Description du sujet de thèse

The research within this project focuses on the growth of thin films of orthoferrite and photoinduced ultrafast spin currents and dynamics in insulating orthoferrite and hexaferrite thin films for spin-current conversion such as THz emitters.
In this PhD project, iron oxide-based thin films (orthoferrites and hexaferrites) will be explored for their potential to generate charge and spin currents using ultra-short femtosecond optical pump pulses. The targeted rare-earth iron perovskite material will be grown on-site using a state-of-the art pulsed laser deposition system. The main characterizations include X-ray diffraction for the structural properties and temperature dependent magneto-optical properties to assess the basic magnetic properties of the materials such as its saturation magnetization and Curie temperature. The materials will then be studied for their transient physical properties such as absorption and reflectivity using available in-house instrumentation with in particular a state-of-the art ultrafast spectroscopy system HARPIA, commercialized by LightConversion. Transient absorption spectroscopy of rare-earth orthoferrites and hexaferrites will help to determine both the electronic band structure and the dynamics of the photoinduced charge current. The associated spin-dynamics will be investigated through time resolved magneto-optical and THz spectroscopy both in-house and in collaboration with partners within this project.
The candidate should have good knowledge regarding optical properties of solids, optics/photonics and growth of materials. Basic skills in experimental physics and material growth are very welcome. In addition, notion of solid-state physics will be also appreciated.
The candidate will be working with a UHV-laser deposition installation, a magneto-optical spectroscopy setup, with time-resolved optical absorption, THz spectroscopy and time-resolved magneto-optic techniques. She/he will be trained in the use of a PLD growth facility, the use of very intense and ultrafast lasers as well as time-resolved spectroscopy. Basic skills in programming are very welcome as the setup is currently implemented with Python.
This project is part of the France 2030 excellence scheme under the ANR MoxSpin project being part of the PEPR SPIN.

Contexte de travail

Applications in the field of THz radiation cover a very broad spectrum from monitoring the irrigation of agricultural crops, non-destructive testing of welds in the automotive industry and extends nowadays towards high-frequency telecommunication beyond 6G. These applications are based on the availability of THz radiation sources and detectors. Very often, sources and detectors are almost identical. But using THz radiation also requires active optical elements to modulate the intensity of the radiation, switch it between different channels or code the information through polarisation. Unlike visible or near infrared optics (optical telecommunications), very few active optical elements are currently available in this spectral range, and the field of research is wide open.
To develop materials whose magnetic properties respond to THz excitations, one needs to focus on materials with very high magnetic anisotropy. Up to date, the highest resonance frequencies close to THz have been observed for iron-based pseudo-perovskite materials, RFeO3 with R a rare-earth element [1,2], whose magnetic order varies between antiferromagnetism and weak ferromagnetism, depending on their chemical composition. In a recent work on the orthoferrite TmFeO3 materials, it has been shown that under excitation with femtosecond pulses not only are spin waves created in this orthoferrite, but also two polaritons of very close frequency coupled themselves to magnons [3].
Today, intensive research is being carried out in the field of spin-to-charge conversion for the creation of broadband THz emitters (e.g. [4]). The basic operating principle is the creation of a spin current by means of an ultra-short femtosecond optical pump pulse in a ferromagnetic material. By placing a non-magnetic metal with a strong spin-orbit interaction in contact with the ferromagnetic material, this spin current is transformed into a charge current by the inverse spin-Hall effect. This charge current is the source of a broadband THz pulse. THz emission with a spectral bandwidth comparable to that of LiNbO3-based pulsed THz sources has also been demonstrated for antiferromagnetic nickel oxide [5]. An indication of the presence of an ultrafast photo-induced spin current can already be observed from a spin dynamics measurement [6]. Such strong spin signals have been observed for a variety of insulating oxides, such as garnets, orthoferrites [7], spinel systems and hexaferrites [8]. Consequently, all oxides can generate photo-induced spin currents.

Supervision:
The thesis will be supervised by N. Keller, O. Popova and R. Bertoni for the research on PLD growth of complex magnetic oxide thin films, time-resolved light-matter interaction and spin-dynamics.
This thesis aims at training a PhD student the material growth of highest possible quality of oxide thin films, in time resolved optical and magneto-optical spectroscopy.
The thesis work will be carried out at the Institute of Physics of Rennes. Our available infrastructure comprises a UHV laser deposition installation, time-resolved absorption spectroscopy setups, with one commercial Harpia systems from Lightconversion and several amplified laser systems for THz generation and time resolved optical and magneto-optical studies.
The candidate will be deeply involved both in PLD based thin film growth and in the femtosecond laser lab built by the materials and light team. Daily work involves strong interactions with all team members working in the laser lab.

Skills and knowledge required
Candidates must have good knowledge in material growth, in optics and solids-state physics. Also, a strong background in optical spectroscopy and the related experimental techniques is recommended. Good skills in programming, interfacing, data treatment and analysis (if possible, by using Python) are strongly appreciated. The PhD candidate will be strongly encouraged to present the scientific results in national and international conferences. English is the working language in the team. We are looking for candidates with a taste for experimental work in an intercultural team with good communication skills and developing a meticulous approach with an eye for detail.

Employment conditions:
The contracts established by the CNRS comprises the monthly salary, retirement contributions and gives right to full social security coverage and unemployment benefits. PhD students have no teaching obligations at University of Rennes. Rennes is a medium size French city less 1h30 train ride from Paris and 1h from the sea coast, offering a relaxing lifestyle with many cultural and sport activities.

The Team:
The candidate will work inside the “Materials and Light Department” at the Institute of Physics of Rennes and will be associated also to the “Nanoscience and Materials Department”.
The Material and Light team is currently a major actor of the international French-Japanese laboratory IRL DYNACOM involving French and Japanese Universities (CNRS, University of Rennes, University of Tokyo, Tohoku University) and has a worldwide recognition in the field of ultrafast science. The working environment is multicultural and international.
Our research is focused on the growth of functional magnetic oxides, ultrafast out-of-equilibrium phenomena in materials and molecules using optical and X-ray techniques with femtosecond to picosecond time resolution.
More information can be found on the website:
https://ipr.univ-rennes.fr/en/materials-and-light-departement

References :
[1] A. V. Kimel et al., Nature 429, 850 (2004).
[2] A. H. M. Reid et al., Appl. Phys. Lett. 106, 082403 (2015).
[3] K. Grishunin et al., ACS Photonics 5, 1375 (2018).
[4] T. S. Seifert et al., Appl. Phys. Lett. 120, 180401 (2022).
[5] E. Rongione et al., Nat Commun 14, 1818 (2023).
[6] T. Moriyasu et al., in Ultrafast Phenomena XIX, edited by K. Yamanouchi et al. (Springer International Publishing, Cham, 2015), pp. 653–656.
[7] A. V. Kimel and A. K. Zvezdin, Low Temperature Physics 41, 682 (2015).
[8] H. Ueda et al., Phys. Rev. Res. 4, 023007 (2022).

Le poste se situe dans un secteur relevant de la protection du potentiel scientifique et technique (PPST), et nécessite donc, conformément à la réglementation, que votre arrivée soit autorisée par l'autorité compétente du MESR.

Contraintes et risques

The candidate will be working with a UHV-laser deposition installation, a magneto-optical spectroscopy setup, with time-resolved optical absorption, THz spectroscopy and time-resolved magneto-optic techniques. She/he will be trained in the use of a PLD growth facility, the use of very intense and ultrafast lasers as well as time-resolved spectroscopy.