Prof. Federico Capasso
School of Engineering and Applied Sciences, Harvard University, Cambridge
Conference room Querzoli - LENS - via Nello Carrara 1 - Sesto Fiorentino (Florence)
Published on-line at 11:15:39 AM on Monday, November 5th, 2012
Classical optics in a new light: flat photonics based on metasurfaces
QSTAR-LENS seminar: introducing phase discontinuities for high-flexibility wavefront engineering.
Conventional optical components such as lenses rely on gradual phase shifts accumulated during light propagation to shape light beams. We have recently shown how new degrees of freedom in optical design can be attained by introducing in the optical path abrupt phase changes over the scale of the wavelength (see 10.1126/science.1210713 and 10.1021/nl300204s). Phase discontinuities enable wavefront engineering with unprecedented flexibility, which is promising for a wide variety of planar optical components such as lenses free of monochromatic aberrations (spherical, astigmatism and coma) (10.1021/nl302516v), axicons (10.1063/1.3673334), background free broadband quarter wave plates and spiral phase plates that create optical vortices (10.1038/NMAT3443).
Finally I will show that the phenomenon of thin film interference, known for hundreds of years, which gives rise to vivid coloring when the thickness is on the scale of the wavelength, under appropriate conditions can persist in ultrathin, highly absorbing films of o few to tens of nanometres in thickness . We have demonstrated a new type of optical coating comprising such a film on a metallic substrate, which selectively absorbs various frequency ranges of the incident light. These coatings have a low sensitivity to the angle of incidence and require minimal amounts of absorbing material that can be as thin as 5-20 nm for visible light.
This technology has the potential for a variety of applications from ultrathin photodetectors and solar cells to optical filters, to labelling, and even the visual arts and jewellery.
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