Adaptive Optics: Optical wavefront sensor samples wavefronts at a nanometric resolution
Wavefront sensors are at the heart of most adaptive optics (AO), which are used widely in systems ranging from astronomical telescopes, optical coherence tomography (OCT), and free-space communications to microscopy and ophthalmological instruments.
Wavefront sensors are at the heart of most adaptive optics (AO), which are used widely in systems ranging from astronomical telescopes, optical coherence tomography (OCT), and free-space communications to microscopy and ophthalmological instruments. Common types of wavefront sensors include Hartmann-Shack and interferometric, both of which are complex in their own way. Any new form of wavefront sensing that comes along, especially a simple one, can capture the optical researcher's or engineer's interest.
Researchers Brian Vohnsen and Denise Valente at University College Dublin (Ireland) have done just that, creating 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 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 requires the measurement of only a single wavefront and thus does not require getting two separate wavefronts to be in phase.
In addition to AO applications, the new technique may be an aid in optical metrology, chemical sensing, and quality-control inspection of planar materials, films, and coatings.
Based on attenuated total internal reflection
The sensor technology is based on the creation of surface-plasmon polaritons (SPPs), which are quasiparticles created when light waves couple with electron oscillations at certain types of solid surfaces—in this case, a 50 nm gold film coated on a glass cover slip with a 2 nm titanium binding layer in between. By measuring how efficiently incoming light creates the SPPs across the gold film, the researchers are able to derive previously undetectable distortions in the wavefronts.
The technique is based on attenuated total internal reflection. The coated cover slip is mounted with index-matching oil onto a 20 mm rectangular glass prism-the film has an onset of total internal reflection at just under 45° incidence. The occurrence of SPPs creates very steep negative and positive slopes in the ATIR spectrum of the assembly (see figure). Placing the incidence angle at the center of one of these slopes creates a situation in which the reflected intensity is extremely sensitive to angle-or essentially wavefront slope.
|The attenuated-total-internal-reflection (ATIR) spectrum for surface-plasmon polariton excitation for a properly oriented prism assembly (containing a gold thin film on the hypotenuse) highlights how sensitive the reflectivity can be to the slightest change in angle for certain angles (left). The optical system in which the prism is placed (right) also contains a deformable mirror (DM; to create a desired wavefront), a Hartmann-Shack wavefront sensor (HS WFS) for comparison, and a CCD camera to capture wavefront-slope images (insets at right). (Courtesy of OSA)|
The variation in intensity is captured using a CCD camera. The result (a map of wavelength slope) is integrated to produce the wavefront shape. To fully reconstruct the wavefront, the system requires two separate measurements made at 90° 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 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 change in that angle, as would occur from a distortion in the wavefront, affects the way the SPPs are formed.
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. B. Vohnsen and D. Valente, Optica, 2, 12, 1024–1027 (2015).