• membrane
• Alq3
• optomechanics

We investigate the optomechanical properties of an optical cavity containing, as a mirror, a multilayer membrane composed of tris(8-hydroxyquinoline)aluminium (Alq3), silver and silicon nitride. Alq3 is an organic semi-conductor compound which shows photoluminescence at 500 nm with absorption peaked at 405 nm. These properties, together with the relative ease with which it can be prepared in thin films, are the reasons why it is already being used in applications such as OLEDs and photovoltaic cells. Alq3 may also prove useful in optomechanics thanks to, for instance, cooling due to forces photo-induced by interaction between electron-hole pairs and mechanical vibrational modes.
The typical experimental setup in optomechanics consists of an optical cavity where one of the end-mirrors can move as an oscillator. Forces are exerted on the oscillator by the light in a resonator like configuration. This forces may have different origins, for example radiation pressure, due to momentum exchange in collisions between photons and the oscillator, and photothermal deformation, due to heating by light absorption. The consequent mechanical response shifts the optical resonance frequency of the resonator, altering the intensity of the intra-cavity field and thus the forces. This way, a coupling between light field in the resonator and mechanical motion is established.
The light-induced forces respond to a displacement of the movable component with a time lag, given by the finite lifetime of photons inside the resonator for the radiation pressure and diffusion time of heat in the oscillator for the photothermal force. In a linearised dynamics regime, this retardation enables cooling/heating of the mechanical modes of motion as extraction/insertion of work by the light field.
Moreover, non-linear dynamical effects arise when the mechanical modes are heated, of which multistability and auto-oscillations are some examples.
In our experiment, the multilayer membrane is clamped but able to bend and vibrate according to vibrational modes due to its geometry and boundary conditions.
An effective temperature for each mode is identified with the amount of its associated thermal energy. Cooling or heating is thus experienced as a decrease or increase of this energy.
We produced software simulations of our system driven by two different wave-lengths, one inside and one outside the absorption spectrum of Alq3. This allows to compute the absorption in each layer and to simulate how it varies due to changes in the membrane’s layered structure. We then show how cooling is induced on two vibrational modes of the membrane through photothermal forces mediated by light absorption in the Alq3 at its absorption wavelength and Ag outside the absorption spectrum. Moreover, we made preliminary investigations on the non-linear dynamics arising by photothermal-induced heating, which yield instabilities depending on the initial conditions of laser power and detuning from resonance.