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New tools for nanoscale optical spectroscopy -
Functional imaging of single nanostructures using antennas

Final Report Summary - NEWNANOSPEC (New tools for nanoscale optical spectroscopy -Functional imaging of single nanostructures using antennas)

In this project, we developed novel optical techniques that provide sub-diffraction spatial and femtosecond time resolution combined with high detection sensitivity. These techniques were then utilized to gain new insights into the optical and optoelectronic properties of single nanoobjects. Our approach is based on the localization and enhancement of light-matter interactions using optical antennas and on ultrafast laser spectroscopic methods in combination with scanning microscopy.
By combining antenna-enhanced near-field optical microscopy with sensitive electronics for current detection, we demonstrated the first photocurrent measurements of working devices with 30 nm spatial resolution [1]. The importance of our results was underlined in a Perspective article in ACS Nano. We also succeeded in recording antenna-enhanced electroluminescence maps for the first time and were able to identify nanoscale field gradients as the common origin of photocurrent and electroluminescence signals. With this, we realized the first comprehensive functional characterization of an optoelectronic device with nanoscale resolution and we are now using this technique to study other nanomaterials. We developed and tested the first optical microscope for antenna-enhanced measurements at low temperatures that provides a numerical aperture (NA) larger than unity, which is essential for studying the photoluminescence (PL) of samples on dielectric substrates. We succeeded in recording antenna-enhanced PL images of carbon nanotubes with ~20 nm resolution at low temperature (manuscript in prep.) and now use the setup for further studies. In antenna-experiments, excitation and emission enhancement can both contribute. To separate thrm, we performed time-resolved PL measurements on rare-earth ion doped nanocrystals as model systems [2]. For the nanocrystals, we have indications of an additional non-local contribution to the PL enhancement that is the result of efficient energy transfer within the crystal. For metallic nanowire antennas, we derived a scheme to visualize and quantify the different radiation channels using back-focal plane (BFP) imaging [3]. BFP detection also allowed us to record the Raman radiation patterns of graphene for the first time [4]. We also developed a new description for the quantitative modelling of polarized Raman scattering that considers the effect of depolarization. This work was highlighted in a Perspectives article in ACS Nano. These results now form the basis for the quantification of plasmon enhanced radiation in other nanomaterials. For ultrafast studies of single nanoobjects, we implemented a pulse shaper that allows to control both spectral amplitude and phase of 15 fs laser pulses in the focus of a high NA objective. In this context, we improved an iterative intra-focus pulse characterization technique for the efficient handling of broadband laser pulses [5]. Its importance was highlighted in a “Spot-light on Optics” article. With this platform, we investigated the ultrafast response of graphene [6], single carbon nanotubes and plasmonic nanoantennas. Ultrafast microscopy of graphene showed layer-dependent electronic relaxation times ranging from ~25 fs for single layer graphene to ~15 fs for bulk graphite that can be attributed to varying substrate induced doping. Transient Raman microscopy probing the G and the 2D phonon relaxation showed a layer dependent-increase of the phonon lifetimes (manuscripts in prep.). By laser phase shaping we found the spectral phase response of conical metal tips to be flat, which means that these tips do not prolong the laser pulse duration. We now use these antenna-tips in femtosecond near-field optical imaging of carbon nanotubes and graphene.
[1] Rauhut et al., ACS Nano 6, 6416 (2012), Mauser et al., Nano Lett. 14, 3773 (2014)
[2] Mauser et al. ACS Nano 9, 3617 (2015), Chem. Soc. Rev 42, 1248 (2014)
[3] Hartmann et al., ACS Nano 7, 10257 (2013), Piatkowski et al., Nanoscale 7, 1479 (2015)
[4] Budde et al. ACS Nano 10, 1756 (2016)
[5] A. Comin et al., J. Opt. Soc. Am. B 5, 1118 (2014) & Optics Express 24, 2505 (2016)
[6] Ciesielski et al. Nano Lett. 15, 4968 (2015), Winzer et al. Nano Lett. 15, 1141 (2015)