Optical Sciences

Biomolecules and nanostructures

The Optical Sciences group studies the interaction of light and matter. Our current focus is on detection and sensing/imaging with an emphasis on the development of integrated photonics. We are part of Twente University's Department of Science and Technology and member of the MESA+ institute.


Single molecule mapping of the optical field distribution of probes for near-field microscopy

(full pdf)

Veerman JA, Garcia-Parajo MF, Kuipers L, Van Hulst NF
vol 194: p477-p482 part 2-3 MAY-JUN 1999

The most difficult task in near-field scanning optical microscopy (NSOM) is to make a high quality subwavelength aperture probe, Recently we have developed high definition NSOM probes by focused ion beam (FIB) milling. These probes have a higher brightness, better polarization characteristics, better aperture definition and a Batter end face than conventional NSOM probes. We have determined the quality of these probes in four independent ways: by FIB imaging and by shear-force microscopy (both providing geometrical information), by far- field optical measurements (yielding throughput and polarization characteristics), and ultimately by single molecule imaging in the near-field. In this paper, we report o­n a new method using shear-force microscopy to study the size of the aperture and the end face of the probe (with a roughness smaller than 1.5 nm). More importantly, we demonstrate the use of single molecules to measure the full. three-dimensional optical near-field distribution of the probe with molecular spatial resolution, The single molecule images exhibit various intensity patterns, varying from circular and elliptical to double arc and ring structures, which depend o­n the orientation of the molecules with respect to the probe. The optical resolution in the measurements is not determined by the size of the aperture, but by the high optical field gradients at the rims of the aperture, With a 70 nm aperture probe, we obtain fluorescence field patterns with 45 nm FWHM. Clearly this unprecedented near-field optical resolution constitutes an order of magnitude improvement over far-field methods like confocal microscopy.
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