Plasmonic superlens based on doped GaAs

Infrared and THz free-electron lasers are interesting sources for near-field investigations as they are tunable in a range where suitable tabletop sources exist only at particular frequencies. The free-electron laser FELBE at Dresden covers the frequency range from 1.3–75 THz with narrowband (~ 1 % spectral width) radiation. We briefly show for the low-frequency region of FELBE (1.3–75 THz) that scattering near-field microscopy can be performed with a constant spatial resolution of 50 nm, which is determined by the diameter of the scattering tip. For the longest wavelength, this corresponds to /4600 [1]. Mainly, we present results on a superlens, which consists of a doped GaAs layer sandwiched between two intrinsic GaAs layers. Superlensing is expected when the condition 〖Re(ε〗_GaAs^doped)=〖-Re(ε〗_GaAs^intrinsic) is met in the spectral vicinity of the plasmonic resonance. Here, the Drude response in the doped layer induces resonant enhancement of evanescent waves accompanied by a significantly improved spatial resolution at radiation wavelengths around 15 THz (see Fig. 1) [2]. The resonance frequency is adjustable by changing the doping concentration. Compared to superlenses based on phononic resonances the plasmonic superlance features a somewhat broader range of the resonant response. Such a tunable superlense consisting of a single semiconductor material is
a versatile device to enhance signal and spatial resolution in near-field imaging of buried structures.