Breaking the Optical Resolution Limit by Near-field Microscopy

Gaining spectroscopic information of a sample by optical image devices (spectrometers, fourier transform spectroscopy) is a daily and important task in science. But it encounters a fundamental problem when used in nanotechnology where sample structures can have dimensions of a few nanometers. Then the optical resolution is limited by diffraction to about half the used wavelength. For the visible spectrum this means a possible resolution of around 300nm but for the infrared (1-50µm) or terahertz (50 - 500µm) regime this allows only µm resolution, far too bad for nanotechnology. Different techniques like AFM, SEM or TEM exist to examine nanoscaled samples but all of them lack to gain directly spectroscopic information. This gap can be closed by near-field microscopy.
The most advanced near-field technique, called s-SNOM for scattering scanning near-field optical microscope, is based onto an AFM in tapping mode. Additionally to an ordinary AFM, a laser is focused onto the cantilever tip apex generating a nano-focus locally illuminating the sample surface. The interaction between the nano-focus and the sample scatters the light which will be modified in amplitude and/ or phase. Scanning the sample and recording for every pixel the scattered light allows obtaining an optical image. Thereby the optical resolution is only determined by the tip apex radius and independent of the illuminating wavelength! This has been demonstrated from the visible to the THz [1]. Routinely, 20nm is achieved but sub 10nm has been demonstrated too. For gaining spectral information the wavelength has to be changed and a new picture to be taken. Comparing the different pictures enables to gain the desired spectral information of the material properties, like the chemical composition, crystal structure or mobile carrier density which is very interesting for the semiconductor industry [2,3]. But also applications in biology are possible as demonstrated with a tobacco virus [4]. Thereby s-SNOM benefits from the advantage that its sample hasn’t to be labeled as it often necessary in biology. Currently, different approaches are underway to replace the tunable laser source by a broadband source to record the spectral information in one measurement attempt. Today, s-SNOM starts to enter several labs of research groups completing their analysis tools for nanotechnology. Hence, keep your eyes open, especially on the nanoscale!

University Groups:
Nanogune, San Sebastian
University of Washington
University of Rochester
ICFO, Barcelona

Company:
Neaspec GmbH

References:
[1] Terahertz Near-Field Nanoscopy of Mobile Carriers in Single Semiconductor Nanodevices, A.J. Huber, Nano Lett., 2008, 8 (11), pp 3766–3770

[2] Simultaneous IR Material Recognition and Conductivity Mapping by Nanoscale Near-Field Microscopy, A.J. Huber, Volume 19 Issue 17, Pages 2209 - 2212

[3] Controlling the near-field oscillations of loaded plasmonic nanoantennas, M. Schnell, Nature Photonics 3, 287 - 291 (2009)

[4] Infrared Spectroscopic Mapping of Single Nanoparticles and Viruses at Nanoscale Resolution, M.Brehm, Nano Lett., 2006, 6 (7), pp 1307–1310