that can vary its position.

Jack Harris, a physics professor at Yale, said that the group’s main interest is in seeing quantum effects in a large object, such as the 1-mm-square membrane.

“It has never been possible to see such an object behave quantum mechanically,” he said.

The cavity helps in three ways: It optimizes laser cooling of the membrane, it is the experimental test bed, and it incorporates the readout mechanism. The key is the adjustable coupling between photons and the membrane. With the membrane at an antinode and with maximum standing wave intensity, the interaction with the photons is highest —- but it also is equal from both sides, so photons do not modify the mechanical modes. At a node, the intensity is essentially zero, leading to no interaction at all. Somewhere between a node and an antinode, the membrane will receive a “kick” from the photons.

The cooling within the cavity is achieved by detuning the laser wavelength — redshifting it from the cavity mode — so that successive kicks reduce the mechanical energy in the membrane. This system demonstrated an unprecedented temperature reduction of nearly 50,000× — from 294 K to 6.8 mK.

The cavity functions as the experimental test bed by coupling photons to the mechanical structure. The membrane must be sensitive to small forces, so it must be light and flexible, and it must have a high mechanical quality factor. Separating the mechanical element from the cavity mirrors allows the design to reach coupling efficiencies that are higher than anything that has been built before.

The final function of the cavity — acting as a readout device — is particularly important because the membrane cools to extremely low temperatures.

In its lowest vibrational mode, the membrane can be regarded as a harmonic oscillator, which has energy proportional to the square of the displacement from equilibrium. The reflectance of the cavity depends upon the position of the membrane, but, near a node, the reflectance is proportional to the square of the distance from the node, which provides a direct readout of the membrane energy. At energies where quantum effects dominate, a direct readout of energy is necessary to avoid problems associated with simultaneously measuring variables that are incompatible, according to the Heisenberg uncertainty principle.

The next step in the work is to reach the 5-μK temperature necessary to put the membrane in its quantum ground state, where, according to Harris, the device will permit the investigators to transfer the quantum properties of the light to the membrane in a way that they can measure.

“This will require making the coupling between the photons and the membrane so strong that they are no longer really independent objects, and the quantum fluctuations of the photons will be transferred to the membrane’s motion,” he said.

Nature, March 2008, pp. 72-76.

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