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 with an intense laser beam. A small scattering centre immediately above the surface transforms them into propagating waves, the scattered light yielding information about the local near-field (NF) of the sample. This scattering center is practically realized by using the cantilever of an atomic-force microscope (AFM).
Hence, the spatial resolution is only limited by the (deeply sub-wavelength) 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 research topics
s-SNIM is a powerful method for investigating the electronic properties of nano-objects like quantum dots, nanocrystals, nanowires, ferroelectric domains, local structures arising from phase transitions etc. Here the FEL provides unique research opportunities allowing us, e.g., to access directly the electronic levels in the conduction band of a single quantum dot.
Another interesting application is the development of superlenses. We have recently demonstrated a GaAs-based superlens, whichs allow for imaging of small objects located on the opposite side of the superlens at mid-infrared and THz wavelengths far beyond the diffraction limit. The superlens consists of a 200 nm / 400 nm / 200 nm sandwich structure made of undoped and n-type GaAs, respectively. The operating wavelength of this superlens is determined by the condition that the dielectric constant of the n-doped GaAs has the same value but opposite sign as the undoped GaAs. In this way, the operating wavelength can be adjusted by choosing an appropriate carrier concentration for the doped GaAs layer.
Images of narrow gold stripes have clearly revealed the sub-wavelength imaging capabilities of such superlenses as well as the resonant character of the wavelength dependence.
D. Lang, L. Balaghi, S. Winnerl, H. Schneider, R. Hübner, S. C. Kehr, L. M. Eng, M. Helm, E. Dimakis, and A. Pashkin, Nonlinear plasmonic response of doped nanowires observed by infrared nanospectroscopy, Nanotechnology 28, 119921 (2018)
D. Lang, J. Döring, T. Nörenberg, A. Butykai, I. Kezsmarki, H. Schneider, S. Winnerl, M. Helm, S. C. Kehr, and L. M. Eng, Infrared nanoscopy down to liquid helium temperatures, Rev. Sci. Instrum. 89, 033702 (2018)
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).