Electrically tunable nanostructures on semiconductors for infrared – nanoELASTIR

This project aims at developping nanostructured spectral filters made of semiconductors, contrary to usual materials (metal and dielectrics). We will address the LWIR spectral range. We follow two goals:

- we will demonstrate monolithic integration of the filters on T2SL photodetectors
- we will demonstrate the ability to realize dynamic spectral filtering thanks to electrical actuation.

The target is the scan of several hundreds-wide spectral range, with a maximum voltage of 5V. To reach these goals, we will rely on semiconductors such as GaSb and heavily doped InAs (HDS - heavily doped semiconductor). Thanks to heavy doping, InAs will have "metallic" properties in the spectral range of interest. This HDS can be thus used instead of metals in usual nanostructured filters architectures. Furthermore, T2SL photodetectors are also made of InAs, GaSb and other material with the same lattice parameter.
Thus, the monolitic integration of the filter on the photodetectors is possible. Then, we will study a mechanism of tunability based on the move of the free carriers in the HDS, when voltage is applied to the component. The concentration of these carriers will then be higher in the vicinity of a barrier, and refractive index will there be modified. If the barrier is properly placed in the nanostructured component, the resonant conditions will lead to a modification of filtering wavelength.

Four components will be fabricated and each of them will be designed, fabricated and characterized. These components are:
- nanostructured spectral filter made of semiconductors : this will validate the conception experimentaly
- a LWIR photodiode with a nanostructured spectral filter on it : this will validate the monolitic integration
- a component without nanostructures which will allow to validate experimentaly the barrier
- a tunable nanostructured spectral filter grown on photodetector, which is the final goal of the project

The work will be divided into three parts:
- conception (electromagnetism to design nanostructures and electronics to design the barrier)
- nanofabrication of components in clean-room
- characterisation (experimental validation of the optical functions)

We believe that these components will be building blocks for the next generation of multispectral imaging system with high frequency acquisition. These building blocks will break the traditionnal compromise between spatial and spectral resolution. Besides, the concept of tunability with barrier could be used in other applications, such as nanostructured thermal sources.

Two laboratories will be part of the consortium :
(1) ONERA, leader, where conception of nanostructures will be done and also electro-optical
characterization of components ;
(2) Montpellier University, where epitaxial growth, most of fabrication process and conception of barrier will be done.

We underline the fact that promising experimental and theoretical results have been obtained by our teams these last months. These results have not been published yet, but a summary is described in the proposal.