First submicron fluorescent pattern obtained with direct writing on LiF films by X-ray microprobe |
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The colour centers (CCs) are lattice vacancies of halogen atoms trapping electrons, which emit in the visible and near IR spectral regions. CCc can be created by irradiation with ionising radiation using charged particles (electrons and ions) or gamma and X-rays. Alkali halide crystals containing CCs are well-known active media in optically pumped tunable solid state lasers1. http://www.frascati.enea.it/fis/lac/solidstate/index-ss.html The continuously shrinking dimension of the photonic structures has stimulated the development of CCs direct-writing methods using irradiation with submicron-sized beams. Such maskless processing techniques open up new opportunities for production of small size active elements in integrated optics, efficient point light sources in near-field optical microscopy and optical memories, novel miniaturised coherent light sources, such as active waveguides and microcavities (http://www.frascati.enea.it/fis/lac/solidstate/index-ss.html) for the optolectronics. Up to now only electron and ion microprobes have been used for direct patterning in LiF. Even though electron beam diameters smaller than 10 nm can be achieved, lateral spreading of the electrons by scattering processes and charge effects enlarges substantially the interaction volume and limits the spatial resolution. Currently the smallest line width obtained with direct electron beam writing on untreated LiF films is of the order of several µm3, whereas using a 35 KeV Ga+ ion beam lines of 1.3 µm have been produced. The main advantages of using X-rays are their lower scattering cross section, which limits the lateral spreading of the X-ray beam, and the neutrality of this kind of radiation. We have patterned LiF films deposited on glass by scanning the samples in front of the X-ray beam in the Scanning Photoemission Microscope at ELETTRA. In this system the photon beam is demagnified to a microspot with a diameter of 100 nm using a zone plate focusing optics. The photon density of the microprobe (>= 109 photons/sec) allowed creation of CCs using short (10-100 ms) exposure times. Different patterns were written using 640 eV photons by x-y scanning of the samples with steps comparable with the spot size. The luminescent patterns were characterised by means of a confocal microscope in fluorescent mode illuminating the samples at 458 nm. Fig.1 shows the Gaussian profile of a fluorescing line with a FWHM of ~500nm. Fig. 2 is obtained using a conventional fluorescence optical microscope and shows the influence of the photon dose on the spectral distribution of the emitted luminescence. [1] L. F. Mollenauer in Tunable Lasers (eds L. F. Mollenauer, Springer Verlag, Berlin, 1987). [2] R. M. Montereali, S. Bigotta, M. Piccinini, M. Giammatteo, P. Picozzi, S. Santucci, Nucl. Instr. Meth. Phys. Res. B 166-167, 764 (2000). [3] Martin, J. Bischoff, L. Wannemacher, R., Opt. Comm. 188, 119-128 (2001). R. Larciprete*,#, L. Gregoratti *, R. M. Montereali #, F. Bonfigli #, M. Danailov * and M. Kiskinova * * Sincrotrone Trieste, S. S. 14, Km.163,5, 34012 Basovizza (TS) Italy,
Correspondence and requests for materials should be addressed to R. Larciprete |
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