"A flat plateau is almost mandatory if you want to precisely locate your modified probe on a sample," Bert Hecht of the University of Würzburg's nanooptics and biophotonics group told nanotechweb.org. "Our plan is to fabricate antennas on the tip surface that confine impinging radiation to a feedgap of around 10 nm for use in direct optical spectroscopy."

Ideally, the tip's flattened surface will run parallel with the sample to make probe-sample alignment as easy as possible. In reality, each range of SPMs has its own mounting geometry, which makes it difficult for suppliers to offer a flattened tip that will be parallel across all systems.

Hecht and his colleagues have solved the problem by shaping their tips in situ, which tailors the flattened surface perfectly to the SPM setup. Their method involves scanning the tip over the focus of a high-numerical-aperture objective illuminated by a Ti:sapphire laser. The unit emits 100 fs pulses of near-infrared radiation (830 nm) at a repetition rate of 80 MHz.

To flatten the tip, the probe is brought into contact with a transparent sample (a glass microscope cover slip) and scanned at the focus position for 1–2 hours. A plateau forms at the tip as the apex material melts and ablates, with an overall success rate of around 50%.

It turns out that scanning the tip over a relatively large area (10–20 µm) plays an essential role in the flattening process. As lead author Paolo Biagioni explains, the large scan size allows the tip to move in and out of the laser's focus. This sets up repeated heating and cooling cycles in the probe that appear to be critical in reshaping the tip.

The researchers presented their work in Review of Scientific Instruments.