Plasmons Enhance Detection of Wavefront Aberrations

A sensor that exploits plasmonics to gauge nanoscale distortions in lightwaves could yield more powerful tools for metrology and chemical sensing, as well as sharper microscopes.

The method detects wavefront aberrations indirectly by measuring changes in the reflectivity of gold films. It may be the first to use plasmons to address a classical optics problem, according to its developers at University College Dublin.

As light travels through water, the atmosphere and even human tissue its wavefront becomes distorted, blurring images and reducing resolution. It's possible to correct for these distortions with adaptive optics by precisely measuring the shape of the wavefront.

Such measurements — albeit on relatively large scales — are used in astronomy to correct for atmospheric distortion.

Conventional wavefront sensors work by either mechanically sampling wavefronts with microlenses or other devices or measuring interference patterns. The latter approach requires the extra step of ensuring that the interacting light waves are in phase — meaning their waveforms overlap precisely.

Cartesian wavefront derivatives can be determined by monitoring intensity variations across the reflected beam of light used to excite surface plasmon polaritons in the Kretschmann configuration. Courtesy of Optica/The Optical Society.

Now, by observing how efficiently incoming light creates surface plasmon polaritons (SPPs) on gold film, it's possible to derive previously undetectable nanoscale distortions in the wavefronts.

SPPs arise when light meets an electrically conducting material, causing electrons to oscillate in a wavelike pulse that travels across the material's surface. Any changes in the angle of incidence — as would occur from a distortion in the wavefront — affects the way the SPPs are formed. This directly affects how much light is reflected back from the surface.

"Since these polaritons are perfectly coupled to the light that forms them, any changes in their behavior would indicate a change in the waveform of light," said Brian Vohnsen, a senior lecturer at University College Dublin. "We make use of the attenuation of the signal from the gold surface to simply convert the wavefront shape — or slope — into an intensity difference in a beam of light."

This change is captured with cameras that are sensitive to minute changes in intensity.

To fully reconstruct the wavefront, the system requires two separate measurements made at 90° to one another. It is then possible to calculate the tiny changes in the actual wavefront based on the orthogonal intensity data points. The speed of the measurement is limited only by the speed of the cameras.

This type of sensor may find applications in the quality inspection of planar materials, films and coatings, the researchers said. It could also replace some wavefront sensors used in astronomy, microscopy and vision science.

The researchers are working to overcome two limitations in the current setup. The first is the requirement for simultaneous measurement of wavefront changes with two cameras. The second is improving the method by which the SPPs are "excited" on the surface of the gold film.

The results were published in Optica (doi: 10.1364/optica.2.001024 [open access]).