Fernando Agulló-López

• Doctor in Physics
• Professor Emeritus at Autonomous University of Madrid

Ionoluminescence measurements of SrTiO3 were performed from 30 to 100 K using 3 MeV H+ and 19 MeV Si6+ ions. Spectral kinetics curves exhibited sharp qualitative and quantitative changes around 65 K. The behavior of the 2.0 eV emission, associated with VO-T3+ centers, was correlated with the large anomalous increase in electron conductivity at temp...

Second-Harmonic Response and Relaxation Behavior of High Glass-Transition Temperature Polyphosphazene Films

Results recently reported on the effect of thermochemical treatments on the (He-Cd) laser-excited emission spectra of strontium titanate (STO) are re-analyzed here and compared with results obtained under ion-beam irradiation. Contributing bands centered at 2.4 eV and 2.8 eV, which appear under laser excitation, present intensities dependent upon p...

A novel model for the non-radiative decay of self-trapped excitons (STEs) in the advanced functional oxide SrTiO 3 is proposed and supported by experimental observations. The study is based on the initial ionoluminescence stage for STE emission at 2.5 eV, and its dependence on temperature and electronic ex-citation rate under energetic heavy-ion ir...

The origin of the blue emission in SrTiO3 has been investigated as a function of irradiation fluence, electronic excitation density, and temperature using a range of ion energies and masses. The emission clearly does not show correlation with the concentration of vacancies generated by irradiation but is greatly enhanced under heavy-ion irradiation...

The combined action of the pyroelectric (PY) and photovoltaic (PV) effects, exhibited by z-cut LiNbO3:Fe substrates, has been investigated for particle trapping and patterning applications. The novel hybrid procedure provides new possibilities and versatility to optoelectronic manipulation on LiNbO3 substrates. It has allowed obtaining periodic and...

Oxide perovskites exhibit fascinating physical properties that identify them as key materials for the next generation of oxide-based functional electronic devices, as well as for catalysis and photochemistry applications. In strontium titanate, substantial efforts have been devoted to elucidate the role of oxygen vacancies and localized electronic...

This review presents an updated summary of the fundamentals and applications of optoelectronic photovoltaic tweezers for trapping and manipulation of nano-objects on the surface of lithium niobate crystals. It extends the contents of previous reviews to cover new topics and developments which have emerged in recent years and are marking the trends...

Evanescent fields photo-generated on the surface of photorefractive Fe-doped LiNbO3 templates have been used to trap and align anisotropic zeolite micro-cylinders. x- and z-cut crystal configurations have been employed to obtain different 1D and 2D zeolite micro-patterns. Inside of the patterns, a substructure of zeolites aligned along the electric...

The paper briefly describes the main features of the damage produced by swift heavy ion (SHI) irradiation. After a short revision of the widely used thermal spike concept, it focuses on cumulative mechanisms of track formation which are alternative to those based on lattice melting (thermal spike models). These cumulative mechanisms rely on the pro...

A theoretical model is presented in order to discuss detailed kinetic data describing the evolution of the two main ionoluminescence bands at 650 nm (1.9 eV) and 460 nm (2.7 eV) in silica as a function of the irradiation fluence at room temperature. The model is based on the generation of self-trapped excitons (STEs), their hopping migration throug...

One- and two-dimensional diffractive optical devices have been fabricated by light-assisted trapping and patterning of nanoparticles. The method is based on the dielectrophoretic forces appearing in the vicinity of a photovoltaic crystal, such as , during or after illumination. By illumination with the appropriate light distribution, the nanopartic...

The application of evanescent photovoltaic (PV) fields, generated by visible illumination of Fe:LiNbO3 substrates, for parallel massive trapping and manipulation of micro- and nano-objects is critically reviewed. The technique has been often referred to as photovoltaic or photorefractive tweezers. The main advantage of the new method is that the in...

Main experimental evidences and theoretical models, currently used to describe the modification of dielectric materials by swift-heavy ion (SHI) beams, operating in the electronic stopping regime, are reviewed. The emphasis is on the interplay and synergy between point defects and amorphous tracks. This implies a change of focus from purely thermal...

Abstract Comparative in situ ionoluminescence (IL) experiments using light (H, He) and heavy ions (C, Si, Br) at MeV energies have been performed on three types of silica with different silanol group content (negligible, medium and high). In all cases the IL spectra consist of two predominant features: a blue band peaked at 460 nm (2.7 eV) and a re...

An overview of the work recently conducted by our group on the development and applications of photovoltaic tweezers is presented. It includes the analysis of the physical basis of the method and the main achievements in its experimental implementation. Particular attention will be paid to the main potential applications and first demonstrations of...

It has been recently demonstrated that space charge fields generated in photorefractive (PR) crystals act as an efficient template to trap and arrange nanoparticles on the crystal surface. In particular, this technique can be used with metal particles that offer very promising capabilities. The obtained structures provide a unique system to investi...

The role of light modulation m and particle size on the morphology and spatial resolution of nano-particle patterns obtained by photovoltaic tweezers on Fe : LiNbO3 has been investigated. The impact of m when using spherical as well as non-spherical (anisotropic) nano-particles deposited on the sample surface has been elucidated. Light modulation i...

The purpose of this work has been to explore the effects of such PV fields in the biology and biomedical areas. A first work was able to show the necrotic effects induced by such fields on He-La tumour cells grown on the surface of an illuminated iron-doped LiNbO3 crystal. In principle, it is conceived that LiNbO3 nanoparticles may be advantageousl...

Combined high precision RBS/channeling, NRA/channeling and perturbed angular correlation studies were used to characterize near-stoichiometric and congruent LiNbO3 crystals. In congruent LiNbO3 two non-equivalent Li sites are distinguished whereas in near-stoichiometric LiNbO3 only one of them exists. A comparison of the experimental results with M...

The ionoluminescence (IL) spectra and yields have been investigated on pristine and pre-damaged crystals of BaMgF4, using a He probe beam at 3 MeV. The lattice previous damage has been produced by irradiation with Br ions at 15 MeV in the electronic stopping regime (Se = 5.8 keV/nm textgreatertextgreater Sn) and has been monitored by Rutherford-bac...

The nitrogen content dependence of the electronic properties for copper nitride thin films with an atomic percentage of nitrogen ranging from 26 ± 2 to 33 ± 2 have been studied by means of optical (spectroscopic ellipsometry), thermoelectric (Seebeck), and electrical resistivity measurements. The optical spectra are consistent with direct optical t...

Lithium niobate (LiNbO 3 ) is a paradigmatic photorefractive (PR) material. It provided the first experimental evidence of the PR effect and still maintains a key position in the field. This position is fostered by the fact that large congruent single crystals with good optical quality are routinely grown, enabling technological applications. A rem...

The kinetics of amorphization in crystalline SiO2 (α-quartz) under irradiation with swift heavy ions (O+1 at 4 MeV, O+4 at 13 MeV, F+2 at 5 MeV, F+4 at 15 MeV, Cl+3 at 10 MeV, Cl+4 at 20 MeV, Br+5 at 15 and 25 MeV and Br+8 at 40 MeV) has been analyzed in this work with an Avrami-type law and also with a recently developed cumulative approach (track...

We have determined the cross-section r for color center generation under single Br ion impacts on amorphous SiO2. The evolution of the cross-sections, s(E) and r(Se), show an initial flat stage that we associate to atomic collision mechanisms. Above a certain threshold value (Se textgreater 2 keV/nm), roughly coinciding with that reported for the o...

Nitrogen sputtering yields as high as 104 atoms/ion, are obtained by irradiating N-rich-Cu3N films (N concentration: 33 ± 2 at.%) with Cu ions at energies in the range 10–42 MeV. The kinetics of N sputtering as a function of ion fluence is determined at several energies (stopping powers) for films deposited on both, glass and silicon substrates. Th...

In this communication, preliminary results regarding the use of photovoltaic fields generated in Fe:LiNbO3 nanoparticles for damaging human tumor cells in culture will be presented. These results are an extension of those we have previously obtained after exposing eukaryotic cell cultures to both macroscopic and microscopic crystals of iron-doped L...

In this communication we will summarize our results in two applications of the PV fields in nanotechnology i) micro/nanoparticle trapping and structuring on the surface of LiNbO3 crystals, and ii) Effects of PV fields of LiNbO3 micro- and nano-particles in tumour cells.

InTechOpen book chapter: Optical Waveguides Fabricated by Ion Implantation/Irradiation: A Review Optical Waveguides Fabricated by Ion Implantation/Irradiation: A Review, Published on: 2012-05-30. Authors: Ovidio Peña-Rodríguez, José Olivares, Mercedes Carrascosa, et

The archaeometric application of IBA is possible because in the majority of cases the working conditions are such that no damage and in particular visible damage is produced on the valuable unique samples. Obviously this does not mean that cultural heritage materials do not experience, like all other, the probe-target interaction and effects, but t...

Soluble metal-free and Cu-, Ni- and Co-substituted 3,4-tetrapyridinoporphyrazines (Tpps)5–8 have been synthesized. Poly(methyl methacrylate) (PMMA) spin coated and Langmuir–Blodgett (LB) films containing these compounds have been prepared. Third-harmonic generation (THG) was used to measure the third-order-non-linear optical susceptibility of both...

The refractive index changes induced by swift ion-beam irradiation in silica have been measured either by spectroscopic ellipsometry or through the effective indices of the optical modes propagating through the irradiated structure. The optical response has been analyzed by considering an effective homogeneous medium to simulate the nanostructured...

The propagation losses (PL) of lithium niobate optical planar waveguides fabricated by swift heavy-ion irradiation (SHI), an alternative to conventional ion implantation, have been investigated and optimized. For waveguide fabrication, congruently melting LiNbO3 substrates were irradiated with F ions at 20 MeV or 30 MeV and fluences in the range 10...

Systematic data on the effect of irradiation with swift ions (Zn at 735 MeV and Xe at 929 MeV) on NaCl single crystals have been analysed in terms of a synergetic two-spike approach (thermal and excitation spikes). The coupling of the two spikes, simultaneously generated by the irradiation, contributes to the operation of a non-radiative exciton de...

The damage induced on quartz (c-SiO2) by heavy ions (F, O, Br) at MeV energies, where electronic stopping is dominant, has been investigated by RBS/C and optical methods. The two techniques indicate the formation of amorphous layers with an isotropic refractive index (n=1.475) at fluences around 1014cm−2 that are associated to electronic mechanisms...

Ionoluminescence (IL) has been used in this work as a sensitive tool to probe the microscopic electronic processes and structural changes produced on quartz by the irradiation with swift heavy ions. The IL yields have been measured as a function of irradiation fluence and electronic stopping power. The results are consistent with the assignment of...

The operation of photovoltaic (PV) tweezers, using the evanescent light-induced PV fields to trap and pattern nano- and micro-meter particles on a LiNbO3 crystal surface, is discussed. The case of a periodic light pattern is addressed in detail, including the role of particle shape and the modulation index of the light pattern. The use of a single...

The elastic strain/stress fields (halo) around a compressed amorphous nano-track (core) caused by a single high-energy ion impact on LiNbO3 are calculated. A method is developed to approximately account for the effects of crystal anisotropy of LiNbO3 (symmetry 3m) on the stress fields for tracks oriented along the crystal axes (X, Y or Z). It only...

Massive cell death in cell cultures due to their exposure to the Bulk Photovoltaic Effect (BPE) has been demonstrated using iron-doped lithium niobate (LiNbO 3 :Fe) crystals or micro-particles under low-intensity visible light.

The thermal annealing of amorphous tracks of nanometer-size diameter generated in lithium niobate (LiNbO3) by Bromine ions at 45 MeV, i.e., in the electronic stopping regime, has been investigated by RBS/C spectrometry in the temperature range from 250°C to 350°C. Relatively low fluences have been used (<1012 cm−2) to produce isolated tracks. Howev...

LiNbO3 is a relevant material to prepare a number of electrooptic (bulk and waveguide) devices. Moreover, its photorefractive (PR) behavior is also potentially useful for optoelectronic applications such as holographic storage and coherent optical amplification. For a full understanding and optimization of the PR effect, the lattice location and va...

A preliminary study of the EPR spectrum of Ni-doped KDP is presented. It is shown that Ni+2 substitutes for K+ ions and three different dipoles are formed depending on the position of associated vacancy.

A number of optical absorption bands have been introduced in initially transparent Rb2ZnCl4 crystals by additive coloring and X-ray irradiation at low temperature. Possible assignements for some ef the bands are temptatively given. No absorption develops during electrolytic coloring.

A simple illustrative physical model is presented to describe the kinetics of damage and amorphization by swift heavy ions (SHI) in LiNbO3. The model considers that every ion impact generates initially a defective region (halo) and a full amorphous core whose relative size depends on the electronic stopping power. Below a given stopping power thres...

This work reports a pioneer application of the bulk photovoltaic effect in the biomedical field. Massive necrotic cell death was induced in human tumour cell cultures grown on a bulk photovoltaic material (iron-doped lithium niobate, LiNbO(3):Fe) after irradiation with visible light. Lethal doses (≈100% cell death) were obtained with low-intensity...

Lithium niobate is a resistant opto-electronic oxide material, which has close similarity with Al2O3 and could be a candidate for potential tritium breeding material. In this work we study the possible radiation enhanced release of He from the material and evaluate the potential to use radioluminiscence as a tool to monitor material modification an...

The non-radiative exciton decay model recently developed to account for swift-ion-beam damage to LiNbO3 is, here, discussed within a general physical perspective, taking previous work on alkali halides as a reference. Some general rules for the validity of excitonic models have been put forward, allowing one to predict the irradiation behaviour of...

The preparation of optical waveguides for visible light in silica (a-SiO2) by means of swift heavy-ion irradiation (F at 5MeV, Cl at 20MeV) is reported. They originate from the refractive index enhancement (compaction) brought about by the electronic excitation. Their main features are discussed in relation to those prepared by light ion irradiatio...

N-rich Cu3N films were irradiated with Cu at 42 MeV in the fluences range from 4 × 1011 to 1 × 1014 cm−2. The radiation-induced changes in the chemical composition, structural phases, surface morphology and optical properties have been characterized as a function of ion fluence, substrate temperature and angle of incidence of the incoming ion by me...

We present a MonteCarlo approach to the non-radiative exciton-decay model recently proposed to describe ion-beam damage in LiNbO3 produced in the electronic excitation regime. It takes into account the statistical (random) spatial distribution of ion impacts on the crystal surface. The MonteCarlo approach is necessary to simulate the evolution of t...

The atomic composition, structural, morphological, and optical properties of N-rich copper nitride thin films have been investigated prior to and after annealing them in vacuum at temperatures up to 300 ° C. Films were characterized by means of ion-beam analysis IBMA, X-ray diffraction XRD, atomic force microscopy AFM, and spectroscopic ellipsometr...

Heavy mass ions, Kr and Xe, having energies in the approximately 10 MeV/amu range have been used to produce thick planar optical waveguides at the surface of lithium niobate (LiNbO3). The waveguides have a thickness of 40-50 micrometers, depending on ion energy and fluence, smooth profiles and refractive index jumps up to 0.04 (lambda = 633 nm). Th...

Heavy mass ions, Kr and Xe, having energies in the ~10 MeV/amu range have been used to produce thick planar optical waveguides at the surface of lithium niobate (LiNbO3). The waveguides have a thickness of 40-50 micrometers, depending on ion energy and fluence, smooth profiles and refractive index jumps up to 0.04 (λ = 633 nm). They propagate ordin...

The influence of nitrogen excess on the optical response of N-rich Cu 3 N films is reported. The optical spectra measured in the wavelength range from 0.30 to 20.00 µm have been correlated with the elemental film composition which can be adjusted in the nitrogen atomic percentage (at%) range from 27 ± 2 up to 33 ± 2. The absorption spectra for the...

The generation of periodically poled structures in waveguides prepared by swift-heavy-ion (SHI) irradiation, i.e. in the electronic stopping power regime, has been achieved following two different strategies. In one of them we have prepared bulk PPLN samples by an applied electrical field, followed by irradiation with F ions at 22 MeV. After the io...

The structural damage induced by ion irradiation on dielectric materials and associated device degradation has been, so far, explained on the basis of collisional processes mostly ignoring the electronic excitation. Recent work, focused on lithium niobate, offers conclusive evidence that at high ion energy and moderate mass (A ≥ 15) electronic exci...

We report on the characterization of planar waveguides formed in the Raman-active crystal KGd(WO₄)₂ using swift carbon, fluorine, and oxygen ion irradiation. The characterization of the waveguiding regions was performed using high-resolution microreflectivity and micro-Raman spectroscopy. The high-resolution microreflectivity...

A theoretical strategy is developed to quantitatively assess the ion-beam damage as measured by Rutherford- backscattering spectrometry under channeling conditions (RBS/C) in LiNbO3 crystals. The approach is based on a recent exciton decay model to calculate the concentration of defects generated by the incoming ions as a function of their stopping...

The morphology of the nanopores obtained by chemical etching on ion-beam irradiated LiNbO3 has been investigated for a variety of ions (F, Br, Kr, Cu, Pb), energies (up to 2300 MeV), and stopping powers (up to 35 keV/nm) in the electronic energy loss regime. The role of etching time and etching agent on the pore morphology, diameter, depth, and sha...

The surface morphology of the ablation craters generated in LiNbO3 by 130fs laser pulses at 800nm has been investigated by AFM/SNOM microscopy. The single pulse fluence corresponding to the ablation threshold has been estimated to be ≈1.8J/cm2.A complex structure including random cone-shaped protrusions is observed inside the ablated crater. The sc...

We develop a concept of charge transport for congruent and stoichiometric LiNbO3 crystals, both doped and nominally pure. The concept is based on well-established experimental facts. It incorporates fragments of the known one-center and two-center models and other important elements including electronic transitions and transport channels. The model...

The lattice disorder generated in LiNbO3 by electronic excitation is quantitatively assessed and compared to that produced by elastic nuclear collisions. In particular, the main differential features between the two mechanisms are clearly established. The analysis is based on a recent non-radiative exciton decay model. For ions with atomic mass ⩾15...

The morphology of the nanopores obtained by chemical etching on ion-beam irradiated LiNbO3 has been investigated for a variety of ions (F, Br, Kr, Cu, Pb), energies (up to 2300 MeV), and stopping powers (up to 35 keV/nm) in the electronic energy loss regime. The role of etching time and etching agent on the pore morphology, diameter, depth, and sha...

Relevant damage features associated with femtosecond pulse laser and swift-ion irradiations on LiNbO3 crystals are comparatively discussed. Experiments described in this paper include irradiations with repetitive femtosecond-laser pulses (800 nm, 130 fs) and irradiation with O, F, Si, and Cl ions at energies in the range of 0.2–1 MeV/amu where elec...

(N-rich) Cu3N polycrystalline films were deposited by DC triode sputtering from a copper target in a mixture of argon and nitrogen atmosphere. Their chemical composition, structure and electrical properties have been studied as a function of deposition parameters: nitrogen partial pressure (PN2) and DC bias. Depending on PN2 and DC bias, the atomic...

A model to account for the defects generated by ion irradiation in the electronic loss regime, and based on non-radiative decay of self-trapped excitons, is discussed and compared to experiments. It takes into account the competing role of the light-emission (radiative) and defect-creation (non-radiative) decay channels. Calculations are applied to...

A novel method to produce optical waveguides is demonstrated for lithium niobate ( Li Nb O 3 ) . It is based on electronic excitation damage by swift ions, i.e., with energies at approximately 1 MeV ∕ amu or above. The new technique uses high-energy medium-mass ions, such as Cl, with electronic stopping powers above the threshold value for amorphiz...

The lattice location of Fe in LiNbO3 single crystals has been investigated by PIXE-channelling techniques. For the first time, it has unambiguously shown that Fe (mostly as Fe3+) lies at or near the Li-site, although a small fraction sitting at the Nb or intrinsic vacant sites cannot be ruled out.

Proton-exchange LiNbO3 planar optical waveguides have been irradiated with swift ions (Cl 30 MeV) at very low fluences in the range 5 × 1010−5 × 1012 cm−2. Large modifications in the refractive index profiles, and therefore in the optical performance, have been obtained due to the generation of amorphous nano-tracks by the individual ion impacts. M...

The damage kinetics induced by irradiation with a diversity of swift ions O at 5 MeV; F at 5.1 MeV; Si at 5, 7.5, and 41 MeV; and Cl at 11 and 46 MeV has been investigated in the range of 10 12 –10 15 at./ cm 2 . It covers from the initial stage where single damage tracks are isolated and well separated, up to the stage where a full amorphous layer...

We review recent results on a novel method to modify crystalline dielectric materials and fabricate optical waveguides and integrated optics devices. It relies on irradiation with medium-mass high-energy ions (2–50 MeV) where the electronic stopping power is dominant over that one associated to nuclear collisions. By exploiting the processing capab...

The formation of buried heavily damaged and amorphous layers by a variety of swift-ion irradiations (F at 22 MeV, O at 20 MeV, and Mg at 28 MeV) on congruent LiNbO3 has been investigated. These irradiations assure that the electronic stopping power Se(z) is dominant over the nuclear stopping Sn(z) and reaches a maximum value inside the crystal. The...

The European Physical Society (EPS) is a not for profit association whose members include 41 National Physical Societies in Europe, individuals from all fields of physics, and European research institutions. As a learned society, the EPS engages in activities that strengthen ties among the physicists in Europe. As a federation of National Physical...

The effect of local rigid rotations of the LiNbO3 structure on the optical response has been theoretically explored and compared to experimental data obtained after irradiation with F (20?22 MeV), O (20 MeV) and Mg (39 MeV) ions. The calculations account for the main trends of the ordinary and extraordinary refractive indices measured after the ini...

A theoretical model is proposed to account for the damage and amorphization induced in LiNbO3 by ion bombardment in the electronic energy-loss regime. It relies on the synergy between the thermal spike generated by electron-phonon interaction and the nonradiative decay of localized (self-trapped) excitons. Calculations have been carried out to desc...

Three-dimensional (3D) profiles of single nanotracks generated by a low impact density of Cl ions at 46 MeV have been determined by optical methods, using an effective-medium approach. The buried location of the maximum stopping power induces a surface optical waveguiding layer even at ultralow fluences (1011–1013 at./cm2) that allows to obtain the...

The surface of LiNbO3 crystals after swift ion irradiation in the low velocity regime has been investigated by optical and atomic force microscopy (AFM). Samples were irradiated with Cu 50MeV, Cl 46MeV and Br 12MeV ions. Before any additional treatment the surface of the sample irradiated with Cu ions shows hillocks of a 5–8nm diameter and of 5nm i...

A general theoretical framework to describe crystal damage and amorphization under swift-ion-beam irradiation is discussed. Main physical channels accounting for electron energy losses are considered: (a) phonon generation (heating) and (b) exciton localization and de-excitation. Two alternative schemes are analyzed. In one of them, intrinsic defec...

Irradiations with F, O at 25 MeV and C at 14 MeV energy have been performed in KGd(WO4)2 (KGW) in order to fabricate optical waveguides by a novel route based on ion irradiation instead of ion implantation. The method relies on selecting the energy high enough so as to place the maximum of the electronic stopping power a few microns inside the crys...

We report the direct observation of waveguides formed in KGd(WO4)2 by microreflectivity measurements. Results show a sharp step in refractive index as well as other index variations previously not detected by the m-line technique.

In this chapter, the standard model accounting for the photorefractive nonlinearity in a material is presented. The mathematical formalism of the rate equations has been introduced and developed under the adiabatic approximation. Through a perturbative treatment, it leads to a single nonlinear equation relating the general space-charge field to lig...

In this paper a new transient effect in the room-temperature F-colouring of NaCl is presented which occurs whenever a X- or γ-irradiation is resumed after an interruption time where in the crystal remains in the dark. A detailed study of the characteristics of the effect is made and a relatively simple theoretical model is proposed to account for t...

This paper presents a Monte Carlo (MC) simulation tool which is applied to describe the ion beam induced damage generated by electronic excitation in LiNbO3. Based on a previously published thermal spike based analytical model, the MC technique allows for a more flexible and accurate treatment of the problem. A main advantage of this approach with...

The effects of high-energy silicon (5MeV, 7.5MeV and 30MeV) irradiations have been optically investigated by the dark-mode m-lines technique. In all cases, an optically isotropic homogeneous layer is created after a certain critical fluence that depends on ion and energy. The structure of the layer has been investigated by micro-Raman spectroscopy...

The correlation effects between excitation and trapping sites after light excitation in a Li-deficient Fe-doped LiNbO3 crystal have been quantitatively analyzed. They appear as a direct consequence of the thermally activated random transport (hopping) of electrons (free polarons) by using a Monte Carlo approach. The physical consequences of those e...

The refractive-index profiles induced by high- energy (5 MeV, 7.5 MeV) silicon irradiation in LiNbO3 have been systematically determined as a function of ion fluence in the range 10 13 -10 15 cm −2 . At variance with irradiations at lower energies, an optically isotropic ('amorphous') homo- geneous surface layer is generated whose thickness increas...