Plasmonic Superlensing in Doped GaAs Markus Fehrenbacher,* ,†,‡ Stephan Winnerl,* ,† Harald Schneider, † Jonathan Dö ring, ‡ Susanne C. Kehr, ‡ Lukas M. Eng, ‡ Yongheng Huo, § Oliver G. Schmidt, § Kan Yao, ∥ Yongmin Liu, ∥ and Manfred Helm †,‡ † Helmholtz-Zentrum Dresden-Rossendorf, Institute of Ion Beam Physics and Materials Research, Bautzner Landstraße 400, 01328 Dresden, Germany ‡ Institut fü r Angewandte Physik, TU Dresden, 01062 Dresden, Germany § Institute for Integrative Nanosciences, IFW Dresden, Helmholtzstrasse 20, 01069 Dresden, Germany ∥ Department of Electrical and Computer Engineering, Northeastern University, Boston, Massachusetts 02115, United States ABSTRACT: We demonstrate a semiconductor based broadband near-field superlens in the mid-infrared regime. Here, the Drude response of a highly doped n-GaAs layer induces a resonant enhancement of evanescent waves accompanied by a significantly improved spatial resolution at radiation wavelengths around λ = 20 μm, adjustable by changing the doping concentration. In our experiments, gold stripes below the GaAs superlens are imaged with a λ/6 subwavelength resolution by an apertureless near-field optical microscope utilizing infrared radiation from a free-electron laser. The resonant behavior of the observed superlensing effect is in excellent agreement with simulations based on the Drude−Lorentz model. Our results demonstrate a rather simple superlens implementation for infrared nanospectroscopy. KEYWORDS: Superlens, diffraction limit, surface plasmons, near-field microscopy, semiconductor I n classical optical microscopy, spatial resolution is con- strained by the wavelength of the applied radiation, limited by diffraction. In 2000, Pendry proposed that a negative refractive-index material 1 could act as a perfect lens, 2 which not only focuses propagating waves but in addition reconstructs information contained in the evanescent fields of an object, thereby creating an image with a resolution beyond the diffraction limit. Pendry also showed that in the near-field regime, negative permittivity alone is sufficient to realize a superlens to recover details on a subwavelength scale, which initiated a number of related experiments. While thin planar silver sheets have been demonstrated to be promising candidates for superlens-based UV nanophotolithography, 3−5 near-field investigations of SiC, 6 perovskites 7,8 and graphene 9 reveal imaging capabilities beyond the diffraction limit at infrared wavelengths. Depending on the material, the spectral position and bandwidth of superlensing is determined by its plasma frequency 3−5,9 and phonon resonances. 6−8 Conse- quently, accessible wavelengths are restricted by the limited availability of suitable materials. Various approaches have been proposed to overcome these restrictions. On the one hand, multilayered systems with different phonon resonances 10 and the concept of an “unmatched superlens” 11 promise to broaden the operation wavelength range of a superlens. On the other hand, the superlensing wavelength can be tailored by manipulating the electronic properties and, thus, the permittivity of a material. Correspondingly it has been suggested to use doped graphene 12 or metal-dielectric composites 13 as frequency-adjustable subdiffractive imaging systems, continuously covering the visible and infrared range by controlling the respective plasma frequencies. Especially relevant to this work, semiconductors have been suggested to be exploited as plasmonic devices 14−17 where the operational spectral range can be adapted by changing the doping level. However, a superlens consisting of doped semiconductor has not been demonstrated yet. Our approach to realize a spectrally adjustable plasmonic superlens in the mid- and far-infrared is to use Si-doped GaAs, taking advantage of precisely controllable charge-carrier concentration by standard semiconductor fabrication techni- ques. Here, the electron density determines the plasma frequency of a conductive layer which in turn determines the superlensing wavelength. In this article, we evidence the performance of such a device by imaging gold stripes below the superlens with a scattering-type scanning near-field optical microscope (s-SNOM) 18 combined with a free-electron laser (FEL). 7,8,19−21 The observed enhancement of both the near- field signal and the spatial resolution is consistent with theoretical considerations based on the Drude−Lorentz model, taking into account free electrons and optical phonons. 22 Superlensing for evanescent electric fields is related to the excitation of strongly localized surface polariton modes 23 at the interface between two media, A and B, with permittivities of Received: October 17, 2014 Revised: January 8, 2015 Published: January 13, 2015 Letter pubs.acs.org/NanoLett © 2015 American Chemical Society 1057 DOI: 10.1021/nl503996q Nano Lett. 2015, 15, 1057−1061