Metasurfaces for Polarization Scramblers

**Metasurfaces for polarization scramblers**:

- Réf
- **ABG-111221**
- Sujet de Thèse- 16/02/2023- Autre financement privé- Institut Fresnel- Lieu de travail- Marseille - Provence-Alpes-Côte d'Azur - France- Intitulé du sujet- Metasurfaces for polarization scramblers- Champs scientifiques- Sciences de l’ingénieur
- Matériaux
- Physique
- Mots clés- Electromagnetism, optics and photonics, Space Optics**Description du sujet**:
Different types of depolarizers are already used but no solution is so far optimal. Babinet depolarizers and temporal scramblers are among the most widespread because they are efficient in terms of depolarizing ratio but they have significant drawbacks at implementation (division of the beam into 2 or 4, generation of movements, etc ).

In this context, the Institut Fresnel has been working with the CNES for several years on the development of innovative technologies for polarization scramblers. First works have highlighted the interest of extending the principle of spatial depolarization issued from speckle analysis to the case of specular beams. The principles have been theoretically confirmed with success and the general conditions for the design of a “perfect” depolarizing device have been defined [1]. The key point is the control of the spectral and spatial variations of the polarimetric phases within the specular beam [2].

Then, the reliability of the model has been confirmed by the realization of prototypes manufactured with multilayer coatings with thickness gradients [2,3]. The corresponding metrology has been developed with the measurement of the spectral variations of the degree of polarization in the visible range [3]. Experimental results are in perfect accordance with the design for low (1%) as well as high (98%) degree of polarization. This confirms the theoretical analysis.

We recall that a metasurface is generally constituted by a quasi-planar array of antennas and/or dielectric resonators of sub-wavelength size and spacing, allowing a local engineering of the properties of a light wave, in terms of phase, amplitude, chromatism, but also polarization. The manipulation of this polarization state can be achieved by using either an antenna array with spatially varying geometrical parameters, or anisotropic subwavelength scatterers with identical geometrical parameters but spatially varying orientations. In the latter case, the polarization state changes are associated with the so-called geometrical (or Pancharatnam-Berry) phase. Moreover, very recently, next to these local approaches, we have also seen the emergence of new approaches, called non-local, where the interactions between adjacent nano-elements make possible the appearance of collective modes with a strong spatial or spectral selectivity. Note that the local approach has already been successfully implemented [4] for the design and fabrication of planar spectro-imaging functions.

The work program of the proposed thesis would be as follows:

- State of the Art of metasurfaces to the manipulation of the polarization state of a polychromatic light wave
- Development of a numeric code adapted to the modeling of these structures
- Design and optimization of metasurfaces ensuring the most efficient scrambling of the polarization state of a polychromatic light wave
- Realization of a demonstrator (in partnership with RENATECH, the French network of high-end facilities in the field of micro & nanotechnology coordinated by CNRS)
- Characterization of the demonstrator
- Comparison with the performances available for a multilayer structure with thickness gradient
- In general, the ability to shape light spectrally or spatially is of strategic interest in precision optics. It is now well established that spectral shaping can be achieved with great precision, especially by using multilayer interference filters. However, these components are of modest interest for the spatial (angular) control of light. To overcome this shortcoming, new concepts such as the metasurface concept have emerged, which allow the design of specific components for the spatial control of light. One of the long-term objectives of this thesis is to combine these two structures (metasurfaces and multilayers) to offer dual control (spectral and spatial) of light._

References

1- M. Zerrad, C. Luitot, J. Berthon, C. Amra. Optical systems for controlled specular depolarization. _Optics Letters_, Optical Society of America - OSA Publishing, 2014, 39, pp.6919-6922.

2- Q. Ailloud, M. Zerrad, C. Amra. Broadband loss-less optical thin-film depolarizing devices. _Optics Express_, Optical Society of America - OSA Publishing, 2018, 26 (10)

3- M. Zerrad, Q. Ailloud, A. Moreau, F. Lemarquis, J. Lumeau, C. Amra.. Broadband loss-less optical thin-film depolarizing devices. _International Conference on Space Optics—ICSO 2020_, 2021, Online, France.

**Prise de fonction**:

- 02/11/2021**Nature du financement**:

- Autre financement privé**Précisi