We prepare NbN thin films by DC magnetron sputtering on [100] GaAs substrates, optimise their quality and demonstrate their use for efficient single photon detection in the near-infrared. The interrelation between the Nb:N content, growth temperature and crystal quality is established for 4−22nm thick films. Optimised films exhibit a superconducting critical temperature of 12.6±0.2K for a film thickness of 22 ± 0.5nm and 10.2 ± 0.2K for 4 ± 0.5nm thick films that are suitable for single photon detection. The optimum growth temperature is shown to be ∼ 475 • C reflecting a trade-off between enhanced surface diffusion, which improves the crystal quality, and arsenic evaporation from the GaAs substrate. Analysis of the elemental composition of the films provides strong evidence that the δ-phase of NbN is formed in optimised samples, controlled primarily via the nitrogen partial pressure during growth. By patterning optimum 4nm and 22nm thick films into a 100nm wide, 369µm long nanowire meander using electron beam lithography and reactive ion etching, we fabricated single photon detectors on GaAs substrates. Time-resolved studies of the photo-response, absolute detection efficiency and dark count rates of these detectors as a function of the bias current reveal maximum single photon detection efficiencies as high as 21 ± 2% at 4.3 ± 0.1K with ∼ 50k dark counts per second for bias currents of 98%I C at a wavelength of 950nm. As expected, similar detectors fabricated from 22nm thick films exhibit much lower efficiencies (0.004%) with very low dark count rates ≤ 3cps. The maximum lateral extension of a photo-generated hotspot is estimated to be 30±8nm, clearly identifying the low detection efficiency and dark count rate of the thick film detectors as arising from hotspot cooling via the heat reservoir provided by the NbN film. PACS numbers: 74.78.-w 74.25.Gz 78.67.Uh 85.25.Oj 85.25.-j 42.50.-p

The ablation of Al2O3 by CO2 laser radiation is investigated both theoretically and experimentally. The model connects the laser-induced phase transition from condensed to vaporized state of the target and the dynamic of the emerging process plasma. The plasma is described in a two-fluid approximation by use of non-dissipative gas-dynamical equations incorporating absorption of laser radiation in the plasma and the dynamic of its ionization state. In the experimental part, the geometry of the luminous process plasma above the target at different instances is detected and the weight loss of the target as a function of the fluence is measured. At an Ar-base pressure below 1 mbar, both calculated and measured results reveal that there exist two zones in the process plasma: one which is directly attached to the target surface throughout the whole process, and another which is recognized as an outward moving shock front. Further, it is seen from both approaches that, due to absorption of laser radiation by the plasma, the weight loss has a local maximum.

The weight loss due to laser-induced vaporization of alumina and zirconia is investigated both theoretically and experimentally. To calculate the weight loss of the target under laser irradiation a model is used describing the laser-induced vaporization and the dynamics of the vapour/plasma state including the absorption of laser radiation. The weight loss is not an increasing function of the fluence over the whole range but has a local maximum due to absorption of laser radiation in the plasma state. The energy deposited in the plasma state due to the absorption of laser radiation is calculated and compared to the weight loss data. Different pulse shapes of laser radiation influence the maximum value of the weight loss but leave the principal dependence on the fluence unaltered. Furthermore, it is shown that increasing the back pressure from 0.06 to 100 mbar leads to considerably smaller weight losses.