Metasurfaces : Optics with interfaces
à 11h en salle C. Brot
Abrupt modifications of the fields across an interface can be engineered by depositing an array of sub-wavelength resonators specifically tailored to address local amplitude, phase and polarization changes. Physically, ultrathin nanostructure arrays (), also called ‘‘optical metasurfaces’’, control light by engineering artificial boundary conditions of Maxwell’s equations. Metasurfaces have been implemented to obtain various sorts of optical functionalities, ranging from the basic control of the transmission and reflection of light, to the control of the radiation patterns for comprehensive wavefront engineering and holography.
After transmission or reflection, however, the amount of propagation phase shift required to achieve any optical function depends on the wavelength, therefore, a specific phase profile imposed at interface will shape the light in a desired manner only for a single wavelength. This basic dispersion effect, which already affects bandwidth of conventional devices, is also limiting the operation of metasurfaces to a narrow bandwidth. We will discuss how to manage dispersion effect directly at interface to create multi-wavelength achromatic metasurfaces for broadband control of light. This approach is applied to fabricate dispersion-free beam deflectors and achromatic flat lenses in the near-infrared.
Rapid developments in the emerging field of stretchable and conformable photonics necessitate analytical expressions for boundary conditions at metasurfaces of arbitrary geometries. In the last part of the seminar, we will discuss the idea of ‘‘conformal boundary optics’’ : a design theory that determines the optical response for designer input and output fields at interfaces of arbitrary shapes. Given any object, one can now realise coatings to achieve exotic effects like optical illusions and anomalous diffraction behaviour.
Metasurface-based optical devices are already finding applications as multi-bandpass filters, lightweight collimators, imaging lenses with chromatic aberration correction, or sensors for integrated on-chip spectrometers. Their design can now be made compatible with silicon-on-insulator technology, and can be realized with well-established semiconductor fabrication processing.
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