Maria Lucia Protopapa

Laser damage dependence on structural and optical properties of ion-assisted HfO2 thin films

Abstract Laser damage studies at 248 nm (KrF excimer laser) have been performed on HfO 2 films of 300 nm thickness deposited on silica substrates by the Xe ion-assisted electron beam evaporation technique. The assistance parameters (ion mass, energy and current density) have been adjusted to investigate the effect of the Xe-ion momentum transfer parameter P on the film optical and structural properties. Then, the dependence of laser damage fluence on film properties has been studied. Higher laser damage fluences have been found for the HfO 2 films deposited at lower P values and characterized by a fully crystalline structure with the grain of smaller size and randomly oriented.

Surface plasmon resonance of metal nanoparticles sandwiched between dielectric layers: theoretical modelling

Composite films consisting of a ceramic matrix with embedded metal nanoparticles have received increased interest due to their numerous potential applications in the field of optics and optoelectronics. Numerous studies have been dedicated to the fabrication of these composite materials and it has been shown that nanocermet films can be obtained by successive deposition of alternate dielectric and metal films of thicknesses opportunely chosen. In this case, stacks of dielectric layers alternated with layers of metal nanoclusters (NCs) are obtained. However, until now, optical characterization of these kinds of multilayer stack has been used to retrieve mainly qualitative information on the dimension, shape, and geometric distribution of nanoparticles inside the dielectric matrix. An easy-to-handle model that quantitatively links the optical properties to the main features of the NCs embedded in the matrix is presented. This model can be applied to multilayer stacks of dielectric layers alternated with metal NC layers and is shown to be a valid alternative to a recently published model [Nanotechnology 19, 125709 (2008)NNOTER0957-448410.1088/0957-4484/19/22/225302] that was applied to the case of a three-layer structure (dielectric/metal:dielectric/dielectric).

Laser damage studies on MgF2 thin films

The results of laser damage studies performed at 248 nm (KrF excimer laser) on MgF2 thin films deposited by different techniques (electron-beam evaporation, thermal boat evaporation, and ion-beam sputtering) on fused silica and CaF2 substrates are presented. We find that the films deposited on CaF2 substrates by the electron-beam evaporation technique present the highest damage threshold fluence (9 J/cm2). The photoacoustic (PA) beam deflection technique was employed, in addition to microscopical inspection, to determine laser damage fluences. We confirm, by scanning electron microscopy analysis of the damaged spots, the capability of the PA technique to provide information on the mechanisms leading to damage. The dependence of both laser damage fluence and damage morphology on the film deposition technique, as well as on the film substrate, is discussed.

Simulation of optical properties of layered metallic nanoparticles embedded inside dielectric matrices: interference method or Maxwell Garnett effective-medium theory?

Optical characterization of composite films consisting of a ceramic matrix with embedded layered metal nanoparticles have recently received increasing interest. In particular, two methods have been mainly proposed in order to obtain optical performances of dielectric matrices containing layered nanoclusters (NCs): the first method is based on the simulation of the layered system as composed of alternated films of dielectric material and effective-medium material. Therefore, the optical response of the multilayer stack is calculated, assigning to the effective-medium layers the dielectric constant epsilon(f)(Yama), obtained by the Yamaguchi theory, and calculating the interference between the beams reflected and refracted at each interface inside the stack. The second method considers the multilayer stack as a single-layer effective-medium film whose dielectric constant is calculated by the Maxwell Garnett (MG) theory. In particular, this second method is recognized to be valid in the case of nanoparticles uniformly distributed inside a dielectric matrix. The present study shows that the interference method, as it has been applied up to now, does not allow reproducing reflectance and transmittance spectra calculated by the MG theory in the case of a uniform distribution of NCs.

Layered silver nanoparticles embedded in a BaF 2 matrix: optical characterization

Multilayer stacks of silver and BaF(2) alternate layers have been deposited by thermal evaporation on a silica substrate with the aim to obtain Ag clusters dispersed in a BaF(2) insulator matrix. The Ag layer thickness was approximately 1.2 nm; the thickness of the BaF(2) layer was approximately 25 nm. The samples were thermally treated for a 1 h thermal annealing process at 500 degrees C. These kinds of multilayer device also have several applications in the field of optics for the realization of antireflection coatings. However, optical characterization of dielectric matrices that contain layered metallic nanoparticles still remains an unsolved problem in the field of nanostructured optical coatings. Therefore, the surface plasmon resonance peak that appears in the optical absorption spectra because of the formation of Ag nanoclusters inside the BaF(2) insulator matrix has been monitored and fitted by numerical codes. In particular, a previously published theoretical model, based on the Maxwell-Garnett effective medium theory, modified to take into account the effects that are due to the particle shapes and the spatial arrangement of the clusters, has been employed to fit the optical absorption spectra.

Correlation between the structural and optical properties of ion-assisted hafnia thin films

The ion beam assistance during the film growth is one of the most useful method to obtain dense film along with improved optical and structural properties. Afnia material is widely used in optical coating operating in the UV region of the spectrum and its optical properties depend on the production method and the physical parameters of the species involved in the deposition process. In this work afnia thin films were evaporated by an e-gun and assisted during the growth process. The deposition parameters, ion beam energy, density of ions impinging on the growing film and the number of arrival atoms from the crucible, have been related to the optical and structural properties of the film itself. The absorption coefficient and the refractive index were measured by spectrophotometric technique while the microstructure has been studied by means of x-ray diffraction. A strictly correlation between the grain size, the optical properties and the laser damage threshold measurements at 248 nm was found for the samples deposited at different deposition parameters.

Ion assistance effects on electron beam deposited MgF2 films

Thin films of MgF2 have been deposited by the ion-assisted electron-beam evaporation technique in order to find out the ion beam parameters leading to films of high laser damage threshold whose optical properties are stable under uncontrolled atmosphere conditions. It has been found that the ion-assisted electron-beam evaporation technique allows getting films with optical properties (refraction index and extinction coefficient) of high environmental stability by properly choosing the ion-source voltage and current. But, the laser damage fluence at 308 nm was quite dependent on the assisting ion beam parameters. Larger laser damage fluences have been found for the films deposited by using assisting ion beams delivered at lower anode voltage and current values. It has also been found that the films deposited without ion assistance were characterized by the highest laser damage fluence (5.9 J/cm2) and the lowest environmental stability. The scanning electron microscopy analysis of the irradiated areas has r...

Ultraviolet-graded coatings for lasers : surface optical performance

Thin film optical coatings play an important role for the design of optical components to be used in the laser field. In particular, optical components with graded coatings allow the improvement of the quality of the laser beam without introducing inside the cavity additional optical elements. A number of coated optical components, useful to this end, are described and thin film multilayer stacks, capable of giving the required performance, are designed taking advantage of the possibility of tailoring the film thickness along the sample surface. Three different components to be used either inside or outside the laser cavity are proposed for a XeCl excimer laser.

Fine trimming of SmS film resistance by XeCl laser ablation

Abstract The laser ablation threshold fluence of thin SmS films has been measured at 308 nm (XeCl excimer laser) by the photoacoustic beam deflection technique. Fine trimming of the SmS film resistance by pulsed XeCl laser ablation has then been demonstrated. In particular, it is shown that the film electrical resistance increases linearly with the number of laser shots at fluences above the ablation threshold fluence and that the increasing rate is quite dependent on the XeCl laser fluence. The resistance growth rate which was of 8 Ω/pulse at the laser fluence of 1.1 J/cm 2 has increased to 280 Ω/pulse at the laser fluence of 6.2 J/cm 2 . The rate of material removal was of 0.8 nm/pulse at 6.2 J/cm 2 and of 0.019 nm/pulse at 1.1 J/cm 2 .

High mechanical-damage-resistant sol-gel coating for high-power lasers

This paper is a work-in progress report on the development of sol-gel coatings for high power laser systems in the near-UV, infrared region. Silica, titania and titania-silica acid catalysed sols were prepared by using tetraethoxysilane and titanium isoproxide as precursors. Single and multi-layer coatings were generated by dipping on fused silica substrates. The single films were heated at 500°C and 900°C after deposition in order to investigate the role of the sintering temperature either on the optical properties and on the film laser-induced damage threshold at 1064 nm (Nd:YAG c.w. laser) and 351 nm (XeF excimer laser). The ageing effects due to the exposure to humidity was investigated by testing the damp heat resistance of the coatings in agreement with the ISO environmental test for optical coatings. The silica coatings have been assessed before and after the damp heat test with regard to their laser-induced-damage resistance, reflectance and transmittance properties. The optical parameters (refractive index and extinction coefficient) have been determined by UV-VIS-NIR spectrometry. A global fit procedure based on the simultaneous characterisation of several samples was used for the evaluation of the optical properties of the materials both as single films and inside multi-layer stacks.