Ioanna Sakellari

Ultra-Low Shrinkage Hybrid Photosensitive Material for Two-Photon Polymerization Microfabrication

Investigations into the structuring by two-photon polymerization of a nonshrinking, photosensitive, zirconium sol-gel material are presented. This hybrid material can be photostructured even when it contains up to 30 mol % of zirconium propoxide (ZPO); by varying the materials inorganic content, it is possible to modify and tune its refractive index. The introduction of ZPO significantly increases the photosensitivity of the resulting photopolymer. The fabricated three-dimensional photonic crystal structures demonstrate high resolution and a clear band-stop in the near-IR region. In contrast to common practice, no additional effort is required to precompensate for shrinkage or to improve the structural stability of the fabricated photonic crystals; this, combined with the possibility of tuning this materials optical, mechanical, and chemical properties, makes it suitable for a variety of applications by two-photon polymerization manufacturing.

Femtosecond laser polymerization of hybrid/integrated micro-optical elements and their characterization

The femtosecond laser-induced multi-photon polymerization of a zirconium–silicon based sol–gel photopolymer was employed for the fabrication of a series of micro-optical elements with single and combined optical functions: convex and Fresnel lenses, gratings, solid immersion lenses on a glass slide and on the tip of an optical fiber. The microlenses were produced as polymer caps of varying radii from 10 to 90xa0 µm. The matching of refractive indices between the polymer and substrate was exploited for the creation of composite glass-resist structures which functioned as single lenses. Using this principle, solid immersion lenses were fabricated and their performance demonstrated. The magnification of the composite solid immersion lenses corresponded to the calculated values. The surface roughness of the lenses was below ~ 30xa0nm, acceptable for optical applications in the visible range. In addition, the integration of micro-optical elements onto the tip of an optical fiber was demonstrated. To increase the efficiency of the 3D laser polymerization, the lenses were formed by scanning only the outer shell and polymerizing the interior by exposure to UV light.

Diffusion-Assisted High-Resolution Direct Femtosecond Laser Writing

We present a new method for increasing the resolution of direct femtosecond laser writing by multiphoton polymerization, based on quencher diffusion. This method relies on the combination of a mobile quenching molecule with a slow laser scanning speed, allowing the diffusion of the quencher in the scanned area and the depletion of the multiphoton-generated radicals. The material we use is an organic-inorganic hybrid, while the quencher is a photopolymerizable amine-based monomer which is bound on the polymer backbone upon fabrication of the structures. We use this method to fabricate woodpile structures with a 400 nm intralayer period. This is comparable to the results produced by direct laser writing based on stimulated-emission-depletion microscopy, the method considered today as state-of-the-art in 3D structure fabrication. We optically characterize these woodpiles to show that they exhibit well-ordered diffraction patterns and stopgaps down to near-infrared wavelengths. Finally, we model the quencher diffusion, and we show that radical inhibition is responsible for the increased resolution.

Three-Dimensional Metallic Photonic Crystals with Optical Bandgaps

The fabrication of fully three-dimensional photonic crystals with a bandgap at optical wavelengths is demonstrated by way of direct femtosecond laser writing of an organic-inorganic hybrid material with metal-binding moieties, and selective silver coating using electroless plating. The crystals have 600-nm intralayer periodicity and sub-100 nm features, and they exhibit well-defined diffraction patterns.

Two-Photon Polymerization of Hybrid Sol-Gel Materials for Photonics Applications

Two-photon polymerization of photosensitive materials has emerged as a very promising technique for the fabrication of photonic crystals and devices. We present our investigations into the structuring by two-photon polymerization of a new class of photosensitive sol-gel composites exhibiting ultra-low shrinkage. We particularly focus on two composites, the first containing a zirconium alkoxide and the second a nonlinear optical chromophore. The three-dimensional photonic crystal structures fabricated using these materials demonstrate high resolution and clear bandstops in the near IR region.

Directed dewetting of amorphous silicon film by a donut-shaped laser pulse

Irradiation of a thin film with a beam-shaped laser is proposed to achieve site-selectively controlled dewetting of the film into nanoscale structures. As a proof of concept, the laser-directed dewetting of an amorphous silicon thin film on a glass substrate is demonstrated using a donut-shaped laser beam. Upon irradiation of a single laser pulse, the silicon film melts and dewets on the substrate surface. The irradiation with the donut beam induces an unconventional lateral temperature profile in the film, leading to thermocapillary-induced transport of the molten silicon to the center of the beam spot. Upon solidification, the ultrathin amorphous silicon film is transformed to a crystalline silicon nanodome of increased height. This morphological change enables further dimensional reduction of the nanodome as well as removal of the surrounding film material by isotropic silicon etching. These results suggest that laser-based dewetting of thin films can be an effective way for scalable manufacturing of patterned nanostructures.

3D Photonic Nanostructures via Diffusion-Assisted Direct fs Laser Writing

We present our research into the fabrication of fully three-dimensional metallic nanostructures using diffusion-assisted direct laser writing, a technique which employs quencher diffusion to fabricate structures with resolution beyond the diffraction limit. We have made dielectric 3D nanostructures by multiphoton polymerization using a metal-binding organic-inorganic hybrid material, and we covered them with silver using selective electroless plating. We have used this method to make spirals and woodpiles with 600u2009nm intralayer periodicity. The resulting photonic nanostructures have a smooth metallic surface and exhibit well-defined diffraction spectra, indicating good fabrication quality and internal periodicity. In addition, we have made dielectric woodpile structures decorated with gold nanoparticles. Our results show that diffusion-assisted direct laser writing and selective electroless plating can be combined to form a viable route for the fabrication of 3D dielectric and metallic photonic nanostructures.

Quantum dot based 3D photonic devices

In this work, we present our most recent results on the fabrication of 3D high-resolution woodpile photonic crystals containing an organic-inorganic silicon-zirconium (Si-Zr) composite and cadmium sulfide (CdS) quantum dots (QDs). The structures are fabricated by combining 3D Direct Laser Writing by two-photon absorption and in-situ synthesis of CdS nanoparticles inside the 3D photonic matrix. The CdS-Zr-Si composite material exhibits a high nonlinear refractive index value measured by means of Z-scan method. 3D woodpile photonic structures with varying inlayer periodicity from 600nm to 500nm show clear photonic stop bands in the wavelength region between 1000nm to 450nm.

3D active photonic nanostructures

We present our most recent results on the fabrication of 3D high-resolution nanostructures containing Cadmium Sulfide (CdS) quantum dots (QDs) and exhibiting higher order diffraction patterns and stop-gaps at visible wavelengths. These are fabricated using direct laser writing (DLW) and novel, organic-inorganic hybrid materials.

Direct laser writing of gain and metallic nanostructures

We present our investigations on the fabrication of three-dimensional nanostructures by direct laser writing using organic-inorganic hybrid materials that (i) can be structured accurately to sub-100nm, (ii) contain quantum dots, and (iii) can be selectively covered with metal.