KEYWORDS: Laser induced fluorescence, X-rays, Luminescence, Near field scanning optical microscopy, Lithium, Spatial resolution, Crystals, Nanostructures, Microscopy, Near field optics
The continuously shrinking of the photonic structures has stimulated the research and development of low-dimensionality photoluminescent patterns based on visible-emitting color centers (CCs) in thin films. In particular, Lithium Fluoride (LiF) treated with ionizing radiation by using versatile lithographic techniques, represents a very promising candidate for the realization of innovative photonic devices. Permanent tluorescent patterns based on CCs have been realized in LiF films by direct writing with a X ray microprobe -of the ESCA Microscopy beamline at ELETTRA synchrotron in different configuration. Luminescent nanostructures, written by scanning the LW specimen with respect to the X-ray beam, have been investigated by near-field optical microscope and a sub-wavelength spatial resolution is demonstrated.
We present a diode side-pumped Nd:YVO4 laser emitting up to 11 W CW output power c axis polarized at 1064 nm in a bounce geometry. We employed a 2x4x12 mm3, 6° wedge, 1.1% Nd3+ doped slab crystal. We set up a compact 2-mirrors cavity design using a cylindrical high reflectivity mirror (curved in the tangential plane) and several plane output coupler. The optimum total internal reflection angle of the designed bounce geometry, leading to the maximum output power, was measured to be about 3.5°. The overall length of the cavity ranges from 8 to 11 cm depending on the investigated configuration. As much as 11 W CW output power was obtained for approximately 26 W of absorbed pump power at 808 nm, with a slope efficiency up to 50% and a near diffraction limited outpu beam in the vertical direction. An overall M2 < 3 was obtained at the maximum output power. We also present a focal thermal lens characterization of the cavity in two different experimental setup. The study was based on a first order dependence of the focal thermal length on the absorbed pump power model fth∞k/Pabs. Our results are in good agreement with the model and the experimental behaviour of the system confirmed the outcome of the focal thermal lens measurements.
Periodic luminescent patterns based on active color centers have been efficiently created in Lithium Fluoride (LiF) crystals and films by Extreme Ultra-Violet (EUV) light and soft x-rays from a laser-plasma source by masking the incoming radiation. Strong visible photoluminescence at room temperature has been measured from colored geometric microstructures, produced with high spatial resolution on large areas in short exposure times. Accurate spectrophotometric measurements allow estimating a significant increase in the real part of the refractive index, locally induced by the formation of high concentrations of stable primary and aggregate electronic defects at the surface of the LiF irradiated crystal, in a very thin layer, whose depth has been evaluated around 50 nm. On the basis of a semi-classical dipole-electromagnetic field interaction model, the contribution of different kinds of point defects to the overall refractive index change has been quantified. Promising opportunities in the fabrication of passive and active devices for integrated optics, such as gratings and distributed lasers, are offered by the use of this kind of radiation.
A new imaging detector for EUV or soft-X-ray radiation based on optically stimulated luminescence (OSL) of lithium fluoride (LiF) films or crystals is presented. The first micro-radiography images of biological samples and of meshes obtained on LiF using a laser-plasma source or an X-ray laser are shown, and (up to now) a resolution better than one micron is demonstrated. The dependence of the coloration density vs the deposited X-ray dose is considered and the advantages of this new diagnostic technique for both coherent and non-coherent EUV sources, compared with CCDs detectors, photographic films and photoresists are discussed. This new detector is extremely suitable for laser plasmas and for X-ray lasers sources.
The miniaturization of optical devices is a key objective in the field of photonics, and a large set of materials and techniques are under investigation. Among the former, lithium fluoride (LiF) is of particular interest because it is almost not hygroscopic and it can host stable color centers (CCs) produced by ionizing radiation and emitting in the visible spectral range even at room temperature (RT) under optical excitation. The increasing demand for low-dimensionality photonic devices imposes the utilization of advanced lithographic techniques for producing luminescent structures with submicrometric spatial resolution. We present an innovative irradiation method producing CCs in LiF crystals and films by using an EUV and soft X-ray laser-plasma source. This technique is able to produce colored patterns with high spatial resolution on large (more than 10 cm2) areas in a short exposure time compared with other irradiation methods. The colored LiF samples have been characterized by optical absorption and photoluminescence measurements for different irradiation fluences.
In order to understand the luminescence mechanism and luminescence centers in LaOCl, LaOBr and LaOBr:Tm, Thermally stimulated luminescence(TSL) and photoluminescence(PL) studies have been made on unirradiated and irradiated samples at room temperature. LaOCl revealed three glow peaks having their maxima at 355,390 and 410k while in LaOBr a shoulder at 335K and two glow peaks at 365 and 420k are observed. Incorporation of Tm in LaOBr resulted in significant changes in intensity. In addition,the shoulder at 335K gets suppressed and the 365 and 420K glow peaks shifted towards high temperatures to 380 and 430K. The shoulders at 355, 335 K in LaOCl and LaOBr have been attributed to impurities while the glow peaks at 390 and 380 K might originated due to radiative electron - hole recombination due to detrapping from chlorine and bromine ion vacancies. The high temperature glow peak at 420 and 430k might belong to F+ centers being formed due to charge transfer between oxygen ion vacancies and excited electrons. The reflectance and photoluminescence studies supported these attributions as they revealed different emissions which may be responsible for color centers as well as luminescence centers.
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