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Lensfree on-chip sensing setup. A plasmonic nanoaperture array, as shown in the SEM image on the top right corner, is illuminated with a quasimonochromatic source (e.g., 550 nm center wavelength with bandwidth) located away from its surface. The lensfree transmission pattern of this plasmonic structure is sampled by a CMOS chip placed at away from the aperture plane. The yellow surface indicates the detector active area . The plasmonic nanoaperture array shown above is composed of uniformly spaced slits each with a length of where the slit width varied (from left to right) between and in discrete steps of . The physical gap between two neighboring slits is kept constant at .
Cross correlation coefficients are calculated between the first lensfree transmission pattern and the subsequent patterns for (a) the detector plane and (b) reconstruction plane, as illustrated with the blue dotted lines. The same plots also show the running averages of these cross correlation coefficients over frames as shown with the black solid lines in (a) and (b). The intensities of the lensfree transmission patterns used in (a) and (b) are normalized to the instantaneous illumination intensity, which is also detected using the same CMOS chip through a large aperture. The initial refractive index of these sensing experiments (n) is 1.333 corresponding to DI water. Top row: (a)–(c) illustrate the raw lensfree diffraction patterns of the nanoaperture array at three different refractive indices within the microfluidic channel (n, , and , respectively), where all the lensfree transmission patterns extend over a width of . Top row (d)–(f) illustrate the reconstructed transmission patterns of the same nanoaperture array at plane, right underneath the aperture region.
Based on FDTD simulation results, the cross correlation coefficients among lensfree transmission patterns of the same nanoaperture array shown in Figs. 1 and 2 are calculated as a function of the refractive index within the microchannel. These calculations were performed for both (solid line) and (dashed line) planes. False color insets depict the 2D transmission patterns for three different refractive index values (1.33, 1.34, and 1.35, respectively), where the upper (lower) ones are calculated at .
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