Colette Turck

Holographic recording with polymer nanocomposites containing silver nanoparticles photogenerated in situ by the interference pattern

A process allowing in situ photochemically assisted synthesis of silver nanoparticles (NPs), in a polymerizable acrylic formulation was developed and the photopolymerization of this formulation was studied. It was found that the presence of NPs does not perturb the polymerization process in the least, making it possible to fabricate metal/polymer nanocomposites by a one-step-one-pot photochemical process. The size distribution of NPs was very narrow and their size could be controlled by the chemical parameters of the reactive formulation. This system was used to record holographic gratings and it was observed that both the holographic sensitivity and the diffraction efficiency at maximum were significantly improved in the presence of particles. Moreover, TEM analysis (Figure 1) revealed a clear-cut segregation of photochemically generated NPs between dark and bright regions of the incident pattern, thus demonstrating the possibility to manipulate nascent particles with light gradients.

Photogenerating silver nanoparticles and polymer nanocomposites by direct activation in the near infrared

This work reports on an improvement of the photochemically assisted synthesis of silver nanoparticles by direct photoreduction of AgNO3 with a laser source emitting in the near infrared range (NIR). For this, polymethine dyes were used as the photoactive agents. Both the effects of central chain structure and activation intensity were investigated. The reduction kinetics was followed up by UV-Vis spectroscopy, and the particles size was evaluated by transmission electron microscopy. The results showed that light intensity affects both the average size and size distribution of Ag nanoparticles generated through this process. The particles can also be generated in situ in a photopolymerizable formulation so that metal/polymer nanocomposites become available through a one-step photoassisted process on the basis of NIR activation. The process described herein is very fast (seconds to a few minutes), and it readily lends itself to automatization for mass production of micro-optical elements implemented directly onto integrated NIR sources.

A microtip self-written on a vertical-cavity surface-emitting laser by photopolymerization

We present the integration of a self-aligned microtip on a vertical-cavity surface-emitting laser (VCSEL) by near infrared photopolymerization. This one-step fabrication process is triggered by the laser source itself. It is based on the use of photopolymers sensitive at the lasing wavelength and can be applied to VCSEL devices after their process fabrication. We have characterized the fabricated microtips and shown that they focus laser light at few micrometers from the device. The applications of this simple method may concern VCSEL beam shaping as well as the fabrication of microprobes for near-field optical microscopy.

Photopatterning of Dry Polymer Films: on the Relative Involvement of Diffusive and Capillary Convection in the Self-Corrugation Process

Dry poly(vinyl alcohol) (PVA) formulations are widely used in the field of micro-optics for holographic recording and fabrication of micro-optical elements, but very little is known about the recording mechanism and even less in systems generating relief elements through a self-developing process. Thus, the recording of relief gratings with pitches ranging from 5 to 120 l/mm was examined with reference to the average weight of PVA and degree of hydrolysis. The slit width to pitch ratio in the amplitude mask used for photopatterning appeared to be also a controlling factor in the response of sensitive materials. A semiquantitative model was introduced to account for the final shape of the photogenerated relief. It determined the respective parts played by diffusion and capillary convection, i.e., convection processes resulting from gradients of chemical potential and surface tension, respectively, and identified the experimental key parameters governing their interrelation. This work sheds new light on the optimization of the patterning conditions of amplitude masks and is expected to open up new vistas in the replication of computer-generated holograms.

Micro-optics on VCSELs using NIR photopolymers

Laser beam shaping is a key issue for the photonic integration of VCSEL sources. Most of the techniques proposed to integrate micro-optics elements onto VCSEL devices imply either a hybrid assembly or a photolithography step, whose precision limits the accuracy of lens alignment relatively to the VCSEL source. We present here a new method for self-fabrication of microtips on Vertical-Cavity Surface-Emitting Lasers (VCSELs) by means of Near Infra- Red (NIR) photo-polymerization. This approach is based on a single fabrication step, implementing novel photopolymers sensitive at the lasing wavelength. Consequently the process is triggered by the laser source itself and can be applied easily to VCSEL devices during their electro-optic characterization. The method we have developed for tips fabrication is detailed as well as corresponding optical properties. The applications of this new and simple method concern laser light focusing and collimation for integrated micro-systems, coupling to fibers for optical communications as well as novel micro-probes fabrication for near-field optical microscopy.

Holographic recording with a near-IR sensitive photopolymer system using a laser diode

Photopolymerizable formulations exhibiting a fair sensitivity in the near IR were developed. These materials were formulated of a mixture of acrylate multifunctional monomers and oligomers with an initiating system working through a photoredox process. Depending on the spectrum of the sensitizers, this system absorbs various laser lines in the red (Kr+ at 676, 752 and 799 nm or laser diode at (lambda) > 750 nm). In particular, it was holographed with a monomode laser diode emitting in the 780 nm region, and bright transmission gratings were obtained. Typically, an exposure of ca. 500 mJ/cm2 is necessary to reach complete inactivation of the material and to produce gratings with diffraction efficiency exceeding 40% with a spatial frequency of 800 lines/mm. The quality of the holographic recording was studied by recording multiple series of holograms of a resolution test target. In addition, these materials that contain crosslinkable monomers and convert to a tridimensional network upon irradiation, show a self-processing character allowing storage of optical information in situ. The applicability of this material to real time holographic interferometry was demonstrated.