Holographic phase-conjugation through a sub-wavelength hole

We investigate a holographic lens [1] as an alternative to high-numerical-aperture microscope objectives. For this, the interference pattern of a plane reference beam R and a spherical signal beam S is recorded in an iron- doped lithium niobate crystal utilizing the photorefractive effect [2]. The signal beam is the light being transmitted through a sub-wavelength hole in a gold surface on top of the crystal. For read-out, the phase- conjugated reference beam R* is used, i.e. a plane wave counter propagating to R. The phase-conjugated signal beam S* is reconstructed. This wave is focused onto the sub-wavelength hole. In principle, light coming from all directions behind the hole can contribute to the focusing, and a numerical aperture close to the index of refraction n = 2.3 may result.