Metrology: quantum-limited measuring method for nanosensors

New manufacturing technologies are making it possible to produce mechanical components on silicon chips with dimensions in only the nanometre range. Use of them is, however, still limited by the fact that sufficiently accurate measuring processes are not available for these tiny components. A team led by Professor Tobias Kippenberg (head of the junior research group / "Laboratory of Photonics and Quantum Measurements" at the Max Planck Institute for Quantum Optics – MPQ – in Garching / Germany and Tenure Track Assistant Professor at the Swiss Technical University in Lausanne) and Professor Jörg Kotthaus (Professor at LMU University in Munich / Germany) has now tested a fundamentally new approach successfully at the MPQ.

Glass cylinders with a diameter of about 50 micrometres, that are grown on silicon chips and are capable of storing light inside for a considerable time, play a key role here. The scientists have demonstrated that nano-oscillators can be not only read out but also encouraged to oscillate with the optical near-field that escapes from the toroid. The accuracy of these measurements is only limited by the quantum fluctuations of the light. Sensitivity levels that are of the order of magnitude of the quantum mechanical basic state noise of the oscillators, i.e. correspond to the standard quantum limit, are therefore already reached at room temperature. The new measuring method is as a result of great interest to the basic research community. Applications like the detection of individual atoms and/or charges or mass spectrometry may benefit from the measurements too, however.

Nanomechanical oscillators are ideal candidates for experimental testing of the quantum limits of mechanical oscillations. They are also the basis for a number of precision measurements and an established feature of magnetic force and atomic force microscopes. In the past 10 years, close attention has been paid to the development of sensitive readout techniques for increasingly small and thus more sensitive oscillators. Optical methods have produced the best results in this context, but they have been limited to objects larger than the wavelength. The precision levels reached so far with electronic methods applicable to nanoscale objects have been limited.

The MPQ and LMU physicists have now for the first time managed to apply optical methods to nanoscale oscillators successfully. This is not normally possible, because diffraction losses occur as soon as the objects are smaller than the wavelength of the light. This problem is avoided in the experiments reported here by operating in the optical near-field. The key component is a cylindrical resonator made of glass with a diameter of about 50 micrometres. This microtoroid is able to store light that has the right wavelength, i.e. if the optical circumference of the resonator is an integer multiple of the wavelength. A small proportion of the light stored, what is known as the near-field, "leaks" out of the resonator and acts as a measuring probe for the nano-oscillators (see illustration) – an arrangement of parallel silicon nitride strings that are typically 100 nanometres x 500 nanometres thick and 15 to 40 micrometres long. (Nanostrings and microtoroids were produced in Professor Kotthaus' clean rooms at the LMU and ETH Lausanne.)

If the nano-oscillators are brought into the near-field, which extends a few hundred nanometres away from the surface of the toroids, they can interact with the microtoroid. The nano-oscillators act on the optical near-field like a dielectric, i.e. they change the refractive index locally. This leads in turn to a change in the optical circumference and thus in the resonance frequencies of the microtoroid.

The change in the optical resonances caused by the nano-oscillators is so large here that their Brownian motion alone has a strong, clearly measurable effect and the movement of the strings can be measured highly sensitively. The sensitivity to changes in distance achieved in this context is of the same order of magnitude as the quantum mechanical fluctuations that are expected for nanomechanical oscillators at absolute zero and that correspond to what is known as the standard quantum limit for distance measurements.

The high sensitivity level with respect to the movement of nanoscale objects is only one aspect of the new process, however, stresses Georg Anetsberger, a PhD student in Professor Kippenberg's group. He says that demonstration for the first time that nanoscale objects can also be influenced directly – i.e. can be encouraged to oscillate when cooled – by the force exerted by photons (radiation pressure) is just as important. "We observe that the dipole force of the optical near-field leads to dynamic backaction, which causes the nanostrings to start coherent, laser-like oscillations."

The method used here can be applied in practice to all nanoscale mechanical oscillators, which could lead to a further improvement in use of them as highly sensitive sensors. To Professor Kippenberg, this is further evidence of the versatility of the microtoroids, which have been the focal point in his research for a number of years now. "We have developed an experimental platform here that could increase the potential applications for nanomechanical components substantially. It also provides an interface at which photons and phonons can interact in such an optimised way that quantum mechanical effects could become measurable at room temperature."

Source: G. Anetsberger et al.: Near-field cavity optomechanics with nanomechanical oscillators, Nature Physics, Advance Online Publication

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