Optical probe enhanced with LED

Materials World magazine
1 Feb 2009
Optical probe diagram

A scanning near-field optical microscope (SNOM) integrated with a microlens and a light emitting diode (LED) could allow the SNOM to become a more routine analytical tool for high resolution materials research and optical lithography.

Scientists at the University of Sheffield, UK, say their device may eventually be retrofitted onto commercial atomic force microscopes for user-friendly analysis. Coupling a single light source to the SNOM cantilever is said to eliminate the need for time-consuming optical alignment.

The integrated probe comprises a fabricated 50µm gallium nitride LED, which has been mounted onto one side of a glass substrate, with a 100µm microlens on the other. The SNOM cantilever (up to a few millimetres high) is then mounted with the aperture. Light from the LED is collected by the microlens and focused at the aperture of the tip.

Professor David Lidzey explains, ‘Scanning near-field optical microscopy gives some of the best resolutions (50nm upwards) available for optical imaging. The main drawback is that it is a difficult technique to master. The light from a remote laser is directed to the SNOM cantilever aperture via a series of lenses and mirrors. In our design, we do not need this external imaging system’.

Unlike previous research into integrated light sources, the LED is not attached directly to the cantilever. ‘Such designs have led to a compromise between the wavelength of light or the resolution that can be achieved, or the practicality of device manufacture,’ says Lidzey. ‘The novelty of this work is that we have taken technologies from the optics and photonics community to make a micro-optics system.’ The components are currently put together by hand, but Lidzey believes the micro-manufacture technique could be automated with a pick-up and place process.

Furthermore, if the user wants to capture an image at an alternative wavelength using this device, rather than investing in another costly external laser system, the manufacturer could develop a range of SNOM chips based on different LEDs.

So far the team has successfully patterned a photosensitive surface at a resolution of 35nm, and hopes to achieve the same magnitude in imaging technology, as well as push it further by improving the light collection efficiency of the microlens.

‘We hope that eventually our probes will become a drop-in replacement for existing tips – with the benefit of SNOM imaging capability,’ adds Lidzey.

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