GNT: Graphene and Nanotubes

Thales Research & Technology (TRT) has recently proposed new concepts of optically driven electron sources based on carbon nanotubes (CNTs).

The envisaged applications are:
- 3D X-ray imaging using stationary CT systems for security and medical applications,
- compact, light and highly efficient microwave amplifiers for satellite communication,
- high power THz sources.

Classical computed tomography scanners incorporate a rotating ring or "gantry" which holds the X-ray source and the X-ray sensors. They exhibit a high resolution but are large, heavy and are characterised by a low throughput. Using multiple X-ray sources, low cost, stationary (gantry free) and efficient scanners could be fabricated. Each electron source can be advantageously a CNT photocathode driven by a laser diode. The emission current is proportional to the laser power and this leads to a direct and accurate control of the emission current and thus of the X-ray source. As the circuit driving the laser power is insulated from the cathode, this cathode can be biased at high voltage and the anode can be grounded. The anode cooling is then facilitated. The long term objective of this project is to develop a new generation of optically switchable multiple X-ray source for compact, efficient and low cost CT scanners for medical applications.

The second application of CNT-based photocathodes relates to microwave amplifiers and particularly to the large bandwidth travelling-wave tubes (TWTs), which are used on satellites for telecommunication. In a TWT, signal amplification is based upon the interaction between a modulated electron beam and an electromagnetic (EM) wave. So far, only thermionic cathodes emitting a continuous electron beam have been used. The beam post modulation, which is required to amplify the microwave signal roughly takes half of the tube length. Furthermore, simulations have shown that directly modulating the electron beam increases the efficiency of the interaction of the beam with the EM wave. Consequently, the use of a photocathode, which is able to deliver a high frequency modulated beam, would enable the fabrication of light, compact and highly efficient amplifiers. These features are particularly interesting for satellite communication.

The third application relates to high power (100 mW) THz sources based on THz cathodes. These cathodes are based on the direct interaction between a dual frequency laser (with THz offset) and a carbon nanotube. To obtain an efficient coupling, the CNTs are grown on nanophotonic devices.

The objective of the postdoc is to design, model and characterise the photocathodes for the three applications presented above.
The postdoc will work in the Nanocarb Lab (Joint team between Ecole Polytechnique and TRT) which includes 6 growth/deposition systems (for CNTs and semiconducting nanowires) and several experimental setups to characterise (optically controlled) solid state and field emission devices. In particular a field emission microscope will be used to study individual CNTs. This will allow the postdoc to validate the proposed theoretical model.
The candidate will collaborate with different Laboratories, including Technological Labs (clean room micro/nano fabrication), Analysis Labs (TEM, Raman,...) and the Nanophotonic Lab.

The candidate will have a doctoral degree, or equivalent, in a relevant discipline (Electronics, Optronics, Nanophotonics) with an emphasis on optoelectronic devices.

Contact: pierre.legagneux@thalesgroup.com