We measure the direct detection effect in a small volume ͑0.15 m ϫ 1 m ϫ 3.5 nm͒ quasioptical NbN phonon cooled hot electron bolometer mixer at 1.6 THz. We find that the small signal sensitivity of the receiver is underestimated by 35% due to the direct detection effect and that the optimal operating point is shifted to higher bias voltages when using calibration loads of 300 K and 77 K. Using a 200 GHz bandpass filter at 4.2 K the direct detection effect virtually disappears. This has important implications for the calibration procedure of these receivers in real telescope systems. © 2005 American Institute of Physics. ͓DOI: 10.1063/1.1887812͔ NbN phonon cooled hot electron bolometer ͑HEB͒ mixers are currently the most sensitive heterodyne detectors at frequencies above 1.2 THz.1,2 They combine a good sensitivity ͑8-15 times the quantum limit͒, an IF bandwidth of the order of 4 -6 GHz, 3-6 and a wide RF bandwidth from 0.7 to 5.2 THz. However, for use in a space based observatory, such as Herschel, it is of vital importance that the local oscillator ͑LO͒ power requirement of the mixer is compatible with the low output power of present day THz LO sources. 7 This can be achieved by reducing the mixer volume and critical current density.5 However, the large RF bandwidth and low LO power requirement of such a mixer result in a direct detection effect, characterized by a change in the bias current of the HEB when changing the RF signal from a black body load at 300 K to one at 77 K. [8][9][10][11] As a result the measured sensitivity using a 300 K and 77 K calibration load differs significantly from the small signal sensitivity relevant for astronomical observations. In this article we describe a set of dedicated experiments to characterize the direct detection effect for a small volume quasioptical NbN phonon cooled HEB mixer.The devices are fabricated on a high purity Si wafer that is covered at MSPU, Moscow with a NbN film with T c = 9.3 K and an expected thickness of 3.5 nm. The fabrication is mostly identical to the process described in Refs. 3 and 12, however, in stead of a spiral antenna we use a twin slot antenna with a center frequency of 1.6 THz and a bandwidth of 0.9 THz. The bolometer length is 0.15 m, the width 1 m, the critical current I c =68 A at 4.2 K and the normal state resistance is 170 ⍀ at 11 K. In the experiment we use a quasi-optical coupling scheme in which the HEB mixer chip is glued to the center of an uncoated elliptical Si lens. The lens is placed in a mixerblock thermally anchored to the 4.2 K plate of a liquid Helium cryostat. We use one Zytex G104® at 77 K as infrared filter and 0.9 mm HDPE as vacuum window. The LO power required to reach the optimal pumping level of the mixer, as determined by the isothermal technique, P LO,iso = 30 nW. The real LO power need P LO , determined from the output power of a calibrated LO source and the known optics losses, has been estimated to be 2.4 times larger for similar mixers, 13 hence P LO = 70 nW. In the first experiment we measure the uncor...

We characterize superconducting antenna-coupled hot-electron bolometers for direct detection of terahertz radiation operating at a temperature of 9.0 K. The estimated value of responsivity obtained from lumped-element theory is strongly different from the measured one. A numerical calculation of the detector responsivity is developed, using the Euler method, applied to the system of heat balance equations written in recurrent form. This distributed element model takes into account the effect of nonuniform heating of the detector along its length and provides results that are in better agreement with the experiment. At a signal frequency of 2.5 THz, the measured value of the optical detector noise equivalent power is 2.0×10 -13 W·Hz -0.5 . The value of the bolometer time constant is 35 ps. The corresponding energy resolution is about 3 aJ. This detector has a sensitivity similar to that of the state-of-the-art sub-millimeter detectors operating at accessible cryogenic temperatures, but with a response time several orders of magnitude shorter.  Index Terms-Radiation detectors, hot electron bolometer, frequency response.

Nonequilibrium interpretation of DC properties of NbN superconducting hot electron bolometersShcherbatenko, M.; Tretyakov, I.; Lobanov, Yu; Maslennikov, S. N.; Kaurova, N.; Finkel, Matvey; Voronov, B.; Goltsman, G.; Klapwijk, Teun

No abstract