3-D microstructuring inside photosensitive glass by femtosecond laser excitation

We show that a femtosecond laser enables us to produce true three-dimensional (3-D) microstructures embedded in a photosensitive glass, which has superior properties of transparency, hardness and chemical and thermal resistances. The photosensitivity arises from the cerium in the glass. After exposure to a focused laser beam, latent images are written. Modified regions are developed by a post-baking process and then preferentially etched away in a 10% dilute solution of hydrofluoric acid at room temperature. We have measured the critical dose for modification of the photosensitive glass, and fabricated 3-D microstructures with microcells and hollow microchannels embedded in the glass based on the critical dose. Received: 12 August 2002 / Accepted: 13 August 2002 / Published online: 4 December 2002 RID="*" ID="*"Corresponding author. Fax: +81-48/468-4682, E-mail: mmasudaw@postman.riken.go.jp     

Glass modification by continuous-wave laser backside irradiation (CW-LBI)

We report on a permanent change in the physical properties inside glass that is rapidly heated and quenched with a continuous-wave (CW) laser beam. The absorption of the glass was enhanced by laser heating, and the heated spot moved by thermal radiation and conduction. To trigger the heating, an absorbent material was placed on the backside of a glass plate and irradiated through the glass. The laser beam can modify borosilicate glass with a high aspect ratio (∼100:1) at a rate of ∼130 mm/s. The modified zone consists of two concentric cylindrical zones and is crack-free.     

Microlens arrays of high-refractive-index glass fabricated by femtosecond laser lithography

Microlens arrays of high-refractive-index glass GeO₂-SiO₂ were fabricated by femtosecond laser lithography assisted micromachining. GeO₂-SiO₂ thin glass films, which were deposited by plasma-enhanced chemical vapor deposition, have a refractive index of 1.4902 and exhibit high transparency at wavelengths longer than 320 nm. Using a femtosecond laser, three-dimensional patterns were written inside resists on GeO₂-SiO₂ films, and then the patterns were transferred to the underlying films by CHF₃ and O₂ plasma treatments. This combined process enabled us to obtain uniform microlens structures with a diameter of 38 μm. The heights of the transferred lenses were approximately one-quarter the height of the resist patterns, due to differences in the plasma etching rates between GeO₂-SiO₂ and the resist. The lens surfaces were smooth. When 632.8-nm-wavelength He-Ne laser light was normally coupled to the lenses, focal spots with a diameter of 3.0 μm were uniformly observed. The combined process was effective in fabricating three-dimensional surfaces of inorganic optical materials.     

Soot volume fraction measurements in aero-engine exhausts using extinction-calibrated backward laser-induced incandescence

Control and reduction of soot particle emissions from aeronautic turbines requires a monitoring system suitable for quantification of these emissions. Currently, such emissions are estimated using the technique of smoke number. This is an extractive method, which is not sensitive enough for the low emission levels of modern gas turbines. Within a recent European project, AEROTEST, part of the project aimed at investigating an alternative soot monitoring technique, laser-induced incandescence (LII) as an in-situ optical diagnostic for quantification of soot emissions. For aero-engine applications, especially those involving large-scale turbines, it is necessary to perform the measurements at long distance from the turbine. The LII technique is favourable in this respect as it provides for non-intrusive measurements and, by detecting the isotropic LII signal along the same axis as the incoming laser beam (so called backward LII), both the laser and the detector can be built inside one system located several meters from the turbine. The concept was initiated in the previous European projects, AEROJET I and II. This paper describes the modified version of the system and the procedure developed to achieve reliable and quantitative soot volume fraction measurements in the exhausts of aero-engines. Application of the backward LII technique is demonstrated in the exhaust of a military turbojet engine for different engine speeds.     

Three-dimensional microfluidic structure embedded in photostructurable glass by femtosecond laser for lab-on-chip applications

A new technology for rapid prototyping of lab-on-chip devices is described. Direct write of a near-infrared femtosecond laser forms three-dimensional (3D) latent images inside photostructurable glass. Modified regions are developed by a post-annealing and then preferentially etched away in dilute hydrofluoric acid solution with an etching selectivity of 40–50 times, resulting in the formation of true 3D hollow microstructures inside the glass. Microfluidic structures with microcells and microchannels embedded in the glass are fabricated by this technique. PACS 42.62.-b; 47.85.Np; 81.05.Kf     

Fabrication of microchannels in glass using focused femtosecond laser radiation and selective chemical etching

We use the combination of femtosecond laser dielectric modification and selective chemical etching to fabricate high-quality microchannels in glass. The photoinduced modification morphology has been studied in fused silica and in borosilicate glass BK7, using ultra-high spatial resolution techniques of selective chemical etching followed by atomic force or scanning electron microscopy. The analysis shows that the high differential etch rate inside the modified regions, is determined by the presence of polarization-dependent self-ordered periodic nanocracks or nanoporous structures. We also investigate the optimum irradiation conditions needed to produce high-aspect ratio microchannels with small symmetric cross-sections and smooth walls. PACS 42.62.-b; 42.65.Re; 81.05.Kf; 87.80.Mj     

Localized desvitrifiation in Er<Superscript>3+</Superscript>-doped strontium barium niobate glass by laser irradiation

Localized desvitrifiation in strontium barium niobate glass doped with Er³⁺ under laser irradiation has been carried out. The samples of this study have been fabricated by the melt quenching method and doped with 5% mol of Er³⁺. A 1.5-W cw Ar laser was focused on the sample to obtain desvitrifiation of the glass. Evidence of the changes induced by the Ar laser has been observed through the analysis of the photoluminescence of the Er³⁺ ions. The transitions corresponding to ²H_11/2→⁴I_15/2, ⁴S_3/2→⁴I_15/2 and ⁴F_9/2→⁴I_15/2 have been studied to analyze structure changes. Microluminescence measurements have been carried out to spatially select positions inside and outside the irradiated area. We have observed changes in the emission bands corresponding to these transitions. The emission bands from Er³⁺ ions in the irradiated zone show a resolved structure while they are broadened outside that area. These changes in the optical properties of the Er³⁺ ions indicate that the Ar-laser irradiation has produced a change in the local structure of the material. These results show that a localized desvitrifiation has been produced after the laser action and the transition from glass to glass ceramic has been completed.     

Femtosecond laser-assisted etching of Pyrex glass with aqueous solution of KOH

A femtosecond laser-assisted etching technique was applied to Pyrex glass, a kind of borosilicate glass, to perform removal processing with three-dimensional arbitrarity in design and micrometer-order spatial resolution. An aqueous solution of potassium hydroxide (KOH) was adopted as a highly selective etchant. The rate and selectivity of etching were evaluated from in situ images, and fabrication of three-dimensional microchannels was demonstrated.     

Local crystallization in an oxyfluoride glass doped with Er3+ ions using a continuous argon laser

Local crystalline formation in erbium doped oxyfluoride glass has been obtained under a cw Argon laser irradiation up to 1.8 W pumping power. By exciting at 514 nm, the emission from 800 nm and 850 nm corresponding to the ⁴S_3/2(²H_11/2)→⁴I_13/2 electronic transitions have been analyzed both inside and outside the irradiated area. The changes in the emission spectra indicate that the high power Ar laser irradiation has resulted in a localized desvitrification process. The temperature dependence of the fluorescence intensity ratio of the 800 nm and 850 nm emission bands has been used to determine the temperature of the irradiated zone. Moreover, the average lifetime of the ⁴S_3/2(²H_11/2) thermalized levels have been measured as a function of the excitation spot position. An important decrease is observed at the irradiated area. These results confirm that a localized cristalline phase has been created by the laser action.     

Self-organization in a chromium thin film under laser irradiation

Self-organization of chromium on glass was observed during laser ablation of the metal film with partially overlapping laser pulses. The beam of a nanosecond pulse laser tightly focused to a line was applied to the back-side ablation of the chromium thin film on a glass substrate. While the line ablated with a single laser pulse had sharp edges on both sides with ridges of the melted metal, the use of partially overlapping pulses formed a complicated structure made of the metal remaining from the ridges. Regular structures of ripples were developed in a certain range of laser fluence and pulse overlap. The ripple period could be controlled from 2.5 to 4 μm by variation of the processing parameters. Various experimental techniques were applied to test the structures, and different models of the ripple formation in the thin metal film were considered. The initial quasi-periodical formation started because of dewetting of thin liquid metal films on the glass substrate after its melting. Similar to the evaporation of liquid films, the small perturbation in the ridge thickness was able to induce instability in evaporation of the thin melted metal film. Freezing of the nonequilibrium state between laser pulses was one of the stabilizing factors in self-organization of the metal.     

Three-dimensional microfluidic channel with arbitrary length and configuration fabricated inside glass by femtosecond laser direct writing

We demonstrate, for the first time to the best of our knowledge, fabrication of three-dimensional microfluidic channels with arbitrary lengths and configurations inside glass by femtosecond laser direct writing. The main fabrication process includes two steps: (1) direct formation of hollow microchannels in a porous glass substrate immersed in water by femtosecond laser ablation and (2) postannealing of the glass substrate at ～1150°C by which the porous glass can be consolidated. We show that a square-wavelike channel with a total length of ～1.4 cm and a diameter of ～64 μm can be easily produced ～250 μm beneath the glass surface.     

Application of small-angle scattering for the identification of small amounts of platinum supported on porous silica

Pt-porous glass catalysts have been studied by small-angle X-ray scattering. The catalysts have been prepared by impregnating two micro- or mesoporous glasses with 0.1 wt% of Pt. Two different pore systems of the catalyst supports have been generated and investigated: on the one hand, the mesoporous structure is formed by finely dispersed colloidal silica inside a macroporous glass. On the other hand, micropores are given by the porous glass itself. The pure supports and the supported catalysts have been investigated via chord length analysis for a selected range order, based on the analysis of the scattering intensity for relatively large scattering vectors (up to the upper limit of SAS). The obtained specific structure parameters of both supported catalysts like dispersion, specific metal surface areas, basic arrangement, size distribution and volume fraction of the metal particles basically differ with both pore systems.     

Fabrication of three-dimensional microfluidic channels in glass by femtosecond pulses

Fabrication of three-dimensional microfluidic channels in glasses by water-assisted ablation with femtosecond laser pulses was investigated. The experimental results showed that formation of the photoinduced microchannels by femtosecond pulses depended on the incident laser power and the scanning speed. For the same scanning speed, the shape of cross-section of channels changed from ellipse to circle with increasing the laser power. Under the optimum condition of laser processing, we fabricated two layers of microfluidic channels with diameter of about 8 μm inside glass. The distance between two layers of microchannels was about 20 μm.     

Generating optical vortex with computer-generated hologram fabricated inside glass by femtosecond laser pulses

We introduce a novel method to generate the optical vortex with computer-generated hologram (CGH) fabricated inside glass by femtosecond laser pulses. The CGH was directly written inside glass by femtosecond laser pulses induced microexplosion without any pre- or post-treatment of the material. We also realized the restructured optical vortex beams of both the transmission and reflection pattern with high fidelity using a collimated He-Ne laser beam. The total diffractive efficiency of both the transmission and reflection pattern is about 4.79%.     

Measurements and simulation of in-cylinder UV-absorption in spark ignition and Diesel engines

Spatially and spectrally resolved in-cylinder absorption measurements were performed in spark-ignited internal combustion engines and in Diesel engines. With UV-broadband illumination it was shown that the UV attenuation occurs throughout the burned gas area with roughly homogeneous absorption cross-sections. Model calculations based on the absorption properties of CO₂ at elevated temperatures show that this species gives the main contribution to in-cylinder UV absorption. A previously suggested technique of assessing UV absorption using O₂ laser-induced fluorescence (LIF) as probe light is successfully applied to in-cylinder measurements of the light absorption inside a fired heavy-duty Diesel engine. Even in this environment, the comparison with model calculations shows that CO₂ is the main contributor to UV light absorption. Since the O₂-LIF absorption technique is based on the identical geometry used for LIF concentration measurements, the results can directly be used for correcting LIF signal data such as that obtained from NO imaging. Received: 20 April 2001 / Published online: 18 July 2001     

Display glass cutting by femtosecond laser induced single shot periodic void array

We propose an idea of fast cutting a display glass plate where the sample is pre-processed micromachining single shot rear-surface and internal void arrays aligned on working plane prior to glass cleaving. Single shot void morphology is investigated varying input pulse energy, focusing depth, and scanning speed. A femtosecond laser with pulse duration of 172 fs, central wavelength of 780 nm, and repetition rate of 1 kHz is used to fabricate voids.     

Engineering a material for biomedical applications with electric field assisted processing

In this work, using multiple co-flows we demonstrate in-situ encapsulation of nano-particles, liquids and/or gases in different structural morphologies, which can also be deposited in a designated pattern by a direct write method and surface modification can be controlled to release encapsulated material. The range of possibilities offered by exposing a material solution to an applied electric field can result in a plethora of structures which can accommodate a whole host of biomedical applications from microfluidic devices (microchannels, loaded with various materials), printed 3D structures and patterns, lab-on-a-chip devices to encapsulated materials (capsules, tubes, fibres, dense multi-layered fibrous networks) for drug delivery and tissue engineering. The structures obtained in this way can vary in size from micrometer to the nanometer range and the processing is viable for all states of matter. The work shown demonstrates some novel structures and methodologies for processing a biomaterial.     

Biospeckle laser spectral analysis under Inertia Moment, Entropy and Cross-Spectrum methods

Biospeckle or dynamic speckle can be used as a method for analysing activity, biologic or not, from materials illuminated with laser beam. The Spatial Temporal Speckle (STS) contains data of time information of dynamic speckle and it is used as input for many techniques allowing the analysis of the activity which is being monitored. One question that rises from the manipulation of the STS is related with the information inside it, in particular, whether it is possible to access different frequency behaviors in the time series presented in the STS pattern. This study presents the Inertia Moment, the Wavelets based Entropy and the Cross-Spectrum analysis as approaches that can be used for evaluating the STS spectral content. In a simulation, STS lines have been created based on many frequencies of the fundamental harmonic. This was done for verifying as each method acts when analysing different frequencies, varying harmonics offset and amplitude. These techniques were applied to real database, to validate their action mechanism in real samples. The results present that all techniques were able to verify the spectral content of different harmonics. Inertia Moment was more efficient on analysing high frequencies, because it is a second order moment, being able to obtain more information from high variations on activity. Entropy and Cross-Spectrum, in turn, were better on differing lower frequencies. This was attributed to the convolution proccess, which is present in both methods, filtering high frequencies. Although, any of them returned informations on both high and low frequencies at the same time, they can be used simultaneously, since Entropy and Cross-Spectrum were complementary to Inertia Moment.     

Nonlinear side effects of fs pulses inside corneal tissue during photodisruption

In order to evaluate the potential for refractive surgery, fs laser pulses of 150-fs pulse duration were used to process corneal tissue of dead and living animal eyes. By focusing the laser radiation down to spot sizes of several microns, very precise cuts could be achieved inside the treated cornea, accompanied with minimum collateral damage to the tissue by thermal or mechanical effects. During histo-pathological analysis by light and transmission electron microscopy considerable side effects of fs photodisruption were found. Due to the high intensities at the focal region several nonlinear effects occurred. Self-focusing, photodissociation, UV-light production were observed, leading to streak formation inside the cornea. Received: 12 October 2001 / Revised version: 13 February 2002 / Published online: 14 March 2002     

Laser-induced incandescence: Development and characterization towards a measurement of soot-volume fraction

Laser-Induced Incandescence (LII) occurs when a high-energy pulsed laser is used to heat soot to incandescent temperatures. Theoretical calculations predict and experimental tests demonstrate the resulting incandescence to be a measure of soot-volume fraction. Practical implementation of the technique is detailed by examining the spectral character, temporal behavior, and excitation-intensity dependence of the resulting thermal emission from the laser-heated soot in both premixed and diffusion flames. Spatial and temporal capabilities of LII are demonstrated by obtaining one- and two-dimensional images of soot-volume fraction via laser-induced incandescence in both types of flames.