Laurent Dussopt

Broadband terahertz imaging with highly sensitive silicon CMOS detectors

This paper investigates terahertz detectors fabricated in a low-cost 130 nm silicon CMOS technology. We show that the detectors consisting of a nMOS field effect transistor as rectifying element and an integrated bow-tie coupling antenna achieve a record responsivity above 5 kV/W and a noise equivalent power below 10 pW/Hz(0.5) in the important atmospheric window around 300 GHz and at room temperature. We demonstrate furthermore that the same detectors are efficient for imaging in a very wide frequency range from ~0.27 THz up to 1.05 THz. These results pave the way towards high sensitivity focal plane arrays in silicon for terahertz imaging.

MEMS-based reconfigurable antennas

In this paper, two frequency reconfigurable antennas using the ON/OFF states of a microelectromechanical system (MEMS), are presented. A multifrequency planar inverted-F antenna (PIFA) is designed with a L-shaped open slot upon its main plate. As a proof-of-principle, an ON state RF MEMS switch, modeled by a piece of copper (1x1 mm/sup 2/), is inserted into the slot to achieve frequency switching (while OFF state is modeled without the piece of copper). According to the switch position alone the slot, the GSM operating frequency is not detuned while the other standards become reconfigurable. To validate our concept, a thermal MEMS switch, designed and developed at CEA-LETI is used. The MEMS switch is presented and its scattering parameters are measured and compared with simulated results from an equivalent RLC circuit. The active device is then inserted in the slot to valid the simulated frequency behaviour of the PIFA. Based on the same concept, a planar antenna is designed. It consists in a printed PIFA on a high-resistivity silicon substrate (10 k/spl Omega/.cm). The antenna resonates in the 2.4 GHz WLAN band but the insertion of two small gaps in its main aim which model the OFF state of a MEMS switch allows the antenna to work in the 5.150-5.875 GHz HIPERLAN 2 frequency bands.

Imaging above 1 THz limit with Si-MOSFET detectors

We demonstrate that a proper antenna and transistor design can provide high responsivity for Terahertz radiation and imaging capability even above the 1 THz limit with a low-cost 130 nm CMOS technology. This result opens the way to CMOS THz imagers working at high frequencies and therefore exhibiting high a spatial resolution — down to ~300 µm.

THz imaging with low-cost 130 nm CMOS transistors

We report on active imaging with CMOS transistors at 300 GHz and 1.05 THz. Two basic focal plane arrays consisting of nMOS transistors and wide-band bow-tie antennas have been implemented in a low-cost 130 nm CMOS technology. Raster scan imaging of objects concealed in a paper envelope has been achieved at 300 GHz with a commercial radiation source. The images clearly reveal the concealed objects with a dynamic range of 35 dB and a resolution of 3 mm. At 1.05 THz, the pixels achieve a responsivity of 50 V/W and a noise equivalent power of 900 pW/Hz0.5.

Towards industrial applications of terahertz real-time imaging

Recent innovations in photonics and nanotechnology are now enabling terahertz (THz) research to be applied in many industrial fields such as homeland security, information and communications technology (ICT), biology and medical science, non-destructive tests or quality control of food and agricultural products. Still many challenges are to be addressed, the main one being to provide THz systems with sufficient signal to noise ratio when operated in real industrials conditions. In addition, cost is a key lock that hampers the spread of this technology but it is clear that cost-effective sources and detectors compatible with standard microelectronics will drive down the overall cost, and in particular will make THz imaging accessible for industrial use. In order to bring THz imaging to industry, Leti has been developing over the past decade complementary CMOS-compatible uncooled imaging 2D-array technologies: antenna-coupled bolometers and Field Effect Transistor detectors. In addition, CEATech built a test platform dedicated to the development of industrial prototypes of photonics technologies. In particular, in collaboration with i2S, this platform includes the TZCAM camera equipped with Leti’s 320×240 bolometric pixel array and gives access to a full industrial THz imaging chain that is essential for maturation of this emerging technology. This paper gives an overview of these developments and illustrates industrial applications with examples of uncooled THz imaging tests, e.g. opaque object 2D inspection or 3D tomography.

Multistandard PIFA antenna

In this paper, a frequency reconfigurable antenna using the ON/OFF states of a MEMS (Micro-Electro-Mechanical System) switch, is presented. This structure is based on a multifrequency planar inverted-F antenna (PIFA), designed with an L-shaped open slot etched upon its main plate. As a proof-of-principle, an ON state RF MEMS switch, modeled by a piece of copper of 1×1mm2, is inserted into the slot to achieve frequency switching (OFF state is modeled without this piece of copper). According to the switch position along the slot, the GSM operating frequency is not detuned while other standards become reconfigurable. To validate our concept, a thermal MEMS switch [1,2], designed and developped at CEA-LETI is used. The MEMS switch is presented and its scattering parameters are measured and compared with simulated results from a simple equivalent RLC circuit. The active device is then inserted in the slot to validate the simulated frequency capabilities of the PIFA.