Large focal plane arrays for terahertz imaging

Abstract

The increasing attention for Terahertz technology is due to the number of potential applications that may benefit from its use (medical, defense, space). The matter in the universe has a rich spectral content in the THz range, therefore, very high resolution cameras that are able to image at these frequencies will contribute to an understanding of our universe, its origin and its formation, deeper than previous cameras working at other frequencies or with a lower resolving power. SPICA, a future Japanese deep space science mission, will host a European far-infrared imaging spectrometer, SAFARI. Focal plane arrays play a key role as part of this instrument, since parallel acquisition is required to reduce integration time. The Kinetic Inductance Detector (KID) is a very promising device for several reasons: it can be very sensitive (NEP of the order of ... it can be easily arranged in an array configuration, allowing a powerful and effective frequency multiplexing scheme, and it can be easily integrated with a lens antenna to couple the THz radiation to the sensor more effectively. A KID is basically a superconducting microwave resonator, whose characteristics are modified by the incoming THz radiation. Specifically, the resonator consists of a superconducting coplanar waveguide (CPW), where electrons are condensed in a macroscopic quantum state, formed by paired electrons: Cooper pairs. Since the energy needed to break a Cooper pair is smaller than the energy of a photon in the THz frequency range, the photons associated to the absorbed THz radiation are able to break these pairs into quasi-particle excitations, subsequently modifying the superconducting properties of the CPW. This results in a slight deviation of the CPW kinetic inductance, and consequently in a modification of the waveguide propagation constant, which produces an appreciable shift in the resonant frequency of the device. This change can be measured by detecting the shift of the resonance frequency induced in the transmittance S21 of the through line the KID is coupled to. A proper antenna design is a key issue to ensure an optimum behavior of the detector. Several single-pixel antenna designs have been proposed. The analysis of each of them has resulted in the identification of the most suited for our application (the in-line X-slot antenna). It has been shown that, above 700 GHz, using antennas fed by corporate lines, and realized with the same superconducting material as the detector, lowers the system responsivity and broadens the array pattern due to incoherent absorption in the branches of the feed line. Therefore, the use of an antenna with a single feed point is suggested. If the antenna is integrated with the resonator line, the quasi-particle efficiency, i.e. the ratio between the power exploited to break Cooper pairs and the total power absorbed by the antenna, is almost doubled. The behavior of the KID, when two antennas are placed in series within the resonator line, has been analyzed and the results show that the responsivity can be doubled, if the two antennas are identical and a proper length of the line between the two is chosen. If the two antennas are orthogonally polarized, by choosing a proper distance between the two, it is possible to obtain a constant responsivity of the KID, whereas this would drop to zero for a complete polarization mismatch between the incoming wave and the antenna. Placing two antennas in the focal plane of the integrated lens, but not in the focal point, is not optimal from the radiation-e±ciency point of view. Therefore, this kind of configuration can be used efficiently if the lens can be shaped to optimize the coupling efficiency, or if the use of lenses is not necessary. Modeling of arrays of lenses in reception has been carried out to analyze the interaction between the incoming wave and the antenna-plus-sensor system and to analyze the coupling between adjacent lenses. The tool is based on a mixed approach that makes use of Geometrical Optics, Physical Optics and a full-wave model. Moreover, the model has not only been implemented for the specific problem of THz imaging, but it is now an instrument for the analysis of arrays of lenses for more general purposes.