Description du sujet de thèse

Liquid crystal-based tunable semiconductor photodetectors for optical sensing and structural health monitoring

Contexte de travail

Context
In most industries such as aeronautic, space, railway or nuclear industry, a failure can create a disaster and asserting structural integrity is thus highly critical. Structural Health Monitoring (SHM) consists in embedding sensors in a structure to monitor its state while in use. It has been of increasing interest for the last decades due to numerous advantages in terms of security, cost, and environmental impacts. Optical fiber sensors are very promising for SHM because of their low intrusivity and adequation to many harsh conditions such as extreme temperatures, radiative or electromagnetic environments. In particular, FBG (fiber Bragg grating) sensors are of great interest for acoustic waves measurement in highly sensitive infrastructures.

Objectives
In the frame of the ANR project “FBG-DETECT”, a PhD position is open at LAAS-CNRS, Toulouse, to work on the demonstration of a new photonic device for FBG sensor interrogation. This device is a tunable Liquid Crystal-based Photo-Detector (LC-PD) recently demonstrated by our consortium [1]. Its tuning principle is based on the variation of the birefringence of a liquid crystal infiltered inside a polymer microcell integrated onto III-V semiconductor photodiodes (InGaAs/InP). A stable and reproducible spectral tuning by more than 80 nm around 1.55 µm was demonstrated and with a very low energy consumption, making LC-PDs good candidates for FBG interrogation. However, to meet the system requirements, their sensitivity and linewidth have to be improved and their individual control into a multiplexed system should be enabled, which is not possible with the current hybrid technology. To tackle these new challenges, we propose to develop an innovative nano-structuration method for directly aligning the LC molecules inside a III-V semiconductor microcell. It is based on the use of self-assembled GaAs quantum dashes grown by droplet epitaxy [2]. We will explore the potentialities of this new approach to demonstrate the first monolithic tunable LC-PD and optimize its design and performances for FBG sensing.

[1] C. Levallois, et al, Opt. Express 26, 25952-25961 (2018) https://doi.org/10.1364/OE.26.025952
[2] H. Villanti et al 2025, Appl. Phys. Express in press, https://doi.org/10.35848/1882-0786/adb3eb

Activities
The PhD will work on the droplet epitaxy of self-assembled GaAs quantum dashes. She/he will optimize the growth and thermal post-treatment conditions to control the dashes' anisotropy factor and LC alignment properties (uniformity, contrast, anchoring energy…). She/will be also implied in several campaigns of technological processes (photolithography, wet and dry etchings, metallization, nanoimprint, LC infiltration, mounting, …) to fabricate III-V semiconductor microcells embedding quantum dashes, as well as complete tunable optical tunable devices (filters, detectors). This work will require a strong involvement in structural and opto-electrical characterizations.
The student will be part of the MPN team of the LAAS-CNRS Laboratory located in Toulouse. This research work will be carried out within the laboratory's technological and characterization platform under the supervision of experienced engineers in TEAM service.The PhD student will benefit from all the facilities available at LAAS-CNRS: micro and nanotechnology and characterization platforms which are part of the RENATECH national technological network.
After training phases, she/he will be in charge of:
- the experimental work
- the analysis and interpretation of the results
- the results presentations at international conferences and in publications.

Skills
1/ Theoretical knowledges (mandatory): material science, nanostructures, photonics and physics of optoelectronic devices.
2/ Candidates should have technical skills in at least one of the following areas:
- Molecular beam epitaxy (nanostructures, optical thin films, …)
- Photonic device processing (photolithography, spin-coating, etching, electrode deposition, ...)
- Characterization of nanostructures (XRD, AFM, SEM, ...)
- Characterization of photonic devices (polarized optical microscopy, FTIR spectroscopy, optical tests under probes, …).
3/ Skills in promoting results by writing reports or presenting at meetings.
4/ Other skills: written/spoken English.

The candidate must have a M2 or engineer degree in applied optics/photonics. The ideal candidate for this project should demonstrate solid experience and motivation in photonic devices growth, technology and characterization. Experience in epitaxy will be an asset. The application must include a detailed CV, a cover letter, and at least two references who can be contacted.

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

We are looking for a student who will be able to get involved in experimental work in clean room and highly motivated. In addition, the candidate must be able to work independently as well as with other project members and collaborators depending on the progress of the project led. This study will be led in strong interaction through regularly scheduled meetings with our partners: IMT Atlantique, Brest (liquid crystals properties), FOTON Institute, Rennes (photodetectors design and fabrication) and CEA-LIST, Saclay (FBG sensors requirements). Short stays at IMT and I. FOTON are planned during the project.