Cavity Optomechanics

From quantum mechanics, we learn that the vibrational excitations of an oscillator are quantised. In classical oscillators in our everyday world, like a pendulum or a Quartz
crystal, the number of these quanta exceeds 107, making it impossible to resolve energy changes on a single quantum level. Thus, to investigate the quantum features of such
macroscopic oscillators, we have to increase our measurement precision while reducing the amount of vibrational quanta (i.e. the temperature). The eld of research, that deals
with this kind of problems is called (cavity-)optomechanics. Here the ultra high precision of optical resonator cavities can not only be used to probe the mechanical features of
the oscillator, but the high intra-cavity intensities also allow for manipulation of the oscillators characteristics via the radiation pressure.

© Steffen Gohlke

In our experiment we use the drum-like motion of the surface of a dielectric membrane as the mechanical oscillator of interest. This membrane is studied inside an optical cavity resonator, by placing it in between two cavity mirrors. Due to the high reflectance of the membrane, the cavity resonances are altered by creating two sub-cavities that are coupled to each other via transmission through the membrane. Since the optical resonances and intra-cavity fields are highly correlated to the exact membrane position within the system, this setup orders an excellent platform for optomechanical experiments. Inside the optomechanical community a setup like this is often referred to as membrane-in-the-middle setup. This more complicated three mirror setup oers many advantages, compared to a setup, where the membrane acts as cavity end mirror. The most notable advantage is given by the reflective property of the membrane. Close to cavity resonance, the intra-cavity field builds up, and the radiation pressure on the membrane increases. In the case, that the membrane acts as an end mirror, the pressure is acting only from one side, the membrane will be pushed from resonance, if the light intensity and the resulting force become too large. Since for example the optomechanical cooling rates are intensity dependent, this limits the possible cooling rates. Thus to achieve double- or triple-digit quanta the environment has to be cooled down with a cryostat in order to also reduce the competing heating rates.

In contrast the radiation pressure in the described membrane-in-the-middle setup acts on the membrane from both sides, such that the pressures balance each other, allowing for more efficient cooling rates promising low membrane temperatures even at room temperature. Additionally, this balanced radiation pressure could be used to levitate the membrane in the cavity freely, without any mechanical support, increasing the mechanical quality factor of the oscillator by orders of magnitude, while also decreasing the heating rates from the environment.

Our experimental apertures features the in-house manufactured fibre-mirrors for the surrounding mirrors, as they can be used for building short cavities with large mirror curvatures, while remaining in the stable cavity regime. This is important, as the manufacturing process of the dielectric mirror coating on the membrane brings a lot of internal stress, which leads to a curved membrane surface, challenging the stability of the formed sub-cavities. With this fibre-mirrors in mind we were able to increase the dielectric mirror coating stack size to reach transmittance as low as 500ppm, bringing a hardly explored optomechanical regime in reach.

© Steffen Gohlke


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