Fabrication of 3D microfluidic structures inside glass by femtosecond laser micromachining

Femtosecond lasers have opened up new avenues in materials processing due to their unique characteristics of ultrashort pulse widths and extremely high peak intensities. One of the most important features of femtosecond laser processing is that a femtosecond laser beam can induce strong absorption in even transparent materials due to nonlinear multiphoton absorption. This makes it possible to directly create three-dimensional (3D) microfluidic structures in glass that are of great use for fabrication of biochips. For fabrication of the 3D microfluidic structures, two technical approaches are being attempted. One of them employs femtosecond laser-induced internal modification of glass followed by wet chemical etching using an acid solution (Femtosecond laser-assisted wet chemical etching), while the other one performs femtosecond laser 3D ablation of the glass in distilled water (liquid-assisted femtosecond laser drilling). This paper provides a review on these two techniques for fabrication of 3D micro and nanofluidic structures in glass based on our development and experimental results.     

Soda-lime glass microlens arrays fabricated by laser: Comparison between a nanosecond and a femtosecond IR pulsed laser

We present the manufacturing of microlens arrays on soda-lime glass substrates by using two different IR pulsed lasers: a nanosecond Nd:YVO₄ laser (1064 nm) and a femtosecond laser based on Ytterbium crystal technology (1030 nm). In both cases, the fabrication technique consists of the combination of a direct-write laser process, followed by a post-thermal treatment assisted by a CO₂ laser. Through the analysis of the morphological characteristics of the generated microlenses, the different physical mechanisms involved in the glass ablation process with a nanosecond and a femtosecond laser are studied. In addition, by analyzing the optical features of the microlenses, a better result in terms of the homogeneity and quality of the spot focuses are observed for those microlenses fabricated with the Nd:YVO₄ nanosecond laser. Microlens arrays with a diameter of 80 and 90 µm were fabricated.     

Materials processing with a tightly focused femtosecond laser vortex pulse

In this Letter we present the first (to our knowledge) demonstration of material modification using tightly focused single femtosecond laser vortex pulses. Double-charge femtosecond vortices were synthesized with a polarization-singularity beam converter based on light propagation in a uniaxial anisotropic medium and then focused using moderate- and high-NA optics (viz., NA=0.45 and 0.9) to ablate fused silica and soda-lime glass. By controlling the pulse energy, we consistently machine micrometer-size ring-shaped structures with <100nm uniform groove thickness.     

Self-organized near-field etching of the sidewalls of glass corrugations

Using soda-lime glass with a nano-stripe pattern as a test specimen, we demonstrated self-organized near-field etching with a continuum-wave laser (λ=532 nm) light source. Atomic force microscopy confirmed that near-field etching decreases the flank roughness of the corrugations as well as the roughness of the flat surface.     

Oscillation of the refractive index at the focal region of a femtosecond laser pulse inside a glass

The temporal evolution of refractive-index change produced by a tightly focused femtosecond (fs) laser pulse inside a soda-lime glass plate was investigated by use of a transient lens method with subpicosecond time resolution. An oscillating behavior of the light intensity in the central region of the probe beam was observed 0-1500 ps after irradiation of the plate. The oscillation was interpreted in terms of a rapid temperature increase and the ensuing propagation of the pressure wave. This study is to our knowledge the first real-time observation of refractive-index change inside a glass induced by a fs laser pulse.     

A microfluidic chip integrated with a microoptical lens fabricated by femtosecond laser micromachining

We report on the integration of microlens and microfluidic channels in fused silica glass chip using femtosecond laser micromachining. The main process includes three procedures: (1) femtosecond laser scanning for forming a hemispherical surface and a Y-shaped channel in the fused silica glass; (2) chemical etching of the sample for removal of the modified areas; and (3) oxyhydrogen (OH) flame polish for smoothening the surface of the microlens. In addition, we demonstrate that the fabricated microlens exhibits good imaging performance with a 5× magnification, showing great potential in future lab-on-a-chip applications.     

Laser Micro-Hole Drilling of Soda-Lime Glass with Femtosecond Pulses

Using tightly focused femtosecond laser pulses, we have drilled micro-holes from the front and rear surface of soda-lime glass in ambient air. The machined holes have small aspect ratio or irregular inner walls. When the drilling is conducted from the rear surface in contact with distilled water, a good quality micro-hole with a high aspect ratio can be obtained. The corresponding formation mechanisms are investigated.     

Micro-channel etching characteristics enhancement by femtosecond laser processing high-temperature lattice in fused silica glass

A fused silica glass micro-channel can be formed by chemical etching after femtosecond laser irradiation, and the successful etching probability is only 48%. In order to improve the micro-channel fabrication success probability,the method of processing a high-temperature lattice by a femtosecond laser pulse train is provided. With the same pulse energy and scanning speed, the success probability can be increased to 98% by optimizing pulse delay.The enhancement is mainly caused by the nanostructure, which changes from a periodic slabs structure to some intensive and loose pore structures. In this Letter, the optimum pulse energy distribution ratio to the etching is also investigated.     

Three-dimensional micromachining of glass using femtosecond laser for lab-on-a-chip device manufacture

Three-dimensional (3D) micromachining of photosensitive glass is demonstrated by photochemical reaction using femtosecond (fs) laser for lab-on-a-chip application. True 3D hollow microstructures embedded in the glass are fabricated by fs laser direct writing followed by heat treatment and successive wet etching. The modification mechanism of the photosensitive glass by the fs laser and advantage of this process are discussed. Various microcomponents for the lab-on-a-chip devices such as microfluidics, microvalves, microoptics, microlasers, etc. are fabricated by using this technique and their performance is examined . PACS 42.62.-b; 82.50.Pt; 87.80.Mj     

Fabrication of glass micro-prisms using ultra-fast laser pulses with chemical etching process

In this study, a new process of glass micro-prism structures is investigated by an ultra-fast laser irradiation with chemical etching process. The ultra-fast laser is employed by an all-in-one femtosecond laser (FS-laser) system with the amplifier as an excitation source for patterning the structures. Here, the center wavelength of laser is frequency-doubled to 517 nm. Besides, the repetition rate and pulse width of laser are 100 kHz and 350 fs, respectively. First, the embedded gratings of glass with different pitches can be fabricated using a FS-laser process. Afterwards, the glass samples are placed in the hydrofluoric acid (HF) solution for 15 min to develop structures. Finally, the results of this study demonstrated that the V-cut micro-prisms are successfully formed by controlling etching concentration between intrinsic glass material and modified areas.     

The formation of different structures in the interaction between a single femtosecond laser pulse and a thin Au film

The interactions between femtosecond (fs) laser pulses and a thin Au film deposited on a silica glass substrate were systematically investigated based on experimental data. Different structures, including microholes, nanoholes, and nanobumps, are obtained when pulses with different energies are incident on the surface of a gold film. The experimental results are discussed according to specific experimental parameters. Two physical models were constructed in order to explain the experimental results. The formation of nanoholes in a silica substrate is attributed to etching by higher order harmonic generations (HHG) when the femtosecond laser pulse interacts with the generated plasma layer, while the formation of nanobumps on the surface of an Au film is attributed to the elastic and plastic characteristics of the metal film under laser pulse irradiation.     

Rapid microfabrication of transparent materials using filamented femtosecond laser pulses

Microfabrication of transparent materials using femtosecond laser pulses has showed good potential towards industrial application. Maintaining pulse energies exceeding the critical self-focusing threshold by more than 100-fold produced filaments that were used for micromachining purposes. This article demonstrates two different micromachining techniques using femtosecond filaments generated in different transparent media (water and glass). The stated micromachining techniques are cutting and welding of transparent samples. In addition, cutting and drilling experiments were backed by theoretical modelling giving a deeper insight into the whole process. We demonstrate cut-out holes in soda-lime glass having thickness up to 1 mm and aspect ratios close to 20, moreover, the fabrication time is of the order of tens of seconds, in addition, grooves and holes were fabricated in hardened 1.1 mm thick glass (Corning $^\circledR$ Gorilla $^\circledR$ glass). Glass welding was made possible and welded samples were achieved after several seconds of laser fabrication.     

Physicochemical Mechanisms of Nanostructuring of Glass by Femtosecond Laser Pulses with the Use of Selective Etching

JETP Letters - Processes occurring at nanostructuring of glass irradiated by tightly focused single femtosecond laser pulses with selective chemical etching in the alkaline solution have been...     

3D integration of microcomponents in a single glass chip by femtosecond laser direct writing for biochemical analysis

3D integration of microcomponents in a single glass chip by femtosecond laser direct writing followed by post annealing and successive wet etching is described for application to biochemical analysis. Integration of microfluidics and microoptics realized some functional microdevices like a μ-fluidic dye laser and a biosensor. As one of practical applications, we demonstrate inspection of living microorganisms using the microchip with 3D microfluidic structures fabricated by the present technique.     

Direct fabrication of freely movable microplate inside photosensitive glass by femtosecond laser for lab-on-chip application

We demonstrate the fabrication of complicated three-dimensional (3D) microstructures embedded in a photosensitive glass by a high-order multiphoton process using a femtosecond (fs) laser. Direct writing of the fs laser followed by a post baking process and preferential etching in a dilute hydrofluoric (HF) acid solution results in a microplate that can freely move in hollow structures embedded in the glass. The fabricated structure functions as a microvalve that can control the flow direction of fluids in the microreactor. PACS 42.62.-b; 81.05.Kf; 82.50.Pt     

Fabrication of microlens arrays in photosensitive glass by femtosecond laser direct writing

This article reports the fabrication of high-fill-factor plano-convex cylindrical and spherical microlens arrays horizontally and vertically embedded in a photosensitive Foturan glass chip by femtosecond (fs) laser micromachining. The microlens arrays were fabricated by modifying the microstructure of Foturan glass using fs laser direct writing followed by thermal treatment, wet etching, and additional annealing. The focusing ability and image quality of the microlens arrays were examined, showing that the lens arrays not only can focus light well but also provide an imaging capability that holds great potential for lab-on-a-chip applications.     

Femtosecond laser microstructuring: an enabling tool for optofluidic lab‐on‐chips

This paper provides an overview of the rather new field concerning the applications of femtosecond laser microstructuring of glass to optofluidics. Femtosecond lasers have recently emerged as a powerful microfabrication tool due to their unique characteristics. On the one hand, they enable to induce a permanent refractive index increase, in a micrometer‐sized volume of the material, allowing single‐step, three‐dimensional fabrication of optical waveguides. On the other hand, femtosecond‐laser irradiation of fused silica followed by chemical etching enables the manufacturing of directly buried microfluidic channels. This opens the intriguing possibility of using a single laser system for the fabrication and three‐dimensional integration of optofluidic devices. This paper will review the state of the art of femtosecond laser fabrication of optical waveguides and microfluidic channels, as well as their integration for high sensitivity detection of biomolecules and for cell manipulation.     

Femtosecond laser assisted etching of quartz: microstructuring from inside

In quartz crystal substrates, microchannels were made by femtosecond laser assisted etching, i.e., irradiation of focused femtosecond laser pulses followed by wet etching. By the use of wet etching, the laser irradiated region was selectively etched out, and a microchannel was formed inside the quartz substrate. The laser irradiated region was found to be amorphous by transmission electron microscopy. Anisotropy in the etching rate inside the quartz was observed. PACS 42.70.Ce; 61.80.Ba; 82.50.-m     

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     

Fabrication of an integrated Raman sensor by selective surface metallization using a femtosecond laser oscillator

We demonstrate that a Raman sensor integrated with a micro-heater, a microfluidic chamber, and a surface-enhanced Raman scattering (SERS) substrate can be fabricated in a glass chip by femtosecond laser micromachining. The micro-heater and the SERS substrate are fabricated by selective metallization on the glass surface using a femtosecond laser oscillator, whereas the microfluidic chamber embedded in the glass sample is fabricated by femtosecond laser ablation using a femtosecond laser amplifier. We believed that this new strategy for fabricating multifunctional integrated microchips has great potential application for lab-on-a-chips.