Infrared and terahertz near-field microscopy
|Schematics of scattering-type near-field microscopy.|
Scattering-type near-field infrared microscopy (s-SNIM) allows the investigation of the optical properties of a given structure with resolution beyond the diffraction limit of E. Abbe. The evanescent near-fields at the sample surface are created by illumination with an intense laser beam and a small scattering centre immediately above the surface transforms them into propagating waves. Hence, the resolution limit is only limited by the size of the scatterer. In combination with the widely tunable free-electron-laser (FEL) at the HZDR this technique allows the investigation of different physical properties of semiconductor structures in the nanometer-regime.
s-SNIM is a powerful method for investigating the electronic properties of single quantum dots. Here, the FEL provides unique research opportunities allowing us to access directly the electronic levels in the conduction band of a single quantum dot.
|Near-field microscopy images of InAs-like quantum dots taken in the mid-infrared at FEL photon energies of 74 meV (left), 86 meV (middle), and 91 meV (right). Note the resonant character of the inter-sublevel resonance at 86 meV.|
Another interesting application is the development of superlenses. We have recently demonstrated a GaAs-based superlens, whichs allow for imaging at mid-infrared and THz wavelengths far beyond the diffraction limit.
M. Fehrenbacher, S. Winnerl, H. Schneider, J. Döring, S. C. Kehr, L. M. Eng, Y. Huo, O. G. Schmidt, K. Yao, Y. Liu, and M. Helm, Plasmonic Superlensing in Doped GaAs, Nano Lett. 15, 1057 (2015)
R. Jacob, S. Winnerl, M. Fehrenbacher, J. Bhattacharyya, H. Schneider, M. T. Wenzel, H.-G. von Ribbeck, L. M. Eng, P. Atkinson, A. Rastelli, O. G. Schmidt, and M. Helm, Intersublevel spectroscopy on single InAs-quantum dots by terahertz near-field microscopy, Nano Lett. 12, 4336-4340 (2012).
S. C. Kehr, Y. M. Liu, L. W. Martin, P. Yu, M. Gajak, S.-Y. Yang, C.-H. Yang, M. T. Wenzel, R. Jacob, H.-G. von Ribbeck, M. Helm, X. Zhang, L. M. Eng and R. Ramesh, Near-field examination of perovskite-based superlenses and superlense-enhanced probe-object coupling, Nature Communications 2, 249 (2011).
R. Jacob, S. Winnerl, H. Schneider, M. Helm, M. T. Wenzel, H.-G. v. Ribbeck, L. M. Eng, and S. Kehr, Quantitative determination of the charge carrier concentration of sub-surface implanted silicon by IR-near-field spectroscopy, Opt. Express 18, 26206-26213 (2010).
S. C. Kehr, M. Cebula, O. Mieth, T. Härtling, J. Seidel, S. Grafström, L. M. Eng, S. Winnerl, D. Stehr, and M. Helm, Anisotropy Contrast in Phonon-Enhanced Apertureless Near-Field Microscopy Using a Free-Electron Laser, Phys. Rev. Lett. 100, 256403 (2008).
H.-G. von Ribbeck, M. Brehm, D. W. van der Weide, S. Winnerl, O. Drachenko, M. Helm, and F. Keilmann, Spectroscopic THz near-field microscope, Opt. Express 16, 3430 (2008).
S. Schneider, J. Seidel, S. Grafström, L. M. Eng, S. Winnerl, D. Stehr, and M. Helm, Impact of optical in-plane anisotropy on near-field phonon polariton spectroscopy, Appl. Phys. Lett. 90, 143101 (2007).