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Nanostructures of Oxides for Terahertz IMaging Exploration

Final Activity Report Summary - NANOTIME (Nanostructures of oxides for terahertz imaging exploration)

The engineering project NANOTIME (Nanostructures of oxides for terahertz imaging exploration) funded ten young researchers, either to prepare a PhD thesis or spend shorter research periods (six or twelve months), on the terahertz (THz) imaging topic at LGEP-SUPELEC.

This field involves multidisciplinary issues both in research fields (materials science, technology, electronics, optics, instrumentation, modelling) and in application fields (astronomy, environment, aeronomy, civil security etc).

NANOTIME involved interrelated groups (ERD-SUPELEC, SME SATIMO, L2E-UPMC Univ Paris 06 and LERMA-Paris Observatory) and external collaborators (CEA-Grenoble, CNES, Alcatel Thales III-V Lab, CNRS-Thales research unit).

The scientific program encompassed the study of two types of innovative imaging devices:
Type 1: Uncooled 2D pixel arrays, made from new semiconducting oxides. The sensing pixels are conventional bolometers (i.e. radiation power sensitive) working in the direct detection mode, based on an active imaging principle (the scene is illuminated by a THz source). This device provides a good compromise between sensitivity and cost. Applications mainly concern civil security.
Type 2: High performance cooled linear pixel arrays made from high temperature superconducting oxides. The sensing pixels are hot electron nanobolometers (HEB) used in the heterodyne mode, based on a passive imaging principle (the scene provides the blackbody emission to be analysed). This device provides a high resolution high sensitivity choice, with moderate cooling cost (80 K lightweight refrigerator). Applications concern new generation of embarked THz heterodyne receivers for satellites or space probes for environment, Earth climate and astronomy studies.

Major scientific results:
- Fabrication of semiconductor YBCO films using a CMOS compatible process. Realisation of trilayer micro-bolometers (metal/YBCO/metal), leading to moderately low impedance devices (a few kohms).
- Thermal modelling of heat diffusion processes inside pixel arrays under illumination. Experimental validation.
- Design of high impedance antennas to couple the incident THz radiation with semiconducting bolometers. Validation with microwave large scale models.
- Design, realisation and test of a readout CMOS chip with feedback free 40 dB fixed voltage gain amplifiers.
- Fabrication of a linear three-HEB array from high-Tc superconducting YBCO ultrathin films.
- Design of wideband THz planar micro-antennas for coupling with HEBs. Validation with microwave large scale models. Final integration of the THz antennas with HEBs.
- Design and realisation of the microwave chain to process signals extracted from HEBs.
- Design of phase gratings for local oscillator power distribution among three, four and five HEB elements of an array. Fabrication and THz test. Realisation using MEMS techniques.

The research training method was based on:
1. Autonomy: Training made the fellows autonomous on experimental platforms. Complementary training (communication, project management...) was also offered by SUPELEC and UPMC Univ Paris 06, as well as training in French language.
2. Thematic mobility: The fellows were strongly encouraged to feel partners of a multipolar, although unique scientific project. A short meeting was organised weekly.
3. Responsibility: The fellows were invited to teach in small classes at UPMC, to mentor undergraduate students or to participate to the committee of the European NTTI summer school (New Trends in THz Imaging, two editions organised within NANOTIME and co-funded by Region Ile-de-France).
4. Employability issue: Visits to industrial / academic sites were organised for discussions. The fellow gave presentations on their activities during annual meetings where potential employers were invited. The fellows worked in a research / education context in line with the requirements of both industry and research, which helped them to find a job easily at the end of their training period within NANOTIME.