Measurements in the infrared wavelength domain allow us to assess directly the physical state and energy balance of cool matter in space, thus enabling the detailed study of the various processes that govern the formation and early evolution of stars and planetary systems in the Milky Way and of galaxies over cosmic time. Previous infrared missions, from IRAS to Herschel, have revealed a great deal about the obscured Universe, but sensitivity has been limited because up to now it has not been possible to fly a telescope that is both large and cold. Such a facility is essential to address key astrophysical questions, especially concerning galaxy evolution and the development of planetary systems.SPICA is a mission concept aimed at taking the next step in mid-and far-infrared observational capability by combining a large and cold telescope with instruments employing state-of-the-art ultrasensitive detectors. The mission concept foresees a 2.5-meter diameter telescope cooled to below 8 K. Rather than using liquid cryogen, a combination of passive cooling and mechanical coolers will be used to cool both the telescope and the instruments. With cooling not dependent on a limited cryogen supply, the mission lifetime can extend significantly beyond the required three years. The combination of low telescope background and instruments with state-of-the-art detectors means that SPICA can provide a huge advance on the capabilities of previous missions.The SPICA instrument complement offers spectral resolving power ranging from R ∼50 through 11000 in the 17-230 µm domain as well as R ∼28.000 spectroscopy between 12 and 18 µm. Additionally SPICA will be capable of efficient 30-37 µm broad band mapping, and small field spectroscopic and polarimetric imaging in the 100-350 µm range. SPICA will enable far infrared spectroscopy with an unprecedented sensitivity of ∼ 5 × 10 −20 W/m 2 (5σ/1hr) -at least two orders of magnitude improvement over what has been attained to date. With this exceptional leap in performance, new domains in infrared astronomy will become accessible, allowing us, for example, to unravel definitively galaxy evolution and metal production over cosmic time, to study dust formation and evolution from very early epochs onwards, and to trace the formation history of planetary systems.

We have developed a model for the resistive transition in a transition edge sensor (TES) based on the model of a resistively and capacitively shunted junction, taking into account phase-slips of a superconducting system across the barriers of the tilted washing board potential. We obtained analytical expressions for the resistance of the TES, R(T, I), and its partial logarithmic derivatives αI and βI as functions of temperature and current. We have shown that all the major parameters describing the resistive state of the TES are determined by the dependence on temperature of the Josephson critical current, rather than by intrinsic properties of the S-N transition. The complex impedance of a pristine TES exhibits two-pole behaviour due to its own intrinsic reactance.

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We report the experimental evidence of the ac Josephson effect in a transition edge sensor (TES) operating in a frequency domain multiplexer and biased by ac voltage at MHz frequencies. The effect is observed by measuring the non-linear impedance of the sensor. The TES is treated as a weakly-linked superconducting system and within the resistively shunted junction model framework. We provide a full theoretical explanation of the results by finding the analytic solution of the non-inertial Langevian equation of the system and calculating the non-linear response of the detector to a large ac bias current in the presence of noise.Superconducting transition-edge sensors (TESs) are highly sensitive thermometers widely used as radiation detectors over an energy range from near infrared to gamma rays. In particular we are developing TESbased detectors for the infrared SAFARI/SPICA 1 and the X-ray XIFU/Athena 2 instruments. TESs are in most cases low impedance devices that operate in the voltage bias regime while the current is generally read-out by a SQUID current amplifier. Both a constant or an alternating bias voltage can be used 3,4 . In the latter case changes of the TES resistance induced by the thermal signal modulate the amplitude of the ac bias current. The small signal detector response is modelled in great details both under dc and ac bias 5,6 . Those models however do not fully explain all the physical phenomena recently observed in TESs. It has been recently demonstrated that TES-based devices behave as weak-links due to longitudinally induced superconductivity from the leads via the proximity effect 7 and a detailed experimental investigation of the weak-link effects in dc biased x-ray microcalorimeters has been reported 8 . Evidence of weaklink effects in ac biased TES microcalorimeters has been given 9 , but an adequate experimental and theoretical investigation is still missing. We previously proposed a theoretical framework 10 based on the resistively shunted junction model (RSJ) that can be used to describe the resistive state of a TES under dc bias. In this letter, we extend the model to calculate the stationary non-linear response of a TES to a large ac bias current in the presence of noise and we compare it to the experimental data obtained with a TES-based bolometer. We report a clear signature of the ac-Josephson effect in the TES biased at MHz frequencies.The general equation for the Frequency Domain Multiplexing (FDM) electrical circuit, simplified for a single a) Electronic mail: l.gottardi@sron.nl resonator is 6(1) where V (t) is the total voltage across the TES, L and C are respectively the inductance and the capacitance of the bias circuit, r s is the total stray resistance in the circuit and Z T ES is the TES impedance, which depends on temperature T and current I(t). As previously reported 8,11 , the superconducting leads proximitize the TES bilayer film over a distance defined by the coherence length ξ. As a result, the superconducting order parameter |Ψ| is spatially dependent over the ...

SRON is developing ultra-low noise Transition Edge Sensors (TESs) based on a superconducting Ti/Au bilayer on a suspended SiN island with SiN legs for the SAFARI instrument aboard the SPICA mission. We successfully fabricated TESs with very narrow (0.5-0.7 µm) and thin (0.25 µm) SiN legs on different sizes of SiN islands using deep reactiveion etching process. The pixel size is 840×840 µm 2 and there are variety of designs with and without optical absorbers. For TESs without absorbers, we measured electrical NEPs as low as <1×10 -19 W/√Hz with response time of 0.3 ms and reached the phonon noise limit. Using TESs with absorbers, we quantified the darkness of our setup and confirmed a photon noise level of 2×10 -19 W/√Hz. All bolometers are 50×50 µm 2 and made of Ti/Au (16/65 nm) bilayer on a 0.25 µm thick suspended SiN island. The absorbers are 200×200 µm 2 and made of 8 nm thick tantalum (Ta). The large SiN islands are 220×280 µm 2 and the small ones 70×70 µm 2 . The SiN legs are about 0.5 µm wide and 350 µm long for large devices and 460 µm for small ones.Fabrication of TES bolometers includes 14 lithography steps and finishes with a deep reactive-ion etching (DRIE) process that is explained in detail by Ridder et al [2]. *P.Khosropanah@sron.nl; phone +31 88777 5678; fax +31 88777 5601; sron.nl Millimeter, Submillimeter, and Far-Infrared Detectors and Instrumentation for Astronomy VIII, edited by Wayne S. Holland, Jonas Zmuidzinas, Proc. of SPIE Vol. 9914, 99140B