Photodetector with subwavelength spatial resolution

A photodetector operating with subwavelength spatial resolution is presented. The active region of this detector is defined by the pn-interface region of a highly doped GaAs homojunction. The cleavage plane normal to the interface defines a line detector with high spatial resolution. The actual resolution was tested using a near-field scanning optical microscope. An Al-coated tapered optical fiber of nominal aperture of 60 nm was scanned in the optical near-field vicinity across the active region of the junction. The photocurrent was measured as a function of the position of the fiber tip relative to the cleaved GaAs pn-junction. The width of the observed peak in photocurrent is found to be highly asymmetric with respect to the p or n side of the junction. The reason for this asymmetry is discussed in terms of diffusion length of minority carriers in the n- and p-GaAs close to the suface.