Recording and retrieving of multiplexed near-field holograms

We present experimental results on the recording and retrieving multiplexed near-field holograms using near-field scanning optical microscopy (NSOM) and a conventional rectangular-parallelpiped or cubic photorefractive crystal. We use the fiber tip of NSOM both as an object and as a probe for scanning (reading) the images. The recording distance between the tapered tip of NSOM and the crystal (i.e. between the object and the recording medium) is a crucial factor determining the size of the stored spot and the angular selectivity since it is dependent on that distance whether the near-field components of the object wave can reach the crystal or not. Experiments on angular multiplexing show that the angular selectivity was about 0.01 degree and the retrieved spot size was smaller than the Rayleigh limit when the recording distance is about 10 nm. In addition, experiments show that near-fields originated from sub-diffraction-limit -size objects could be stored in a photorefractive crystal at 2 mm apart from the crystal surface resulting in the retrieval of sub-diffraction-limit- size spots which means that our scheme can provide a way of multilayer (stack-wise) near-field storage and, thus, contribute to the significant enhancement of the storage capacity of the near-field optical memory.