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Molecular Orientation by Two-Photon Absorption: Towards 3 Dimensional Multiplexed Nano-Data Storage

Z. Sekkat
University of Ifrane, MA

Keywords:
photon, absorption, storage

Abstract:
Two-photon absorption (TPA) is being studied for a variety of applications, including three-dimensional (3D) microfabrication, ultra-high-density optical data storage, biological imaging by TPA-excited fluorescence microscopy, and the controlled release of biologically relevant species. TPA occurs in the presence of intense laser pulses; molecules can simultaneously absorb two photons, and the transition probability for absorption of two identical photons is proportional to the square of the intensity of the laser pulse (I2). In TPA, virtual excited states are created with photons of half the nominal one-photon excitation energy, which provides better penetration in absorbing or scattering media, and the I2 dependence of the absorption process allows for high 3D spatial selectivity through the use of a tightly focused laser beam. Under tight focusing conditions, the absorption is confined at the focus to a volume of order l3 (where l is the laser wavelength), and any subsequent process, such as fluorescence or photoinduced chemical reactions, is localized in this small volume (voxel); a feature which is the key of the high spatial resolution of TPA by a tightly focused light. I present experimental evidence of two-photon photo-selection, and I show that polarized two-photon excitation of photoisomerizable chromophores orients them in a nonpolar manner. I discuss the case of a diarylethene chromophore which is oriented by two-photon selective isomerization, and that of an azobenzene derivative which is selectively bleached by two-photon polarized excitation. The studies are done in thin films of poly(methylmethacrylate). Polarization Reflection confocal microscope images demonstarate light encoding of polarization sensitive bit-data by means of two-photon induced birefringence in the samples.

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