Cold-Electron Bolometer with Superconductor-Insulator-Normal Metal Tunnel Junctions

Licentiate thesis, 2006

The Cold-Electron Bolometer (CEB) is an ultrasensitive device designed for the detection of radiation in the terahertz region of the electromagnetic spectrum. The key to its sensitivity is the electron cooling of the absorber by the superconductor-insulator-normal metal (SIN) tunnel junction. At a voltage near and below the superconducting gap, the electrons in the absorber are cooled well below the phonon temperature of the normal metal. This translates to the enhanced sensitivity of the CEB. This thesis describes the work we have done on the optimization of electron cooling of the normal metal absorber, and our measurement of the sensitivity of the CEB. We have optimized the electron cooling of the absorber by SIN tunnel junctions. The best electron cooling was achieved when normal metal traps were added in proximity to the superconducting electrodes in addition to the advanced geometry of the superconducting electrodes. With these modifications, we have decreased the electron temperature by 198 mK. With just the advanced geometry, the electron temperature drop was 129 mK. With just a simple geometry, the drop in temperature was 56 mK. The noise equivalent power (NEP) of the CEB was also measured at 100 mK to be better than 10-18 W/Hz1/2 for frequencies above 100 Hz. The NEP was obtained by measuring the noise of the CEB, and then dividing that by the voltage response to a small applied modulated heating power to the absorber. The main limitation in our measurements was the noise component from the amplifier. Finally, we have made measurements on the temperature sensitivity of the SIN tunnel junctions. We have compared the sensitivity between single and ten SIN junctions in series and found that it increases proportionally to the number of junctions. The best responsivity obtained for 10 junctions was approximately 15 μV/mK. Using such thermometer, we have been able to measure the temperature stability of the Oxford Instruments cryogenfree refrigerator. The short time temperature sensitivity was measured to be 30 μK, limited only by noise due to the amplifier. The increase in the number of junctions improves the temperature sensitivity and decreases the noise component due to the amplifier. Our future work would thus include sensitivity measurements of 30 to 100 SIN junctions.