Aurélie Gensbittel

Radiofrequency Characterization of Gold/Ferroelectric SrBi 2 Nb 2 O 9 Heterostructures for Tunable Devices

In order to show the potential interest of SrBi 2 Nb 2 O 9 (SBN) ferroelectric films to develop microwave agile devices, we have undertaken the radiofrequency characterization of structures patterned in Au/SBN bilayers deposited on LaAlO 3 and MgO single-crystals. Both coplanar waveguide transmission lines and 10 GHz half-wavelength resonators were characterized in the 100 kHz to 10 GHz frequency range and in the 300 K to 100 K temperature range.

Broadband dielectric and IR pyroelectric response of amorphous Y-Ba-Cu-O oxygen depleted thin films

We report on the study of dielectric properties of amorphous semiconducting YBa2Cu3O6+x (x < 0.5, YBCO) thin films with two different thicknesses measured in a broad frequency range (from 40 Hz to 2 GHz) and in the 210 to 430 K temperature range, using a coaxial discontinuity technique. At all temperatures, dielectric permittivity and conductivity spectra exhibit typical features of dielectric relaxation processes. A first relaxation at low frequency (300 Hz - 2 kHz), observed only at high temperature, might be attributed to interfacial effects. Grain boundaries of the YBCO thin films could explain the second relaxation observed at higher frequency (800 Hz - 660 kHz). The higher relaxation frequencies increase with temperature and film thickness and follow a thermally activated behavior of the Arrhenius-type. Application to IR detection with a film deposited on silicon has been targeted: the observed pyroelectric behavior of the detector exhibits a fast response in the μs range.

Ageing and embedding issues for high-Tc superconducting hot-electron bolometers for THz imaging

Superconducting hot electron bolometer (HEB) mixers are a competitive alternative to other technologies in the terahertz frequency range because of their ultrawide bandwidth, high conversion gain, and low intrinsic noise level. A process to fabricate stacked YBaCuO / PrBaCuO ultra-thin films (in the 15 to 40 nm range) etched to form 0.4 μm × 0.4 μm constrictions, elaborated on MgO (100) substrates, has been previously described. HEB structures were fabricated on such stacks, covered by log-periodic planar gold antennas, aiming at spanning the 0.9 to 7 THz range. Ageing effects were observed, however, with the consequence of increased electrical resistance, significant degradation of the regular bolometric response, so preventing HEB mixing action. Several measures have been attempted to address these problems, mainly by considering the embedding technological issues related to the YBaCuO constriction electrical coupling to the antenna and the intermediate frequency (IF) circuitry. For this purpose, the YBaCuO impedance was analyzed, and mismatch to antenna and IF strip was considered. Besides, THz antenna simulations were performed and validated against experiments on scaled models at GHz frequencies. Electromagnetic coupling to the incoming radiation was also studied, including crosstalk between neighbor antennas forming a linear imaging array.

YBCO hot-electron bolometers dedicated to THz detection and imaging: Embedding issues

High-Tc hot-electron bolometers (HEB) are an interesting alternative to other superconducting heterodyne mixers in the terahertz frequency range because of low-cost cooling investment, ultra-wide instantaneous bandwidth and low intrinsic noise level, even at 80 K. A technological process to fabricate stacked yttrium-based (YBCO) / praseodymium-based (PBCO) ultra-thin films (in the 15 to 40 nm thickness range) etched to form 0.5 μm × 0.5 μm constrictions, elaborated on (100) MgO substrates, has been previously described. Ageing effects were also considered, with the consequence of increased electrical resistance, significant degradation of the regular THz response and no HEB mixing action. Electron and UV lithography steps are revisited here to realize HEB mixers based on nano-bridges covered by a log-periodic planar gold antenna, dedicated to the 1 to 7 THz range. Several measures have been attempted to reduce the conversion losses, mainly by considering the embedding issues related to the YBCO nano-bridge impedance matching to the antenna and the design of optimized intermediate frequency circuitry. Antenna simulations were performed and validated through experiments on scaled models at GHz frequencies. Electromagnetic coupling to the incoming radiation was also studied, including crosstalk between neighbour antennas forming a linear imaging array.

In-plane tunability of coplanar microwave devices by SrBi2Nb2O9 ferroelectric thin films

In order to investigate the dielectric properties of SrBi 2 Nb 2 O 9 (SBN) ferroelectric thin films in the microwave frequency range, we have characterized coplanar waveguide transmission lines patterned in Au/SBN heterostructures deposited on MgO single-crystals. Various line widths and ferroelectric film thicknesses were considered. From measurements performed in the 500 MHz to 40 GHz range, the values of SBN dielectric permittivity and loss tangent were extracted. Results showed the potential interest of SBN films to develop microwave agile devices. The ferroelectric behaviour of the SBN films was also studied.