Cationic-induced two-photon photo-polymerization is demonstrated at 710 nm, using an isopropylthioxanthone / diarylidonium salt initiating system for the cationic polymerization of an epoxide. In-situ monitoring of the polymer conversion using interferometry allows for determination of the polymerization threshold J2th, polymerization rate R and its dependence of initiator's concentration z. Best J2th achieved is 1 GW/cm 2 , with a dynamic range of > 100, i.e. the material can be fully polymerized at intensities > 100 times the threshold level without damage. The R is found to be proportional to the m=1.7 power of the intensity, or R =[C(J-J2th)]m =[C(J-J2th)]1.7 , which implies a significantly stronger localization of the photochemical response than that of free radical photoinitiators. Both R and J2th significantly improve when the concentration z of the initiator (onium salt) increases, reduction of J2th exhibiting z -m trend.

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We describe an instrumented biopsy needle that provides physicians the capability to sense interaction forces directly at the tip of the needle’s inner stylet. The sensors consist of optical fiber Bragg gratings (FBGs), and are unaffected by electromagnetic fields; hence the needle is suitable for MR-guided procedures. In comparison to previous instrumented needles that measure bending strains, the new design has additional sensors and a series of micro-machined holes at the tip. The holes increase strain sensitivity, especially to axial forces, without significantly reducing the stiffness or strength. A comparison of the dynamic forces measured with the new needle and those obtained using a force/torque sensor at the needle base shows that the enhanced tip sensitivity is particularly noticeable when there is significant friction along the needle sleeve.

In this paper, we present a technology that can be used for random accessed mass optical data storage. The technology uses a two-photon recordable plastic disk medium that can fluoresce when recorded and that can support multiple stored data layers inside one disk. This technology is capable of achieving 200GB data capacity with a 120mm diameter, 10mm thick disk, and a data transfer rate of Gigabits/Sec by using parallel readout. IntroductionSince the amount of digital information is growing very fast and access to this information is made via faster and faster networks, data storage systems needs to keep pace with this explosive growth. High capacity data storage devices with high-speed data transfer rate and capable of random access are already in high demand. One approach to satisfy this demand is to increase the areal storage density. Unfortunately, for many of the conventional optical storage technologies this is becoming increasingly costly and technically challenging. Another direction to increase the data capacity is to stack multiple layers in one disk. This requires a new means of recording data in multiple layers closely packed together but with minimum interlayer crosstalk. The two-photon absorption based recording approach pioneered by Call/Recall Inc. uses the volume of suitable disk media enabling recording of highly packed multiple data layers at desired locations achieving very high volumetric densities [1,2,3,4,5]. Call/Recall Inc. has developed several kinds of two-photon recordable fluorescent WORM (Write Once Read Many) media. When recorded by two-photon absorption the recorded bits fluoresce when excited by a suitable readout laser beam. The recorded area in our materials is non-reflective and has no noticeable index change for both readout and recording enabling optical beams to access the volume of the disk without significant aberrations and losses. These properties allow the recording and readout beam to access multiple layers even in parallel. We should note that there are also possible approaches that use the index change [6,7,9] or photon bleaching [8] to store information by twophoton absorption but these approaches generally limit optical access to only a few layers. This paper presents the recording and readout systems and associated performances for single channel and parallel channels implementations.

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