Improvement of titanium film absorption with antireflection coatings

One of the materials widely used in the fabrication of transition-edge sensor for photon counting is titanium. In the visible–near infrared spectral region the photon absorption occurs directly in the film and the quantum efficiency of the detector is limited by the reflection losses at the film surface, which are around 60%. For many applications like quantum cryptography these losses are too high. One way to reduce the reflection is to apply an antireflection coating on the films. In this work we present different solutions based on the deposition of few layers of amorphous silicon-nitrogen alloys, a material with a tunable refraction index. The design of multilayer is based on an optimisation program which adopts a genetic algorithm to find out the global minimum of the reflectivity among a wide range of thickness possibilities for the different layers. We present solutions that minimize the reflectivity simultaneously at different telecommunication wavelengths or in a continuum band. The simulations are compared with experimental results.