Camilo Florian

Direct Laser Printing of Tailored Polymeric Microlenses

We report a laser-based approach for the fast fabrication of high-optical-quality polymeric microlenses and microlens arrays with controllable geometry and size. Our strategy consists of the direct laser printing of microdroplets of a highly viscous UV prepolymer at targeted positions, followed by photocuring. We study the morphological characteristics and imaging performance of the microlenses as a function of the substrate and laser parameters and investigate optimal printing conditions and printing mechanisms. We show that the microlens size and focusing properties can be easily tuned by the laser pulse energy, with minimum volumes below 20 fL and focal lengths ranging from 7 to 50 μm.

Sub-wavelength Laser Nanopatterning using Droplet Lenses

When a drop of liquid falls onto a screen, e.g. a cell phone, the pixels lying underneath appear magnified. This lensing effect is a combination of the curvature and refractive index of the liquid droplet. Here, the spontaneous formation of such lenses is exploited to overcome the diffraction limit of a conventional laser direct-writing system. In particular, micro-droplets are first laser-printed at user-defined locations on a surface and they are later used as lenses to focus the same laser beam. Under conditions described herein, nanopatterns can be obtained with a reduction in spot size primarily limited by the refractive index of the liquid. This all-optics approach is demonstrated by writing arbitrary patterns with a feature size around 280 nm, about one fourth of the processing wavelength.

Fabrication of amorphous micro-ring arrays in crystalline silicon using ultrashort laser pulses

We demonstrate a simple way to fabricate amorphous micro-rings in crystalline silicon using direct laser writing. This method is based on the fact that the phase of a thin surface layer can be changed into the amorphous phase by irradiation with a few ultrashort laser pulses (800 nm wavelength and 100 fs duration). Surface-depressed amorphous rings with a central crystalline disk can be fabricated without the need for beam shaping, featuring attractive optical, topographical, and electrical properties. The underlying formation mechanism and phase change pathway have been investigated by means of fs-resolved microscopy, identifying fluence-dependent melting and solidification dynamics of the material as the responsible mechanism. We demonstrate that the lateral dimensions of the rings can be scaled and that the rings can be stitched together, forming extended arrays of structures not limited to annular shapes. This technique and the resulting structures may find applications in a variety of fields such as ...

Coherent scatter-controlled phase-change grating structures in silicon using femtosecond laser pulses

Periodic structures of alternating amorphous-crystalline fringes have been fabricated in silicon using repetitive femtosecond laser exposure (800 nm wavelength and 120 fs duration). The method is based on the interference of the incident laser light with far- and near-field scattered light, leading to local melting at the interference maxima, as demonstrated by femtosecond microscopy. Exploiting this strategy, lines of highly regular amorphous fringes can be written. The fringes have been characterized in detail using optical microscopy combined modelling, which enables a determination of the three-dimensional shape of individual fringes. 2D micro-Raman spectroscopy reveals that the space between amorphous fringes remains crystalline. We demonstrate that the fringe period can be tuned over a range of 410 nm – 13 µm by changing the angle of incidence and inverting the beam scan direction. Fine control over the lateral dimensions, thickness, surface depression and optical contrast of the fringes is obtained via adjustment of pulse number, fluence and spot size. Large-area, highly homogeneous gratings composed of amorphous fringes with micrometer width and millimeter length can readily be fabricated. The here presented fabrication technique is expected to have applications in the fields of optics, nanoelectronics, and mechatronics and should be applicable to other materials.

Author Correction: Coherent scatter-controlled phase-change grating structures in silicon using femtosecond laser pulses

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Optical spectroscopy study of nano- and microstructures fabricated by femtosecond laser pulses on ZnO based systems

The formation of laser induced periodic surface structures (LIPSS) upon irradiation with ultrashort laser pulses at the surface of polycrystalline ZnO based samples, and the potential use of irradiated areas as growth patterns for the production of highly ordered nanostructures have been studied. In particular, pure ZnO, ZnO:Al (5 wt% Al2O3) and ZnO:Mg (5 wt% MgO) samples have been investigated. The surface morphology of the laser fabricated structures depends on the processing parameters, such as energy, number of pulses, repetition rate and laser polarization. Three main types of periodic structures have been formed: LSF (low spatial frequency) and HSF (high spatial frequency) LIPSS and pseudo-spikes (PS). After irradiation, the samples have been used as substrates for vapor–solid growth. It has been found that the growth of micro- and nanostructures in the irradiated regions is much faster than in the non-irradiated ones. Strong differences in the final morphology of the deposited regions are appreciated, not only between irradiated and non-irradiated areas, but also between areas irradiated under different conditions. Scanning electron microscopy (SEM) and cathodoluminescence (CL) measurements at room temperature have been used to monitor the morphology and luminescence properties of the generated structures. For all processed samples, the CL analysis shows a shift of the band edge emission towards higher energies and a lower relative intensity of the defect band in the areas irradiated at higher fluence. The first effect might be attributed to the incorporation of dopants, as already observed for Al and Mg. Yet, the redistribution of defects associated with shallow levels might also be responsible for the observed shift, playing a major role in undoped samples. The decrease in the relative intensity of the defect band can be ascribed to the partial recovery of defects caused by the deposited energy (annealing effect) during the irradiation process.

Controlling the wettability of steel surfaces processed with femtosecond laser pulses

The wettability of a material surface is an essential property that can define the range of applications it can be used for. In the particular case of steel, industrial applications are countless but sometimes limited because of the lack of control over its surface properties. Although different strategies have been proposed to tune the wetting behavior of metal surfaces, most of them require the use of processes such as coatings with different materials or plasma/chemical etching. In this work, we present two different laser-based direct-write strategies that allow tuning the wetting properties of 1.7131 steel over a wide range of contact angles using a high repetition rate femtosecond laser. The strategy consists in the writing of parallel and crossed lines with variable spacing. A detailed morphological analysis confirmed the formation of microstructures superimposed with nanofeatures, forming a hierarchical surface topography that influences the wetting properties of the material surface. Contact angle measurements with water confirm that this behavior is mostly dependent on the line-to-line spacing and the polarization-dependent orientation of the structures. Moreover, we demonstrate that the structures can be easily replicated in a polymer using a laser-fabricated steel master, which enables low-cost mass production. These findings provide a practical route for developing user-defined wetting control for new applications of steel and other materials functionalized by rapid laser structuring.

Biomimetic structures on steel via self-organization processes in multiple-scan, fs-laser irradiated surfaces

Weakly focused fs-pulses have the potential to produce nanometer structures that self-organize under certain irradiation conditions [1]. Such structures, known as LIPSS [2] (laser induced periodic surface structures), are potential candidates for mimicking surface textures found in nature, on materials of technological interest. This enables surface functionalization for a variety of applications including wettability, optical response, and wear resistance [3, 4].

Fabrication of amorphous-crystalline micro- and nanostructures in silicon using ultrashort laser pulses

Silicon is one of the key-materials of nowadays technology. This is partly related to its special phase change dynamics characteristics. In particular, upon pulsed laser irradiation, silicon can re-solidify either in the crystalline (c-) or amorphous (a-) phase depending on the local supercooling achieved [1]. This feature has recently been used to produce non-ablative LIPSS (Laser Induced Periodic Surface Structures [2]) in crystalline silicon upon multiple laser pulse exposure. The induced structures consist of a-c-fringes with subwavelength period [3].