Efficient evaluation of Sommerfeld integrals for the optical simulation of many scattering particles in planarly layered media.

A strategy for the efficient numerical evaluation of Sommerfeld integrals in the context of electromagnetic scattering at particles embedded in a plane parallel layer system is presented. The scheme relies on a lookup-table approach in combination with an asymptotic approximation of the Bessel function in order to enable the use of fast Fourier transformation. Accuracy of the algorithm is enhanced by means of singularity extraction and a novel technique to treat the integrand at small arguments. For short particle distances, this method is accomplished by a slower but more robust direct integration along a deflected contour. As an example, we investigate enhanced light extraction from an organic light-emitting diode by optical scattering particles. The calculations are discussed with respect to accuracy and computing time. By means of the present strategy, an accurate evaluation of the scattered field for several thousand wavelength scale particles can be achieved within a few hours on a conventional workstation computer.

Sensitivity improvement in fluorescence-based particle detection.

Microfluidic flow cytometers are highly interesting candidates for biomedical point-of-care applications. However, the sensitivity, reliability, and throughput of these systems must be improved to provide the full functionality of established flow cytometric systems. One proposed method to improve fluorescence detection systems is to use spatial modulation techniques. We derive the noise-related statistics and calculate the coefficient of variation for a detection system with and without spatial modulation. We measure the noise properties of a nonmodulated microfluidic fluorescence particle detection system and analyze the possible performance gains using spatial modulation.

Hybrid lithography: combining UV-exposure and two photon direct laser writing.

We demonstrate a method for the combination of UV-lithography and direct laser writing using two-photon polymerization (2PP-DLW). First a dye doped photoresist is used for UV-lithography. Adding an undoped photoresist on top of the developed structures enables three-dimensional alignment of the 2PP-DLW structures by detecting the spatially varying fluorescence of the two photoresists. Using this approach we show three dimensional alignment by adding 3D structures made by 2PP-DLW to a previously UV-exposed structure. Furthermore, a fluidic system with an integrated total internal reflection mirror to observe particles in a microfluidic channel is demonstrated.

Direct fabrication of PDMS waveguides via low-cost DUV irradiation for optical sensing.

We demonstrate the fabrication of single mode optical waveguides by irradiating polydimethylsiloxane (PDMS) with a low cost Hg lamp through a conventional quartz mask. By increasing the refractive index of the irradiated areas, waveguiding is achieved with an attenuation of 0.47 dB/cm at a wavelength of 635 nm. The refractive index change is stable in ambient air and water for time periods of more than 3 months. The excitation of water-dispersed fluorescent nanoparticles in the evanescent field of the waveguide is demonstrated.

Flexible planar microfluidic chip employing a light emitting diode and a PIN-photodiode for portable flow cytometers.

Detection of fluorescence particles is a key method of flow cytometry. We evaluate the performance of a design for a microfluidic fluorescence particle detection device. Due to the planar design with low layer thicknesses, we avoid optical components such as lenses or dichroic mirrors and substitute them with a shadow mask and colored film filters. A commercially available LED is used as the light source and a PIN-photodiode as detector. This design approach reduces component cost and power consumption and enables supplying the device with power from a standard USB port. From evaluation of this design, we obtain a maximum particle detection frequency of up to 600 particles per second at a sensitivity of better than 4.7 × 10(5) MESF (molecules of equivalent soluble fluorochrome) measured with particles for FITC sensitivity calibration. Lowering the flow rate increases the instrument sensitivity by an order of magnitude enabling the detection of particles with 4.5 × 10(4) MESF.