PhD Defense by Andreas Næsby Rasmussen – Niels Bohr Institute - University of Copenhagen

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PhD Defense by Andreas Næsby Rasmussen

The topic of this talk will be the work we have done with micromechanical membrane structures made out of gallium arsinide. The interaction of coherent light with the membranes was found to have a strong coupling to the motional degrees of freedom, and exceptionally high cooling of the mechanical modes was shown. The cooling effect was determined to be caused by a photothermal effect, that is due to local heating of the membrane by the laser light. This heating induces a slow change in the shape and motion of the membrane. Because this expansion is not instantaneous, the membrane moves before the force of the local heating has reached equillibrium. This delay makes the force viscous, and removes energy from the membrane motion. This is denoted vibrational cooling. A very strong vibrational cooling down to an effective 4 K from room temperature is shown for the (4,3) mode of a large GaAs membrane. Additionally the GaAs membrane is shown to have had a record mechanical quality factor of Q > 2 million at room temperature, which is a remarkable result for a GaAs membrane.


Another source of vibrational cooling was discovered, and the use of deformation potential coupling to the vibrations is described. This effect is due to the coherent excitation and subsequent relaxation of electrons and holes in the membrane, which couples to the lattice spacing, and thus to the vibrational modes of the membrane. This method has the potential of enabling coherent coupling to the vibrational modes. New GaAs membrane systems created with thin quantum wells for confinement of the electron-hole excitations were investigated using vibrational methods and techniques measuring photoluminescence. These devices did not show a high mechanical quality factor and a next generation of GaAs membrane system was proposed. This setup has two quantum wells for better confinement and additional tunability added by an electrical degree of freedom, and the current work on these samples is described in this thesis. As work is going on with this next generation of GaAs micromembranes, it is hoped, that a strong deformation potential cooling of vibrations will be shown in the immediate future.