Adaptive control of tip-enhanced near-field optical signals in carbon nanotubes
Zusammenfassung der Projektergebnisse
This project focused on the control of near-field optical phenomena in plasmonic nanostructures and nanocarbons using ultrafast laser pulses. As a central outcome we developed a very powerful microscope platform and the expertise to compress and flexibly shape broadband 15 fs laser pulses in the tight focus of a high numerical aperture (NA) objective. This allows us to study the weak optical response of single nanostructures on ultrafast timescales with optimum spatial resolution and detection sensitivity. Shaping is achieved using the spatial light modulator purchased through this grant together with different laser sources and a home-built confocal / near-field optical microscope. The complexity of the setup and procedures as well as the experimental difficulties resulting from combining broadband pulses and high NA objectives are both enormous. Consisting of specifically optimized components our platform provides full flexibility, for which no commercial solution is available. At present only very few groups worldwide have comparable possibilities. Working towards the initial aim, the control of tip-enhanced near-field optical signals in carbon nanotubes, the ultrafast physics of both signal-enhancing metal nanostructures and nanocarbons turned out to be surprinsingly rich and complex. Using our platform we achieved the following important results in the field of ultrafast nanooptics: As starting point we demonstrated the possibility of adaptive control of near-field intensities in plasmonic nanostructures by pulse shaping using simulations. After studying different model systems we showed that the location of “hotspots” in rough films can be adaptively controlled. Our results indicated that efficient control requires large spectral bandwidth, substantially exceeding the initially available bandwidth of 15 nm. We developed an improved method to determine and control the spectral phase of strongly focused broadband laser pulses. We demonstrated the viability of our approach and we think that it will be valuable also for other groups working in the field of ultrafast nanooptics facing similar challenges. We demonstrated the control of second harmonic generation (SHG) for single gold nanorods by pulse shaping using MIIPS. Since SHG reflects the local near-field, the recorded MIIPS maps amount to controlling both its magnitude and spatial distribution, a key step described in the proposal. We further showed that the spectral phase varies from nanorod to nanorod, probably due to different spectral resonances. We demonstrated that the intensity and polarization of the non-linear photoluminescence (PL) of graphene and its coupling to surface plasmon polaritons can be effectively controlled by chirping the excitation pulse. These are the first reports on the ulrafast pulse control of graphene’s properties and are thus important for our understanding of the multi-particle processes in this material and its applications. We showed the first studies on the sub-ps dynamics of single carbon nanotubes. Excitation with two 250 fs pulses lead to a substantial reduction of the PL that recovers within few picoseconds. We then went further using 15 fs pulses controlled by the pulse shaper. Splitting the excitation spectrum in half, we obtained two pulses with tunable delay. We find that the delay controls the PL after non-resonant excitation within 30 fs. With our results on the ultrafast control of the optical signals of nanocarbons and plasmonic particles and our established expertise in pulse shaping, we can now start ultrafast tip-enhanced measurements using our existing microscope setup.
Projektbezogene Publikationen (Auswahl)
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“Compression of Ultrashort Laser Pulses via Gated Multiphoton Intrapulse Interference Phase Scans”
A. Comin, R. Ciesielski, G. Piredda, K. Donkers, A. Hartschuh
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“Controlling near-field optical intensities in metal nanoparticle systems by polarization pulse shaping”, Appl. Phys. B 100, 195 (2010)
G. Piredda, C. Gollub, R. de Vivie-Riedle, A. Hartschuh
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“Optical Investigations of Charge Carrier Dynamics in Organic Semiconductors and Graphene for Photovoltaic Applications ”, PhD thesis (2014), LMU München, Germany
M. Handloser
