Structural Modification of Amorphous Fused Silica under Femtosecond Laser Irradiation

Non-linear absorption of femtosecond laser pulses enables the induction of structural changes in the interior of bulk transparent materials without affecting their surface. This property can be exploited for the transmission welding of transparent dielectrics, three dimensional optical data storages and waveguides. In the present study, femtosecond laser pulses were tightly focused within the interior of bulk fused silica specimen. Localized plasma was formed, initiating rearrangement of the network structure. The change in material properties were studied through employment of spatially resolved Raman spectroscopy, atomic force microscopy and optical microscopy. The nature of the physical mechanisms responsible for the alteration of material properties as a function of process parameters is discussed. INTRODUCTION In recent times, advances in femtosecond (fs) laser development have lead to a wide spectrum of new applications [2]. One of main advantages of femtosecond lasers in comparison to nanosecond ones is that they induce much smaller collateral damage due to heat conduction [29]. Furthermore, non-linear absorption, an unique property of femtosecond lasers, makes them particularly suitable for treatment of dielectric materials [4]. Laser assisted transmission welding using conventional nanoseconond (ns) lasers can be utilized when top material is transparent to the laser and bottom material is opaque to it. The laser beam transmits trough the top layer and is absorbed by the opaque material beneath. The subsequent heat accumulation helps to create the weld. When using the ultrafast laser, however, non-linear absorption enables us to induce structural changes into the interior of the target material without affecting the surface. Miyamoto et al. [40] used fs laser to simulate fusion welding by inducing damage within the interior of borosilicate glass. Watanabe et al. [30] investigated conditions to join two substrates using the fs laser. A laser beam was focused on the interface of two transparent substrates resulting in melting and quenching of the surrounding material. Rapid solidification was responsible for joining. Tamaki et al. [32] studied the effect of localized heat accumulation in transmission micro welding. Efforts described above mainly dealt with problems specifically related to welding from a parametric point of view and did not go into details of structural changes that take place within the affected region. Our initial motivation was to gain insight into the welding process. However, this led us to a more fundamental study of the glass – ultrafast laser interaction. Previous studies on this topic are reviewed in the following paragraph. The potential of creating three dimensional optical storages [25] has led to the study of the interaction of various transparent materials with ultra-fast laser pulses. Glezer and Mazur [26] showed that femtosecond laser pulses tightly focused into the interior of the glass with an energy level near to the threshold produce sub-micron cavities or voxels. These structures are characterized by a change in refractive index and consist of a centrally placed void accompanied by a denser surrounding region. Bellouard et al. [24] studied change in morphology of fused silica through effects of etching selectivity, increase of internal stresses and densification. 1 Copyright © 2008 by ASME Schaffer et al. [27], [28], [29] explored different aspects of ultra-fast laser fused silica interaction. Topics covered included the potential of micromachining bulk glass with use of nanojoule energy, bulk heating of transparent materials and morphology of structural changes induced by femtosecond lasers. Kucheyev and Demos [33] used photoluminescence (PL) and Raman spectroscopy to characterize defects created in the amorphous fused silica irradiated by nanosecond and femtosecond lasers with different wavelengths. Their PL results have shown that laser irradiation causes forming of nonbridging oxygen hole centers (NBOHC) and oxygen-deficiency centers (ODC). Raman spectroscopy revealed densification of irradiated area. Chan et al. [21] used femtosecond laser to irradiate the interior of a fused silica slab and performed in situ Raman spectroscopy measurements. Atomic-scale structural changes were observed that lead to the densification of affected material. Damage induced was related to changes in other properties obtained for vitreous silica using different treatments. The work reviewed above addresses change in properties and morphology which are a consequence of structural alterations due to the ultra-fast laser irradiation of interior of transparent materials. It also somewhat explains the physical process that leads to those changes. In the study presented here, the focus is on the atomistic level and rearrangement of the ring structures of which amorphous fused silica consists. Spatially resolved Raman spectroscopy is employed as a non-destructive characterization technique to show local densification and relative volume fraction change of ring structures within focal volume and in the surrounding region. Moreover, the contribution of different mechanisms in feature formation under more complex process conditions is examined. In particular, thermal and explosive plasma expansion mechanisms are studied. FEMTOSECOND LASER IRRADIATION OF TRANSPARENT DIELECTRIC MATERIALS The mechanism of femtosecond laser interaction with both metals and dielectric materials has been studied intensively in recent times [4], [1], [34]. Unlike linear photon absorption that follows Beer-Lambert law seen in nanosecond pulse regime, the absorption of laser energy with pulse duration in order of femtoseconds is done through nonlinear absorption, and it is independent from the laser wavelength. This phenomenon enables the creation of features within interior of the bulk specimen without affecting its surface. In the next paragraph the principle of interaction of ultrafast lasers with dielectrics will be briefly reviewed. At femtosecond pulse regime (pulse duration ~ 100 fs) ionization of dielectric material occurs at the beginning of the pulse followed by the absorption of laser energy by free electrons through inverse Brehmstrahlung and resonance absorption mechanism [1]. The pulse duration is shorter than plasma expansion and heat conduction time, resulting in the feasibility to ionize any material. Furthermore, the main process that occurs during that time is heating of the electrons by the laser field. Depending on the laser intensity, two competing mechanisms are responsible for the absorption of the femtosecond laser energy. At lower laser intensities, below 10 W/cm, avalanche ionization takes place, whereas at intensities above 10 W/cm multiphoton ionization is responsible for absorption. In the case of avalanche ionization, upon absorption of photons at lower intensity, kinetic energy of free electrons becomes sufficiently high to trigger collision impact ionization of the target material. That leads to the generation of more free electrons through the process in which free electron density increases. On the other hand, multiphoton ionization occurs when a number of electrons with energy hν and wavelength collide with the bound electron at the same time. If the total energy of absorbed photons is greater than ionization potential, electron breaks free from the valence. Through processes described above, free electron density rises until it reaches critical value. Ionization of transparent dielectric material occurs at the beginning of the pulse followed by the absorption of laser energy by free electrons via inverse Brehmstrahlung and resonance absorption mechanism [1]. Due to this transparent materials become completely opaque [4]. Furthermore, material in the focal volume is transformed into plasma with metallic properties through laser induced breakdown. Subsequent processes are the consequence of plasma-matter interaction. In the field of femtosecond glass interaction, a lot of work has been conducted by Mazur’s group at Harvard University. The initial motivation for their work was the potential of creating three dimensional data storages as well as waveguides. They showed [25], [26] that single femtosecond laser pulse can create a void inside of bulk transparent specimen surrounded by the densified shell. The mechanism responsible for creation of these structures is explosive plasma expansion. When ultrafast laser pulse energy is deployed into the sample, high temperature and pressure initiate formation of optically dense electron-ion plasma. The material in the center of the focal volume is then ejected into the surrounding area creating a voxel or less dense region. On the other hand, when multiple pulses applied at the same position nature of laser-transparent dielectric interaction becomes thermal [28]. For high-repetitionrate pulsing (~MHz), and nanojoule energy of the pulses, characteristic thermal diffusion time in glass is longer than the time between two pulses leading to thermal accumulation, which has the consequence of denisfying affected region. A similar approach can be used at lower repetition rates (~kHz) and microjoule pulse energies [37]. Another important parameter in determining morphology, structure and size of the altered region is the numerical aperture (NA) of the objective used to focus incoming laser beam [28], [29]. For Gaussian beams focal volume has the shape of Gaussian ellipsoid and its size is a function of numerical aperture [5]. Intensity distribution within the focal volume is: 2 2 2 2 2 exp[( 2( ) / )]exp[ 2 / ] xy I x y z z ω ω = − + − (1) Where x, y and z are coordinates and ωxy and ωz are diameters of the ellipsoid defined by