Nanoscale optical resonators enable a new class of flat optical components called metasurfaces. This approach has been used to demonstrate functionalities such as focusing free of monochromatic aberrations (i.e., spherical and coma), anomalous reflection, and large circular dichroism. Recently, dielectric metasurfaces that compensate the phase dispersion responsible for chromatic aberrations have been demonstrated. Here, we utilize an aperiodic array of coupled dielectric nanoresonators to demonstrate a multiwavelength achromatic lens. The focal length remains unchanged for three wavelengths in the near-infrared region (1300, 1550, and 1800 nm). Experimental results are in agreement with full-wave simulations. Our findings are an essential step toward a realization of broadband flat optical elements.

Achromatic Metasurface Lens at Telecommunication Wavelengths

The so-called ‘flat optics’ that shape the amplitude and phase of light with high spatial resolution are presently receiving considerable attention. Numerous journal publications seemingly offer hope for great promises for ultra-flat metalenses with high efficiency, high numerical aperture, broadband operation… We temperate the expectation by referring to the current status of metalenses against their historical background, assessing the technical and scientific challenges recently solved and critically identifying those that still stand in the way.

Metalenses at visible wavelengths: past, present, perspectives

We have proposed, designed, manufactured and tested low loss dielectric micro-lenses for infrared (IR) radiation based on a dielectric metamaterial layer. This metamaterial layer was created by patterning a dielectric surface and etching to sub-micron depths. For a proof-of-concept lens demonstration, we have chosen a fine patterned array of nano-pillars with variable diameters. Gradient index (GRIN) properties were achieved by engineering the nano-pattern characteristics across the lens, so that the effective optical density of the dielectric metamaterial layer peaks around the lens center, and gradually drops at the lens periphery. A set of lens designs with reduced reflection and tailorable phase gradients have been developed and tested, demonstrating focal distances of a few hundred microns, beam area contraction ratio up to three, and insertion losses as low as 11%.

All-dielectric subwavelength metasurface focusing lens

https://iopscience.iop.org/article/10.1088/2040-8978/18/10/103001/pdf

Resonant dielectric nanostructures: a low-loss platform for functional nanophotonics

Metasurfaces have opened a new frontier in the miniaturization of optical technology by allowing exceptional control over the wavefront. Here, we demonstrate off-axis meta-lenses that simultaneously focus and disperse light of different wavelengths with unprecedented spectral resolution. They are designed based on the geometric phase via rotated silicon nanofins and can focus light at angles as large as 80°. Due to the large angle focusing, these meta-lenses have superdispersive characteristics (0.27 nm/mrad) that make them capable of resolving wavelength differences as small as 200 pm in the telecom region. In addition, by stitching several meta-lenses together, we maintain a high spectral resolution for a wider wavelength range. The meta-lenses have measured efficiencies as high as 90% in the wavelength range of 1.1 to 1.6 μm. The planar and compact configuration together with high spectral resolution of these meta-lenses has significant potential for emerging portable/wearable optics technology.

Super-Dispersive Off-Axis Meta-Lenses for Compact High Resolution Spectroscopy.

Surface topography dictates the deterministic functionality of diffraction by a surface. In order to maximize the efficiency with which a diffractive optical component, such as a grating or a diffractive lens, directs light into a chosen order of diffraction, it is necessary that it be "blazed". The efficiency of most diffractive optical components reported so far varies with the wavelength, and blazing is achieved only at a specific nominal energy, the blaze wavelength. The existence of spurious light in undesirable orders represents a severe limitation that prevents using diffractive components in broadband systems. Here we experimentally demonstrate that broadband blazing over almost one octave can be achieved by combining advanced optical design strategies and artificial dielectric materials that offer dispersion chromatism much stronger than those of conventional bulk materials. The possibility of maintaining an efficient funneling of the energy into a specific order over a broad spectral range may empower advanced research to achieve greater control over the propagation of light, leading to more compact, efficient and versatile optical components.

Broadband and Efficient Diffraction

The goal of cost effective, high performance optical systems capable of pattern matching tasks has fuelled research for over two decades. It is only recently with the introduction of novel optical/electronic architectures that such systems are becoming viable.Although conceptually elegant, the single output from an optical correlator is of restricted application due to its sensitivity to image distortions. A multichannel hybrid optical/electronic architecture has therefore been devised, exhibiting greatly enhanced flexibility and performance. This hybrid processor is being used in conjunction with a robotic handling system as part of an ESPRIT programme.The video signal from a robot's TV camera is relayed to an electrically addressed spatial light modulator in the processor. This input image is then replicated optically and fed into each of the system's parallel channels where it is cross-correlated with a set of differing reference patterns. The multiple correlation signals are detected and compiled into an electronic descriptor code. A 'Euclidean1 search through a look-up table of codes then extracts object identification and rotation. Scale information and out-of-class recognition is also possible since the system can be taught to memorise new codes in real time.

Brigitte Loiseaux

Papers

Broadband and Efficient Diffraction

Central de Recherches