Femtosecond laser-induced microstructures in glasses and applications in micro-optics

Femtosecond laser has been widely used in microscopic modifications to materials due to its ultra-short laser pulse and ultrahigh light intensity. When a transparent material e.g. glass is irradiated by a tightly focused femtosecond laser, the photo-induced reaction is expected to occur only near the focused part of the laser beam inside the glass due to the multiphoton processes. We observed various induced structures e.g. color center defects, refractive index change, micro-void and micro-crack, in glasses after the femtosecond laser irradiation. In this paper, we review the femtosecond laser induced phenomena and discuss the mechanisms of the observed phenomena. We also introduce the fabrication of various micro-optical components, e.g. optical waveguide, micro-grating, micro-lens, fiber attenuator, 3-dimensional optical memory by using the femtosecond laser-induced structures. The femtosecond laser will open new possibilities in the fabrication of micro-optical components with various optical functions.     

Sol-Gel Prepared Glass for Refractive and Diffractive Micro-Optical Elements and Arrays

The fast sol-gel method enables facile preparation of siloxane-based glassy matrices, in which polymerization is completed within minutes and volume changes <5% take place upon curing. Single-face and two-face replication of micro-optical arrays have been obtained, as well as crack-free elements >10 mm thick. Minimizing shrinkage and enabling relaxation of the drying sol-gel are key factors in the elimination of cracking. These features and the resulting optical quality of the glass make this method technologically and economically attractive for replication-produced micro-lenses and micro-optical arrays.     

Low-Loss Photopatternable Hybrid Sol–Gel Materials

Hybrid sol–gel technology has proved its usefulness in the field of micro-optics. Indeed, it makes it possible to manufacture optical microcomponents by processes much simpler than those currently used, particularly when photopatternable materials are employed. However, for wavelengths in the near infrared window (NIR), this process is confronted with a major problem which results from the significant optical absorption of this type of materials, in particular around 1550 nm. The aim of this paper is to describe the origin of the losses of some hybrid sol–gel materials of direct interest in micro-device fabrication in order to propose solutions to achieve a greater transparency in the NIR region. The intrinsic absorption of the precursors was first of all considered since it is crucial to pursue the final goal of decreased losses. Then, the principal parameters of the sol–gel synthesis were studied. Among them, the thermal treatment conditions, the hydrolysis ratio and the controlled addition of transition metal alkoxides allow an improvement of the material transparency, at 1550 nm most particularly. This last point was considered with the highest attention, and NIR and NMR spectroscopies were employed to explain how the addition of zirconium alkoxides at the end of the reaction makes it possible to considerably decrease the losses in the material.     

Microscale patterning of thermoplastic polymer surfaces by selective solvent swelling

A new method for the fabrication of microscale features in thermoplastic substrates is presented. Unlike traditional thermoplastic microfabrication techniques, in which bulk polymer is displaced from the substrate by machining or embossing, a unique process termed orogenic microfabrication has been developed in which selected regions of a thermoplastic surface are raised from the substrate by an irreversible solvent swelling mechanism. The orogenic technique allows thermoplastic surfaces to be patterned using a variety of masking methods, resulting in three-dimensional features that would be difficult to achieve through traditional microfabrication methods. Using cyclic olefin copolymer as a model thermoplastic material, several variations of this process are described to realize growth heights ranging from several nanometers to tens of micrometers, with patterning techniques include direct photoresist masking, patterned UV/ozone surface passivation, elastomeric stamping, and noncontact spotting. Orogenic microfabrication is also demonstrated by direct inkjet printing as a facile photolithography-free masking method for rapid desktop thermoplastic microfabrication.     

Membrane oscillator as a chemical sensor

A rhythmic, sustained, stable oscillation was reproducibly observed for a lipid membrane supported by a micropore of a thin membrane tip micropipet (TM pipet). The construction of the TM pipet was accomplished by implementing a microfabrication method that allowed the transference of a Si₃N₄ film with a hole from the Si substrate to the glass tube tip. The main part of the fabrication method is the sealing process: a mix between thermal and amodic bonding. The TM pipet fabrication is described in detail with emphasis on the thermal-anodic bonding process. In addition, a general account of the new device's main features, including various applications, is given.     

Fabrication of micro-lens arrays built in photosensitive hybrid films by UV-cured imprinting technique

Photosensitive TiO₂-contained organic–inorganic hybrid films were prepared by combining a low-temperature sol–gel process with a spin-coating technique. Optical properties and photochemical activities of the as-prepared hybrid sol–gel films were characterized by prism coupling technique, thermal gravimetric analysis, UV–Visible spectroscopy, and Fourier transform infrared spectroscopy. Advantages for fabrication of micro-lens arrays based on the as-prepared photosensitive hybrid films were demonstrated by a direct-contact lithography technique and a reflow technique, followed by an UV-cured imprinting technique. Results indicate that the as-prepared photosensitive hybrid materials have great applicability for the fabrication of photonic components. Micro-sphere lens arrays and micro-ellipsoid lens arrays with the diameter from 20 to 100 μm and built in the as-prepared hybrid films were obtained. Morphological and surface profile properties of the as-fabricated micro-lens arrays were characterized by scanning electron microscopy and surface profiler, respectively. Results indicate that the fabrication process of the micro-lens arrays is a simple, cost-effective and mass production process, and the as-prepared photosensitive hybrid materials have great potential applications for the fabrication of the micro-optical elements.     

Hydrogel microparticles as dynamically tunable microlenses

Tunable micro-optical elements were prepared by aqueous free-radical polymerization and electrostatic self-assembly techniques. Stimuli-responsive poly(N-isopropylacrylamide-co-acrylic acid) (pNIPAm-AAc) microgels were used as lenses to generate dynamically tunable optical elements. By using optical microscopy to investigate the micrometer-scale dynamics of the self-assembled microlenses, we demonstrate focal length tuning through modulation of the solution pH and/or temperature.     

Use of NIR spectroscopy in the production of modified industrial resins

Natural resins are scarcely used, but after appropriate modification processes they acquire characteristics of viscosity, point of softening, stability, etc. that facilitate their application in fields such as paintings, varnishes, cosmetic, etc. The complexity of resins makes it very difficult to monitor the reactions involved in their modification, the extent of which is usually determined via more experimentally accessible parameters. However, the methods typically used to determine such parameters are slow and produce environmentally unfriendly waste.In this work, we assessed the potential of NIR spectroscopy, as an alternative to the traditional analytical methods, for monitoring the industrial processes involved in the production of modified resins. To this end, we developed PLS calibration models that were used to quantify physical (viscosity and cloud point) and chemical parameters (acid and hydroxyl numbers), with a view to characterize the evolution of the resins during the reaction that take place throughout the fabrication process.Samples were withdrawn at different times stages of the process for analysis with the proposed quantitation models; the data thus obtained were compared with those provided by reference methods. Based on the results, NIR spectroscopy is an effective choice for the accurate, expeditious monitoring of industrial resin modification processes.     

Isoquinoline-based lanthanide complexes: bright NIR optical probes and efficient MRI agents

In the objective of developing ligands that simultaneously satisfy the requirements for MRI contrast agents and near-infrared emitting optical probes that are suitable for imaging, three isoquinoline-based polyaminocarboxylate ligands, L1, L2 and L3, have been synthesized and the corresponding Gd(3+), Nd(3+) and Yb(3+) complexes investigated. The specific challenge of the present work was to create NIR emitting agents which (i) have excitation wavelengths compatible with biological applications and (ii) are able to emit a sufficient number of photons to ensure sensitive NIR detection for microscopic imaging. Here we report the first observation of a NIR signal arising from a Ln(3+) complex in aqueous solution in a microscopy setup. The lanthanide complexes have high thermodynamic stability (log K(LnL) =17.7-18.7) and good selectivity for lanthanide ions versus the endogenous cations Zn(2+), Cu(2+), and Ca(2+) thus preventing transmetalation. A variable temperature and pressure (17)O NMR study combined with nuclear magnetic relaxation dispersion measurements yielded the microscopic parameters characterizing water exchange and rotation. Bishydration of the lanthanide cation in the complexes, an important advantage to obtain high relaxivity for the Gd(3+) chelates, has been demonstrated by (17)O chemical shifts for the Gd(3+) complexes and by luminescence lifetime measurements for the Yb(3+) analogues. The water exchange on the three Gd(3+) complexes is considerably faster (k(ex)(298) = (13.9-15.4) × 10(6) s(-1)) than on commercial Gd(3+)-based contrast agents and proceeds via a dissociative mechanism, as evidenced by the large positive activation volumes for GdL1 and GdL2 (+10.3 ± 0.9 and +10.6 ± 0.9 cm(3) mol(-1), respectively). The relaxivity of GdL1 is doubled at 40 MHz and 298 K in fetal bovine serum (r(1) = 16.1 vs 8.5 mM(-1) s(-1) in HEPES buffer), due to hydrophobic interactions between the chelate and serum proteins. The isoquinoline core allows for the optimization of the optical properties of the luminescent lanthanide complexes in comparison to the pyridinic analogues and provides significant shifts of the excitation energies toward lower values which therefore become more adapted for biological applications. L2 and L3 bear two methoxy substituents on the aromatic core in ortho and para positions, respectively, that further modulate their electronic structure. The Nd(3+) and Yb(3+) complexes of the ligand L3, which incorporates the p-dimethoxyisoquinoline moiety, can be excited up to 420 nm. This wavelength is shifted over 100 nm toward lower energy in comparison to the pyridine-based analogue. The luminescence quantum yields of the Nd(3+) (0.013-0.016%) and Yb(3+) chelates (0.028-0.040%) are in the range of the best nonhydrated complexes, despite the presence of two inner sphere water molecules. More importantly, the 980 nm NIR emission band of YbL3 was detected with a good sensitivity in a proof of concept microscopy experiment at a concentration of 10 μM in fetal bovine serum. Our results demonstrate that even bishydrated NIR lanthanide complexes can emit a sufficient number of photons to ensure sensitive detection in practical applications. In particular, these ligands containing an aromatic core with coordinating pyridine nitrogen can be easily modified to tune the optical properties of the NIR luminescent lanthanide complexes while retaining good complex stability and MRI characteristics for the Gd(3+) analogues. They constitute a highly versatile platform for the development of bimodal MR and optical imaging probes based on a simple mixture of Gd(3+) and Yb(3+)/Nd(3+) complexes using an identical chelator. Given the presence of two inner sphere water molecules, important for MRI applications of the corresponding Gd(3+) analogues, this result is particularly exciting and opens wide perspectives not only for NIR imaging based on Ln(3+) ions but also for the design of combined NIR optical and MRI probes.     

Development-free vapor photoetching effect of azide-type photopolymers

A new dry photoetching process that dose not required development step had been developed, utilizing the development-free vapor photoetching effect of cinnamate-type photopolymers.^1–3 In contrast to obtaining positive patterns with cinnamate-type photopolymers, this process with azide-type photopolymers, which consist of axide crosslinking agent and polymers, gives negative patterns. Some preliminary studies on the development-free vapor photoetching effect of azide-type photopolymers were made. The results indicate that the polymers in this process only play the role of forming a film on the surface of silicon dioxide; and the crosslinking agent is the most important factor, which leads the difference of etching rate between the exposed and unexposed areas.     

Optofluidic integrated cell sorter fabricated by femtosecond lasers

The main trend in optofluidics is currently towards full integration of the devices, thus improving automation, compactness and portability. In this respect femtosecond laser microfabrication is a very powerful technology given its capability of producing both optical waveguides and microfluidic channels. The current challenge in biology is the possibility to perform bioassays at the single cell level to unravel the hidden complexity in nominally homogeneous populations. Here we report on a new device implementing a fully integrated fluorescence-activated cell sorter. This non-invasive device is specifically designed to operate with a limited amount of cells but with a very high selectivity in the sorting process. Characterization of the device with beads and validation with human cells are presented.     

A rapid and economical method for profiling feature heights during microfabrication

Quality control is an important and integral part to any microfabrication process. While the widths of features often can be easily assessed by light microscopy, the heights of the fabricated structures are more difficult to determine. Here, we present a rapid, accurate, and low-cost method to measure the heights of microfabricated structures during and after the fabrication process. This technique is based on white-light interferometry, which offers accuracy on the submicrometre scale.     

Responsive photonic crystals

This Review summarizes recent developments in the field of responsive photonic crystal structures, including principles for design and fabrication and many strategies for applications, for example as optical switches or chemical and biological sensors. A number of fabrication methods are now available to realize responsive photonic structures, the majority of which rely on self-assembly processes to achieve ordering. Compared with microfabrication techniques, self-assembly approaches have lower processing costs and higher production efficiency, however, major efforts are still needed to further develop such approaches. In fact, some emerging techniques such as spin coating, magnetic assembly, and flow-induced self-assembly have already shown great promise in overcoming current challenges. When designing new systems with improved performance, it is always helpful to bear in mind the lessons learnt from natural photonic structures.     

Synthesis and liquid crystal alignment properties of new cinnamate-based photocrosslinkable polymers

Because optical buffing can achieve aligning patterns with different azimuthal angles of the liquid crystal directors, photoalignment appears to be a very hopeful technique for designing complex LCD structures. Until recently, most photopolymers provided low anchoring and/or low tilt angles which are unsuitable for industrial applications. In this work, four new photocrosslinkable polymers based on biphenyl and naphthalene moieties have been prepared and characterized. These photopolymers have been irradiated with linearly polarized UV light, and liquid crystal cells have been made. The tilt angles generated by oblique irradiation have been measured as well as the anchoring energies. Finally, the stability of the liquid crystal alignment properties have been studied.     

Synthesis and liquid crystal alignment properties of new cinnamate-based photocrosslinkable polymers

Because optical buffing can achieve aligning patterns with different azimuthal angles of the liquid crystal directors, photoalignment appears to be a very hopeful technique for designing complex LCD structures. Until recently, most photopolymers provided low anchoring and/or low tilt angles which are unsuitable for industrial applications. In this work, four new photocrosslinkable polymers based on biphenyl and naphthalene moieties have been prepared and characterized. These photopolymers have been irradiated with linearly polarized UV light, and liquid crystal cells have been made. The tilt angles generated by oblique irradiation have been measured as well as the anchoring energies. Finally, the stability of the liquid crystal alignment properties have been studied.     

Fabrication of a glass-implemented microcapillary electrophoresis device with integrated contactless conductivity detection

Glass microdevices for capillary electrophoresis (CE) gained a lot of interest in the development of micrototal analysis systems (microTAS). The fabrication of a microTAS requires integration of sampling, chemical separation and detection systems into a microdevice. The integration of a detection system into a microchannel, however, is hampered by the lack of suitable microfabrication technology. Here, a microfabrication method for integration of insulated microelectrodes inside a leakage-free microchannel in glass is presented. A combination of newly developed technological approaches, such as low-temperature glass-to-glass anodic bonding, channel etching, fabrication of buried metal interconnects, and deposition of thin plasma-enhanced chemical vapour deposition (PECVD) silicon carbide layers, enables the fabrication of a CE microdevice with an integrated contactless conductivity detector. The fabrication method of this CE microdevice with integrated contactless conductivity detector is described in detail. Standard CE separations of three inorganic cations in concentrations down to 5 microM show the viability of the new microCE system.     

Fabrication of large-area ferromagnetic arrays using etched nanosphere lithography

Nanosphere lithography (NSL) is a simple, cost-effective, and powerful technique capable of producing large-area arrays of ferromagnetic nanostructures with dimensions below 100 nm. These properties make NSL an attractive process for the fabrication of arrays of magnetic elements with applications in magnetic data storage. The main disadvantage with conventional NSL is that the monolayer of spheres always contains imperfections that are transferred to the resulting nanostructures. This can significantly affect the structural and magnetic properties of the fabricated array. In this paper we present a novel adaptation of NSL that reduces the effect of such defects on the resulting nanostructures. The technique also offers excellent control over the diameter, aspect ratio, and pitch of the fabricated elements. These properties are demonstrated through the fabrication of arrays of Ni elements of 210 nm diameter and arrays of Co elements with diameters between 200 and 320 nm.     

Applications of Near Infrared Spectrometry Technique in Petroleum Products Analysis

Near infrared (NIR) spectroscopy has become a popular technique for process analytical chemistry and is being studied extensively in the petrochemical industry fields. NIR spectroscopy has several attractive properties:hardly any sample preparation is required,it is a nondestructive method, and it has a high signal-to-noise ratio. Furthermore, NIR spectroscopy has the possibility of remote sensing using optical fibers. All these advantages make NIR spectroscopy very suitable for on-line quality control in process analytical chemistry. In this paper some recent applications of NIR in analysis of petroleum products are reviewed.     

Contemporary Synthesis of Ultrasmall (sub-10 nm) Upconverting Nanomaterials

Due to their unique photophysical properties, upconverting nanoparticles (UCNPs), i. e. particles capable of converting near-infrared (NIR) photons into tunable emissions in the range of ultraviolet (UV) to NIR, have great potential for use in various biomedical fields such as bioimaging, photodynamic therapy and bioanalytical applications. As far as biomedical applications are concerned, these materials have a number of advantageous properties such as brilliant luminescence and exceptional photostability. Very small “stealth” particles (sub-10 nm), which can circulate in the body largely undetected by the immune system, are particularly important for in vivo use. The fabrication of such particles, which simultaneously have a defined (ultrasmall) size and the required optical properties, is a great challenge and an area that is in its infancy. This minireview provides a concise overview of recent developments on appropriate synthetic methodologies to produce such UCNPs. Particular attention was given to the influence of both surfactants and dopants used to precisely adjust size, crystalline phase and optical properties of UCNPs.     

Sacrificial layer microfluidic device fabrication methods

Over the past 15 years, research in the field of microfluidics has experienced rapid growth due to significant potential advantages such as low cost, short analysis times, and elimination of sources of contamination. Although etched and thermally bonded glass substrates have seen widespread use and offer solid performance, device fabrication still remains cumbersome. Recent advances in sacrificial layer microfabrication methods for microfluidics have overcome many disadvantages of conventional fabrication approaches. Phase-changing sacrificial layers have been implemented in making inexpensive and high-performance polymer microchips for electrophoretic analysis, protein focusing, and sample preconcentration. In addition, novel channel fabrication methods based on standard thin-film processes, which are readily integratable with microfabrication techniques used for electrical components, are being applied increasingly for the creation of microfluidic devices. These new sacrificial layer fabrication approaches will be instrumental in making low-cost and high-quality polymer microchips, and in interfacing electrical and fluidic systems on glass or semiconductor substrates.