The accurate determination of the azimuth of a given direction, e.g., the true (geographic) North, is of fundamental importance in many fields. Just as few examples, it guides buildings construction in civil engineering, supports environmental and cartographic surveys, allows the correct positioning and stability control of concentrating solar power plants, as well as of airport installations, provides the geographic North reference for geomagnetic measurements, contributes to the interpretation of the orientation choices of ancient constructions in archeoastronomy, can be the primary benchmark to calibrate other compasses or gyroscopes. When aiming at reaching azimuth measurements with accuracies well below 1°, magnetic compasses are unreliable: firstly, they indicate the magnetic North rather than the geographic one; secondly, they are heavily influenced by possible surrounding ferromagnetic items.
The main characteristics of the ENEA Discharge Produced Plasma (DPP) Extreme Ultraviolet (EUV) source are presented together with results of irradiations of various materials. The DPP EUV source, based on a Xe–plasma heated up to a temperature of 30 ÷ 40 eV, emits more than 30 mJ/sr/shot at 10 Hz rep. rate in the 10 ÷ 18 nm wavelength spectral range. The DPP is equipped with a debris mitigation system to protect particularly delicate components needed for patterning applications. The ENEA source has been successfully utilized for sub–micrometer pattern generation on photonic materials and on specifically designed chemically amplified resists. Details down to 100 nm have been replicated on such photoresists by our laboratory–scale apparatus for contact EUV lithography. Preliminary EUV irradiations of graphene films aimed at modifying its properties have been also performed.
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.
Within a National Project on nanotechnologies, at the ENEA Research Centre in Frascati a micro-exposure tool for
projection lithography at 14.4 nm has been developed. The laser-plasma soft X-ray source is equipped with a patented
debris mitigation system developed in the frame of a European Integrated Project, in order to preserve the collecting
optics. A 90-nm-resolution patterning has been achieved on resist by this laboratory-scale tool based on a Schwarzschildtype
projection optics.
Within a National Project on nanotechnologies, a Micro-Exposure Tool (MET) for projection lithography at 14.4 nm,
based on a laser-produced plasma source, is being developed at the Frascati ENEA Center. The choice of this "exotic"
wavelength is due to the higher efficiency of a Debris Mitigation System (DMS) working in the interval of
approximately 14 nm < λ < 15 nm. It has to be noted that Mo/Si multilayer mirrors (MLM) can still have a high
reflectivity also at these wavelengths.
The solid-tape-target laser-generated plasma is driven by a XeCl excimer laser, with an optimized intensity of about
3•1010 W/cm2, generating an extreme ultraviolet (EUV) source with a diameter of about 0.2 mm. Clearly, this kind of
source emits a lot of debris (both atomic and particulate types) and the 7-cm-far collector mirror must be protected
against them.
The paper is mostly devoted to the accurate and systematic study of these debris and to their reduction. The results of
mitigation efficiency obtained with a DMS prototype are very encouraging and lead to the design and patenting of its
improved version.
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.
C. Vaccarezza, D. Alesini, M. Bellaveglia, S. Bertolucci, M. Biagini, R. Boni, M. Boscolo, M. Castellano, A. Clozza, L. Cultrera, G. Di Pirro, A. Drago, A. Esposito, M. Ferrario, D. Filippetto, V. Fusco, A. Gallo, A. Ghigo, S. Guiducci, M. Migliorati, L. Palumbo, L. Pellegrino, M. Preger, C. Sanelli, M. Serio, F. Sgamma, B. Spataro, A. Stella, F. Tazzioli, M. Vescovi, C. Vicario, F. Ciocci, G. Dattoli, A. Doria, F. Flora, G. Gallerano, L. Giannessi, E. Giovenale, G. Messina, P. Ottaviani, G. Parisi, L. Picardi, M. Quattromini, A. Renieri, C. Ronsivalle, S. Cialdi, C. Maroli, V. Petrillo, M. Romè, L. Serafini, L. Catani, E. Chiadroni, A. Cianchi, C. Schaerf, P. Musumeci, F. Alessandria, A. Bacci, F. Broggi, C. De Martinis, D. Giove, M. Mauri, L. Ficcadenti, M. Mattioli, A. Mostacci, P. Emma, S. Reiche, J. Rosenzweig
KEYWORDS: Magnetism, Free electron lasers, Electron beams, Particles, Stanford Linear Collider, Superconductors, Diagnostics, S band, Lanthanum, Energy efficiency
The SPARX project consists in an X-ray-FEL facility jointly supported by MIUR (Research Department of Italian
Government), Regione Lazio, CNR, ENEA, INFN and Rome University Tor Vergata. It is the natural extension of the
ongoing activities of the SPARC collaboration. The aim is the generation of electron beams characterized by ultra-high
peak brightness at the energy of 1 and 2 GeV, for the first and the second phase respectively. The beam is expected to
drive a single pass FEL experiment in the range of 13.5-6 nm and 6-1.5 nm, at 1 GeV and 2 GeV respectively, both in
SASE and SEEDED FEL configurations. A hybrid scheme of RF and magnetic compression will be adopted, based on
the expertise achieved at the SPARC high brightness photoinjector presently under commissioning at Frascati INFNLNF
Laboratories. The use of superconducting and exotic undulator sections will be also exploited. In this paper we
report the progress of the collaboration together with start to end simulation results based on a combined scheme of RF
compression techniques.
We present the design elements and the preliminary experimental results of an Extreme-UltraViolet (EUV) exposure tool driven by the high-output-energy excimer laser-facility Hercules, which is aimed at printing a sub-100-nm-pattern on a photoresist in a single-shot irradiation.
D. Alesini, S. Bertolucci, M. Biagini, R. Boni, M. Boscolo, M. Castellano, A. Clozza, G. Di Pirro, A. Drago, A. Esposito, M. Ferrario, V. Fusco, A. Gallo, A. Ghigo, S. Guiducci, M. Incurvati, C. Ligi, F. Marcellini, M. Migliorati, A. Mostacci, L. Palumbo, L. Pellegrino, M. Preger, R. Ricci, C. Sanelli, M. Serio, F. Sgamma, B. Spataro, A. Stecchi, A. Stella, F. Tazzioli, C. Vaccarezza, M. Vescovi, C. Vicario, F. Alessandria, A. Bacci, I. Boscolo, F. Broggi, S. Cialdi, C. De Martinis, D. Giove, C. Maroli, V. Petrillo, M. Romè, L. Serafini, D. Levi, G. Medici, M. Mattioli, P. Musumeci, L. Catani, E. Chiadroni, D. Moricciani, F. Ciocci, G. Dattoli, A. Doria, F. Flora, G. Gallerano, L. Giannessi, E. Giovenale, G. Messina, L. Mezi, P. Ottaviani, L. Picardi, M. Quattromini, A. Renieri, C. Ronsivalle, A. Cianchi, C. Schaerf, J. Rosenzweig
SPARC and SPARX are two different initiatives toward an X-ray FEL SASE source at LNF. SPARC is a high gain FEL
project devoted to provide a source of visible and VUV radiation while exploiting SASE mechanism. An advanced
Photo-Injector system, emittance self-compensating RF-gun plus a 150 MeV Linac, will inject a high quality e-beam into
the undulator to generate high brilliance FEL radiation in the visible region at the fundamental wavelength, (530 nm).
The production of flat top drive laser beams, high peak current bunches, and an emittance compensation scheme will be
investigated together with the generation of higher harmonic radiation in the VUV region. SPARX is the direct evolution
of such a high gain SASE FEL towards the 13.5 and 1.5 nm operating wavelengths, at 2.5 GeV. The first phase of the
SPARX project, fiinded by Government Agencies, will be focused on R&D activity on critical components and
techniques for future X-ray facilities as described in this paper.
The results of a numerical simulation of a conventional and a modified Schwarzschild objective are illustrated in relation with their use as imaging systems in an extreme ultraviolet lithography setup. It is demonstrated that the degradation of the resolution on the wafer due to the unavoidable tilt of the mask to the axis can fairly be vanished by a counter tilt of the wafer. In particular, it has been analysed the Schwarzschild objective setup under implementation at the ENEA Frascati Center within the context of the Italian FIRB project for EUV lithography.
The XeCl excimer laser Hercules at ENEA Frascati, thanks to its peculiar features, is a unique laser-facility for many irradiation experiments. Among these applications, most machine-time is devoted to drive a laser-plasma source. This laser-plasma source is used for high-resolution atomic spectroscopy, in vivo contact microscopy of biological samples, induced DNA damage, micro-radiography, generation of sub-micron luminescent pattern on Lithium Fluoride films for miniaturized active optical devices.
Recently, we equipped the Hercules-driven laser plasma source with a novel Debris Mitigation System to drastically reduce the amount of hot debris emitted by the solid target (potentially dangerous for optics, filters and detectors put near the plasma). Here we will discuss the challenging attempt of stopping debris emitted by our solid-target laser plasma, in order to achieve a clean and efficient EUV source.
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.
New approaches of a spectrally tunable backlighting schemes based on a spherically bent crystal are considered. In a contrary to the traditional backlighting scheme, in which the investigated objects should be placed between the backlighter and the crystal, for the considered schemes an object is placed downstream of the crystal, before the tangential or after the sagittal focus and an image of the object is recorded at the distance from the object corresponded to the needed magnification. The magnification is defined by the ratio of the distances form the sagittal focus to the detector and from the object to the sagittal focus. A ray tracing modeling and experimental images of test meshes, obtained at an incidence angles of the backlighter radiation of 10° and 22°, are presented. It is demonstrated that, at incident angles up to 22°, a linear transformation of the obtained astigmatic images allows to reconstruct them with an accuracy (5 - 15%). A spatial resolution around 10 μm in a field of view of some mm2 is achieved, for the spectral range around 9 Å. It is also demonstrated that spherically bent crystals could be used for X-ray imaging of a self emitting plasma structures with a spatial resolution at least 50 μm in a field of view of some square millimeters for angles of incidence up to 22°.
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.
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.
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.
The effect of the transverse-direction electrical fields on the stability and dynamics of a capillary discharge Z-pinch, at conditions for which soft x-ray lasing in Ne-like Ar has been demonstrated, is studied. It is shown that the transverse electrical fields of the sliding surface discharge provide the instability-free compression and heating of the plasma. The stable and homogeneous heating and compression allows achievement of the appropriate conditions for the soft x-ray lasing in Ne-like Ar. Numerical calculations using the MHD model of the discharge yield new predictions for dynamics and stability of the plasma collapse in the presence of the transverse electrical fields and explain details of experimental observations without artificial adjustments.
The life time of the first multilayer mirror (the condenser) in optical systems for projection Extreme Ultra-Violet (EUV) lithography based on Laser Produced Plasmas (LPP) is limited both by the debris bombardment and by the EUV radiation photoablation. These problems can be dramatically reduced by using krypton as debris stopper, exploiting its high atomic weight (21 times the helium value) and its high transparency in the EUV region only between 60 and 90 eV. We demonstrate both theoretically and experimentally that by filling the laser-target interaction chamber with krypton at low pressure (a few mbar) it is possible to stop efficiently the small debris (having a size smaller than 1 micron, including ions, neutrals and clusters) while keeping a reasonable transmission of the EUV radiation in the 60-90 eV range. In particular, this is experimentally demonstrated for a solid target LPP based on a tantalum or copper tape target and pumped by a large energy (4 J - 120 ns) XeCl excimer laser. The poor transmission of krypton, out of the above mentioned spectral window, significantly reduces the exposition of the condenser mirror to useless radiation. This new technique for stopping the debris has recently been patented by our group. As far as the larger size debris is regarded, a dramatic reduction of their flux has been obtained by decreasing the laser intensity on the target, still keeping a laser to EUV energy conversion as high as 0.7 percent/eV/sr at 70 eV, from a Ta target, together with a shot to shot EUV pulse energy stability better than 1 percent.
We report on a fast soft x-ray source consisting in a high temperature small diameter plasma column produced by electric discharge in a ceramic capillary. This source was developed to produce pulses of few hundred nanosecond duration for EUV lithography, x-ray microscopy applications and also with the aim of developing a soft x-ray amplifier. We obtained experimental results concerning the intensity and spectral analysis of the emitted x radiation pumped by a 30-40 kA, 100-200 ns, electric discharge at 1 Torr pressure in Ar gas. We refer also on the spectra obtained using CO2, as plasma medium, after the optimization of the discharge setup and electrical parameters.
A novel experimental setup for transmission x-ray microscopy is presented. It is based on the use of a point isotropic x- ray source and a single spherical crystal. The x-ray beam intensity is modulated by the object attenuation, then monochromatized and enlarged using a spherical crystal and, lastly, imaged using a detector downstream of the crystal. We demonstrate by ray tracing technique and experimental testing that this system allows microscopy studies with image resolution better than the dimensions of the source, high magnification ratios, and great field of view. Microscopes using this model ca be easily built using different micro x-ray sources, like conventional x-ray tube generators, x-rays emitted by laser generated plasmas or synchrotron radiation. Utilization of spherically bent crystals to obtain high-resolution, large field, monochromatic images in a wide range of Bragg angles is demonstrated for the first time. High quality monochromatic images with high magnification about 15-35 times and spatial resolution over a large field of view were obtained. Some possible applications and preliminary experimental verification of the feasibility of the setup are also presented.
In this paper we summarise the results of the annealing of a-Si films done at ENEA Frascati by the XeC1 laser facility Hercules and the preliminary results of the characterisation work done on the first Italian industrial high-energy excimer laser, named Hercules L. Some information will be also given on a novel process to obtain homogeneous, large grain poly-Si and on a new homogeniser with zoom.
Propagation of the soft x-rays, which have been generated by a small-diameter incoherent source, through the straight and tapered plane capillaries is studied using the Fresnel- Kirchhoff diffraction theory. The diffraction and interference are manifested by appearance of the diffraction fringes in the far-field zone of the capillary output. The experimental data presented in the paper confirms such behavior of the x-ray radiation.
We present the results of beam quality measurements of an XeCl ((lambda) equals 0.308 micrometer) laser, equipped with a generalized self-filtering unstable resonator (GSFUR), while operating in the burst mode at repetition rates of up to 50 Hz. In particular, we have measured the behavior of the laser- energy distribution (both in the near- and far-field) and of the beam-angular-stability (BAS) vs. the repetition rate. The time-evolution of the divergence within the single laser pulse was also measured. The GSFUR is able to achieve a nearly diffraction-limited divergence since the beginning of the laser pulse, and to maintain the values of the times- diffraction-limit number, of the M2 parameter and of the BAS independent of the repetition rate. The BAS was measured by using two different techniques, and the results suggest that the reliability of the standards commonly used may depend upon the experimental set-up.
In the frame of a large project on new materials technologies for photovoltaic and microelectronic applications (FOTO), the process of amorphous silicon (a-Si) transformation into polycrystalline silicon (poly-Si) by means of laser irradiation has been tested with a long-pulse (160 ns), 8 J/p XeCl source. Following the positive results, a laser source, having design parameters of 10 J/p, 120 ns, 10 Hz, has been designed and built, with the aim of realizing a laboratory line for the production of thin film transistors (TFTs) devices.
Sarah Bollanti, Paolo Di Lazzaro, Francesco Flora, Tommaso Letardi, Alessandro Marinai, Alessandro Nottola, Kostandia Vigli-Papadaki, A. Vitali, Francesca Bonfigli, Nicola Lisi, Libero Palladino, Armando Reale, Cheng En Zheng
We present the exciting results obtained by using a natural (i.e. as long as the active medium gain) 120 ns-duration excimer laser pulse focused on relatively thick targets (100 micrometer): a conversion efficiency exceeding 20% has been obtained in the 40 - 70 eV (170 - 300 Angstrom) spectral interval from Cu and Ta targets, with more than 100-ns-FWHM X- ray pulses and low speed (less than 100 m/s) emitted debris. A fast CCD camera is used to reveal the debris and to measure their speed for different laser parameters. These values of debris speed are compatible with the use of a mechanical device to separate them from the X-ray beam and hence to protect the optics of a projection-microlithography system.
The combination of a table-top laser produced plasma X-ray source and spherically bent crystals for the soft X-ray region is used in traditional X-ray microscopy schemes. The X-ray source is well localized both spatially (approximately 20 micrometer) and temporally (1 ps - 10 ns, it depends on the used laser) and it is spectrally tunable in a relatively wide range (6 - 19 angstrom). High quality monochromatic ((delta) (lambda) /(lambda) approximately 10-5 - 10-3) images with high spatial resolution (up to approximately 4 micrometer) and in a large field of view (few mm) are presented. For many applications, these low-cost compact systems can offer a simple alternative to the larger installations which are usually used. It was demonstrated that the spherically bent crystals can be efficiently used in a wide ((Theta) equals 40 - 90 degrees) range of reflection angles, thus allowing wide wavelength selection. A very efficient concentration of monochromatic X-Ray radiation into different spot shapes (line, circle spot, etc) is demonstrated.
Propagation of the x-rays generated by a small incoherent source through the capillary, is studied using the Fresnel- Kirchhoff diffraction theory. The influence of the diffraction on the propagation is demonstrated. The diffraction is manifested by the appearance of the diffraction fringes in both the guide channel and the far- field zone of the capillary output. The experimental data which confirms such a behavior of the x-rays is also presented. The result give a confirmation for the interference effects recently observed in capillary optics.
Patrizia Albertano, Mauro Belli, Sarah Bollanti, Paolo Di Lazzaro, Anatoly Faenov, Francesco Flora, Gualtiero Giordano, Antonio Grilli, Fiorenza Ianzini, Sergei Kukhlevsky, Tommaso Letardi, Alessandro Marinai, Alessandro Nottola, Libero Palladino, Tatiana Pikuz, Armando Reale, Lucia Reale, Anna Scafati, Giovanni Schina, Maria Tabocchini, I. C. Edmond Turcu, Kostandia Vigli-Papadaki
A large volume excimer laser, HERCULES, has been successfully applied as pump for a soft x-ray plasma source. The laser pulse duration has been varied from the natural value of 120 ns down to 10 ns, reaching different emission spectra from the plasma which have resulted to be optimum for different applications of the plasm source itself. Some experimental results on the applications of the source to different fields are presented; the choice of the best laser parameters in relation to the applications of the plasma source are also discussed.
We have tested a high-repetition-rate X-ray diode working in reflection geometry, which uses commercial spark-plugs as plasma cathode. The results show that the spark-plug plasma cathode is a reliable, high-current and long-lifetime electron gun. After more than 106 shots, the dose, uniformity and ionization rate of the emitted X-ray guarantee an effective preionization of XeCl excimer laser discharges. Due to the low cost, the long lifetime and the substantial absence of maintenance, this X-ray diode seems suitable to preionize commercial gas lasers, such as excimer and TEA CO2 lasers.
We present a measurement of the time-behavior of the divergence of pulsed laser beams, joining the time- resolution capability of photodetectors and the space- resolved information of the knife-edge technique. This method has been tested by using HERCULES, a high output energy XeCl laser facility, equipped with two different unstable resonators. The time-resolved beam divergence was much higher than the time-integrated one at the leading edge of the laser pulse duration, and it had a minimum before the peak of the laser pulse. The consequences on the optimization of laser-matter interaction processes are discussed.
A high brightness x-ray source has been obtained by focusing the large aperture excimer laser HERCULES on a solid target with a laser intensity of more than 1013W/cm2. The characteristics of the x-ray source for different laser time evolution in the range 6-120 ns are investigated. Experimental result on specific applications of the source, like the soft x-ray contact microscopy and the low photon energy radiobiology will be presented.
We propose to enhance the gain of Ne-like ion capillary lasers by the utilization of two discharge pulses. A first pulse working in the pinch regime mode is designed to create the cold-plasma column with the required ionization degree, followed by a second fast high-voltage pulse to do the effective collision excitation. The second pulse, which heats the electrons to a kinetic temperature Te greater than Tecold in times shorter than the ionization time, increases the laser gain. In the two stage pumping scheme, both the amplitude of each of the pulses and the delay between them can be adjusted for independent control of the ionization degree and excitation rate. Numerical estimations performed for the low-Z-ion (Na+1, . . ., Ar+8) lasers with the double pulse pumping predict enhancement in gain by a factor of approximately 10. In addition, the extension of two-stage electrical pumping to the high-Z-ion collision and recombination laser schemes is discussed. Description of the experimental set-up constructed for such experiments and some related measurement data are also presented.
A simple technique of the generation of high-density metal- vapor columns for x-ray capillary lasers is demonstrated. The method is based on vaporization of the electrode material in the low-pressure (P less than 1 torr) capillary microsecond- discharge. High-density (NAl greater than 1017 cm-3), small-diameter (1 divided by 6 mm), long (5 divided by 20 cm) Al-vapor columns have been produced. The Al- vapor density along the capillary and the time-resolved spectra from Al-vapor plasmas are presented. The Al-vapor density and profile were controlled by varying the discharge parameters.
Patrizia Albertano, Mauro Belli, Paolo Di Lazzaro, Anatoly Faenov, Francesco Flora, Antonio Grilli, Fiorenza Ianzini, Tommaso Letardi, Alessandro Nottola, Libero Palladino, Tatiana Pikuz, Armando Reale, Lucia Reale, Anna Scafati, Maria Tabocchini, I. C. Edmond Turcu, Kostandia Vigli-Papadaki
A large volume non-conventional XeCl excimer laser (HERCULES) emitting long pulses (from 10 ns up to 120 ns at a wavelength of 308 nm) has been used to drive a soft x-ray plasma source. The x-rays pulse duration and the energy conversion efficiency in different spectral regions have been measured; x-rays emission lasting up to 100 ns has been obtained in the 70 eV region. The dependence of x-ray pulse duration on the size of the laser spot is discussed. The x- ray source can be operated both in vacuum and in helium at atmospheric pressure. This allows irradiating over a large area both for contact microscopy of living specimens (up to 1 mm2 windows) and for radiobiology (up to some cm2 windows). The experimental results obtained for these two applications as well as for radiographic images of living insects are discussed.
Aldo Conti, Dimitri Batani, Cesare Botto, Alessandra Masini, A. Bernardinello, Fulvia Bortolotto, M. Moret, G. Poletti, S. Piccoli, F. Cotelli, C. Lora Lamia Donin, Anthony Stead, A. Marranca, Klaus Eidmann, Francesco Flora, Libero Palladino, Lucia Reale
The use of a high energy laser source for soft x-ray contact microscopy is discussed. Several different targets were used and their emission spectra compared. The x-ray emission, inside and outside the Water Window, was characterized in detail by means of many diagnostics, including pin hole and streak cameras. Up to 12 samples holders per shot were exposed thanks to the large x-ray flux and the geometry of the interaction chamber. Images of several biological samples were obtained, including Chlamydomonas and Crethidia green algae, fish and boar sperms and Saccharomyces Cerevisiae yeast cells. A 50 nm resolution was reached on the images of boar sperm. Original information concerning the density of inner structures of Crethidia green algae were obtained.
A comparison is made between the beam quality parameter M2 and a new times-diffraction-limit-factor (TDL) for two high peak power XeCl lasers. The laser systems, equipped with unstable resonators, emit beams with different geometrical characteristic and optical quality. The results show that the M2-factor is always larger than the TDL value and this discrepancy increases if the beam is almost diffraction- limited. A novel method to obtain the far-field profile of coherent superGaussian-like beams and a simple technique to estimate the beam angular stability of a pulsed laser are also presented.
We present the results of a compact three-electrode XeCl laser operated in a power oscillator-power amplifier (POPA) configuration with transverse mode selection. The smaller of two laser heads is an oscillator, equipped with a generalized self-filtering unstable resonator (GSFUR). The oscillator output beam has an energy of 9 mJ in a 75 ns FWHM pulse and is only 1.1 times over its diffraction limit. The single-pass amplified beam has an output energy of 90 mJ in a pulse 85 ns long and, with a times diffraction limit (TDL) equal to 1.6, reaches a brightness of 3 (DOT) 1014 Wcm-2ster-1. Operating the laser at low repetition rate (1 Hz), the beam pointing stability (BPS) of both oscillator and amplifier output beams has been measured. The angular fluctuations of the focused beams are each within 1/6 of their own total beam divergence.
A large volume long pulse excimer laser ((lambda) equals 308 nm) is used to generate a soft x- ray plasma source with long heating time at a power density in the order of 1012 - 1014 W/cm2. The characteristics of the plasma source for different laser pulse time evolution in the range 10-120 ns and for different target materials are investigated. In particular the most suitable source conditions (spectral energy distribution, time duration, etc.) for specific applications are analyzed.
Soft x-ray contact microscopy (SXCM), using a pulsed x-ray source, offers the possibility of imaging the ultrastructure of living biological systems at sub-50nm resolution. We have developed a pulsed plasma x-ray source for this application, generated by the large volume XeCl laser 'Hercules'. Various unstable optical resonator configurations were employed to achieve a high laser intensity to increase the conversion efficiency to 'water window' x-rays (280-530eV). Optimum plasma conditions for SXCM are discussed, including the effect of pulse duration on image resolution. Soft x-ray contact images of Chlamydomonas dysosmos (unicellular alga) and the cyanobacteria Leptolyngbya are shown. In addition, the potential of producing a 'movie film' of the development of x-ray images within the photoresist (acting as the recording medium) is discussed, following the resist development while viewing by atomic force microscopy.
An analytical technique based on a generalization of the Stappaerts method is proposed for discharge electrode designing. It allows one to consider asymmetry elements, such as current return bars, or to approach new laser geometries, e.g., the double-head laser.
A novel discharge configuration has been realized with the XeCl laser IANUS, which has been developed at the ENEA Frascati center as a 1:1 laser head prototype for the 1 kW average output power XeCl system EOLUS (EUREKA EU211 Project). The patented design of the discharge region consists of two active zones which are geometrically in parallel and electrically in series. The obvious advantages of this configuration are, the presence of the intermediate high impedance electrode, suitable for prepulse application, then the absence of current return bars, so improving the recirculating gas flow uniformity, and the automatic synchronization of the two discharges very useful for a compact oscillator-amplifier configuration (POPA). A number of measurements have been done on IANUS, to have an almost complete characterization; based on these results a generalized self-filtering unstable resonator (GSFUR) has been designed and realized on the smaller gap, leaving the other active medium for the high optical quality beam amplification, in the scheme of a POPA system. The GSFUR output beam results are 1.5 times the diffraction limit, with an energy of 9 mJ in a 87 ns FWHM pulse.
A large volume XeCl laser has been equipped with a confocal positive branch unstable resonator (CPBUR), obtaining a near diffraction-limited beam, with a brightness of > 1014W/(cm2sr). Focusing the radiation, an intensity of about 1012W/cm2 is reached on a copper target, generating an x-ray emitting plasma in the region of 100 eV, with pulse length up to 100 ns.
Short and continuously tunable (2 ns - 100 ns) UV laser pulsewidths have been obtained for the first time by applying the intracavity injection technique to a long pulse, x-ray preionized XeCl laser with remarkable stability and reproducibility characteristics.
In this paper we intend to present some recent result obtained in our laboratory on excimer laser development and applications. Even if the activity has been initiated as long as 15 years ago after 1986, following the European Eurolaser Initiative, the programs were strengthened and enlarged to accommodate more ambitious goals, according to the increased attention to this field. Actually the activity includes the following: (1) development of source, (2) computer models, 3) special devices, and (4) applications. The description of our activity is presented according to the above mentioned order, with special attention to the most recent results.
The 1-D numerical code developed to model the performance of a high power excimer laser is described. The spectral method and the Rigrod equation are basic to the code, which is quite general for application to any two-mirror resonator geometry. The specific features of the code make it appropriate for analyzing laser resonators with high Fresnel numbers and accounting for gain medium nonhomogeneity and instability phenomena, as those arising in discharge pumped lasers.
Increasing the interest in high-power coherent UV light sources, the ENEA Frascati Centre is developing 1 kW average power XeCl laser systems. In order to analyze the physical behavior of these sources, net gain measurements spatially and temporally resolved have been done on a large-volume prototype and compared with the results of a numerical 2-D simulation code.
In a large volume XeCl laser with longitudinal gas recirculation system, the tecnique of discharge photo-triggering has been successfully applied. Flow characteristics and laser performances, included gain profiles spatially and temporally resolved, are here described. Moreover, the results of a simplified simulation code are compared with the experimental ones.
A 1 0 liter active volume, x-ray preionized XeC1 laser has been operated with both a
conventional spark-gap switched discharge and a x-ray triggered one. In the last case, 45 W
average output power was obtained. A simple model has been developed to study the minimum
conditions on the preionization source to drive a uniform switchiess discharge.
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