Panjawat Kongsuwan

Femtosecond Laser-Induced Simultaneous Surface Texturing and Crystallization of a-Si:H Thin Film: Absorption and Crystallinity

Hydrogenated amorphous silicon (a-Si:H) thin films have been considered for use in solar cell applications because of their significantly reduced cost. Their overall efficiency and stability, however, are less than that of their bulk crystalline counterparts. Limited work has been performed on solving the efficiency and stability issues of a-Si:H simultaneously. In this study, both surface texturing and crystallization on a-Si:H thin film are achieved through one-step femtosecond laser processing. The nanoscale conical and pillar-shaped spikes formed on the surface of a-Si:H films by femtosecond laser irradiation in both air and water are presented and enhanced light absorption is observed due to light trapping based on surface geometry changes, while the formation of a mixture of hydrogenated nanocrystalline silicon (nc-Si:H) and a-Si:H after crystallization suggests that the overall material stability can potentially be increased. The relationship among crystallinity, fluence, and scan speed is also discussed. Furthermore, a comparison of absorptance spectra for various surface morphologies is developed. Finally, the absorptance measurement across the solar spectrum shows that the combination of surface texturing and crystallization induced by femtosecond laser processing is very promising for a-Si:H thin film solar cell applications. [DOI: 10.1115/1.4006548]

Characterization of Morphology and Mechanical Properties of Glass Interior Irradiated by Femtosecond Laser

Femtosecond laser pulses were focused in the interior of a single fused silica piece. Proper use of optical and laser processing parameters generated structural rearrangement of the material through a thermal accumulation mechanism, which could be potentially used for the transmission welding process. The morphology of generated features was studied using differential interference contrast optical microscopy. In addition, the predictive capability of the morphology is developed via a finite element analysis. The change in mechanical properties was studied through employment of spatially resolved nanoindentation. The specimen was sectioned and nanoindents were applied at the cross section to examine mechanical responses of the laser-modified region. Fracture toughness measurements are carried out to investigate the effects of the laser treatment on strength of the glass.

Transmission Welding of Glass by Femtosecond Laser: Mechanism and Fracture Strength

Femtosecond laser pulses were focused on the interface of two glass specimens. Proper use of optical and laser processing parameters enables transmission welding. The morphology of the weld cross section was studied using differential interference contrast optical microscopy. In addition, a numerical model was developed to predict the absorption volumes of femtosecond laser pulses inside a transparent material. The model takes into account the temporal and spatial characteristics and propagation properties of the laser beam, and the transmission welding widths were subsequently compared with the absorption widths predicted by the model. The model can lead to the achievement of a desirable weld shape through understanding the effects of laser pulse energy and numerical aperture on the shape of the absorption volume. The changes in mechanical properties of the weld seams were studied through spatially resolved nanoindentation, and indentation fracture analysis was used to investigate the strength of the weld seams.

Ultrafast Laser Induced Structural Modification of Fused Silica—Part II: Spatially Resolved and Decomposed Raman Spectral Analysis

Nonlinear absorption of femtosecond laser pulses enables the induction of structural changes in the interior of bulk transparent materials without affecting their surface. 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 random network structure. Cross sections of the induced features were examined via decomposition of spatially resolved Raman spectra and a new method for the quantitative characterization of the structure of amorphous fused silica was developed. The proposed method identifies the volume fraction distribution of ring structures within the continuous random network of the probed volume of the target material and changes of the distribution with laser process conditions. Effects of the different process conditions and the material response to different mechanisms of feature generation were discussed as well.

Laser Induced Porosity and Crystallinity Modification of a Bioactive Glass Coating on Titanium Substrates

Functionally graded bioactive glass coatings on bioinert metallic substrates were produced by using continuous-wave (CW) laser irradiation. The aim is to achieve strong adhesion on the substrates and high bioactivity on the top surface of a coating material for load-bearing implants in biomedical applications. The morphology and microstructure of the bioactive glass from the laser coating process were investigated as functions of processing parameters. Laser sintering mechanisms were discussed with respect to the resulting morphology and microstructure. It has been shown that double layer laser coating results in a dense bond coat layer and a porous top coat layer with lower degree of crystallinity than an enameling coating sample. The dense bond coat strongly attached to the titanium substrate with a 10 lm wide mixed interfacial layer. A highly bioactive porous structure of the top coat layer is beneficial for early formation of a bone-bonding hydroxycarbonate apatite (HCA) layer. The numerical model developed in this work also allows for prediction of porosity and crystallinity in top coat layers of bioactive glass developed through laser induced sintering and crystallization. [DOI: 10.1115/1.4029566]

Ultrafast Laser Induced Structural Modification of Fused Silica—Part I: Feature Formation Mechanisms

Nonlinear 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 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. Features were generated through employment of single pulses as well as pulse trains using various processing conditions. The change in material properties were studied through employment of differential interference contrast optical microscopy and atomic force microscopy. The morphology of the altered material as well as the nature of the physical mechanisms (thermal, explosive plasma expansion, or in-between) responsible for the alteration of material properties as a function of process parameters is discussed.

FEMTOSECOND LASER-INDUCED SURFACE TEXTURING AND CRYSTALLIZATION OF A-SI:H THIN FILM

Hydrogenated amorphous silicon (a-Si:H) thin films have been considered for use in solar cell applications because of their significantly reduced cost, however, the overall efficiency and stability are less than that of their bulk crystalline counterparts. Limited work has been performed on solving the efficiency and stability issues of a-Si:H simultaneously. In this study, both surface texturing and crystallization on a-Si:H thin film are achieved through one-step femtosecond laser processing in water. Light absorption is enhanced by light trapping based on surface geometry changes, and the formation of a mixture of hydrogenated microcrystalline silicon (µc-Si:H) and a-Si:H after crystallization suggests that the overall stability may be increased. Furthermore, the formation mechanism for the surface spikes is discussed. A comparison of absorptance spectra for various surface morphologies and crystallinities shows that the combination of surface texturing and crystallization induced by femtosecond laser processing is very promising for a-Si:H thin film solar cell applications.

Effects of Laser Radiation on the Wetting and Diffusion Characteristics of Kovar Alloy on Borosilicate Glass

The purpose of this study was to investigate the advantages of laser surface melting for improving wetting over the traditional approach. For comparison, kovar alloy was preoxidized in atmosphere at 700 C for 10 min, and then wetted with borosilicate glass powder at 1100 C with different holding time in atmosphere. The proposed approach used a Nd:YAG laser to melt the surface of the kovar alloy sample in atmosphere, then wetted with borosilicate glass powder at 1100 C with the same holding time. The laser melted surface shows a decrease in contact angle (CA) from 47.5 deg to 38 deg after 100 min. Xray photoelectron spectroscopy (XPS) analysis shows that the surface and adjacent depth have higher concentration of FeO for laser treated kovar (Kovar(L)) than that on traditional thermal treated kovar (kovar(P)). This is attributed to the following improved wetting and diffusion process. The adhesive oxide layer formed on kovar (L) may enhance the oxygen diffusion into the substrate and iron diffusion outward to form an outside layer. This is an another way to enhance the wetting and diffusion process when compared to the delaminated oxide scales formed on kovar (P) surface. The diffusion mechanisms were discussed for both approaches. Scanning electron microscope (SEM) revealed that an iron oxide interlayer in the joint existed under both conditions. Fayalite nucleated on the iron oxide layer alloy and grew into the glass. In both cases, neither Co nor Ni were involved in the chemical bonding during wetting process. The work has shown that laser surface melting can be used to alter the wetting and diffusion characteristics of kovar alloy onto borosilicate glass. [DOI: 10.1115/1.4037426]

Investigation of the morphology of the features generated via femtosecond lasers in the interior of a bovine cornea sections

Nonlinear absorption of femtosecond laser pulses enables the induction of bubble cavities in the interior of eye cornea without affecting other parts of an eye, a phenomena utilized for flap formation in laser assisted corneal surgery. In the present study laser pulses were focused in the interior of the sections of bovine cornea. Tight focus of the laser pulses results in the plasma formation followed by its explosive expansion, which drives cavity formation. The morphology of the generated features as well as the nature of the physical mechanisms of the phenomenon as a function of process parameters is discussed. Numerical model is proposed to develop predictive capabilities for the feature size and shape and the results are compared against the experimental findings.

Single Step Channeling in Glass Interior by Femtosecond Laser

Channeling inside a transparent material, glass, by femtosecond laser was performed by using a single step process rather than hybrid processes that combine the laser irradiation with an additional tool or step to remove the material. Tightly focusing of a single femtosecond laser pulse using proper optical and laser processing parameters could induce the micro-explosion and could create voids inside transparent materials, and the effects of these parameters on the resultant feature geometry and channel length were studied. Understanding of the channel length variation at different locations from the specimen surface could enhance prediction capability. Taking into account of the laser, material, and lens properties, numerical models were developed to predict the absorption volume shape and size at different focusing depths below the surface of a specimen. These models will also be validated with the variation in feature and channel lengths inside the specimen obtained from the experiments. Spacing betwee...