Fabrication of 3D microoptical lenses in photosensitive glass using femtosecond laser micromachining

We describe the fabrication of microoptical cylindrical and hemispherical lenses vertically embedded in a photosensitive Foturan glass by femtosecond (fs) laser three-dimensional (3D) micromachining. The process is mainly composed of four steps: (1) fs laser scanning in the photosensitive glass to form curved surfaces (spherical and/or cylindrical); (2) postannealing of the sample for modification of the exposed areas; (3) chemical etching of the sample for selective removal of the modified areas; and (4) a second postannealing for smoothening the surfaces of the tiny lenses. We examine the focusing ability of the microoptical lenses using a He-Ne laser beam, showing the great potential of using these microoptical lenses in lab-on-a-chip applications. 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.     

Three‐dimensional femtosecond laser micromachining of photosensitive glass for biomicrochips

Internal modification of transparent materials such as glass can be carried out using multiphoton absorption induced by a femtosecond (fs) laser. The fs‐laser modification followed by thermal treatment and successive chemical wet etching in a hydrofluoric (HF) acid solution forms three‐dimensional (3D) hollow microstructures embedded in photosensitive glass. This technique is a powerful method for directly fabricating 3D microfluidic structures inside a photosensitive glass microchip. We used fabricated microchips, referred to as a nanoaquarium, for dynamic observations of living microorganisms. In addition, the present technique can also be used to form microoptical components such as micromirrors and microlenses inside the photosensitive glass, since the fabricated structures have optically flat surfaces. The integration of microfluidics and microoptical components in a single glass chip yields biophotonic microchips, in other words, optofluidics, which provide high sensitivity in absorption and fluorescence measurements of small volumes of liquid samples.     

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     

Three-dimensional integration of microoptical components buried inside photosensitive glass by femtosecond laser direct writing

We report the three-dimensional (3D) integration of microoptical components such as microlenses, micromirrors and optical waveguides in a single glass chip by femtosecond (fs) laser direct writing. First, two types of microoptical lenses were fabricated inside photosensitive Foturan glass by forming hollow microstructures using fs laser direct writing followed by thermal treatment, successive wet etching and additional annealing. One type of lens is the cylindrical microlens with a curvature radius R of 1.0 mm, and the other is the plano-convex microlens with radius R of 0.75 mm. Subsequently, by the continuous procedure of hollow microstructure fabrication, a micromirror was integrated with the plano-convex microlens in the single glass chip. Further integration of waveguides was performed by internal refractive index modification using fs laser direct writing after the hollow structure fabrication of the microlens and the micromirror. A demonstration of the laser beam transmission in the integrated optical microdevice shows that the 3D integration of waveguides with a micromirror and a microoptical lens in a single glass chip is highly effective for light beam guiding and focusing. PACS 42.62.-b; 81.05.Kf; 42.82.Cr; 82.50.Pt; 42.79.Gn     

Three-dimensional micro-optical components embedded in photosensitive glass by a femtosecond laser

We show that three-dimensional micro-optical components can be embedded in a photosensitive glass by a femtosecond (fs) laser. After exposure to the tightly focused fs laser beam, latent images are written inside the sample. Modified regions are developed by a postbaking process and then preferentially etched away in a 10%-dilute solution of hydrofluoric acid. After this process, hollow internal structures are formed that act as a mirror and a beam splitter. Furthermore, we find that postannealing smoothes the surfaces of the fabricated hollow structures, resulting in great improvement of their optical properties.     

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.     

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.     

Threshold effect in femtosecond laser induced nanograting formation in glass: influence of the pulse duration

Recently, femtosecond laser direct writing in porous glass is emerging as a powerful technique for building arbitrary 3D hollow micro/nanostructures in bulk glass materials. In this study, we investigate the pulse duration dependence of laser intensity window for inducing a single nanocrack inside porous glass by femtosecond laser direct writing. We find that the window for a single nanocrack increases with the pulse duration, while the roughness of side walls in the nanocracks becomes higher for pulses longer than ~300 fs. When the femtosecond laser pulses of an optimized duration of ~200 fs are chosen, a sufficiently broad range of laser intensity (~44 % of the structuring threshold) for creating a single nanocrack can be obtained, while smooth sidewalls required by nanofluidic applications can still be maintained. The reported results will be beneficial not only for the development of the 3D femtosecond laser micro/nanostructuring techniques, but also for gaining a deeper understanding of the physical mechanism behind the nanograting formation induced by femtosecond laser irradiation in glass and other transparent materials.     

Femtosecond laser application for high capacity optical data storage

A femtosecond (fs) laser application for multi-layer optical recording is investigated. Information patterns at different layer depths were written inside a transparent glass substrate due to micro-void formation by fs laser ablation, which causes re-distribution in glass materials and a refractive index modification. The information bits recorded in a single layer can be retrieved clearly without interference from the neighboring layers. A fs laser irradiation of a transparent polymer matrix (doped with fluorescent materials for use as low-cost recording media) is also studied. A fs laser induced photo-chemical reaction changes the chemical properties of the fluorescent materials and records information bits inside the matrix. With an ultra-fast laser as a new light source, 3D optical recording can be available for high capacity data storage up to 1 TB per disc. PACS 82.50.-m; 42.65. Re; 72.70.Jk.     

Fabrication of surface relief gratings on transparent dielectric materials by two-beam holographic method using infrared femtosecond laser pulses

Fabrication of surface relief-type gratings in transparent dielectrics, which are hard to machine, has been achieved by a holographic technique using two infrared femtosecond (fs) pulses from a mode-locked Ti:sapphire laser. The present method can be applied for a variety of transparent dielectrics, Al₂O₃ (sapphire), TiO₂, ZrO₂, LiNbO₃, SiC, ZnO, CdF₂, MgO, CaF₂ crystals, and SiO₂ glass. It is found that the grating formation is due primarily to laser ablation processes. Planar surface relief gratings can be fabricated by colliding two fs laser pulses on the surface of substrates which move at a constant speed, synchronized with the laser repetition rate. Received: 1 March 2000 / Published online: 7 June 2000     

Single-pass waveguide amplifiers in Er-Yb doped zinc polyphosphate glass fabricated with femtosecond laser pulses

We have investigated the direct fabrication of subsurface waveguide amplifiers in Er-Yb zinc polyphosphate glass by utilizing the relationship between the initial glass composition and the resulting changes to the network structure after modification by fs laser pulses. Waveguides, exhibiting internal gain of 1 dB/cm at 1.53 μm when pumped with 500 mW at 976 nm, were directly fabricated using a regenerative amplified Ti:sapphire 1 kHz, 180 fs laser system. Optical properties as well as insertion losses and internal gain are reported.     

Infrared femtosecond laser-induced great enhancement of ultraviolet luminescence of ZnO two-dimensional nanostructures

Two-dimensional (2D) complex nanostructures on the surface of ZnO crystal are fabricated by the interference of three 800 nm fs laser beams. The 2D nanostructures exhibit a great enhancement of UV emission excited by infrared fs laser with central wavelengths ranging from 1,200 nm to 2,000 nm. We propose that the defect states in the band gap of 2D nanostructures induced by 800 nm fs laser ablation cause the great enhancement of UV emission. We make theoretical calculations and explain well with the experimental results.     

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     

Modification of the fused silica glass network associated with waveguide fabrication using femtosecond laser pulses

Atomic-scale structural changes have been observed in the glass network of fused silica after modification by tightly focused 800-nm, 130-fs laser pulses at fluences between 5 and 200 J cm^-2. Raman spectroscopy of the modified glass shows an increase in the 490 and 605-cm^-1 peaks, indicating an increase in the number of 4- and 3-membered ring structures in the silica network. These results provide evidence that densification of the glass occurs after exposure to fs pulses. Fluorescence spectroscopy of the modified glass shows a broad fluorescence band at 630 nm, indicating the formation of non-bridging oxygen hole centers (NBOHC) by fs pulses. Waveguides that support the fundamental mode at 633 nm have been fabricated inside fused silica by scanning the glass along the fs laser beam axis. The index changes are estimated to be approximately 0.07×10^-3. Received: 17 December 2001 / Accepted: 9 July 2002 / Published online: 25 October 2002 RID="*" ID="*"Corresponding author. Fax: +1-925/423-2463, E-mail: dmkrol@ucdavis.edu     

飞秒激光湿法刻蚀微纳制造

飞秒激光微加工作为一种新型微纳制造技术,在复杂三维构型制作方面具有其独特的优势,但激光加工效率问题严重制约了飞秒激光微加工技术走向实际工程应用,提出一种飞秒激光湿法刻蚀微纳制造方法,以提高飞秒激光微加工的效率为突破口,通过调控激光与物质相互作用获得材料的目标靶向改性,进而结合化学湿法刻蚀实现硬质材料上的高效和高精度三维微加工,采用这一方法制作出的微透镜尺寸为80 m,球冠高6.7 m,表面粗糙度小于10 nm。利用这种方法,实现了不同结构与特性的高质量微透镜阵列的超精密制备,在石英内部也实现了螺旋微通道的复杂三维结构,螺旋通道直径为20 m,长径比超过100。     

Structural changes induced on strontium barium niobate glass by femtosecond laser irradiation

Localized modification of the optical properties of erbium doped strontium barium niobate (SBN) glass has been performed using femtosecond laser irradiation. The samples, with composition SrO–BaO–Nb₂O₅–B₂O₅ and doped with 5%mol of Er³⁺, were fabricated using a melt-quenching method. The samples were irradiated with different number of pulses per spot (1–50 pulses) at two different laser fluences (2.6 and 5.6 J/cm²) by using an fs laser amplifier operating at 800 nm and generating pulses with a duration of 120 fs. Micro-luminescent microscopy, using an Ar⁺ laser as excitation source, has been used to analyze the modifications of the luminescent properties of the sample upon fs laser exposure. The emissions of the Er³⁺: ⁴I_11/2→⁴I_15/2 and ⁴I_13/2→⁴I_15/2 transitions allow appreciating the structural modifications caused by femtosecond laser exposure. The lifetimes of the levels involved in these transitions were measured inside and outside the laser irradiated region. These measurements have been compared with those obtained in bulk glass ceramic sample, which is obtained from the glass precursor by a thermal treatment in order to estimate the optimal conditions to produce nanocrystals in a localized region by ultrafast laser irradiation.     

Control of the cross-sectional shape of a hollow microchannel embedded in photostructurable glass by use of a femtosecond laser

Theoretical and experimental investigations have been made of the three-dimensional microchannel fabrication of photostructurable glass by use of a femtosecond (fs) laser. Generally, a microchannel fabricated inside glass by the scanning focal spot of a fs laser perpendicular to the direction of laser propagation assumes an elliptical shape with a cross section of large aspect ratio. We demonstrate that one can greatly reduce the aspect ratio merely by inserting a slit, which is oriented parallel to the laser's scanning direction, before the focusing lens. Computer simulations show that a more symmetrical pattern is obtained in the vicinity of the focal point with the help of such a slit, owing essentially to a diffraction effect.     

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     

Spectroscopic investigations of femtosecond laser irradiated polystyrene and fabrication of microstructures

We report microfabrication of structures in bulk and thin films of polystyrene (PS) using femtosecond (fs) laser pulses. For the first time to our knowledge, we report emission from the fs laser modified regions of bulk and thin films of PS when excited at 458, 488, and 514 nm. Moreover, we report the existence of peroxide type free radicals, for the first time, in fs laser irradiated bulk PS. We observed the suppression of Raman modes in case of structures fabricated at higher energies and the same effect was noticed in central portion of the structures fabricated. No appreciable broadening was observed in the case of structures fabricated at low energies. Possible applications resulting from such structures are discussed briefly.