Abstract |
DLW by multiphoton polymerization (MPP) has become a powerful tool for the
fabrication of fully three-dimensional micro- and nano-structures for microfluidic,
biomedical, metamaterial, and photonic applications. In DLW, the beam of an ultrafast
laser is tightly focused into the volume of a photosensitive material, initiating
multiphoton polymerization within the focused beam volume (namely, voxel). By
moving the focus of the beam in the three-dimensions, arbitrary 3D, high-resolution
structures can be written into the volume of the material. By simply immersing the
sample in an appropriate solvent, the non-polymerized area can be dissolved, revealing
the 3D structure.
The materials developed in this work are photostructurable organic-inorganic hybrid
materials, prepared using the sol-gel process. This versatile technique has been
exploited for the incorporation of inorganic networks into polymer matrices, using as
monomers molecules that carry an inorganic part (which serves as the precursor to the
inorganic network) and a polymerizable organic group (which acts as the precursor to
the organic polymer). For the fabrication of 3D structures by DLW, it is often necessary
to add a photoinitiator, that is a molecule which, upon multiphoton absorption,
generates the active species which initiate the polymerization process.
In this work, we present for the first time the fabrication of excellent quality 3D
structures by photoinitiator-free multiphoton polymerization. The process relies on the
synthesis of a novel vanadium-based hybrid material, containing vanadium (V)
triisopropoxide oxide, which self-generates radicals via a light-induced redox reaction.
In particular, upon multiphoton absorption the composite generate radicals by the
photoinduced reduction of vanadium (V) to vanadium (IV). We exploit this material
for the fabrication of fully 3D structures by multiphoton polymerization with 200 nm
resolution, employing a femtosecond laser operating at 800 nm, in the absence of a
photoinitiator. Nonlinear absorption measurements indicate that the use of a 800 nm
laser initiates the photopolymerization due to a three-photon absorption of the
vanadium alkoxide. The laser power required to induce this three-photon process is
comparable to that required for inducing two-photon polymerization in materials using
standard two-photon absorbers, most likely due to the high content of vanadium in the
final composite (up to 50% mole). In the second part of the present study hybrid organic-inorganic materials were
modified by the addition of a quantum dot precursor molecule, which becomes
chemically attached onto the fabricated 3D structures during the photo-polymerization
process. Next, the 3D structures are reacted with sodium sulfide (Na2S) to form CdS
quantum dots within the structures. Such semiconductor nanoparticles enrich the
fabricated structures with third order non-linear properties. 3D printed active photonic
devices, of a woodpile geometry with an inlayer periodicity as low as 500 nm, are
successfully fabricated at high resolution and exhibit clear photonic stop bands in the
visible spectral region, while for the first time, evidence of ultrafast dynamic tuning of
the photonic band gap properties in the visible, is also demonstrated.
In the final part of this thesis, pre-synthesized highly fluorescent CdSe-CdS quantum
dots, bearing appropriate functionalities, were permanently bound onto the surface of
3D photonic crystal structures, following chemical functionalization of the surface of
the particles or the surface of the 3D structures. Woodpile 3D photonic crystal
structures, with an inlayer periodicity of 550 nm, were fabricated, using the Direct Laser
Writing technique, exhibiting photonic stopgaps at visible wavelengths. Next, the
structures were coated with the synthesized quantum dots that can act as a gain medium.
Near the band edge of these gaps, the group velocity, υg, of photons localized in the
structure, approaches zero and photons undergoing multiple reflections in the lattice
experience longer interaction with the gain material, thus resulting in an enhanced
effective gain. By matching the photoluminescence of the nanoparticles with the
bandedges of the photonic crystals a a functional device that can act as a nanolaser was
developed.
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