We present the study of optical and spectral properties of radiation-induced stable point defects, known as color centers (CCs), in lithium fluoride (LiF) for the detection of 10 keV XFEL beam at Spring-8 Angstrom Compact free electron LAser (SACLA) in Japan. A thick LiF crystal was irradiated in four spots with 10 keV XFEL beam (pulse duration = 10 fs) with different number of accumulated shots. After irradiation the colored-LiF spots were characterized with an optical microscope in fluorescence mode and their photoluminescence intensity and spectra were analyzed.
An Aluminum coated FBG was heated to 650 °C in order to study the regeneration process at low temperature. When compared to the standard regeneration process (at 800 °C -1000 °C) the regeneration occurred at longer times, the onset of the process took around 150 days to appear. The reflected signal after regeneration also is weaker than that obtained in the usual process, although it presents a good stability after the recovery phase – measured for a time span of ~200 days.
In this work we tested use of thin film aluminium coating to enhance the temperature sensing features of FBG sensors. Thin film coating was evaporated on the Bragg grating after removal of the original acrylate recoating, using a modified vacuum evaporation system that allows uniform radial deposition around the fibre. Results show that recoating thickness up to 0.5 μm do not produce appreciable effects, whereas thickness of 2.3 μm appreciably increases temperature sensitivity without increasing time response constant.
We present a recently patented apparatus which consists of an extreme ultraviolet radiation source writing invisible
patterns on thin tags of alkali halides. The tags patterned using this method are almost impossible to counterfeit, and
offer a much better protection against fakes than available anti-counterfeiting techniques. We will discuss if this novel
technology is ready for industrial production of anti-counterfeiting tags.
Lithium Fluoride (LiF) presents interesting optical properties and it has been proposed as an active optical material for
colour centre (CC) lasers and amplifiers. The use of a grating to reduce the laser threshold and to narrow the line-width
has also been demonstrated. More recently, LiF gained increased attention due to its unique characteristics as x-ray
imaging detector for high resolution microscopy and for the development of CC lasers using gratings induced by high
energy femtosecond laser pulses. We present a review of our recent work in the production of photonic structures, like
photo-induced Bragg gratings and wave-guides either by laser CW illumination or by femtosecond laser irradiation.
CW laser illuminated gratings were written in both coloured LiF crystals and films. Gratings and wave-guides are
characterized using confocal microscopy, optical absorption and emission spectroscopy, near field scanning, scattering
and insertion loss measurements.
Broad-band light-emitting color-center active waveguides induced at the surface of Lithium Fluoride, LiF, crystals by
direct writing with low-energy electron beams show great potentialities for the realization of innovative miniaturized
solid-state light sources, optical amplifiers and lasers operating in the green-red wavelength interval under optical
pumping in the blue spectral range. Their full spectral characterization, hence the optimization of LiF-based single-mode
active waveguides, still remains a difficult task. Color-center micro-strip waveguides induced by electron-beam
lithography in LiF crystals were successfully characterized via fluorescence imaging microscopy and optical
transmittance measurements performed in a versatile spectrophotometer that can measure the transmittance through
microstructures down to dimensions of about 50 μm. The irradiation gave rise to the stable formation of primary and
aggregate color centers within a thin surface layer of the crystal, whose refractive index is locally modified with respect
to the surrounding blank material. The electronic defect volume concentrations and the wavelength-dependent refractive
index modifications were evaluated as functions of the irradiation dose for three active micro-strips produced by 12 keV
electrons on the same LiF crystal.
Among insulating materials containing point defects, Lithium Fluoride, LiF, is a radiation sensitive material well known in dosimetry and as active medium in optically pumped optolectronic devices. Primary and aggregate electronic defects, known as colour centres (CCs), can be efficiently produced in LiF by low-penetrating radiation. A novel imaging detector for soft X-ray microscopy, based on photoluminescence from laser active CCs, is currently under development. The continuous shrinking dimensions of photonic devices has prompted us to use thin LiF films, directly grown by thermal evaporation on different substrates, as recording media in Extreme Ultra-Violet contact-lithography experiments for the fabrication of permanent, regular, light-emitting microstructures, produced with high spatial resolution on large areas in short exposure times. The experiments were performed by using geometrical masks in an excimer-pumped laser-plasma source and the samples analyzed by confocal laser scanning microscopy. Strong visible photoluminescence at room temperature was measured from very thin surface layers. A preliminary comparison between the optical response of CCs in thin LiF films grown on glass and silicon substrates with respect to LiF crystals was performed. The polycrystalline LiF films show a higher sensitivity, which is discussed taking into account light confinement effect in the investigated planar structures.
Much attention is currently being paid to the materials and processes that allow one to directly write or to imprint waveguiding structures and/or diffractive elements for optical integrated circuits by exposure from a source of photons, electrons or ions. Here a brief overview of the results achieved in our laboratories is presented, concerning the fabrication and characterization of optical guiding structures based on different materials and exposure techniques. These approaches include: electron and ion beam writing of waveguides in (poly)-crystalline lithium fluoride, uv-laser printing of waveguides and gratings in photorefractive glass thin films, and fs-laser writing in tellurite glasses. Properties and perspectives of these approaches are also discussed.
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.
Nowadays communications require a continuous increase of data rates and, as far as the single customer is concerned, there is a severe bottleneck in the last kilometer of the transmission line. The only viable solution to this problem is the Fiber to the Home (FTTH) system, which requires an amplification of the incoming signal in the optical fiber. However, the device of election, the Erbium Doped Fiber Amplifier (EDFA), is still very expensive, so that new optical waveguide amplifiers based on Lithium Fluoride (LiF) thin layers containing color centers have been recently proposed. Such electronic defects are characterized by broad absorption bands in the visible and near ultra-violet (UV) spectral range and by efficient, broad-band emissions extending from the visible to the near-infrared (NIR). One of the important properties of these light emitters arises from the short lifetime, several nanoseconds, which is a fundamental feature for high gain laser media. Several basic and technical aspects of novel solid-state amplifier configurations based on active waveguides in LiF will be presented and discussed.
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 recent progress in the development of the tabletop soft x-ray lasers pumped by capillary discharges has opened the possibility of the widespread use of these coherent sources for applications. In this paper, we present the latest results achieved in our laboratory concerning the optimization of a 46.9nm capillary discharge pumped soft x-ray laser and the imaging of the laser beam on Lithium Fluoride (LiF, films), which appears to be a new promising x-ray detector. The use of LiF allows a high spatial resolution (smaller than 1μm) on large areas, high-contrast imaging and simple manipulation. The laser, which is pumped by 18-20kA, 150-180ns long current pulses at a repetition rate of 0.1Hz, is produced in 3.2mm diameter up to 45cm length alumina capillary tubes filled with Ar at the initial gas pressure of 300-600mTorr. At the capillary lengths examined, the laser is operating in the saturation regime giving an output energy of about 300μJ per pulse and high coherence degree.
Electron-beam lithography techniques allow realizing channel waveguides based on laser-active color centers in lithium fluoride (LiF) emitting in the visible. Amplified spontaneous emission (ASE) of red light from F2 defects has been observed in these confining structures: the appreciable values of the gain coefficient, several cm-1, with an exciting power density of few mW/cm2, makes LiF a good candidate for the realization of active integrated optical devices.
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.
Lithium fluoride (LiF) films irradiated by low energy electrons were employed as active spacers in all-solid, dielectric optical microcavities emitting in the visible spectral range. We present the results of optical characterization of the spontaneous emission from F2 color centers embedded in a LiF layer confined inside a planar microcavity. These structures seem promising for the realization of novel kinds of solid-state miniaturized emitting devices.
The efficient coloration of LiF material, in the form of bulk and films, by EUV and soft X-rays emitted by a laser-plasma source is demonstrated. The short penetration depth of soft-X-rays is exploited to obtain high spatial resolution luminescent patterns while the high dynamic range of proportionality between X-ray dose and coloration is exploited for using LiF as image detector in micro-radiography and soft X-ray microscopy applications.
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.
A brief overview of some recent results in the fabrication and characterization of optical guiding structures based on thin films of lithium fluoride is presented. These LiF structures are being developed with the main aim of obtaining an integrated broad-band tunable lasers operating in the visible spectrum. Properties and perspectives of this approach are also discussed.
Impurity and stress distribution in diamond films have been studied by microRaman and microphotoluminescence. Raman and PL lateral profiles reveal the existence of a large anisotropic stress at the grain boundary resulting in frequency splitting and linewidth variations of the TO phonon. The detection of an additional peak at 1326 cm-1 related to exagonal diamond phases and the high intensity reached by the 1.68 eV PL band at the coalescence region between two grains give evidence of preferential incorporation of defects at the grain boundary regions. Similarly, Raman and PL depth profiles indicate that the non-diamond intergrain tissue, abundant close to the substrate, induces compressive stress and represents a preferential site for Si diffusion.
Low energy electron beam irradiation of LiF single crystals and polycrystalline films induces efficient formation of stable laser active defects emitting in the visible spectral range at room temperature, together with a consistent increase of the real part of the refractive index in the same wavelength interval. The use of electron lithography techniques look promising for the realization of active channel waveguides.
Lithium fluoride thin films represent an innovative material for integrated optics. Passive waveguides may be produced by using a LiF/NaF two-layer structure deposited on any substrate material. On the other hand, low-energy electron beam irradiation of LiF polycrystalline films gives rise to the efficient formation of laser active lattice defects showing intense photoluminescence and sizable optical gain in the visible spectral range from green to red at room temperature. This irradiation at the same rime induces an increase of the real part of the refractive index, thus allowing one to exploit electron lithography technique to directly write integrated optical amplifiers and lasers in LiF films. Experimental results of the characterization of both passive and active waveguides are reported, demonstrating the feasibility of more complex circuits in this material.
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