New type of optical wavefront sensor is based on quasiparticles
Technique uses attenuated total internal reflection to see across the wavefront at the nanoscale level.
|The optical setup (DM = deformable mirror; HS WFS = Hartmann–Shack wavefront sensor, which the setup includes for comparison). (Image: OSA)|
Researchers Brian Vohnsen and Denise Valente at University College Dublin, Ireland, have created a new type of wavefront sensor based on sensing of plasmonic "quasiparticles" that can measure wavefront slopes across a beam of light down into the nanoscale region.1 This is in contrast to Hartmann-Shack (HS) wavefront sensors that sample the wavefront at lateral distances of 100 μm or more. And, in contrast to interferometric methods of wavefront sensing, the quasiparticle method is requires the measurement of only a single wavefront and thus does not require getting two separate wavefronts to be in phase.
The technique is potentially useful for adaptive-optical (AO) applications such as microscopy and biomedicine, as well as in other applications in metrology, chemical sensing, and quality-contol inspection of planar materials, films, and coatings.
Using quasiparticles practically
The sensor technology is based on a curious phenomenon: a quasiparticle that emerges when light waves couple with electron oscillations at certain types of solid surfaces. By measuring how efficiently incoming light creates these quasiparticles, the researchers are able to derive previously undetectable distortions in the wavefronts.
Based on attenuated total internal reflection, the technique sees wavefront slopes as intensity differences across the beam produced by surface-plasmon polariton (SPP) excitation at the surface of a gold film. The result (a map of wavelength slope) is integrated to produce the wavefront shape.
The resonance behavior of the SPP quasiparticles responds to even extremely small-scale wavefront distortions. SPPs arise when a wavefront meets an electrically conducting surface at a specific angle; at the point where they interact, electrons oscillate, forming a wave-like pulse that travels across the surface. Any changes in that angle, as would occur from a distortion in the wavefront, would affect the way the SPPs are formed. This then directly effects how much light is reflected back from the surface.
It is this change in reflected intensity that the researchers measure. To fully reconstruct the wavefront, the system requires two separate measurements made at 90º to one another, which are then integrated to produce the wavefront. The speed of the measurement is only limited by the speed of the cameras.
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.
1. Brian Vohnsen and Denise Valente, Optica, Vol. 2, Issue 12, pp. 1024-1027 (2015); https://www.osapublishing.org/optica/fulltext.cfm?uri=optica-2-12-1024&id=333151