Para-excitons in image—a new approach

We propose a non-resonant two-photon absorption as a method to create a high-density gas of para-excitons uniformly distributed in a whole volume of a crystal. In non-linear pump–probe experiments we measure changes of the IR absorption induced by the presence of optically excited para-excitons. This way, we directly determine the energy of the para-excitonic image transition at 129 meV. On the basis of temporal evolution of the induced absorption signal we find the lifetime of the para-excitons to be of the order of 10 ms.