Thèse

General framework

SEEDs team has a long standing know-how in synthesis and characterisation of group-IV semiconductor. At the moment we are especiallyinvestigating different ways to synthesized the very promising hexagonalGe-2H phase(an allotrope of the standard diamond cubic 3C structure) which exhibits a direct band gapand an excellent light emission capabilities [1] with a tuneableemission wavelength in the mid infrared between 1.8-4.2 μm bya concentration range of 0-40% of Si.This material may therefore provide additional photonic functionality to the silicon technologyand fill the gap between electronics and photonicsindustry. We aim at integratingSiGe-2H light sources on silicon on insulator in a CMOS compatible way.

Recently, we have pioneered an original method to achieve ashear induced phase transformationin Ge and Si nanowires NWsresulting in unprecedented heterostructureswith quasi periodic embedded 2H domains distributed all along the <111>-oriented nanowire(fig).The process is described in refs [2,3]. Our studies show the influence of temperature, diameter and crystallographic direction on the phase transformation.We propose to combine this shear strain-induced process with selective epitaxial growth on the transformed SiGe-2H seed. To make this approachfully compatible with CMOS one purpose is to optimizethe process to enable phase transformation along <100>-oriented SiGe nanostructures(nanofins or nanowires).The second challenge is to develop a highly selective etching method to remove the 3C top part and uncover the 2H transformed domain which is expected to serve as seed in order tosubsequently increase the volume of the 2Hsegment.This project is part of Opto-silicon project funded by the EU H2020 FET-OPEN.

[1] E.M.T. Fadaly et al. Nature580, 205–209 (2020).

[2] L. Vincent et al. Nanoletters 14 (2014) p.4828

[3] L. Vincent et al. Nanotechnology 29 n°12 (2018)

Objectives and work plan

The PhD student is expected to contribute to :

-technological developments for integration of SiGe-2H on SOI (achieve phase transformation in <100> nanofins, feasibility of selective etching)

-modelling strain field inducing phase transformation in nanostructuresand exploring the influence of the system geometry (influence of crystal orientation, aspect ratio, density of nanofins...)

-understanding phase transformation mechanismsin SiGe nanostructures

-optimisation of epitaxial regrowth on SiGe-2H seeds and characterisation of structural and physical properties of the synthesized structures.

Profile and skills required 

We are looking for applicants with a Masters degree in Physics,Physical Chemistry, materials science or related areas with an interest in nanotechnologies.Experience with microfabrication in cleanroomwould be a clear advantage.The candidate should have :

-strong knowledge in crystallography,crystal growth and plasticity of materials

-ability to work in cleanroom environment

-ability to work in ateamand good communication skills

-upper intermediate level in English

Application

Applicants must provide by email the following documents :

-Curriculum vitae

-Details of grades obtained at university from the 1st year to present

-Motivation letter from the applicant-Short description of the master internship

-Two recommendation letters

Deadline application 05/28/2021

Funded offer

Fully funded PhD by EU H2020 FETOPEN

Employer : University of Paris Saclay

Duration 3 years starting on 09/01/21

Contact

Laetitia VincentC2N, CNRS /Université Paris-Saclay, Département Matériaux / Equipe SEEDsLaetitia.vincent@c2n.u-psaclay.fr

Thèse

Centre de Nanosciences et nanotechnologies

 UMR 9001 du CNRS,

université Paris-Sud, université Paris-Saclay

91 405 Orsay – France

  

PhD research proposal

All-Dielectric Metamaterials for Terahertz metadevices

 Éric Akmansoy 

Département Photonique

__________________ 

 

General framework

Metamaterials have opened a new field in physics and engineering. Indeed, these artificial structured materials give rise to unnatural fascinating phenomena such as negative index, sub-wavelength focusing and cloaking. Metamaterials also exhibit near-zero refractive index [1]. These open a broad range of applications, from the microwave to the optical frequency domain. Metamaterials have now evolved towards the implementation of optical components[2]. 

We consider All-Dielectric Metamaterials which are the promising alternative to metallic metamaterials, because they do not suffer from ohmic losses and consequently benefit of low energy dissipation and because they are of simple geometry. They consist of high permittivity dielectric resonators involving Mie resonances. We have experimentally demonstrated negative effective permeability and/or permittivity by the means of all-dielectric metamaterials [3]. Previously, we have also demonstrated a negative index all-dielectric metamaterial [4]. Recently, we have numerically demonstrated a metadevice, namely, a metalens that focuses an incident plane wave, is less than one and a half wavelength thick. Its focal length is only a few wavelength and the spot in the focal plane is diffraction - limited.[5]. We also study role of the coupling of the modes of Mie resonances in an all-dielectric metamaterial so as to achieve negative index at terahertz frequencies (see fig. 1)[6].

Work Plan

During this thesis, all dielectric metamaterials will be designed, fabricated and characterized for the terahertz range. Negative index will be studied in this frequency domain, which is a challenge. Various devices which will be investigated such as flat lens, gradient devices, etc. This work takes place within a group of scientists of different disciplines (chemists, material scientists, physicists). 

 

 

Fig. 1: Spatial mode coupling : frequency of the first two modes of Mie resonances in function of the distance  between two resonators which is half the lattice period . The shaded area corresponds to negative value of the effective index . It evidences the mode degeneracy [6].

Bibliography

[1]    I. Liberal and N. Engheta, “Near-zero refractive index photonics,”  Nature Photonics, vol. 11, pp. 149 EP –, 03 2017.

[2]    N. I. Zheludev and Y. S. Kivshar, “From metamaterials to metadevices,”  Nat Mater, vol. 11, pp. 917–924, 11 2012.

[3]    T. Lepetit, E. Akmansoy, and J.-P. Ganne, “Experimental evidence of resonant effective permittivity in a dielectric metamaterial,” Journal of Applied Physics, vol. 109, no. 2, p. 023115, 2011.

[4]    T. Lepetit, É. Akmansoy, and J.-P. Ganne, “Experimental measurement of negative index in an all-dielectric metamaterial,” Applied Physics Letters, vol. 95, no. 12, p. 121101 (, 2009.

[5]    F. Gaufillet, S. Marcellin, and E. Akmansoy, “Dielectric metamaterial-based gradient index lens in the terahertz frequency range,” IEEE J Sel Top Quant, vol. 10. 1109/JSTQE. 2016. 2633825, 2017.

[6]    S. Marcellin and É. Akmansoy, “Negative index and mode coupling in all-dielectric metamaterials at terahertz frequencies,” in 2015 9th International Congress on Advanced Electromagnetic Materials in Microwaves and Optics (Metamaterials), pp. 4–6, Sept 2015.

Thèse

  

Centre de Nanosciences et nanotechnologies

UMR 9001 du CNRS,

université Paris-Sud, université Paris-Saclay

91 405 Orsay – France

  

PhD research proposal

Graded Photonic Crystals Devices for Graded Index Optics

 Éric Akmansoy 

Département Photonique

__________________ 

 

General framework

On the one hand, Graded Photonic Crystals (GPC) allow to efficiently control the flow of light thanks to the shape of their photonic bands, which we verified by demonstrating a “photonic mirage” at wavelength scale[1]. In the other hand, GRaded INdex (GRIN) optics is undergoing a renewal because it allows to downsize optical systems, opening a new way to optical design[2]. But GRIN optics was limited by a lack of easy to implement fabrication techniques. Nanotechnology enables to efficiently fabricate photonic crystals, which we will implement to fabricate GPC for GRIN optics. 

We are designing, numerically simulating and characterising GPC devices (see fig.1)[3, 4]. More specifically, we have conceived a GRIN flat lens which we have characterised in the microwave[5]. Recently, we have designed and simulated a Luneburg lens[6] (see fig.1). We conceive thin and thick flat GRIN lenses[7]. For the sake, we engineer the equi-frequency curves which convey the dispersion properties of photonic crystals. Our aim is to go further up to the optical domain. The experimental evidence of such devices is an end in itself because it is a technological challenge to fabricate such GPC. Nevertheless, we will investigate the great variety of applications of GPC flat lens, which concerns Photonic Integrated Circuits, Lab on Chip, optical pumping of organic materials, OLED, biophotonics, fluorescence of cells, etc.

PhD work plan

GPC devices will be designed, fabricated and characterised at the Centre de Nanosciences et Nanotechnologies (C2N). This work will concern numerical simulation and optical design, nanotechnological processes and optical characterisation. These devices will operate at telecommunication wavelengths (1,55 μm). The Silicium On Insulator (SOI) channel of the C2N will be involved, because “state of the art" photonic crystal -based devices have already been fabricated in its clean-room [8]. These devices require the technological mastery of the position, of the size and of the shape of the patterns, which is adapted for the realisation of GPC devices. The size of the patterns of these latter is of several tens of nanometers, which will be challenging. The devices will be also characterised at the C2N. GPC devices operating in the near infrared (NIR) spectrum will also investigated (from 1 to 20μm) because, in this domain, lies a lot of molecular signatures, which is fruitful for NIR spectroscopy, chemical sensing, polluant detection, etc. 

      

 

Fig. 1: Right : Focusing by a GPC GRIN flat lens[3] ; left : focusing by a Luneburg lens[6] 

Bibliography

[1]    Éric Akmansoy, Emmanuel Centeno, Kevin Vynck, David Cassagne, and Jean-Michel Lourtioz, Appl. Phys. Lett. 92, 133501 (2008) 

[2]    Predrag Milojkovic, Stefanie Tompkins, Ravindra Athale, Gradient Index Optics, Optical Engineering, November 2013/Vol. 52(11), p. 112101-1 

[3]    F. Gaufillet, É. Akmansoy, Graded photonic crystals for graded index lens, Optics Communications, Volume 285, 2638 (2012). 

[4]    F. Gaufillet, É. Akmansoy, Design of flat graded index lenses using dielectric Graded Photonic Crystals, Optical Materials, Vol. 47, 555-560 (2015) 

[5]    F. Gaufillet and E. Akmansoy. Design and experimental evidence of a flat graded-index photonic crystal lens. Journal of Applied Physics, 114(8) :083105, 2013. 

[6]    F. Gaufillet and E. Akmansoy. Graded photonic crystals for Luneburg lens, IEEE Photonics Journal, vol. 8, 11 (2016) 

[7]    CVI Melles Griot. Gradient-Index Lenses – http://pdf.directindustry.com/pdf/cvi-melles-griot/gradient-index-lenses/12567-66963.html. 

[8]    Z. Han, X. Checoury, D. Néel, S. David, M. El Kurdi, P. Boucaud, Optimized design for ultra-high Q silicon photonic crystal cavities, Optics Communications 283 (21) (2010) 4387 – 4391.