A novel platform based on evanescent wave sensing in the 6.5 to 7.5 µm wavelength range is presented with the example of toluene detection in an aqueous solution. The overall sensing platform consists of a germanium-on-silicon waveguide with a functionalized mesoporous silica cladding and integrated microlenses for alignment-tolerant back-side optical interfacing with a tunable laser spectrometer. Hydrophobic functionalization of the mesoporous cladding allows enrichment of apolar analyte molecules and prevents strong interaction of water with the evanescent wave. The sensing performance was evaluated for aqueous toluene standards resulting in a limit of detection of 7 ppm. Recorded adsorption/desorption profiles followed Freundlich adsorption isotherms with rapid equilibration and resulting sensor response times of a few seconds. This indicates that continuous monitoring of contaminants in water is possible. A significant increase in LOD can be expected by likely improvements to the spectrometer noise floor which, expressed as a relative standard deviation of 100% lines, is currently in the range of 10⁻²A.U.

A waveguide Bragg grating temperature sensor implemented using a hybrid inorganic-organic material (Ormocer) with a 25-times higher temperature sensitivity than a typical silica fiber is presented. The sensor consists of second order gratings (1010-nm pitch) in 5 μm × 5 μm waveguides fabricated on a planar substrate using the replication-based methods. The gratings were imprinted in the under-cladding layer, and the waveguide cores were patterned on top by capillary filling of microchannels, which were defined in a transparent and flexible mold. The somewhat larger, slightly multimode waveguides facilitate pigtailing with an optical fiber but lead to three reflection peaks corresponding to the different excited waveguide modes. The peak at the longest wavelength (Bragg wavelength at 1539 nm, corresponding to the fundamental mode) was tracked during temperature testing, and a sensitivity of -249 pm °C ^-1 was found.

Thin and flexible sensor foils are very suitable for unobtrusive integration with mechanical structures and allow monitoring for example strain and temperature while minimally interfering with the operation of those structures. Electrical strain gages have long been used for this purpose, but optical strain sensors based on Bragg gratings are gaining importance because of their improved accuracy, insusceptibility to electromagnetic interference, and multiplexing capability, thereby drastically reducing the amount of interconnection cables required. This paper reports on thin polymer sensor foils that can be used as photonic strain gage or temperature sensors, using several Bragg grating sensors multiplexed in a single polymer waveguide. Compared to commercially available optical fibers with Bragg grating sensors, our planar approach allows fabricating multiple, closely spaced sensors in well-defined directions in the same plane realizing photonic strain gage rosettes. While most of the reported Bragg grating sensors operate around a wavelength of 1550 nm, the sensors in the current paper operate around a wavelength of 850 nm, where the material losses are the lowest. This was accomplished by imprinting gratings with pitches 280 nm, 285 nm, and 290 nm at the core-cladding interface of an imprinted single mode waveguide with cross-sectional dimensions 3 × 3 µm². We show that it is possible to realize high-quality imprinted single mode waveguides, with gratings, having only a very thin residual layer which is important to limit bend losses or cross-talk with neighboring waveguides. The strain and temperature sensitivity of the Bragg grating sensors was found to be 0.85 pm/µε and −150 pm/°C, respectively. These values correspond well with those of previously reported sensors based on the same materials but operating around 1550 nm, taking into account that sensitivity scales with the wavelength.

We present a polymer optical waveguide integration technology for the detection of nanoparticles in an evanescent field (EF)-based biosensor. Polymer waveguides together with their light coupling structures are designed to be integrated in a novel cartridge concept which will eventually lead to cost-effective, rapid, and easy-to-use point-of-care testing (POCT). The selected slab waveguides generate a homogeneous and well-defined EF, illuminating magnetic nanoparticles that are used as optical contrast labels and are measured using dark-field microscopy. The nanoparticles quantitatively bind to the sensor surface in the presence of target molecules, mediated by antibodies. Compatibility of the waveguide materials with these biomolecules is therefore a strong requirement. When designing a biosensor for POCT, it is also necessary to consider fabrication and manipulation tolerances, targeting the compatibility with mass-production technologies. In this context, polymer optics offer unique advantages, and in combination with the optical contrast labels a very high sensitivity can be achieved. The sensing concept was assessed by comparing various commercially available polymer materials (LightLink, Ormocer, Epocore/Epoclad) in terms of waveguide design and fabrication, optical performance (detection of nanoparticles), and biological compatibility and performance as a sensor surface in an immuno-assay for cardiac troponin I.

In this work, the reverse replication of circular micro grating structures on glass substrates is implemented using an ultra-violet curable resin and a polydimethylsiloxane (PDMS) mold which has the same structure as the original circular grating master. Two different techniques (“double PDMS replication” and “polymer- PDMS replication”) are employed to fabricate those reversed circular micro grating structures. Surface profiling measurements show that in case of the polymer-PDMS replication the dimensions of the resulting circular grating structures closely approximate those of the master, while the grating height is slightly decreased in case of the double PDMS replication technique, mainly due to the use of the releasing agent. For both methods, the grating slopes of the circular gratings are almost unchanged, leading to the desired optical performance. The two techniques are quite useful for more accurate reverse replications of micro optical and photonic structures.