Spectrometric Determination of the Refractive Index of Optical Wave Guiding Materials Used in Lab-on-a-Chip Applications

Abstract

The design and optimization of light-based analytical devices often require optical characterization of materials involved in their construction. With the aim of benefiting lab-on-a-chip applications, a transmission spectrometric method for determining refractive indices, n, of transparent solids is presented here. Angular dependence of the reflection coefficient between material-air interfaces constitutes the basis of the procedure. Firstly, the method is studied via simulation, using a theoretical algorithm that describes the light propagation through the sample slide, to assess the potentially attainable accuracy. Simulations also serve to specify the angles at which measurements should be taken. Secondly, a visible light source and an optical fiber spectrometer are used to perform measurements on three commonly used materials in optical lab-on-a-chip devices. A nonlinear regression subroutine fits experimental data to the proposed theoretical model and is used to obtain n. Because the attainable precision using this method of refractive index determination is dictated by the uncertainty in the transmission measurements, the precision (with 95% confidence) for mechanically rigid samples, namely glass and poly(methyl methacrylate) (PMMA), is higher than those estimated for the elastomer sample (in-house-molded poly(dimethylsiloxane) (PDMS)). At wavelengths with the highest signal-to-noise ratio for the spectrometer setup, the estimated refractive indices were 1.43+/-0.05 (580 nm) for PDMS, 1.54+/-0.02 (546 nm) for glass, and 1.485+/-0.005 (656 nm) for PMMA. Accurate refractive index estimations with an average precision equal to 0.01 refractive index units (RIU) were obtained for PMMA and glass samples, and an average precision of 0.09 RIU for the PDMS molded slide between 550 and 750 nm was obtained.

Spiral microchannel with rectangular and trapezoidal cross-sections for size based particle separation

“…To overcome this, we mixed dimethyl sulfoxide (DMSO) with ethanol in a 1:1 volume ratio, which produces a mixture with refractive index of 1.42, density of 0.9805 g/ml and viscosity of 0.978 mPa·s at 298.15 K50. The refractive index of the mixture is similar to that of PDMS (1.43) and enhances imaging by elimination refraction based dispersion51. The solution was shown to dissolve the polystyrene (PS) particles (Bangs Laboratories, Inc. USA) and Tygon tubing after prolonged immersion of 1 week.…”

Precision patterning of PDMS membranes and applications

“…These microlenses were fabricated with a maximum spinning speed of 5500 rpm and partial curing time of 0 s. Due to PDMS reflow, the thickness of the membrane changes from 15 µm to 5.52 µm. As the refractive index of PDMS in air is 1.43 ± 0.05 (580 nm) [15], the focal length of the microlenses in air was calculated to be 1.06 mm. The parylene C lift-off approach permits the creation of microlenses with a bottom diameter ranging from 5 µm to several centimeters with different focal lengths.…”

Adding colors to polydimethylsiloxane by embedding vertical silicon nanowires

“…The refractive index of background medium (PDMS) is taken to be 1.43. 20 Perfectly matched layers are used on all boundaries, and the total-field scattered-field (TFSF) configuration is employed. The scattering, absorption, and extinction cross sections are calculated ( Fig.…”