K. L. van der Molen | Searcharxiv

arXiv2006

A spatial refractive index sensor using whispering gallery modes in an optically trapped microsphere

We propose the use of an optically trapped, dye doped polystyrene microsphere for spatial probing of the refractive index at any position in a fluid. We demonstrate the use of the dye embedded in the microsphere as an internal broadband excitation source, thus eliminating the need for a tunable excitation source. We measured the full width at half maximum of the TE and TM resonances, and their frequency spacing as a function of the refractive index of the immersion fluid. From these relations we obtained an absolute sensitivity of 5e-4 in local refractive index, even when the exact size of the microsphere was not a priori known.

physics.optics↗

arXiv2006

Recognition of paper samples by correlation of their speckle patterns

Buchanan et al. [Nature 436, p. 475 (2005)] have shown that it is possible to recognize paper samples via their speckle pattern by using a line-shaped laser focus, four photo detectors and a scanning mechanism. In this report recognition of five out of ten paper samples is presented. The sample was illuminated by a 2.62 +- 0.02 mm circular spot and the reflected light was measured by a static CCD. We have formulated a criterion for recognition that limits the probability of false recognition to 0.1 % for the experiment with ten samples. We obtained results that show that the probability of false recognition will be negligible for a large amount of samples. The properties of speckle originating from a line and a spot illumination source have been compared to see whether the use of a line results in major advantages for the recognition of samples.

physics.ed-ph↗