A high surface to bulk fluorescence ratio is very useful in bioimaging, sensing, sequencing, and physical chemistry characterization. We used the evanescent field of a photonic waveguide for highly localized excitation and collection of molecular fluorescence. As both near-field excitation and collection are strongly distance dependent, we were able to increase the surface to bulk fluorescence ratio significantly. We have also experimentally investigated the combined excitation and collection efficiency as a function of the position of the molecule in the near field. Finally, we formulated and experimentally verified a general condition for the waveguide−molecule interaction length for maximum optical efficiency of the device.
Most fluorescent immunoassays require a wash step prior to read-out due to the otherwise overwhelming signal of the large number of unbound (bulk) fluorescent molecules that dominate over the signal from the molecules of interest, usually bound to a substrate. Supercritical angle fluorescence (SAF) sensing is one of the most promising alternatives to total internal reflection fluorescence for fluorescence imaging and sensing. However, detailed experimental investigation of the influence of collection angle on the SAF surface sensitivity, i.e., signal to background ratio (SBR), is still lacking. In this Letter, we present a novel technique that allows to discriminate the emission patterns of free and bound fluorophores simultaneously by collecting both angular and spectral information. The spectrum was probed at multiple positions in the back focal plane using a multimode fiber connected to a spectrometer and the difference in intensity between two fluorophores was used to calculate the SBR. Our study clearly reveals that increasing the angle of SAF collection enhances the surface sensitivity, albeit at the cost of decreased signal intensity. Furthermore, our findings are fully supported by full-field 3D simulations.
To address challenges in ultrasound detection for photoacoustic computed tomography, an optomechanical ultrasound sensor (OMUS) was developed in silicon photonic microchip technology. Such sensors are small (20 µm), sensitive (NEP 1.3 mPa Hz −1/2 ), broadband (measured 3 -30 MHz), and scalable to a fine-pitch matrix. This optical sensor has extreme sensitivity by combining an acoustic vibrating membrane with an innovative optomechanical waveguide. In this work, we test this sensor for photoacoustic computed tomography (PACT) by measuring and imaging the photoacoustic response of small 10 µm diameter sutures. Sensor signal-to-noise ratio (SNR), image contrast-to-noise ratio (CNR), and image resolution for different sensor geometries are characterized. We conclude that the sensor behaviour is in line with theory and meets the requirements for future applications in photoacoustic tomography.
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