Cavity cooling of mechanical modes of an optically active nanomembrane

The field of cavity optomechanics explores the interaction between the photonics modes of an optical cavity and the phononic modes of a mechanical resonator. In this thesis work, we demonstrate an optomechanical cavity that exploits a membrane with a layer of photoactive molecules, in which the optomechanical coupling parameters are modulated by exploiting an external laser. As a photoactive molecule the spiropyran (SP) is used, a photochromic molecule that undergoes conversion in merocyanine (MC) upon ultraviolet (UV) irradiation. Four different membranes are designed, fabricated, and characterized. They are made of three different layers: one of Silicon Nitride, one of poly(methyl-methacrylate) (PMMA), and one of Gold. In three membranes, the PMMA layer has been doped with the dye 5,5-dichloro-11-diphenylamino-3,3-diethyl-10,12-ethylenethiatricarbocyanine perchlorate (IR-140) with different weight ratios of dye with respect to the PMMA matrix. In the fourth membrane, PMMA has been doped with IR-140 and SP. The membranes have been fabricated at the National Enterprise for nanoScience and nanoTechnology facilities, and they have been characterized in terms of cooling efficiency of several mechanical modes, damping coefficient, and measurement of the characteristic delay time of the photothermal force. The possibility to tune the optomechanical coupling by optical signal has been investigated for the membrane with the photoactive molecule, by studying the optomechanical cooling efficiency and measuring the delay time of the photothermal force before and after illumination with a blue laser at various intensities, which induces the photoisomerization of the SP to MC. The results of this Thesis work open interesting perspectives for the development of novel mechanical resonators, which can be exploited for the creation of an optomechanical sensor in which the dynamic range can be tuned by light