Two Leiden University (The Netherlands) researchers have demonstrated that at a thickness of 4.5 nm, niobium nitride (NbN) is able to absorb nearly 100% of light. Previously, they say, the best light absorption was 50%. The material promises to enable "the ideal light detector"; a cell made from the material has already proven able to collect light and convert it to an electrical signal. The researchers' work, reported in Applied Physics Letters, was among the journal's top 20 most downloaded articles in May.
Materials that could potentially absorb significant percentages of light have the problem that they reflect the incident light; they are generally very good mirrors. But how much light is reflected and how much is absorbed depends on two factors: the angle at which the light falls onto the material, and the polarization (the direction of oscillation) of the light. Light has two kinds of polarization: s and p. Polaroid sunglasses make good use of this characteristic. The light absorption of a thin slice of NbN is at its maximum if the light falls on it at an angle of 35º and only consists of s-polarized light. The absorption achieved is then 94%. The p-polarized light is reflected in full. At an angle of 46º the absorption for both polarization directions is 80%, which is still extremely good.
This discovery gave Eduard Driessen, MSc, and Dr Michiel de Dood the idea for building a detector able to view individual photons. To date this has been very difficult because the absorption was not high enough.
The most important part of the detector is a lattice of ultra-absorbent NbN filaments, the researchers say. When an s-light particle falls on the lattice, it is absorbed. A p-particle is reflected. This p-particle can then in turn be collected by a second detector so that all the light is detected. Calculations show that the wavelength of the light particle has hardly any influence. The detector can therefore also be used for particles with completely different wavelengths, such as detection systems for telecommunications and infra-red equipment.
The research is being carried out in collaboration with the TU Delft and will be part-funded by the Netherlands Organization for Scientific Research (NWO) and the Foundation for Fundamental Materials Research (FOM).
For more information see the paper, The perfect absorber, published by Applied Physics Letters.
