We report on the successful fabrication of low leakage aluminium
superconducting tunnel junctions with very homogeneous and transparent
insulating barriers. The junctions were tested in an adiabatic demagnetisation
refrigerator with a base temperature of 35 mK. The normal resistance of the
junctions is equal to ~7 uohm cm2 with leakage currents in the bias voltage
domain as low as 100 fA/um2. Optical single photon counting experiments show a
very high responsivity with charge amplification factors in excess of 100. The
total resolving power (including electronic noise) for 500 nm photons is equal
to 13 compared to a theoretical tunnel limited value of 34. The current devices
are found to be limited spectroscopically by spatial inhomogeneities in the
detectors responseComment: 3 pages, 5 figure