F. Bonfigli

Soft x-ray submicron imaging detector based on point defects in LiF

The use of lithium fluoride (LiF) crystals and films as imaging detectors for EUV and soft-x-ray radiation is discussed. The EUV or soft-x-ray radiation can generate stable color centers, emitting in the visible spectral range an intense fluorescence from the exposed areas. The high dynamic response of the material to the received dose and the atomic scale of the color centers make this detector extremely interesting for imaging at a spatial resolution which can be much smaller than the light wavelength. Experimental results of contact microscopy imaging of test meshes demonstrate a resolution of the order of 400nm. This high spatial resolution has been obtained in a wide field of view, up to several mm2. Images obtained on different biological samples, as well as an investigation of a soft x-ray laser beam are presented. The behavior of the generated color centers density as a function of the deposited x-ray dose and the advantages of this new diagnostic technique for both coherent and noncoherent EUV so...

High-contrast photoluminescent patterns in lithium fluoride crystals produced by soft x-rays from a laser-plasma source

A technique using soft x-rays and extreme ultraviolet light generated by a laser-plasma source has been investigated for producing low-dimensionality photoluminescent patterns based on active color centers in lithium fluoride (LiF) crystals. Strong visible photoluminescence at room temperature has been observed in LiF crystals from fluorescent patterns obtained by masking the incoming radiation. This technique is able to produce colored patterns with high spatial resolution on large areas and in short exposure times as compared with other coloration methods.

Hard x-ray contact microscopy with 250nm spatial resolution using a LiF film detector and a tabletop microsource

An innovative route for deep-submicrometer spatial resolution hard x-ray microscopy with tabletop x-ray source is proposed. A film of lithium fluoride (LiF) was used as imaging detector in contact mode. We present here the x-ray images recorded on LiF films of a Fresnel zone plate with submicrometer gold structures and of an onion cataphyll. The images were read with an optical confocal microscope in fluorescence mode. The measured spatial resolution was about 250nm, i.e., close to the resolution limit of the confocal microscope. The advantages and drawbacks, and the possible improvements, of this route are discussed.

Scanning near-field optical microscopy images of microradiographs stored in lithium fluoride films with an optical resolution of λ∕12

Here we show a new, simple method to observe soft x-ray microradiographs of biological material. A thin film of lithium fluoride (LiF) works as image detector, storing the microradiograph obtained exposing biological samples to extreme ultraviolet and soft x-ray radiations. To read the stored image, collecting the optically stimulated visible luminescence emitted by the LiF active color centers locally produced by the x rays, a scanning near-field optical microscope is used with an optical aperture of 50nm, i.e., λ∕12, where λ is the wavelength of the collected photoluminescence.

Direct writing of fluorescent patterns on LiF films by x-ray microprobe

Fluorescent patterns with submicron dimensions have been obtained by creating stable F3+ and F2 color centers in LiF films using a focused x-ray beam provided at the ELETTRA synchrotron radiation facility. The patterns were written by scanning the LiF specimen with respect to the x-ray microprobe. In these attempts, using an x-ray microspot with a diameter of 100 nm and a flux density ⩾109 photons/s, we generated ∼500-nm-wide lines efficiently emitting in the visible spectral region when excited by blue light at 458 nm. Preliminary results indicate that the spectral distribution of the emitted luminescence can be changed by varying the photon dose delivered to the sample.

Damage and ablation of large bandgap dielectrics induced by a 46.9 nm laser beam

We applied a 0.3 mJ, 1.7 ns, 46.9 nm soft-x-ray argon laser to ablate the surface of large bandgap dielectrics: CaF2 and LiF crystals. We studied the ablation versus the fluence of the soft-x-ray beam, varying the fluence in the range 0.05-3 J/cm2. Ablation thresholds of 0.06 and 0.1 J/cm2 and ablation depths of 14 and 20 nm were found for CaF2 and LiF, respectively. These results define new ablation conditions for these large bandgap dielectrics that can be of interest for the fine processing of these materials.

Shadow monochromatic backlighting: Large-field high resolution X-ray shadowgraphy with improved spectral tunability

The shadow monochromatic backlighting (SMB) scheme, a modification of the well-known soft X-ray monochromatic backlighting scheme, is proposed. It is based on a spherical crystal as the dispersive element and extends the traditional scheme by allowing one to work with a wide range of Bragg angles and thus in a wide spectral range. The advantages of the new scheme are demonstrated experimentally and supported numerically by ray-tracing simulations. In the experiments, the X-ray backlighter source is a laser-produced plasma, created by the interaction of an ultrashort pulse, Ti:Sapphire laser (120 fs, 3–5 mJ, 10 16 W/cm 2 on target) or a short wavelength XeCl laser (10 ns, 1–2 J, 10 13 W/cm 2 on target) with various solid targets (Dy, Ni + Cr, BaF 2 ). In both experiments, the X-ray sources are well localized spatially (∼20 μm) and are spectrally tunable in a relatively wide wavelength range (λ = 8–15 A). High quality monochromatic (δλ/λ ∼ 10 −5 –10 −3 ) images with high spatial resolution (up to ∼4 μm) over a large field of view (a few square millimeters) were obtained. Utilization of spherically bent crystals to obtain high-resolution, large field, monochromatic images in a wide range of Bragg angles (35° < Θ < 90°) is demonstrated for the first time.

High concentrations of aggregate colour centres in heavily irradiated LiF crystals

Abstract The stable formation of several colour centres (CCs) has been investigated in lithium fluoride (LiF) single crystals irradiated at room temperature (RT) and at 213 K by 5 MeV electrons with doses from 1019 to 1023 eV/cm3. The temperature during irradiation influences the production of aggregate defects, in particular the ratio between the F3+ and F2 laser active centres and the amount of parasitic complex defects. Optical absorption and photoluminescence spectra allow clarifying the role of different aggregate defects on the emission properties of the F3+ and F2 centres at concentrations up to 1018 cm−3.

Luminescent patterns based on color centers generated in lithium fluoride by extreme ultraviolet radiation and soft X-rays

Primary and aggregate color centers in lithium fluoride (LiF) crystals and polycrystalline LiF films were produced by an innovative irradiation technique using extreme ultraviolet radiation and soft X-rays generated by a laser-plasma source. This irradiation facility allowed the efficient formation of active color centers on luminescent patterns with submicron spatial resolution on large areas and short exposure times. The method looks promising for the realization of low-dimensionality photonic devices. The optical characterization of the colored structures was performed by means of absorption and photoluminescence measurements on LiF samples colored under different irradiation conditions.

Photo-induced gratings in thin color center layers on lithium fluoride

We study the recording of permanent Bragg gratings on surface-colored lithium fluoride (LiF) crystals by using the interference pattern of a continuous-wave UV argon-ion laser operating at 244 nm. Gratings with spatial periodicity ranging from 400 to 1000 nm are written by using a phase-mask interferometer and are stable for several months after the writing process. Absorption and photoluminescence spectra show the bleaching of primary F and F -aggregate laser-active color centers as a result of the process. Confocal microscopy is used to determine the pitch and the profile of the fluorescent gratings. The UV laser-induced optical bleaching in highly colored LiF ultrathin layers is responsible for the periodic spatial modulation of absorption and photoemission properties that characterize the gratings. In the colored surface layer, a reduction of as much as 50% of the initial color-center-induced refractive-index increase has been estimated in the bleached areas.