Alessandro Carpentiero

Novel fabrication method for three-dimensional nanostructuring: an application to micro-optics

We propose a 3D micro and nanofabrication method with potential applications to several nanotechnology-related fields. Our approach is based on the combination of lithographic steps and isotropic wet etchings performed on a quartz or glass substrate to form 3D structures with very accurate shape control and nanometer scale surface roughness. The resulting concavities at the quartz surface are converted into convex plastic elements by hot embossing or casting techniques. Complex all-polymer refractive optical elements have been realized by this method. Upon illumination, such micro-optics focus the light into predetermined 3D distributions of focal lines and spots. The general fabrication scheme explored here is illustrated through a series of examples in optics, but is expected to offer new solutions to other fields such as medicine, microfluidics and nano-optics.

Three-dimensional digital scanner based on micromachined micromirror for the metrological measurement of the human ear canal

Manufacturers of hearing aids have made initial testing of rapid prototyping of hearing aid shells using laser scans of ear impressions, but they have not performed any actual scans of the human ear canal. We report the direct scanning of the human external auditory canal by using an electromagnetically actuated torsion micromirror fabricated by using a micromachining technique as the scanner. This demonstrates the actual scanning of the human external auditory canal by a single integral microelectro-optical-mechanical system (MEOMS). A prototype three-dimensional (3D) scanning system was developed: It is based on the acquisition of optical range data by a conoscopic holographic laser interferometer using an electromagnetically actuated scanning MEOMS micromirror. A fabrication process, based on a poly(methylmethacrylate) sacrificial layer for the fabrication of a free-standing micromirror was used. Micromirror actuation was achieved by using a magnetic field generated with an electromagnetic coil stick. ...

SnO2 lithographic processing for nanopatterned gas sensors

Tin dioxide (SnO2) is widely used as sensing material in metal-oxides gas sensors. In this work we present two lithographic approaches for SnO2 patterning, an additive process and a subtractive one. In the first case patterns of SnO2 nanowires are successfully fabricated and exploited as sensing element in working devices; responses to several testing gases are satisfactorily improved with respect to continuous film devices. Regarding the subtractive process, we present reactive ion etching of SnO2 based on CF4∕H2 gas mixture. Dependence of etch rate upon H2 concentration and effects due to Ar additions to plasma are investigated; results are discussed and a possible etching reaction is proposed, but further developments are required to increase the etch rate.

Cross beam lithography (FIB+EBL) and dip pen nanolithography for nanoparticle conductivity measurements

Focused ion beam lithography is a very powerful technique for directly writing patterns on many substrates, it is a maskless and resistless technique that allows a very wide range of applications, providing a resolution down to 10nm. Using a system composed by a 30keV gallium ion beam column plus a 30keV electron beam, nanogaps for electrical measurements of nanoparticle were fabricated with a resolution down to the nanometer scale, by exploiting FIB milling (FIBM) and electron beam lithography (EBL). Starting from prepatterned samples a square pattern reduces the width of the gold wire and a narrow line pattern opens a gap of less than 7nm. Electrical measurements and AFM tapping mode imaging were performed on the gaps. We patterned the ends of the gold leads with dip pen nanolithography using mercapto-undecanol (MUD) to form a bond between the nanoparticle and the alcohol group attached to the gold surface. After this assembly, devices showed an increase in conductivity (10–100-fold increase). Measuring...

Fabrication of three-dimensional stamps for embossing techniques by lithographically controlled isotropic wet etching

The excellent characteristics that nanoimprint lithography (NIL) shows as a two-dimensional patterning technique, such as resolution and throughput, are shared with its three-dimensional (3D) structuring version. Nevertheless, the potentialities of NIL for 3D structuring remain largely unexplored and unexploited, hindered by the difficulties in the fabrication of high quality masters with throughput suitable for practical purposes. We present a technique that allows production of stamps with tightly controlled three-dimensional profiles in the 10nm–100μm scale range. This technique consists of a sequence of lithographic steps alternated with isotropic wet etching processes performed on a quartz substrate. Scanning electron microscopy and atomic force microscopy characterization shows that 3D structures with very accurate shape control and nanometer scale surface roughness can be formed with this method. Quartz stamps have been employed in nanoimprint, hot embossing, or casting processes to shape complex p...

Electromagnetically Actuated Surface Micromachined Free Standing Torsion Beam Micromirror Made by Electroplated Nickel

In this paper we report extremely simple 2-mask exposure fabrication process using photoresist as sacrificial layer to fabricate electromagnetically actuated torsion beam micromirror. The important feature of this reported include that the torsion beam and micromirror body are made of low stress electroplated nickel, and the torsion beam micromirror is resting on a free standing structural frame also made of electroplated copper. This device structure has resulted in a simplified fabrication process. The electroplated, low stress nickel piece having a dimension 1200×1400×4 µm3 constitutes the mirror body and is used as reflective surface as well for magnetic interaction with magnetic field. Electromagnetic actuation in mirror is realized by control of current through a set of 500 turns copper coils on 7 cm long iron pole to which free standing mirror fixture was integrated. This structure is capable to have bi-directional actuation and a maximum deflection >24° has been obtained in static mode for the investigated electromagnets coil current. The electromagnetically induced deflection of our torsion mirror were modeled and shown to agree with experimental measurements.

Experimental setup for lensless imaging via soft x-ray resonant scattering.

We have developed a setup for measuring holographically formed interference patterns using an integrated sample-mask design. The direct space image of the sample is obtained via a two-dimensional Fourier transform of the X-ray diffraction pattern. We present the details of our setup, commenting on the influence of geometrical parameters on the imaging capabilities. As an example, we present and discuss the results of test experiments on a patterned Co film.

Tailored spectroscopic and optical properties in rare earth-activated glass-ceramics planar waveguides

Glass ceramic activated by rare earth ions are nanocomposite systems that exhibit specific morphologic, structural and spectroscopic properties allowing to develop interesting new physical concepts, for instance the mechanism related to the transparency, as well as novel photonic devices based on the enhancement of the luminescence. At the state of art the fabrication techniques based on bottom-up and top-down approaches appear to be viable although a specific effort is required to achieve the necessary reliability and reproducibility of the preparation protocols. In particular, the dependence of the final product on the specific parent glass and on the employed synthesis still remain an important task of the research in material science. Glass-ceramic waveguides overcome some of the efficiency problems experienced with conventional waveguides. These two-phase materials are composed of nanocrystals embedded in an amorphous matrix. The respective volume fractions of the crystalline and amorphous phases determine the properties of the glass ceramic. They also represent a valid alternative to widely used glass hosts such as silica as an effective optical medium for light propagation and luminescence enhancement. Looking to application, the enhanced spectroscopic properties typical of glass ceramic in respect to those of the amorphous structures constitute an important point for the development of integrated optics devices, including optical amplifiers, monolithic waveguide laser, novel sensors, coating of spherical microresonators, and up and down converters for solar energy exploitation.

Nanoelectrochemical Immunosensors for Protein Detection

Nanoelectrochemical immunosensors fabricated by templated electrodeposition of gold nanoelectrodes inside the pores of polycarbonate (PC) track-etched membranes, followed by the immobilization of the biorecognition elements on the surrounding PC, have proven high sensitivity and specificity for protein detection. The signal transduction scheme involves a suitable redox mediator added to the sample solution to shuttle electrons from the gold nanoelectrodes to the biorecognition layer, both elements being in strict spatial proximity. Highly improved signal-to-background current ratio, which are peculiar of NEEs with respect to other electrochemical transducers, can be exploited in this way. Two detection schemes were tested: one based on the direct immobilization of the target protein on the PC of the NEE (approach A) and the other based on the immobilisation on PC of an antibody to capture the target protein (approach B). The biorecognition process was completed by adding a primary antibody and a secondary antibody with horse radish peroxidase (HRP) as enzyme label; methylene blue was the redox mediator added to the electrolyte solution. Typical target analytes were single chain fragment variable proteins, for approach A, and trastuzumab (also known as Herceptin®), for approach B. NEE-based capture sensors were tested successfully to detect small amounts of the receptor protein HER2 in biological samples. Finally, motivated by the target of a better control of the geometrical characteristics of ensembles of nanoelectrodes (size, density, geometrical arrangement, and degree of recession), and by the positive results obtained with track-etch membranes of PC from the standpoint of protein immobilization, we demonstrated the fabrication of nanobiosensors by patterning ordered arrays of nanoelectrodes (NEAs) by electron beam lithography (EBL) on polycarbonate. EBL results perfectly suitable for the top-down fabrication of arrays of nanobiosensors on thin PC films deposited on gold coated silicon.

Fiber optic trapping of low-refractive-index particles

Since the low index particles are repelled away from the highest intensity point, trapping them optically requires either a rotating Gaussian beam or optical vortex beams focused by a high numerical microscope objective. However, the short working distance of these microscope objectives puts a limit on the depth at which these particles can be manipulated. Here, we show that axicon like structure built on tip of a single mode optical fiber produces a focused beam that is able to trap low index particles. In fact, in addition to transverse trapping inside the dark conical region surrounded by high intensity ring, axial trapping is possible by the balance of scattering force against the buoyancy of the particles. The low-index particle system consisted of an emulsion of water droplets in acetophenone. When the fiber was kept horizontal, the low index spheres moved away along the beam and thus could be transported by influence of the scattering force. However in the vertical position (or at an angle) of the fiber, the particles could be trapped stably both in transverse and axial directions. Chain of such particles could also be trapped and transported together by translation of the fiber. Using escape force technique, transverse trapping force and thus efficiency for particle in Mie regime was measured. Details of these measurements and theory showed that trapping of Raleigh particle is possible with such axicon-tip fibers. This ability to manipulate low-index spheres inside complex condensed environments using such traps will throw new insights in the understanding of bubble-bubble and bubble-wall interactions, thus probing the physics behind sonoluminescence and exploring new applications in biology and medicine.