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 demonstrate the phase locking of a 2.7 THz metal-metal waveguide quantum cascade laser (QCL) to an external microwave signal. The reference is the 15th harmonic, generated by a semiconductor superlattice nonlinear device, of a signal at 182 GHz, which itself is generated by a multiplier chain ͑ϫ12͒ from a microwave synthesizer at ϳ15 GHz. Both laser and reference radiations are coupled into a bolometer mixer, resulting in a beat signal, which is fed into a phase-lock loop. The spectral analysis of the beat signal confirms that the QCL is phase locked. This result opens the possibility to extend heterodyne interferometers into the far-infrared range. To proceed with the applications of terahertz QCLs as LOs in a heterodyne spectrometer, the stabilization of the frequency or phase is required to either eliminate the frequency jitter or to reduce the phase noise. For a heterodyne interferometer either on the earth [4] or in space [5], the phase locking of multiple LOs to a common reference at low frequency is essential. Phase locking [6] a laser to a reference means to control the phase of the laser radiation field precisely. This serves not only to stabilize the frequency but also to transfer the line profile of the reference to the laser. In the case of frequency locking, the laser's average frequency is fixed, but its linewidth remains equal to the laser's intrinsic linewidth. Several experiments to stabilize a terahertz QCL have been published. They are the frequency locking of a 3.1 THz QCL to a far-infrared (FIR) gas laser [7], the phase locking of the beat signal of two lateral modes of a terahertz QCL to a microwave reference [8], and the phase locking of a 1.5 THz QCL to a multiplier chain LO source [9]. Since the multiplier chain source has been demonstrated only up to about 2 THz, for a practical solution for the use of a QCL as LO beyond this frequency, the phase needs to be locked to an external reference that can be generated conveniently and should preferably be far below the QCL frequency. Therefore, an important challenge is the demonstration of the phase locking of a singlemode terahertz QCL to a microwave reference signal (MRS), which is the scheme commonly used in existing solid-state LOs. The MRS should be multiplied to a terahertz frequency in the vicinity of the laser frequency to obtain a beat note or an intermediate frequency (IF). As demonstrated in the measurements of FIR gas laser frequency, the harmonics of MRS at 3.8 THz [10] and at 4.3 THz [11] can be generated by Josephson junction harmonic mixers, resulting in a beat between the laser and the upconverted frequencies. Another commonly used harmonic generator is a Schottky diode [12].In this Letter, we report the phase locking of a 2.7 THz QCL to a harmonic generated from an MRS by a semiconductor superlattice (SL) nonlinear device in combination with a multiplier chain. We demonstrate a practical phase-locking scheme that is extendable to a much higher frequency and quantify the phaselocked power.The QCL used is based on the doubl...

We have measured the noise temperature of a single, sensitive superconducting NbN hot electron bolometer (HEB) mixer in a frequency range from 1.6 to 5.3 THz, using a setup with all the key components in vacuum. By analyzing the measured receiver noise temperature using a quantum noise (QN) model for HEB mixers, we confirm the effect of QN. The QN is found to be responsible for about half of the receiver noise at the highest frequency in our measurements. The β-factor (the quantum efficiency of the HEB) obtained experimentally agrees reasonably well with the calculated value.

We report on low noise terahertz mixers (1.4-1.9 THz) developed for the heterodyne spectrometer onboard the Herschel Space Observatory. The mixers employ double slot antenna integrated superconducting hot-electron bolometers (HEBs) made of thin NbN films. The mixer performance was characterized in terms of detection sensitivity across the entire rf band by using a Fourier transform spectrometer (from 0.5 to 2.5 THz, with 30 GHz resolution) and also by measuring the mixer noise temperature at a limited number of discrete frequencies. The lowest mixer noise temperature recorded was 750 K [double sideband (DSB)] at 1.6 THz and 950 K DSB at 1.9 THz local oscillator (LO) frequencies. Averaged across the intermediate frequency band of 2.4-4.8 GHz, the mixer noise temperature was 1100 K DSB at 1.6 THz and 1450 K DSB at 1.9 THz LO frequencies. The HEB heterodyne receiver stability has been analyzed and compared to the HEB stability in the direct detection mode. The optimal local oscillator power was determined and found to be in a 200-500 nW range.

We have studied the sensitivity of a superconducting NbN hot electron bolometer mixer integrated with a spiral antenna at 4.3 THz. Using hot/cold blackbody loads and a beam splitter all in vacuum, we measured a double sideband receiver noise temperature of 1300 K at the optimum local oscillator ͑LO͒ power of 330 nW, which is about 12 times the quantum noise ͑h / 2k B ͒. Our result indicates that there is no sign of degradation of the mixing process at the superterahertz frequencies. Moreover, a measurement method is introduced which allows us for an accurate determination of the sensitivity despite LO power fluctuations. © 2007 American Institute of Physics. ͓DOI: 10.1063/1.2819534͔Superconducting mixers 1 play a key role in astrophysics at terahertz frequencies, where the early universe radiates strongly. The availability of low noise superconductorinsulator-superconductor ͑SIS͒ mixers and hot electron bolometer ͑HEB͒ mixers has made the realization of highly sensitive spectrometers on ground, airborne, and space telescopes possible. An example of this is the heterodyne instrument for far infrared on the Herschel space telescope, 2 to be launched in 2008, where the heterodyne spectrometers are operated up to 1.3 THz using SIS mixers and further up to 1.9 THz using HEB mixers. For the next generation of space telescopes, it becomes highly desirable to demonstrate sensitive mixers in the frequency range between 2 and 6 THz. HEB mixers, which are currently the only devices suitable for this frequency range, have been reported up to 5.3 THz. [3][4][5] However, only few experiments have so far been done at the frequencies above 3 THz, namely, superterahertz frequencies, and the performance is relatively poor. 3,4 The noise temperature of a receiver is a crucial parameter that defines the ultimate sensitivity of the heterodyne spectrometer and the observation time. To achieve the low noise at superterahertz, several challenges are expected either in the mixer itself or in the testing technique. First, it is unclear whether the performance of HEBs will degrade. The relaxation of highly excited electrons due to increased photon energy can be complicated by cascade processes of emission and absorption of phonons. This can compete with the electron-electron interaction and thus may decrease the mixing efficiency. 6 Also, there is a concern of the quantum noise. 7 Second, it becomes more difficult to couple terahertz radiation to the HEB. Third, there is lack of local oscillators ͑LOs͒. Optically pumped far infrared ͑FIR͒ gas lasers are commonly used, but achieving stable output power is cumbersome. Terahertz quantum cascade lasers 8 ͑QCLs͒ are promising, stable solid-state LOs, 9 but still in a development stage. Finally, there is an increase in the air loss due to the absorption of terahertz radiation by water vapor, which can increase the receiver noise temperature and may also cause instability.In this letter, we report the measurement of a quasioptical NbN HEB mixer at 4.3 THz using a hot/cold load built in vacuum an...