Sergiu Amarie

Nano-FTIR Absorption Spectroscopy of Molecular Fingerprints at 20 nm Spatial Resolution

We demonstrate Fourier transform infrared nanospectroscopy (nano-FTIR) based on a scattering-type scanning near-field optical microscope (s-SNOM) equipped with a coherent-continuum infrared light source. We show that the method can straightforwardly determine the infrared absorption spectrum of organic samples with a spatial resolution of 20 nm, corresponding to a probed volume as small as 10 zeptoliter (10(-20) L). Corroborated by theory, the nano-FTIR absorption spectra correlate well with conventional FTIR absorption spectra, as experimentally demonstrated with poly(methyl methacrylate) (PMMA) samples. Nano-FTIR can thus make use of standard infrared databases of molecular vibrations to identify organic materials in ultrasmall quantities and at ultrahigh spatial resolution. As an application example we demonstrate the identification of a nanoscale PDMS contamination on a PMMA sample.

Ultrafast and Nanoscale Plasmonic Phenomena in Exfoliated Graphene Revealed by Infrared Pump–Probe Nanoscopy

Pump-probe spectroscopy is central for exploring ultrafast dynamics of fundamental excitations, collective modes, and energy transfer processes. Typically carried out using conventional diffraction-limited optics, pump-probe experiments inherently average over local chemical, compositional, and electronic inhomogeneities. Here, we circumvent this deficiency and introduce pump-probe infrared spectroscopy with ∼ 20 nm spatial resolution, far below the diffraction limit, which is accomplished using a scattering scanning near-field optical microscope (s-SNOM). This technique allows us to investigate exfoliated graphene single-layers on SiO2 at technologically significant mid-infrared (MIR) frequencies where the local optical conductivity becomes experimentally accessible through the excitation of surface plasmons via the s-SNOM tip. Optical pumping at near-infrared (NIR) frequencies prompts distinct changes in the plasmonic behavior on 200 fs time scales. The origin of the pump-induced, enhanced plasmonic response is identified as an increase in the effective electron temperature up to several thousand Kelvin, as deduced directly from the Drude weight associated with the plasmonic resonances.

Sub-micron phase coexistence in small-molecule organic thin films revealed by infrared nano-imaging

Controlling the domain size and degree of crystallization in organic films is highly important for electronic applications such as organic photovoltaics, but suitable nanoscale mapping is very difficult. Here we apply infrared-spectroscopic nano-imaging to directly determine the local crystallinity of organic thin films with 20-nm resolution. We find that state-of-the-art pentacene films (grown on SiO2 at elevated temperature) are structurally not homogeneous but exhibit two interpenetrating phases at sub-micrometre scale, documented by a shifted vibrational resonance. We observe bulk-phase nucleation of distinct ellipsoidal shape within the dominant pentacene thin-film phase and also further growth during storage. A faint topographical contrast as well as X-ray analysis corroborates our interpretation. As bulk-phase nucleation obstructs carrier percolation paths within the thin-film phase, hitherto uncontrolled structural inhomogeneity might have caused conflicting reports about pentacene carrier mobility. Infrared-spectroscopic nano-imaging of nanoscale polymorphism should have many applications ranging from organic nanocomposites to geologic minerals.

High-power femtosecond mid-IR sources for s-SNOM applications

We demonstrate two high-power femtosecond mid-infrared (mid-IR) sources that can be combined with a scattering-type scanning near-field optical microscope (s-SNOM). The first one is based on difference frequency generation (DFG) between the two signal wavelengths of a high-power dual-signal-wavelength periodically poled lithium niobate (PPLN) optical parametric oscillator (OPO) and covers the spectral range from 10.5 μm to 16.5 μm. The second one is an AgGaSe2 OPO pumped by the PPLN OPO. With this mid-IR OPO we obtained up to 113 mW average idler power at 4857 nm with more than 40 cm−1 FWHM spectral width. We demonstrate mid-IR near-field spectra and near-field images that we obtained by combining the broadband femtosecond mid-IR DFG source with an s-SNOM.

Visualisation of methacrylate-embedded human bone sections by infrared nanoscopy

A recently developed ultra-resolving near-field infrared nanoscope is applied to investigate methyl methacrylate embedded, un-decalcified human bone sections. Results show detail at a resolution of 30 nm. Specific contrasting of mineral components is enabled by choosing an appropriate infrared wavelength, here 9.47 μm, in the phosphate vibrational band. The method is surface-sensitive, probing to a depth of about 30 nm into the surface. The obtained infrared images are presented in direct comparison with optical and electron micrographs of the identical specimen. Lamellar bone organization, peri-cellular mineral deposition, and regional differences in mineral content are clearly detectable. Individual fibrils are resolved. - Infrared nanoscopy requires just standard hard tissue preparation techniques combined with section surface polishing. It can be integrated into existing laboratory environments without impeding subsequent routine staining and evaluation methods.

Infrared Pump-Probe Spectroscopy of Plasmons in Graphene and Semiconductors

1. University of California San Diego, Department of Physics, La Jolla, California 92093. 2. The University of Texas at Austin, Microelectronics Research Center, Austin, TX 78758. 3. University of Maryland, Materials Research Science and Engineering Center, College Park, Maryland 20742. 4. University of California, Department of Physics and Astronomy, Riverside, California 92521. 5. California State University, Department of Physics, San Marcos, San Marcos, California 92096. 6. University of California San Diego, Department of Chemistry and Biochemistry, La Jolla, California 92093. 7. Graphene Research Centre and Department of Physics, National University of Singapore, 117542, Singapore. 8. Neaspec GmbH, Bunsenstr. 5, 82152 Martinsried, München, Germany. 9. Ludwig-Maximilians-Universität and Center for Nanoscience, 80539 München, Germany.

Biomedical imaging by infrared nanoscopy (nano-FTIR)

Summary form only given. Fourier-transform infrared (FTIR) spectroscopy is an established technique for characterization and recognition of inorganic, organic and biological materials by their far-field absorption spectra in the infrared fingerprint region. However, due to the diffraction limit conventional FTIR spectroscopy is unsuitable for measurements with nanoscale spatial resolution.