The interaction of light with nanostructures exhibits two remarkabale features. The incident electromagnetic field can be enhanced by up to several orders of magnitude. Furthermore, these enhanced near-fields are confined to and vary on a scale of a few nanometers around the nanostructure, well below the diffraction limit of the exciting light. These effects form the basis for applications of nanostructures in science and technology. We aim at combining the spatial resolution provided by nanostructures with the temporal resolution offered by the short pulse laser sources in our laboratory, reaching into the attosecond domain. We study strong-field interactions with nanostructures and nanodevices, which exhibit many interesting differences to well-known strong-field atomic physics. We study the dynamics of light-generated electrons to unravel the effects of light-electron and collective charge interactions. Attosecond streaking, a pump-probe technique with synchronized attosecond extreme ultraviolet pulses and optical laser pulses, is one of the powerful techniques we employ to reconstruct nanolocalized electric fields and electron dynamics with attosecond temporal and nanometer spatial resolution. This ability will be instrumental in developing ultrafast plasmonic circuitry, which can overcome current limitations in resistive electronics and might open an avenue towards quantum computing at ambient temperature.
Illumining a metallic nanotip with laser pulses, optical electric fields are enhanced at the tip apex. These enhanced fields permit coherent electron emission from the tip apex. Varying the laser polarization can substantially change the spatial distribution of the local electric fields on the tip apex. In effect, an ultrafast pulsed coherent electron source with emission site selectivity of a few ten nanometers can be realized. Using such an electron source, we are developing a new method to observe ultrafast electron and phonon dynamics of nanoobjects, including molecule-nanotip heterostructures. The animation shows an example of the photoemission from nanotips in laser fields with rotating polarization. The electrons are recorded by a two-dimensional detector, which allows to measure the emission patterns from the tip. An electron analyzer can be used to measure energy spectra of electrons emitted from the tip apex. We are currently investigating the interference of the matter waves emitted from such nanotips in intense few-cycle laser pulses.
Strong laser fields and attosecond pulses can be employed to manipulate and observe molecular processes. The control of reactions with intense laser fields offers manipulation of strongly-coupled nuclear and electronic motion, which opens new photon-based reaction pathways. Tailoring the electric-field waveform of optical pulses on sub-cycle timescales opens the door to the control of electron and nuclear dynamics in molecules. In this project we use 3D-momentum imaging techniques such as velocity-map imaging (VMI) and reaction microscopy (REMI) to investigate the sub-cycle control of strong-field processes in molecules of increasing complexity. In close collaboration with theory we gain deep insight into the related coherent electron and nuclear dynamics. We are currently exploring how fundamental molecular processes can be influenced by intense nanolocalized fields in the vicinity of nanostructures.
Selected recent publications:
I. Yavuz et al., Phys. Rev. A 93, 033404 (2016)
M. Kübel et al., Phys. Rev. Lett. 116, 193001 (2016)
H. Li et al., Struct. Dyn. 3, 043206 (2016)
H. Li et al., Phys. Rev. Lett. 114, 123004 (2015)
A. Alnaser et al., Nat. Communic. 5, 3800 (2014)
Within this new project we currently develop the infrastructure to perform ultrafast microscopy on nanostructures for nanoelectronics and medical applications. Both applications are based on a novel passively carrier-envelope phase (CEP) stable laser system, that is based on optical-parametric-chirped-pulse-amplification (OPCPA) and can operate at high repetition rates. We currently obtain CEP-stable few-cycle pulses close to 2 µm with pulse energies in the µJ range at 0.1 MHz, which are applied in photoemission experiments on nanostructures. We aim to employ our 4th generation femtosecond OPCPA laser system in non-linear microscopy studies, where the relatively high pulse energy permits pulse shaping with full amplitude and phase control, counteracting dispersion of the microscope objective. We aim to develop and explore ultrafast nanodevices using this platform.
In biological applications, we can also counteract dispersion of organic matter for deep-tissue imaging. Ultrashort pulses are highly beneficial in terms of reduced cell damage and also provide the required increased contrast for label-free imaging. Multi-modal, non-invasive imaging, as explored in this project, aims at cancer cell molecular analysis enabling targeting cancer down to the single cellular level.
P. Wnuk et al., Opt. Exp. 24, 18551 (2016)