Search

Expected visual acuity (VA) and depth of focus (DOF) of both spheri-cal and aspheric intraocular lenses (IOLs) are calculated from the in vitro Modulation Transfer Function. The resulting VAs were similar for both types of design but DOF were larger in eyes implanted with spherical IOLs.

This work describes the phase transitions occurring at the film-substrate interface of amorphous germanium films upon nanosecond laser-pulse-induced melting of the surface. Films with thickness ranging from 50 to 130 nm deposited on glass substrates were studied. Real-time reflectivity measurements with subnanosecond time resolution performed both at the air-film and film-substrate interfaces were used to obtain both surface and in-depth information of the process. In the thicker films (≥80 nm), the enthalpy released upon solidification of a shallow molten surface layer induces a thin buried liquid layer that self-propagates in-depth towards the film-substrate interface. This buried liquid layer propagates with a threshold velocity of 16±1 ms and causes, eventually, melting at the film-substrate interface. In the thinnest film (50 nm) there is no evidence of the formation of the buried layer. The presence of the self-propagating buried layer for films thicker than 80 nm at low and intermediate laser fluences is discussed in terms of the thermal gradient in the primary melt front and the heat released upon solidification. © 2005 American Institute of Physics.

We study the feasibility of femtosecond laser writing of optical waveguides in bulk 35PbO·35 Bi2 O3·15 Ga2 O3·15 GeO2 glass, motivated by the extended transparency interval of heavy metal oxide glasses in the mid-infrared regime. Its large linear and nonlinear refractive indices cause critical self-focusing to occur even at low laser energies, leading to filamentary propagation and material damage. However, the vicinity of the laser-damaged region shows a considerable increase in the refractive index, which we attribute to a collateral, stress-induced densification due to the high pressures generated in the focal region. These regions of increased refractive index are strongly birefringent and sufficiently large to support efficient light propagation in transversally written structures. Optical waveguides with a refractive index increase ≥ 10-3 and minimal mode ellipticity have been obtained. © 2005 American Institute of Physics.

We report on the mechanisms responsible for the formation of optical waveguides upon femtosecond laser irradiation of an alkaline lead-oxide silicate glass. MicroRaman spectroscopy and nonlinear fluorescence were employed to probe the local glass network structure and the formation of optically active defects respectively. At low laser pulse energies, the laser modified region is formed by a single light guiding region, whereas for pulses above 14 µJ the modified region is formed by a central dark zone, which does not guide light, accompanied by light guiding zones located in the surrounding of the dark one. This behavior is different from that observed in common silica glass systems but agrees with recent results obtained in phosphate and heavy metal oxide glasses. However, our results show that, unlike the latter glass, local densification of the glass occurs in the whole laser modified region, i.e., in the dark and the guiding zones. The suppression of light guiding in the dark region is explained by a high density of absorbing color centers.

Rapid solidification dynamics in amorphous germanium films melted by nanosecond laser pulses was analyzed. Single-shot subnanosecond time resolved reflectivity measurements were carried out. Results showed that once a minimum depth was induced, a bulk solidification process followed by the release of solidification enthalpy was found.

The dynamics of melting-rapid solidification of amorphous Ge films on transparent substrates upon irradiation with nanosecond laser pulses has been analyzed by means of real time reflectivity measurements performed both at the air-film and film-substrate interfaces. The effect of the heat flow conditions on the rapid solidification process has been studied by comparing the behavior of films with thicknesses ranging from 50 to 180 nm on substrates with different thermal conductivities like glass, quartz, and sapphire. The films deposited onto substrates of low thermal conductivity (glass and quartz) undergo a local delayed melting process in the vicinity of the film-substrate interface, the process being dependent on the film thickness and/or the laser fluence. This delayed melting process is never observed in films deposited on sapphire. The comparison of the results suggests that the solidification heat released from the primary melt is responsible for the delayed melting process at the film-substrate interface whenever the heat-transfer ratio to the substrate is low enough.

Rapid solidification phenomena have been studied in amorphous germanium films on silicon substrates by means of real time reflectivity measurements performed during irradiation with nanosecond laser pulses. The influence of the thermal response of the film/substrate system has been investigated by comparing the behavior of films with thicknesses in the range of 30-180 nm. Two different solidification scenarios are observed depending on the ratio between film thickness (d) and the thermal diffusion length (l) of amorphous germanium (l≈80 nm for 12 ns laser pulses). In the thinner films (d

Melting has been induced in amorphous Ge films upon irradiation with both nano- and pico-second laser pulses. The role of undercooling and heat flow in the subsequent rapid solidification process has been investigated by analyzing the behavior of films with different thicknesses (30-180 nm) grown on Si(100) substrates by means of real time reflectivity measurements in the ns timescale. Recalescence is observed in films with a thickness above a threshold value which depends on the pulse duration. An additional solidification scenario, i.e. surface initiated solidification, is observed upon ps pulse irradiation in films with intermediate thicknesses.

Laser-induced transformations (optical and compositional) of amorphous MOx thin films are studied as a function of the oxygen content and the nature of the M element, M being either a semiconductor (Ge) or a semi-metal (Sb). A high optical contrast in reflectivity is always found associated to the first laser pulses, this process being unaffected by the presence of oxygen. Next pulses lead to a process which may involve optical changes depending on the environmental conditions. Where-as the former optical changes are most likely related to a structural process which is completed before oxidation starts, the latter ones occur when irradiating at high oxygen pressures (1.2 bar) and thus they are easily related to an oxidation process. At low oxygen pressures (close to the oxygen partial pressure in air), the films show a good compositional stability upon irradiation together with a high optical contrast.

The process of oxide removal in crystalline Ge using a pulsed ultraviolet laser has been studied by means of real-time reflectivity measurements with nanosecond resolution. The interaction of laser radiation with a clean, oxide-free surface has been characterized and the inhomogeneous and homogeneous energy density melting thresholds of c-Ge for 193 nm radiation have been determined. The values are 180 and 370 mJ/cm2, respectively. We have demonstrated that it is possible to remove an oxide overlayer by irradiation in vacuum and to produce a surface that shows the same response to laser radiation as a smooth, oxide-free, chemically cleaned surface. Under certain specific irradiation conditions it is even possible, after removing the oxide overlayer, to produce an enhanced crystalline quality in the near-surface region compared to that obtained upon chemical cleaning as evidenced by Rutherford backscattering/channeling measurements.

The dynamics of the species ejected by excimer laser ablation of a BiSrCaCuO target in different gas environments is studied by spatially resolved, real-time optical emission spectroscopy. The evolution of the velocity and the emission intensity of the excited species versus the distance and the pressure of the foreign gas present a similar behavior both in reactive (oxygen) and inert (argon) environments. Furthermore, the results show that the plume expansion process is dominated by the interaction of the ejected species and the gas background atoms or molecules through collisional interactions rather than by reactions in the gas phase. The dynamics of the plume expansion is analyzed in the frame of the shock wave and drag models, the latter leading to a very good agreement with the experimental results and the dependence of the slowing coefficient with the gas pressure is established. Finally, the influence of the excitation energy of the considered transition on the observed emission features is discussed. © 1995 American Institute of Physics.

The composition of BiSrCaCuO films deposited by laser ablation in different gas environments (Ar or O2) is related to the interaction processes observed in the plasma by means of optical emission spectroscopy. It is found that films deposited in Ar or O2 exhibit an increase of Bi and a decrease of Sr, Ca and Cu contents with respect to films deposited in vacuum. These changes in composition are discussed in terms of the processes that may affect the film growth such as collisional interaction processes in the gas phase and re-sputtering. © 1995 Elsevier Science B.V. All rights reserved.

The modification of the thermal conductivity and melting temperature of unrelaxed amorphous Ge films on Si substrates upon laser-induced relaxation and crystallization is presented. Real-Time Reflectivity (RTR) measurements are used to determine experimentally both the melting threshold and the melt durations, and the finite element method is used to simulate the laser-induced heat-flow process. A thermal conductivity of k=0.010 W dem K is determined for the unrelaxed material by fitting the experimental melting thresholds of unrelaxed films of different thicknesses. A similar procedure applied to the amorphous relaxed and crystallized materials lead to a shift to higher values of both the thermal conductivity and the melting temperature. In order to achieve a good fit of the experimental melt durations, it was necessary to assume a large degree of undercooling prior to solidification. The role of undercooling in the solidification process is finally discussed in terms of its dependence on the faser energy density and the high thermal conductivity of the substrate. © 1995 Springer-Verlag.

The correlation between the optical properties and composition of lithium niobate films is investigated in films deposited by laser ablation of single-crystal LiNbO3 and Li enriched LiNbO3 sintered targets under different oxygen pressures and laser fluences. The results show that the variation of the film refractive index is in many cases related to the deposition process itself, rather than to a variation of the film composition. The experimental conditions to produce stoichiometric films with optical properties similar to those of the bulk material are discussed.

Ultraviolet (UV) laser pulses are used to irradiate c-Ge and c-Sb both in vacuum and in an oxygen pressure while a HeNe laser beam is used to record the real time reflectivity (RTR) transients. It will be shown that the removal of the native oxide and therefore surface cleaning is produced when irradiating in vacuum while an oxidation process occurs when irradiating in an oxygen environment. The optical coupling between the growing oxide layer with the bulk material underneath is responsible of the high sensitivity of the RTR measurements to the presence or transformation of small thickness oxide layers at the surface. The RTR transients can be used to optically control pulse-to-pulse the laser energy density in order to induce surface oxidation at a constant rate and to minimize material losses during the oxidation process. © 1994.

Relaxation and crystallization of amorphous germanium films on silicon are induced by nanosecond laser pulses. Real time reflectivity measurements and Raman spectroscopy show that amorphous regrowth occurs upon melting and rapid solidification of the film because the thermal conductivity of the silicon substrate is high enough to extract the laser energy absorbed by the film in a very efficient way. The amorphous regrown film is in a relaxed state when compared to the as-grown amorphous material. Further pulses induce fast crystallization of the film. An increase of the melting threshold is found upon relaxation and crystallization of the film.

The optical constants of Sb and SbOx films grown by d.c. sputtering are determined over a wide spectral (270-870 nm) and composition (x = 0.12-2.0) range. Spectroscopic ellipsometry is used to determine the film optical constants. The composition of the films is obtained by combining nuclear reaction analysis and Rutherford backscattering spectrometry. Optical properties of the films change drastically at a critical oxygen content (x = 0.19) which is related to the transition from a crystalline Sb-like film with optical properties similar to crystalline Sb to an amorphous Sb-like film. It will also be shown that amorphous SbOx films behave optically as a mixture of amorphous Sb-like material and a stoichiometric Sb oxide (Sb2O4, x = 2.0). © 1994.

The dynamics of the plume induced during laser excimer ablation of a BiSrCaCuO target was studied by real time emission spectroscopy with spatial resolution. It is found that the presence of oxygen (7 × 10-1 mbar) enhances the emission line intensity. Although a decrease in species velocity is observed at large distances from the target surface, no clear evidence of the formation of oxide molecules in the gas phase is found. It is also found that the laser energy density plays a significant role in the ablation process since the ratio of ion to neutral populations increases sharply as the energy density is increases. © 1994.

Laser-induced oxidation of single-crystalline antimony and polycrystalline Sb films is studied. The samples are irradiated in an oxygen atmosphere (1.2 bar) by means of UltraViolet (UV) laser pulses while reflectivity measurements are used to monitor the oxidation process in real time. It is shown that there is an optical coupling between the growing oxide layer and the bulk material underneath which produces a dynamical change of the optical properties and leads to a non-constant growth rate. Depending on the laser energy density used, there is a critical oxide thickness above which a material-loss process starts, limiting the ultimate growth of the oxide layer. © 1994 Springer-Verlag.

The dynamics of the gas phase induced by excimer laser ablation of Ge is investigated by analyzing the light emitted by the plume. Space and time-resolved optical spectroscopy measurements indicate the presence of both, neutral (Gei) and ionized (Geii) species. Two populations of neutrals with different velocities, which are related to the neutral atoms directly ejected from the target and those which are produced by recombination of ions, are observed. The velocities of the species remain unchanged for oxygen pressures up to 1 mbar, which suggest that the expansion of the plume occurs without further collisions with the foreign gas in this pressure range. The spectral emission characteristics are consistent with plume dynamics initiated by charged species.

Ultraviolet pulsed laser induced oxidation kinetics of crystalline germanium (c-Ge) is determined from real-time optical measurements in conjunction with absolute measurements of the oxygen incorporation performed by nuclear reaction analysis. Although the oxidation process can be triggered at laser fluences initially below the melting threshold of c-Ge, it is strongly activated when surface melting occurs and therefore the fast oxidation process observed is mainly a thermally activated process. Because an optical coupling between the oxide layer and the c-Ge underneath, the growth kinetics is complex and leads to nonconstant rates. The oxygen incorporation reaches a saturation value which depends both on the laser fluence and the oxygen pressure. The results show that the oxygen incorporation is limited by an overlapped laser-induced material removal process rather than by the diffusion length of oxygen species.

Resonance ionization spectroscopy combined with time of flight mass spectrometry is used to analyze the ejection of ions and neutrals from the surface of a Ge target irradiated with ultraviolet XeCl excimer laser pulses. The ejection of ions and neutrals has been observed to occur at temperatures well below the melting point of the target and, therefore, through a nonthermal mechanism that may be dominant for fluences below the threshold for visible plasma formation. Within this regime, the velocities of the ejected neutrals in the ground state are in the order of 3×104 cm/s.

The dynamics of the plume produced during excimer laser deposition of germanium oxide films by excimer laser ablation of a Ge target in the presence of an oxygen atmosphere is studied by real time emission spectroscopy. It is found that the velocity of the species in the plume (both neutral and ionized) remains unaffected under the presence of an oxygen pressure ranging from 10-4 up to 1 mbar, whereas the oxygen content of the films increases with the oxygen pressure up to the stoichiometric value (GeO2) for pressures higher than 5 × 10-3 mbar. The spectral emission features show no evidence of oxidized species in the gas phase even at high oxygen pressures. It is then concluded that the incorporation of oxygen to the growing film occurs mainly by reactions at the substrate site. © 1993.

Time resolved reflectivity and transmission measurements with nanosecond time resolution are used to follow the evolution of ablation and material transport during Ar+ and ArF excimer laser induced blow-off of W and Cr films deposited onto glass substrates. The analysis of the optical transients combined with heat flow calculations provides evidence that solid phase material removal is the dominant mechanism for the ablation of W films, while Cr is ablated in the form of vapor and small molten droplets. © 1993.

Laser ablation of single-crystal LiNbO3 targets is performed by means of an ArF laser. Films are grown onto glass, quartz and carbon-coated mica substrates held at room temperature. Amorphous and particulate-free films with the refractive index and optical energy gap close to those of bulk single-crystal LiNbO3 are obtained depending on the oxygen pressure applied during deposition and the laser fluence. Optical diagnostics of the laser induced plume show that the complex dynamics followed by lithium species in the gas phase are responsible for the variation of the film refractive index as a function of the laser fluence. © 1993.

We have studied the particulates formed on single-phase copper oxide thin films obtained by laser deposition in a variable oxygen atmosphere. Two different targets have been used: a thick copper foil and a sintered copper powder. We conclude that there are two very different kinds of particulates. The first have very irregular morphology and very similar average size. They are due to solid debris coming from the surface target. The second kind of particulate observed has a round perimeter and is formed in the liquid phase. The morphology of the latter round particulates, which varies considerably, is dependent on the substrate and the oxygen content. Thus, we have been able to determine the conditions for growing particulate-free single-phase copper oxides. © 1992.

UV-laser-induced oxidation of crystalline Ge (c-Ge) is analyzed by means of real-time optical measurements. Irradiations are performed in a controlled oxygen atmosphere with laser fluences above the melting threshold. The melt duration determined from the reflectivity transients depends strongly on the oxygen pressure and on the number of laser pulses. The reflectivity of the material tends to decrease as the number of pulses is increased. The results indicate that the irradiation induces the growth of a surface oxide layer at a non-constant rate. The optical coupling between the oxide layer and the bulk material underneath at the irradiation wavelength explains the results and is found responsible for an enhancement of the oxide growth rate. © 1992.

The growth of Ge oxide films by laser ablation of Ge in an oxygen atmosphere is reported for the first time. Time and space resolved optical measurements of the light emitted by the laser induced plume are performed in order to analyze the plasma reactivity. It is found that the velocity of the plasma species is not influenced by the presence of oxygen up to pressures of 10-3 mbar, whereas the oxygen content of the films grown in this oxygen pressure range is always below 40%. This low oxygen content is discussed and compared to that obtained with other reactive deposition techniques. In the range of pressures studied, the stoichiometry of the films seems to be mainly determined by the ratio of Ge and oxygen atoms impinging on the substrates rather than by reactions in the plume. © 1992.

This work reports a systematic study of the growth of copper oxide thin films by laser evaporation from a Cu foil or a Cu sintered powder target in an oxygen atmosphere. The experimental conditions to deposit Cu2O and CuO thin films are determined. Particle-free single-phase films are obtained from the foil target. The laser-deposited films have good optical properties. For comparison, DC-sputtered films deposited from the same Cu foil were studied. To obtain the same phase the former method requires higher oxygen pressure during deposition. The microstructure formed in the laser-deposited samples is always polycrystalline with a homogeneous grain size whereas sputtering can form columnar growth. © 1992.

Sb/Ge multilayer films with 2, 3 and 4 alternate Sb and Ge layers are irradiated with nanosecond pulses from an excimer laser. Time-resolved optical measurements are used to monitor the kinetics of the interfacially initiated mixing process. Transmission electron microscopy is used to characterize the microstructures upon irradiation and the unique features of Sb/Ge multilayer films allow us to use TEM as a depth profile technique. Amorphous phases are formed upon irradiation and the role of the film configuration will be discussed. © 1991.