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Near-field Spectroscopic Nanotomography at Infrared and Terahertz Frequencies

Final Report Summary - TERATOMO (Near-field Spectroscopic Nanotomography at Infrared and Terahertz Frequencies)

Fundamental understanding and engineering of composite materials, biological structures and building blocks for electrical and optical devices of nanoscale dimensions necessitate the availability of advanced microscopy tools for mapping their local chemical, structural and free-carrier properties. But while optical spectroscopy, particularly in the infrared (IR) and terahertz (THz) frequency range, has tremendous merit in measuring such properties optically, the diffraction-limited spatial resolution has been preventing IR and THz microscopy applications for the longest time to be used in nanoscale materials and device analysis, bio-imaging, industrial failure analysis and quality control. Scattering-type scanning near-field optical microscopy (s-SNOM) overcomes this limitation, by enabling two-dimensional IR and THz imaging of sample surfaces with nanoscale spatial resolution, independent of the wavelength. In s-SNOM, the light scattered at a scanning probe is recorded as function of its position. The core objectives of this project were to develop novel methods for both spectroscopic and three-dimensional (3D) imaging. The project involved the development of instrumentation, reconstruction algorithms as well as their demonstration at various samples.
Instrumental developments
We developed Fourier transform IR nanospectroscopy (nano-FTIR) with a spatial resolution of about 20 nm, which is more than a 100 times improved compared to conventional far-field FTIR spectroscopy. In nano-FTIR the IR radiation from thermal or laser sources is used for illuminating the tip. Spectroscopic recording of the tip-scattered light yields broadband IR near-field spectra [Nat. Mater. 10, 352 (2011), Nano Lett. 12, 3973 (2012)]. In case of organic films, nano-FTIR spectra yield in good approximation the local IR absorption in the sample. We envision nano-FTIR to open a new era in IR spectroscopy and to become a highly valuable to tool for chemical nanoanalytics. As a photonic application of the nano-FTIR setup, we demonstrated nanoscale-resolved time-domain interferometry of hyperbolic phonon polaritons in thin boron nitride slabs, revealing the polaritons´ phase and group velocities, as well as their lifetimes [Nat. Photon. 9, 674 (2015)]. We also invented and developed synthetic optical holography (SOH), allowing for rapid near-field imaging [Nature Commun. 5:3499 (2014)]. To enhance the near-field signals, we developed novel near-field probes based on resonant IR antennas [Nano Lett. 13, 1065 (2013)].
Nano-FTIR applications and permittivity reconstructions
We demonstrated and applied nano-FTIR for secondary structure analysis of protein nanostructures (viruses, membranes, insulin fibrils) [Nat. Commun. 4:2890 (2013)]. We also developed a simple, fast and robust method for the quantitative reconstruction of the complex-valued permittivity and thickness of thin organic films on bulk substrates from s-SNOM and nano-FTIR data [J. Phys. Chem. Lett. 4, 1562 (2013), ACS Nano 8, 6911 (2014)]. The method is based on the finite-dipole model of s-SNOM and can be applied to nanostructures of transverse dimensions larger than a few tip diameters, i.e. about 200 nm for commercial metalized scanning probe tips with nominal apex radius of 50 nm.
Infrared nanoimaging of graphene plasmons
By exploring graphene as potential test sample, we discovered that the metal s-SNOM tip can excite IR plasmons in doped graphene. The interference between the plasmons excited by the tip and the plasmons backreflected from the graphene edges yielded patterns in the near-field images from which we could directly measure in real space the graphene plasmon wavelength and propagation length. [Nature 487, 77 (2012]. Employing silicon tips, we were able to map the plasmon wavefronts, demonstrating the focusing and refraction of graphene plasmons [Science 344, 1369 (2014)]. Because of their dramatically reduced wavelength and strong field confinement compared to IR radiation in free space, graphene plasmons enable manifold novel sensing and detector applications, among others.