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Soft X-ray imaging by optically stimulated luminescence from color centers in lithium fluoride
Spectrochimica Acta Part B 62 (2007) 631 – 635 www.elsevier.com/locate/sab
Soft X-ray imaging by optically stimulated luminescence from color centers in lithium fluoride ☆ F. Bonfigli a,⁎, S. Almaviva a , G. Baldacchini a , S. Bollanti a , F. Flora a , A. Lai a , R.M. Montereali a , E. Nichelatti b , G. Tomassetti c , A. Ritucci c , L. Reale c , A. Ya. Faenov d , T.A. Pikuz d , R. Larciprete e , L. Gregoratti f , M. Kiskinova f a ENEA, C.R. Frascati, Via E. Fermi, 45, 00044 Frascati (Rome), Italy ENEA, C.R. Casaccia, Via Anguillarese 301, 00060 S.Maria di Galeria (Rome), Italy c Università de L'Aquila e INFN, Dip. di Fisica, Coppito, L'Aquila, Italy d MISDC of VNIIFTRI Mendeleevo, Moscow region, 141570, Russia ISC-CNR, Sezione Montelibretti, Via Salaria, Km. 29.3, 00016 Monterotondo Scalo (Rome), Italy f Sincrotrone Trieste, S. S. 14, Km. 163.5, 34012 Basovizza (TS), Italy b
e
Received 21 November 2005; accepted 20 February 2007 Available online 2 March 2007
Abstract An innovative X-ray imaging detector based on Optically Stimulated Luminescence from color centers in lithium fluoride is presented. Regular photoluminescent patterns produced on LiF samples by different intense X-ray sources, like synchrotrons, laser plasma sources and a capillary discharge laser have been investigated by a Confocal Laser Scanning Microscope. The use of a LiF-based imaging plate for X-ray microscopy is also discussed showing microradiographies of small animals. © 2007 Elsevier B.V. All rights reserved. Keywords: Lithium fluoride; Color Center; Photoluminescence; Soft X-ray; Imaging detector
1. Introduction The recent scientific and technological interest for soft X-ray applications in different fields makes still more attractive the development of novel detectors. Among radiation sensitive materials exhibiting active optical properties, lithium fluoride (LiF), in the form of bulk and film, is a very promising candidate for the fabrication of miniaturized light sources for innovative photonic devices. Significant results have been obtained in the manufacture and characterization of integrated active optical devices based on color centers (CCs) in LiF, ☆
This paper was presented at the “18th International Congress on X-ray Optics and Microanalysis" (ICXOM-18) held in Frascati, Rome (Italy), 25-30 September 2005, and is published in the Special Issue of Spectrochimica Acta Part B, dedicated to that conference". ⁎ Corresponding author. Tel.: +39 06 9400 5567; fax: +39 06 9400 5400. E-mail address: [email protected] (F. Bonfigli). 0584-8547/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.sab.2007.02.016
mainly produced by using low-energy electron-beam lithography, like active waveguides [1], microcavities [2], point light sources [3,4] and optical memories [5]. The aggregate CCs are characterized by broad absorption and emission bands in the visible spectral range. Particular attention has been devoted to F3+ and F2 defects, which consist of two electrons bound to two and three close anion vacancies, respectively. These centers have almost overlapping absorption bands (M band) centered at about 450 nm [6] and, therefore, can be simultaneously excited with a single laser pump wavelength. On the other hand, they exhibit two different broad emission bands in the green (F3+) and red (F2) spectral ranges. The formation of CCs induces an increase in the refractive index of the colored volume with respect to that of an uncolored one, in the same spectral interval where the defects photoemissions are located. So it is possible to create an active waveguiding region, or other devices, that exhibit a periodic modification of the refractive index and of the gain, such as a Distributed Feedback
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(DFB) laser. Very recently simultaneous fabrication of laseractive F2 and F3+ color centers in LiF and permanent periodic gratings with fringe spacings as fine as sub-micron size has been obtained by two interfering infrared femtosecond laser pulses [7]. As far as the techniques are concerned, the increasing demand for low-dimensionality photonic devices imposes the utilization of advanced lithographic systems for producing optical structures with submicrometric spatial resolution. In LiF stable CCs production is possible only by ionizing radiation, like neutral and charged particles (neutrons, electrons and ions), gamma and X-rays, and UV light. By using electron beam irradiation, the spatial resolution of the colored volume is limited by beam enlargement due to charge effects in insulating materials and also by lateral spreading of the electrons due to scattering processes in the interaction volume. On the other hand, highly penetrating radiation, like gamma and X-rays, are not suitable to color thin layers with controlled depth and in producing luminescent patterns with high spatial resolution (the resolution is limited by the photoelectrons blurring, as well known in the proximity X-ray lithography experiments [8]). The low penetration, the short wavelength and neutrality of soft X-rays (0.3b hν b 8 keV) and EUV light (20b hν b 300 eV) [8] make both types of electromagnetic radiation particularly attractive for localized surface coloration [9] of LiF single crystals and thin films. Regular photoluminescent patterns based on the stable generation of CCs in LiF samples obtained by different irradiation techniques and by several soft X-ray sources will be presented. The simplicity and the efficiency of the Optically Stimulated Luminescence (OSL) reading technique allows for an extremely large field of view without limiting the final detectable resolution if advanced optical microscopes (like Confocal Laser Scanning Microscope “CLSM” or Scanning Near Field Optical Microscope “SNOM”) are used. Luminescent structures on LiF have been investigated by a CLSM, achieving high spatial resolution images. Recently, the idea of using a LiF film as imaging plate for X-ray microscopy and micro-radiography based on OSL from CCs has been proposed [10] and developed [11] and an example will be presented and discussed.
angle following roughly a cos(θ) angular distribution, it is possible to irradiate the whole surface of the sample placed in its proximity. A significant advantage of this technique is that a large area, up to 4 × 4 cm2 for a typical distance of LiF from the source of 10 cm, can be uniformly irradiated in a single shot. Direct writing of fluorescent patterns on LiF films has been obtained by using a focused X-ray beam provided by the ELETTRA synchrotron facility of Trieste (ESCA Microscopy beamline) [14]. Photoluminescent patterns of CCs in LiF have been obtained by scanning the LiF specimen with respect to the X-ray microprobe [15]. The X-ray source is optimized for the energy range (400–700) eV and can be spatially limited by a pinhole ≈80 μm wide, or can be demagnified to a microspot with a diameter of ≈100 nm by using a zone plate focusing optic, which has the advantage to keep the beam size constant, independently from the chosen energy. In the first operation mode the photon flux is ≈5 × 1014 photons/cm2 s and rises to 2 × 1018 photons/cm2 s in the microprobe configuration, high enough to allow local creation of CCs using short exposure times between 1 and 100 ms. The LiF specimens were mounted on the ESCA positioning and scanning stage, equipped with stepper (accuracy 1 μm) and piezo-driven (accuracy 10 nm) motors, controlled by a computer program. By moving the LiF specimens
2. Experimental details on irradiation sources For testing LiF crystals and films as an imaging detector, as well as for photonic device developments, permanent photoluminescent structures were generated by several intense soft Xrays sources using different lithographic irradiation techniques. A laser plasma source, obtained by focusing a powerful excimer laser beam on a solid target and developed at C.R. ENEA Frascati [12], has been used to irradiate LiF material by masking the incoming radiation [13]. The X-ray emission of this source covers the spectral interval of 0.8–60 nm (1.5 keV– 20 eV), that is the full EUV region and part of the soft X-rays region. The emission spectral distribution can be adjusted by changing the laser parameters and the target material. The typical repetition rate is 1–10 Hz and the pulse duration is 10, 30 or 120 ns depending on the characteristic of the selected excimer laser. As the plasma source emits X-rays in a 2π solid
Fig. 1. a) CLSM optical image in fluorescence mode of a periodic photoluminescent structures at the surface of a LiF crystal, subsequent to irradiation by a laser plasma soft X-rays by masking the LiF sample. b) Gray-level scan of an irradiated zone.
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in front of the beam with steps comparable with the spot size, periodic luminescent patterns of CCs were written. Periodic luminescent lines on LiF can be also obtained by an interferometric technique based on irradiation with an intense and high spatially coherent soft-X-ray laser beam [16]. We used a compact 46.9-nm capillary-discharge-pumped laser developed at University of L' Aquila [17]. The amplification of the 46.9 nm laser line is produced inside a high-temperature and high density Ar plasma, on the 3p–3s transition of the Ne-like Ar ions. The plasma column is formed by the Z-pinch compression of preionized Ar gas, induced by a 50-ns-rise time current pulse with amplitude of 20 kA. The discharge is produced inside a 3-mm diameter and 45-cm length capillary tube of ceramic material, which initially contains the Argon at the pressure of 400– 450 mTorr. The soft X-ray beam is formed by the single pass amplification of the lasing radiation through the plasma medium. For a capillary length of 45 cm, the laser operates in a full saturation regime with an energy per pulse of about 300 μJ, pulse duration of 1.7 ns and output intensity of 4 × 107 W/cm2 (brilliance about 1024 ph/s mm2 mrad in 0.001%bw). The laser pulses are produced at the repetition rate of 0.2 Hz. The beam was characterized by an annular intensity distribution having the peakto-peak divergence of 5 mrad. The short wavelength and the high peak-power of soft-X-ray lasers make them attractive for many types of single-shot applications in the field of nano-science and -technology, such as X-ray time resolved interferometry, holography, microscopy and X-ray focusing. A confocal microscope (NIKON Eclipse 80i-C1) equipped with an Argon laser (Coherent Innova 90) was used to obtain and investigate the images of CCs based structures realized on LiF
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samples irradiated in the above mentioned soft X-ray sources with different methods. The OSL signal has been detected under illumination with an Ar laser operating on the 458 nm line, by exciting in the M absorption band spectral region. The photoluminescence is due to the broad emission bands of F2 and F3+ centers peaked at 678 nm and at 541 nm, respectively. The powerful microscopic technique offered by CLSM can be used for sub-wavelength spatial resolution imaging and to gain details about CCs depth distribution in irradiated LiF crystals and films, by using its focal plane-sectioning property. 3. Results and discussion To obtain geometric colored patterns on LiF crystals, a copper mesh with typical wire dimension of 10 μm and characterized by 2000 lpi (lines per inch), corresponding to a period of 12.7 μm, was placed in close contact with the LiF surface. The X-ray incoming radiation was produced by the laser plasma source described above, equipped with a copper target and exposing for 9600 shots (total exposure time 23 min, total delivered dose on LiF about 1.4 J/cm2). Fig. 1a shows the CLSM image of a regular luminescent pattern obtained in a LiF crystal: the uncolored black parts represent the dark shadow of the mesh, while the gray ones correspond to the colored areas that look as luminescent points. The intensity of the emitted visible luminescence is proportional to the transparency of the object, which filters the X-ray exposure of the LiF. Fig. 1b shows the gray-level scan of a zone containing colored areas. The periodic photoluminescent structures were produced on large areas, due to the point-like behavior of the used laser plasma source. The high sensitivity of the imaging method is
Fig. 2. a) Optical image of a dragonfly wing radiograph stored on a LiF film and detected with an optical microscope in fluorescence mode. b) CLSM optical image of a detail of (a) (black dashed square). c) Intensity profile along one half of the white line seen in (b).
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Fig. 3. a) CLSM optical image in fluorescence mode of a permanent grating of luminescent CCs written directly on a LiF crystal by using a focused soft X-ray beam provided at the ELETTRA synchrotron radiation facility (ESCA Microscopy beamline). b) Intensity profile of part of the grating, consisting of regularly spaced lines whose FWHM has been reported in (c).
related to the elevated values, close to unity, of the quantum photoemission efficiencies for F2 and F3+ defects [18], whose intense photoluminescence spectra fall in an extended spectral range, where the light detector sensitivity is maximum. Inhomogeneities in the emitted light are due to damage in the masking mesh. Similarly, the micro-radiography of a dragonfly (Pyrrhesoma nymphula) wing was recorded by placing the biological sample in close contact with a 2 μm thick LiF film that was thermally
Fig. 4. Lloyd mirror interferometer utilized for producing periodic luminescent lines on LiF by using a compact 46.9-nm capillary-discharge-pumped laser.
Fig. 5. a) Photoluminescent regular grating on a LiF crystal obtained by an interferometric technique based on irradiation with an intense and high spatially coherent soft-X-ray laser beam. The fluorescent image has been detected by CLSM. b) Intensity profile of some lines of the structure. c) Intensity profile of a single line.
evaporated on a glass substrate [19] and by exposing it to soft X-rays. The X-ray dose delivered to the sample was about 160 mJ/cm2 in 1100 shots. The biological sample was dried in vacuum and it should not suffer from distortions due to heating during the X-ray irradiation, even for long exposure times. As a matter of fact, high irradiation doses are usually required in X-ray microscopy, in order to obtain high-resolution images. With this technique the whole X-ray fluence is delivered in a very short time (few nanoseconds), thus reducing damage and modification in the irradiated biological samples during the exposure. Fig. 2a shows
F. Bonfigli et al. / Spectrochimica Acta Part B 62 (2007) 631–635
the entire OSL image of the wing obtained with a conventional optical microscope in fluorescence mode. A detail of the wing was observed by CLSM and is reported in Fig. 2b. Fig. 2c shows the intensity profile along one half of the white line of the rib in Fig. 2b. A resolution of 764 nm was obtained. Fig. 3a shows a regular nanometric structure of CCs on a LiF crystal obtained by direct writing with the focused soft X-ray beam provided at the ELETTRA synchrotron radiation facility, with energy of 650 eV and for an exposure time of 100 ms per pixel of the scan (total exposure time = 6.8 min). The colored area (56× 62 μm2) forms a regular grating whose intensity profile is reported in Fig. 3b. It consists of lines regularly spaced by 1 μm, with FWHM of 340 nm, as shown in Fig. 3c. The high lateral spatial resolution of the confocal system allows measuring this value, which is well below the wavelength of the collected emitted radiation. Periodic luminescent lines on LiF have also been obtained by using the compact 46.9-nm capillary-discharge-pumped laser described-above. Due to the full spatial coherence of the laser beam, interferometric methods for a direct formation of a periodic structure can be utilized. The fringe encoding was obtained by a Lloyd mirror interferometer schematically illustrated in Fig. 4. As shown in the figure, the LiF crystal is placed just after the mirror, oriented perpendicularly to the beam propagation. The overlapping between the direct and the reflected beams generates fringes on the LiF surface up to more than 1 mm from the mirror edge (that is from the beam axis). Irradiating the LiF samples with 10 laser shots, we achieve on the LiF surface the estimated energy flux of 25 mJ/cm2, which is sufficient to obtain well observable photo-luminescent patterns on an area up to 10 mm2. Fig. 5a shows the interferometric pattern produced on a LiF crystal by this encoding technique. The structure consists of a regular grating whose intensity profile is reported in Fig. 5b. Fig. 5c shows the intensity profile of a single line with a FWHM of 625 nm. 4. Conclusions We have demonstrated that LiF, in the form of polished crystal and thin films, can be exploited to host sub-micrometric active optical structures, based on CCs, generated by several soft X-ray sources. Their detection and investigation were performed by means of an advanced fluorescence microscope, the CLSM, which reads the visible photoluminescence of CCs produced through the soft X-ray irradiation and acquires their high optical resolution images. We demonstrated, also, that LiF radiation-sensitivity can be employed in the field of X-ray micro-radiography. LiF, as a novel imaging detector for soft X-rays, shows interesting peculiarities (like high spatial resolution, high dynamic range, efficiency and simplicity of the reading system), which make it very attractive for different applications in the field of photonic devices and for X-ray microscopy for biological investigations. References [1] R.M. Montereali, M. Piccinini, E. Burattini, Amplified spontaneous emission in active channel waveguides produced by electron beam lithography in LiF crystals, Appl. Phys. Lett. 78 (2001) 4082–4084.
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[2] A. Belarouci, F. Menchini, H. Rigneault, B. Jacquier, R.M. Montereali, F. Somma, P. Moretti, Spontaneous emission properties of color centers based optical microcavities, Opt. Commun. 189 (2001) 281–287. [3] P. Adam, S. Benrezzak, J.L. Bijeon, P. Royer, S. Guy, B. Jacquier, P. Moretti, R.M. Montereali, M. Piccinini, F. Menchini, F. Somma, C. Seassal, H. Rigneault, Fluorescence imaging of submicrometric lattices of colour centres in LiF by an apertureless scanning near-field optical microscope, Opt. Express 9 (2001) 353–359. [4] S.K. Sekatskii, V.S. Letokhov, Single fluorescence centers on the tips of crystal needles: first observation and prospects for application in scanning one-atom fluorescence microscopy, Appl. Phys., B Lasers Opt. 63 (1996) 525–530. [5] E.J. Caine, S.D. Miller, Optical data storage in LiF using electron beam encoding, J. Vac. Sci. Technol., B 16 (1998) 3232–3236. [6] J. Nahum, D.A. Wiegand, Optical properties of some F-aggregate centers in LiF, Phys. Rev. 154 (1967) 817–830. [7] T. Kurobori, K. Kawamura, M. Hirano, H. Hosono, Simultaneous fabrication of laser-active colour centres and permanent microgratings in lithium fluoride by a single femtosecond pulse, J. Phys., Condens. Matter 15 (2003) L399–L405. [8] David Attwood: Soft X-rays and Extreme Ultraviolet Radiation: Principles and Applications, Cambridge University Press, Trumpington Street, Cambridge, UK, 1999. [9] E. Nichelatti, F. Bonfigli, A.Ya. Faenov, F. Flora, T. Marolo, M. Montecchi, R.M. Montereali, T. Pikuz, G. Baldacchini, Surface generation of color centers in lithium fluoride by EUV irradiation, J. Non-Cryst. Solids 351 (2005) 1774–1779. [10] ENEA Patent TO2002A000575, G. Baldacchini, F. Bonfigli, A. Faenov, F. Flora, R.M. Montereali, A. Pace, T. Pikuz, L. Reale (International No. WO 2004/005906 A1). [11] G. Baldacchini, F. Bonfigli, A. Faenov, F. Flora, R.M. Montereali, A. Pace, T. Pikuz, L. Reale, Lithium fluoride as a novel X-ray image detector for biological μ-world capture, J. Nanosci. Nanotechnol. 3 (6) (2003) 483–486. [12] S. Bollanti, P. Albertano, M. Belli, P. Di Lazzaro, A. Ya. Faenov, F. Flora, G. Giordano, A. Grilli, F. Ianzini, S.V. Kukhlevsky, T. Letardi, A. Nottola, L. Palladino, T. Pikuz, A. Reale, L. Reale, A. Scafati, M.A. Tabocchini, I.C.E. Turcu, K. Vigli-Papadaki, G. Schina, Soft X-ray plasma source for atmospheric pressure microscopy, radiobiology and other applications, Il Nuovo Cim. 20D (1998) 1685–1701. [13] G. Baldacchini, F. Bonfigli, F. Flora, R.M. Montereali, D. Murra, E. Nichelatti, A. Faenov, T. Pikuz, High-contrast photoluminescent patterns in lithium fluoride crystals produced by soft X-rays from a laser-plasma source, Appl. Phys. Lett. 80 (2002) 4810–4812. [14] L. Gregoratti, M. Marsi, M. Kiskinova, Recent developments of the scanning photoelectron microscope at ELETTRA, Synchrotron Radiat. News 12 (1999) 40–42. [15] R. Larciprete, L. Gregoratti, M. Danailov, R.M. Montereali, F. Bonfigli, M. Kiskinova, Direct writing of fluorescent patterns on LiF films by X-ray microprobe scanning, Appl. Phys. Lett. 80 (2002) 3862–3864. [16] G. Tomassetti, A. Ritucci, A. Reale, L. Arizza, F. Flora, R.M. Montereali, A. Faenov, T. Pikuz, Two-beam interferometric encoding of photoluminescent gratings in LiF crystals by high-brightness tabletop soft X-ray laser, Appl. Phys. Lett. 85 (2004) 4163–4165. [17] G. Tomassetti, A. Ritucci, A. Reale, L. Palladino, L. Reale, S.V. Kukhlevsky, F. Flora, L. Mezi, J. Kaiser, A. Faenov, T. Pikuz, Capillary discharge soft X-ray lasing in the Ne-like Ar pumped by long current pulses, Eur. Phys. J. D 19 (2002) 73–77. [18] T.T. Basiev, S.B. Mirov, V.V. Osiko, Room temperature color center laser, IEEE J. Quantum Electron. 24 (1988) 1052–1069. [19] R.M. Montereali, G. Baldacchini, L.C. Scavarda do Carmo, LiF films: absorption and luminescence of colour centres, Thin Solid Films 201 (1991) 106–108.
Soft X-ray imaging by optically stimulated luminescence from color centers in lithium fluoride ☆ F. Bonfigli a,⁎, S. Almaviva a , G. Baldacchini a , S. Bollanti a , F. Flora a , A. Lai a , R.M. Montereali a , E. Nichelatti b , G. Tomassetti c , A. Ritucci c , L. Reale c , A. Ya. Faenov d , T.A. Pikuz d , R. Larciprete e , L. Gregoratti f , M. Kiskinova f a ENEA, C.R. Frascati, Via E. Fermi, 45, 00044 Frascati (Rome), Italy ENEA, C.R. Casaccia, Via Anguillarese 301, 00060 S.Maria di Galeria (Rome), Italy c Università de L'Aquila e INFN, Dip. di Fisica, Coppito, L'Aquila, Italy d MISDC of VNIIFTRI Mendeleevo, Moscow region, 141570, Russia ISC-CNR, Sezione Montelibretti, Via Salaria, Km. 29.3, 00016 Monterotondo Scalo (Rome), Italy f Sincrotrone Trieste, S. S. 14, Km. 163.5, 34012 Basovizza (TS), Italy b
e
Received 21 November 2005; accepted 20 February 2007 Available online 2 March 2007
Abstract An innovative X-ray imaging detector based on Optically Stimulated Luminescence from color centers in lithium fluoride is presented. Regular photoluminescent patterns produced on LiF samples by different intense X-ray sources, like synchrotrons, laser plasma sources and a capillary discharge laser have been investigated by a Confocal Laser Scanning Microscope. The use of a LiF-based imaging plate for X-ray microscopy is also discussed showing microradiographies of small animals. © 2007 Elsevier B.V. All rights reserved. Keywords: Lithium fluoride; Color Center; Photoluminescence; Soft X-ray; Imaging detector
1. Introduction The recent scientific and technological interest for soft X-ray applications in different fields makes still more attractive the development of novel detectors. Among radiation sensitive materials exhibiting active optical properties, lithium fluoride (LiF), in the form of bulk and film, is a very promising candidate for the fabrication of miniaturized light sources for innovative photonic devices. Significant results have been obtained in the manufacture and characterization of integrated active optical devices based on color centers (CCs) in LiF, ☆
This paper was presented at the “18th International Congress on X-ray Optics and Microanalysis" (ICXOM-18) held in Frascati, Rome (Italy), 25-30 September 2005, and is published in the Special Issue of Spectrochimica Acta Part B, dedicated to that conference". ⁎ Corresponding author. Tel.: +39 06 9400 5567; fax: +39 06 9400 5400. E-mail address: [email protected] (F. Bonfigli). 0584-8547/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.sab.2007.02.016
mainly produced by using low-energy electron-beam lithography, like active waveguides [1], microcavities [2], point light sources [3,4] and optical memories [5]. The aggregate CCs are characterized by broad absorption and emission bands in the visible spectral range. Particular attention has been devoted to F3+ and F2 defects, which consist of two electrons bound to two and three close anion vacancies, respectively. These centers have almost overlapping absorption bands (M band) centered at about 450 nm [6] and, therefore, can be simultaneously excited with a single laser pump wavelength. On the other hand, they exhibit two different broad emission bands in the green (F3+) and red (F2) spectral ranges. The formation of CCs induces an increase in the refractive index of the colored volume with respect to that of an uncolored one, in the same spectral interval where the defects photoemissions are located. So it is possible to create an active waveguiding region, or other devices, that exhibit a periodic modification of the refractive index and of the gain, such as a Distributed Feedback
632
F. Bonfigli et al. / Spectrochimica Acta Part B 62 (2007) 631–635
(DFB) laser. Very recently simultaneous fabrication of laseractive F2 and F3+ color centers in LiF and permanent periodic gratings with fringe spacings as fine as sub-micron size has been obtained by two interfering infrared femtosecond laser pulses [7]. As far as the techniques are concerned, the increasing demand for low-dimensionality photonic devices imposes the utilization of advanced lithographic systems for producing optical structures with submicrometric spatial resolution. In LiF stable CCs production is possible only by ionizing radiation, like neutral and charged particles (neutrons, electrons and ions), gamma and X-rays, and UV light. By using electron beam irradiation, the spatial resolution of the colored volume is limited by beam enlargement due to charge effects in insulating materials and also by lateral spreading of the electrons due to scattering processes in the interaction volume. On the other hand, highly penetrating radiation, like gamma and X-rays, are not suitable to color thin layers with controlled depth and in producing luminescent patterns with high spatial resolution (the resolution is limited by the photoelectrons blurring, as well known in the proximity X-ray lithography experiments [8]). The low penetration, the short wavelength and neutrality of soft X-rays (0.3b hν b 8 keV) and EUV light (20b hν b 300 eV) [8] make both types of electromagnetic radiation particularly attractive for localized surface coloration [9] of LiF single crystals and thin films. Regular photoluminescent patterns based on the stable generation of CCs in LiF samples obtained by different irradiation techniques and by several soft X-ray sources will be presented. The simplicity and the efficiency of the Optically Stimulated Luminescence (OSL) reading technique allows for an extremely large field of view without limiting the final detectable resolution if advanced optical microscopes (like Confocal Laser Scanning Microscope “CLSM” or Scanning Near Field Optical Microscope “SNOM”) are used. Luminescent structures on LiF have been investigated by a CLSM, achieving high spatial resolution images. Recently, the idea of using a LiF film as imaging plate for X-ray microscopy and micro-radiography based on OSL from CCs has been proposed [10] and developed [11] and an example will be presented and discussed.
angle following roughly a cos(θ) angular distribution, it is possible to irradiate the whole surface of the sample placed in its proximity. A significant advantage of this technique is that a large area, up to 4 × 4 cm2 for a typical distance of LiF from the source of 10 cm, can be uniformly irradiated in a single shot. Direct writing of fluorescent patterns on LiF films has been obtained by using a focused X-ray beam provided by the ELETTRA synchrotron facility of Trieste (ESCA Microscopy beamline) [14]. Photoluminescent patterns of CCs in LiF have been obtained by scanning the LiF specimen with respect to the X-ray microprobe [15]. The X-ray source is optimized for the energy range (400–700) eV and can be spatially limited by a pinhole ≈80 μm wide, or can be demagnified to a microspot with a diameter of ≈100 nm by using a zone plate focusing optic, which has the advantage to keep the beam size constant, independently from the chosen energy. In the first operation mode the photon flux is ≈5 × 1014 photons/cm2 s and rises to 2 × 1018 photons/cm2 s in the microprobe configuration, high enough to allow local creation of CCs using short exposure times between 1 and 100 ms. The LiF specimens were mounted on the ESCA positioning and scanning stage, equipped with stepper (accuracy 1 μm) and piezo-driven (accuracy 10 nm) motors, controlled by a computer program. By moving the LiF specimens
2. Experimental details on irradiation sources For testing LiF crystals and films as an imaging detector, as well as for photonic device developments, permanent photoluminescent structures were generated by several intense soft Xrays sources using different lithographic irradiation techniques. A laser plasma source, obtained by focusing a powerful excimer laser beam on a solid target and developed at C.R. ENEA Frascati [12], has been used to irradiate LiF material by masking the incoming radiation [13]. The X-ray emission of this source covers the spectral interval of 0.8–60 nm (1.5 keV– 20 eV), that is the full EUV region and part of the soft X-rays region. The emission spectral distribution can be adjusted by changing the laser parameters and the target material. The typical repetition rate is 1–10 Hz and the pulse duration is 10, 30 or 120 ns depending on the characteristic of the selected excimer laser. As the plasma source emits X-rays in a 2π solid
Fig. 1. a) CLSM optical image in fluorescence mode of a periodic photoluminescent structures at the surface of a LiF crystal, subsequent to irradiation by a laser plasma soft X-rays by masking the LiF sample. b) Gray-level scan of an irradiated zone.
F. Bonfigli et al. / Spectrochimica Acta Part B 62 (2007) 631–635
in front of the beam with steps comparable with the spot size, periodic luminescent patterns of CCs were written. Periodic luminescent lines on LiF can be also obtained by an interferometric technique based on irradiation with an intense and high spatially coherent soft-X-ray laser beam [16]. We used a compact 46.9-nm capillary-discharge-pumped laser developed at University of L' Aquila [17]. The amplification of the 46.9 nm laser line is produced inside a high-temperature and high density Ar plasma, on the 3p–3s transition of the Ne-like Ar ions. The plasma column is formed by the Z-pinch compression of preionized Ar gas, induced by a 50-ns-rise time current pulse with amplitude of 20 kA. The discharge is produced inside a 3-mm diameter and 45-cm length capillary tube of ceramic material, which initially contains the Argon at the pressure of 400– 450 mTorr. The soft X-ray beam is formed by the single pass amplification of the lasing radiation through the plasma medium. For a capillary length of 45 cm, the laser operates in a full saturation regime with an energy per pulse of about 300 μJ, pulse duration of 1.7 ns and output intensity of 4 × 107 W/cm2 (brilliance about 1024 ph/s mm2 mrad in 0.001%bw). The laser pulses are produced at the repetition rate of 0.2 Hz. The beam was characterized by an annular intensity distribution having the peakto-peak divergence of 5 mrad. The short wavelength and the high peak-power of soft-X-ray lasers make them attractive for many types of single-shot applications in the field of nano-science and -technology, such as X-ray time resolved interferometry, holography, microscopy and X-ray focusing. A confocal microscope (NIKON Eclipse 80i-C1) equipped with an Argon laser (Coherent Innova 90) was used to obtain and investigate the images of CCs based structures realized on LiF
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samples irradiated in the above mentioned soft X-ray sources with different methods. The OSL signal has been detected under illumination with an Ar laser operating on the 458 nm line, by exciting in the M absorption band spectral region. The photoluminescence is due to the broad emission bands of F2 and F3+ centers peaked at 678 nm and at 541 nm, respectively. The powerful microscopic technique offered by CLSM can be used for sub-wavelength spatial resolution imaging and to gain details about CCs depth distribution in irradiated LiF crystals and films, by using its focal plane-sectioning property. 3. Results and discussion To obtain geometric colored patterns on LiF crystals, a copper mesh with typical wire dimension of 10 μm and characterized by 2000 lpi (lines per inch), corresponding to a period of 12.7 μm, was placed in close contact with the LiF surface. The X-ray incoming radiation was produced by the laser plasma source described above, equipped with a copper target and exposing for 9600 shots (total exposure time 23 min, total delivered dose on LiF about 1.4 J/cm2). Fig. 1a shows the CLSM image of a regular luminescent pattern obtained in a LiF crystal: the uncolored black parts represent the dark shadow of the mesh, while the gray ones correspond to the colored areas that look as luminescent points. The intensity of the emitted visible luminescence is proportional to the transparency of the object, which filters the X-ray exposure of the LiF. Fig. 1b shows the gray-level scan of a zone containing colored areas. The periodic photoluminescent structures were produced on large areas, due to the point-like behavior of the used laser plasma source. The high sensitivity of the imaging method is
Fig. 2. a) Optical image of a dragonfly wing radiograph stored on a LiF film and detected with an optical microscope in fluorescence mode. b) CLSM optical image of a detail of (a) (black dashed square). c) Intensity profile along one half of the white line seen in (b).
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Fig. 3. a) CLSM optical image in fluorescence mode of a permanent grating of luminescent CCs written directly on a LiF crystal by using a focused soft X-ray beam provided at the ELETTRA synchrotron radiation facility (ESCA Microscopy beamline). b) Intensity profile of part of the grating, consisting of regularly spaced lines whose FWHM has been reported in (c).
related to the elevated values, close to unity, of the quantum photoemission efficiencies for F2 and F3+ defects [18], whose intense photoluminescence spectra fall in an extended spectral range, where the light detector sensitivity is maximum. Inhomogeneities in the emitted light are due to damage in the masking mesh. Similarly, the micro-radiography of a dragonfly (Pyrrhesoma nymphula) wing was recorded by placing the biological sample in close contact with a 2 μm thick LiF film that was thermally
Fig. 4. Lloyd mirror interferometer utilized for producing periodic luminescent lines on LiF by using a compact 46.9-nm capillary-discharge-pumped laser.
Fig. 5. a) Photoluminescent regular grating on a LiF crystal obtained by an interferometric technique based on irradiation with an intense and high spatially coherent soft-X-ray laser beam. The fluorescent image has been detected by CLSM. b) Intensity profile of some lines of the structure. c) Intensity profile of a single line.
evaporated on a glass substrate [19] and by exposing it to soft X-rays. The X-ray dose delivered to the sample was about 160 mJ/cm2 in 1100 shots. The biological sample was dried in vacuum and it should not suffer from distortions due to heating during the X-ray irradiation, even for long exposure times. As a matter of fact, high irradiation doses are usually required in X-ray microscopy, in order to obtain high-resolution images. With this technique the whole X-ray fluence is delivered in a very short time (few nanoseconds), thus reducing damage and modification in the irradiated biological samples during the exposure. Fig. 2a shows
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the entire OSL image of the wing obtained with a conventional optical microscope in fluorescence mode. A detail of the wing was observed by CLSM and is reported in Fig. 2b. Fig. 2c shows the intensity profile along one half of the white line of the rib in Fig. 2b. A resolution of 764 nm was obtained. Fig. 3a shows a regular nanometric structure of CCs on a LiF crystal obtained by direct writing with the focused soft X-ray beam provided at the ELETTRA synchrotron radiation facility, with energy of 650 eV and for an exposure time of 100 ms per pixel of the scan (total exposure time = 6.8 min). The colored area (56× 62 μm2) forms a regular grating whose intensity profile is reported in Fig. 3b. It consists of lines regularly spaced by 1 μm, with FWHM of 340 nm, as shown in Fig. 3c. The high lateral spatial resolution of the confocal system allows measuring this value, which is well below the wavelength of the collected emitted radiation. Periodic luminescent lines on LiF have also been obtained by using the compact 46.9-nm capillary-discharge-pumped laser described-above. Due to the full spatial coherence of the laser beam, interferometric methods for a direct formation of a periodic structure can be utilized. The fringe encoding was obtained by a Lloyd mirror interferometer schematically illustrated in Fig. 4. As shown in the figure, the LiF crystal is placed just after the mirror, oriented perpendicularly to the beam propagation. The overlapping between the direct and the reflected beams generates fringes on the LiF surface up to more than 1 mm from the mirror edge (that is from the beam axis). Irradiating the LiF samples with 10 laser shots, we achieve on the LiF surface the estimated energy flux of 25 mJ/cm2, which is sufficient to obtain well observable photo-luminescent patterns on an area up to 10 mm2. Fig. 5a shows the interferometric pattern produced on a LiF crystal by this encoding technique. The structure consists of a regular grating whose intensity profile is reported in Fig. 5b. Fig. 5c shows the intensity profile of a single line with a FWHM of 625 nm. 4. Conclusions We have demonstrated that LiF, in the form of polished crystal and thin films, can be exploited to host sub-micrometric active optical structures, based on CCs, generated by several soft X-ray sources. Their detection and investigation were performed by means of an advanced fluorescence microscope, the CLSM, which reads the visible photoluminescence of CCs produced through the soft X-ray irradiation and acquires their high optical resolution images. We demonstrated, also, that LiF radiation-sensitivity can be employed in the field of X-ray micro-radiography. LiF, as a novel imaging detector for soft X-rays, shows interesting peculiarities (like high spatial resolution, high dynamic range, efficiency and simplicity of the reading system), which make it very attractive for different applications in the field of photonic devices and for X-ray microscopy for biological investigations. References [1] R.M. Montereali, M. Piccinini, E. Burattini, Amplified spontaneous emission in active channel waveguides produced by electron beam lithography in LiF crystals, Appl. Phys. Lett. 78 (2001) 4082–4084.
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