Optical detection techniques based on surface enhanced Raman spectroscopy (SERS) can provide relevant information on molecular and protein composition of biological samples, thus enabling to discriminate between physiological and pathological conditions. With the attempt to develop point-of-impact diagnostics devices, in this study we combine lowcost fabrication processes with surface functionalization strategies for the fabrication of SERS-active polymeric substrates engineered to selectively detect specific biomarkers. By reversibly coupling these devices with the distal end of portable Raman instruments, SERS measurements could be potentially implemented for the early diagnosis of widespread pathologies by SERS analysis of liquid biopsies.
Coating of high-Q whispering gallery mode micro-resonators is typically performed in order to add the functionalities of the coating material to the unique properties of this type of resonators. Silica microspheres or microtoroids are typically used as high-Q cavity substrate on which a functional film is deposited. In order to effectively exploit the coating properties a critical step is the efficient excitation of WGMs mainly contained inside the deposited layer. We developed a simple method able to assess whether or not these modes are selectively excited. The method is based on monitoring the thermal shift of the excited resonance, which uniquely depends on the thermo-optic coefficient and on the thermal expansion coefficient of the material in which the mode is embedded. We applied this technique to the case of a SU-8 layer deposited on a silica microsphere. Main tests were performed around the wavelength of 770 nm because of potential application in biochemical sensing requiring low light absorption in aqueous environment. We show that by using integrated waveguides made with SU-8 polymer (rather than silica fiber tapers) we can fulfill the proper phase matching conditions thus exciting the fundamental WGM mainly confined in the coating. A further proof of the validity of the approach is obtained assessing the free spectral range of the excited modes which depends on the refractive index of the material in which the mode is confined.
Optical detection techniques based on surface enhanced Raman spectroscopy (SERS) and capable of providing relevant information on molecular and protein composition of biological samples, are gaining rising attention in clinical research as alternatives to traditional detection assays. Meanwhile, due to the technological advances in compact instrumentation as well as in nanofabrication processes, SERS probes based on portable guided-wave systems, have been implemented thus providing for easier accessibility into complex environments and enabling for real-time in situ detection of low concentrated target analytes. In the present study, low-cost fabrication processes were successfully combined with surface functionalization strategies for the fabrication of disposable SERS-active substrates, engineered to tightly fit the distal end of portable Raman instruments. Being based on a polymer casting fabrication process, the overall design of the substrates can be easily adapted to the varied geometry of the probes to be fit, thus guaranteeing high design versatility. SERS-functionality was achieved by immobilization of gold nanoparticles whose size and shape directly affect the plasmonic properties of the substrates. Moreover, SERS substrates can be further modified by covalently binding molecules acting as baits to selectively fish target biomarkers within heterogeneous samples thereby increasing the specificity of SERS signals. Finally, these sensors represent a powerful tool potentially implementable for the early diagnosis of widespread pathologies by real-time SERS analysis of liquid biopsy.
We engineered an effective GO/Ag substrate with a highly controlled and uniform bilayer structure. The substrate was obtained by fine-tuning in the Ag nanocubes (AgNCs) self-assembly, followed by a controlled adsorption of GO sheets on the silver layer. Here the GO coating supplies the signal with additional amplification. The developed GO/AgNCs assemblies show a highly uniform signal distribution, which is ascribed to a homogeneous and soft arrangement of GO sheets over the AgNCs surface. The above characteristics make the GO/AgNCs substrate a reliable tool for detecting different compounds ranging from small molecules to complex biomolecules.
KEYWORDS: Near field scanning optical microscopy, Atomic force microscopy, Near field optics, Optical microscopy, Imaging systems, Photodiodes, Atomic force microscope, Liquids, Transmittance, Optical fibers
Fiber-top and ferrule-top cantilevers (FTC) are a new generation of all optical, monolithic, self-aligned microdevices.
They are obtained by carving a cantilever on the cleaved end of an optical fiber (fiber-top) or on a ferrule terminated
fiber (ferrule-top). FTCs rely on Fabry-Perot interferometry to measure the deflection of the cantilever with
subnanometer deflection sensitivity. FTCs specially developed for scanning probe microscopy are equipped with a sharp
tip that has the dual function of probing the topography and collecting/emitting light. We perform the scanning probe
microscopy using these probes in air, liquid and at low temperature (12°K). The light emission/collection functionality of
FTC probes also allows one to combine scanning near field optical microscopy (SNOM) and optical transmission
microscopy with contact and non-contact mode atomic force microscopy (AFM). This makes FTCs ideal for
AFM+SNOM on soft samples, polymers and biological specimen, where bent fiber probes and tuning fork based
systems would not be recommended because of the high stiffness of those probes. We demonstrate here the capability of
fiber-top cantilevers to measure deflection and collect near field optical signal, and also the capability of ferrule-top
cantilevers for simultaneous optical transmission microscopy and topography of SNOM gratings. Thanks to their unique
features, FTCs also open up possibilities for UV nanolithography and on-demand optical excitation at nanoscale.
Homogeneous polymeric thin layers have been used as functionalising agents on silica microresonators in view of
immunosensing applications. We have characterised the microspheres functionalised with poly-L-lactic acid and
Eudragit® L100, as an alternative to the commonly used silanes. It is shown that after polymeric functionalization the
quality factor of the silica microspheres remains around 107, and that the Q factor is still about 3x105 after chemical
activation and covalent binding of immunogammaglobulin. This functionalising process of the microresonator
constitutes a promising step towards the achievement of a highly sensitive immunosensor.
The structural modifications in the stromal matrix induced by low-temperature corneal laser welding were investigated by atomic force microscopy (AFM). This procedure consists of staining the wound with Indocyanine Green (ICG), followed by irradiation with a near-infrared laser operated at low-power densities. This induces a local heating in the 55-65 °C range. In welded tissue, extracellular components undergo heat-induced structural modifications, resulting in a joining effect between the cut edges. However, the exact mechanism generating the welding, to date, is not completely understood.
Full-thickness cuts, 3.5 mm in length, were made in fresh porcine cornea samples, and these were then subjected to laser welding operated at 16.7 W/cm2 power density. AFM imaging was performed on resin-embedded semi-thin slices once they had been cleared by chemical etching, in order to expose the stromal bulk of the tissue within the section. We then carried out a morphological analysis of characteristic fibrillar features in the laser-treated and control samples.
AFM images of control stromal regions highlighted well-organized collagen fibrils (36.2 ± 8.7 nm in size) running parallel to each other as in a typical lamellar domain. The fibrils exhibited a beaded pattern with a 22-39 nm axial periodicity. Laser-treated corneal regions were characterized by a significant disorganization of the intralamellar architecture. At the weld site, groups of interwoven fibrils joined the cut edges, showing structural properties that were fully comparable with those of control regions. This suggested that fibrillar collagen is not denatured by low-temperature laser welding, confirming previous transmission electron microscopy (TEM) observations, and thus it is probably not involved in the closure mechanism of corneal cuts. The loss of fibrillar organization may be related to some structural modifications in some interfibrillar substance as proteoglycans or collagen VI.
Furthermore, AFM imaging was demonstrated to be a suitable tool for attaining three-dimensional information on the fibrillar assembly of corneal stroma. The results suggested that AFM analyses of resin-embedded histological sections subjected to chemical etching provide a rapid and cost-effective response, with an imaging resolution that is quite similar to that of TEM.
Nonlinear optical microscopy is a relatively new and rapidly growing field of optical engineering, where Ti:sapphire ultrafast laser sources and technologies are finding a wide application. Diagnostic techniques addressed to this kind of application have been widely developed in the last few years. Research efforts have been focused on the evaluation and eventual correction of laser pulse duration widening due to group velocity dispersion of microscope optics, and devices have been specially designed to perform second-order autocorrelation measurements at the objective focal plane. In the present work, innovative, simple setups and procedures are reported that make the best use of all the facilities and characteristics of the microscope itself, so that only a few optical components are needed to temporal characterize the laser pulse at the specimen plane.
Photonic crystals are attractive optical materials for controlling and manipulating light. They are of great interest for both fundamental and applied research, and are expected to find commercial applications soon. In this work digital holography, white light interferometry and atomic force microscopy have been applied to the inspection and characterization of 1D and 2D nanofabricated LiN photonic crystals. Periodic pattern with periods ranging form several microns to a fraction of micron have been accurately analysed. Optical methods allow exploring relatively large areas while atomic force microscopy is well suited for high-resolution inspection of the small features.
We report on the fabrication and characterization of the first periodic sub-micron scale one- and two-dimensional surface structures in congruent 500 μm thick lithium niobate crystal samples. Structures with periods from 2 μm down to 500 nm, lateral feature sizes down to 200 nm and depths around 10 μm, largely compatible with conventional waveguide fabrication, have been obtained. Such structures are fabricated by selective wet etching of ferroelectric domain engineered samples obtained by electric field poling performed at an overpoling regime. Holographic lithography is here used to obtain sub-micron periodic insulating gratings to be used for selective ferroelectric domain reversal. The short-pitch fabricated structures are attractive in a wide range of applications, such as nonlinear short-wavelength conversion processes, backward second-harmonic generation, fabrication of novel tunable photonic crystal (PC) devices, electro-optically modulated Bragg gratings. Moreover moire beating effect is used in the photolithographic process to fabricate even more complex structures which could find applications in complicated photonic bandgap devices involving for example micro-ring resonators. In order to investigate the possibility to utilize these structures for PC applications, accurate and complete topographic characterization has been performed by using different techniques. Atomic force microscope provides high-resolution information about the lateral and depth feature size of the structures. Interferometric techniques, based on digital holography, have been used for wide field information about the homogeneity and periodicity of the structures.
Multiphoton microscopy is a relatively new and rapidly growing field of applied optics where Ti:Sapphire ultrafast laser sources and related technology find a wide application. Laser beam diagnostic techniques specially devoted to this kind of application has been widely developed in these last years. Research efforts have been addressed to the evaluation and the eventual correction of the laser pulse duration widening due to group velocity dispersion caused by the microscope optics. Temporal characterization is thus a fundamental task when operating a ultrafast laser system for multiphoton microscopy applications and it is carried out by means of autocorrelators specially designed to perform pulse width measurements at the focus of the microscope objective. In the present communication, an innovative autocorrelator set-up and a simple metrological procedure are reported.
High energy cosmic rays and neutrinos may be detected by observing the fluorescence showers induced after interaction with Earth's atmosphere. A high energy cosmic rays observatory would benefit from being lifted into space as a larger portion of atmosphere will be observable. Such a system should have a better performance than existing and future ground based observatories, detecting up to 103 - 104 events per year. However, only a system with large field of view, and large collecting aperture can achieve the requested high sensitivity and acceptable event statistics. Several optical designs for the optics of a cosmic ray space observatory have been proposed so far. Amongst them, the Schmidt telescope, one of the best known reflectors, well matches both those characteristics, and appears as an appropriate solution to solve the problem.
Planar waveguides have been realized in lithium fluoride crystals by ion-beam irradiation. Ion bombardment produces color centers in the LiF crystal, increasing locally the refractive index. Confocal microscopy is applied to the characterization of the waveguides in order to assess the uniformity and distribution of color centers through the measurement of the photoluminescence emission.
Optical waveguides in lithium fluoride (LiF) crystals have been obtained by He+ ion beam irradiation. The waveguides
have been characterized by several techniques. In particular, we describe here the application of confocal microscopy to their characterization and show the first results obtained. We have also carried out a preliminary evaluation of the potential of this technique for the assessment of structural and spectroscopic characteristics of the waveguides.
An optical system consisting of a reflecting mirror with a Schmidt corrector plate is presented as a possible design of a space-based observatory for high energy (up to 1020 eV) cosmic rays, by monitoring the fluorescence showers induced after interaction by cosmic rays with the Earth atmosphere. An instrument of that kind is currently into the evaluation phase as an external payload for the International Space Station. The basic requirements demand a system with large field of view, up to ±30°, and large collecting aperture, ≥2 m diameter, to achieve a sufficient sensitivity and event statistics. Among several possible optical systems for this purpose, the Schmidt camera is the simplest, matching most of the optical technical requirements, with some problem for the obscuration due to the focal plane at such extreme field of view. This paper presents ray-tracing simulations for different designs of large aperture (> 2m) Schmidt cameras with FOV from 40° to 50°, with F/# ≈ 0.7 and ground resolution from 1 to 2 km from a LEO. Better performances are achieved with an aspheric mirror, but performances using of a spherical mirror are acceptable with some compromise in resolution. The overall geometrical transmission ranges from 40% to 78%, according to the selected geometry and FOV. Possible technologies for the construction of the main mirror and all other components, including supporting mechanics will be also discussed.
The basic elements of a fairly complete optomechanical kit based on the use of LEGOTM is presented. Through a careful exploitation of the many standard LEGO elements, and adding a few new simple components made of plexiglass, we demonstrate that almost all of the mechanical parts of an optical setup can be built with little effort and at an extremely reduced cost. Several systems and experiments are presented, mainly in the fields of optical filtering and interferometry, to show that the proposed mountings are perfectly suitable for didactic purposes, and can often be employed even in more demanding scientific applications.
An optical configuration for the detection of faults was developed and tested. The optical fourier transformation is the basic working principle of the system. When good fabric passes in front of the optical system the Fourier image, captured by the camera, shows well defined spots corresponding to the spatial frequencies of the tissue. If a defect occurs during production on the loom, the pattern changes significantly and a defect is easily detected in real time. A very simple electronic image processing based on thresholding and binary histograms allows to obtain very encouraging performance for its applicability to the looms. A compact device has been realized and tested in real working conditions on the loom.
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