Terahertz multiband imaging obtained by thermophoto conversion to the infrared – Imhotep

Terahertz band (THz) band covers wavelengths between 100 µm (thermal infrared) and 1 mm (microwave). The THz radiation can penetrate a wide variety of non-conducting materials (clothing, cardboard, wood, plastic...) thus justifying its strong interest in safety applications and nondestructive testing. Developing commercial THz sources (photoconductive antennas sources, Gunn diodes or quantum cascade lasers) has expanded the scope of applications of THz detection and imaging.

The first prototypes of THz sensors operating at room temperature have been developed, and for most of them, have been obtained thanks to heavy developments from the infrared (IR) bolometers. NeTHIS markets an imager based on the principle of conversion of THz toward the infrared (IR). A membrane converts the THz radiation into heat, and again converts this heat into infrared radiation, the observation being made at the end by a commercial IR camera. The latter, if it profits from the considerable advances of IR imaging (cooled or not) is still strongly limited by the membrane scaling performance. Indeed, it must be thick enough (50 microns) to absorb THz radiation, which penalizes the response time and increases the detection noise.

Finally, note that all current solutions THz detection suffer from inherent limitations:
1) The accessible spectral ranges are very limited;
2) There are no spectral multi-band imaging devices.

In this favorable context and still in search of efficient solutions, the IMHOTEP project proposes to develop an effective conversion device of THz radiation to the IR with a membrane both finer (5 µm) and allowing multiband imaging. For this, we will use recent advances in nanophotonics to control absorption and emissivity of the structure, and involve them in the thermal behavior of the diffusion transient.

The device consists of a membrane with THz-absorbing antennas on one side, and on the other side, antennas convert back heat radiation to infrared. The mechanism of the device is as follows:
1) An antenna absorbs THz radiation in the membrane, which leads to a local heating of the membrane in a volume small compared with the wavelength. In the multi-band case, several antenna are absorbing different wavelengths while being spatially distant (typically of one wavelength).
2) The temperature rises in the membrane by thermal conduction. The key point is that the membrane is very thin compared to the wavelength, thus its temperature is almost uniform along its thickness, while the lateral transfer is much slower.
3) Antennas on the other side of the membrane are heated almost instantly and will be re-emitting in the infrared in a narrow band.

The proposed system is based on the results we have obtained in the thesis of Quentin Lévesque (funded by the DGA and defended on 10/16/14). We have demonstrated experimentally thermal emission on a narrow spectral band thanks to IR antennas with a 100% efficiency. We also showed that it was possible in a membrane system to overcome the loss by conduction and convection.

The project will validate, in a representative environment TeraHertz imaging situation, the basic features that are thermoconversion and multi-band spectral detection. These basic building blocks are the first step towards the development of matrices for imaging according to several scenarios (IR detector cooled or not, active or passive imagery, multi-spectral imaging).