Ultrafast electron dynamics: From fundamentals of photoexcitation to lightwave-driven electronics

Time: Thu 2020-01-30 09.15 - 10.00

Lecturer: Prof. Anders Mikkelsen, Department of Physics & NanoLund, Lund University, Sweden

Location: FB52

Abstract

During the past decade we achieved unprecedented abilities in designing light pulses with extreme temporal precision and materials with extreme spatial precision. This allows us to probe the fundamentals of light-matter interaction at a whole new level of detail as well as to rationally design a variety of novel functional materials and devices with applications for energy harvesting and computing. In particular, the local enhancement of laser pulses by nanostructures open up for spatiotemporal control of optical interactions down to the nanometer and sub-femtosecond scale. In this work, we use light pulses with durations from less than a hundred attoseconds up to tenths of femtoseconds with a wide variety of spectral profiles to explore the electron dynamics in metal and semiconductor nanostructures. Two topics will be covered:

First, to achieve simultaneous spatiotemporal characterization, we combine the sub-femtosecond time resolution of advanced laser systems with the nanoscale spatial resolution of PhotoEmission Electron Microscopy (PEEM). This allows for very sensitive measurements of plasmonics fields, surface chemistry and pump-probe experiments on ultrafast time scales - all in the same picture. Using <6 fs laser pulses in an interferometric time-resolved PEEM setup, we observe differences in near-field enhancement and electron excitation inside a variety of metal and semiconductor nanostructures [1-4] already within the first few optical cycles. Measurements are supported by ab-initio and finite-difference time-domain modelling. We now design novel experiments to directly image hot electron dynamics inside semiconductor nanostructures. Our studies indicate new opportunities for spatiotemporal control of light inside nanostructures using easily accessible parameters such as crystal structure and geometric placement. Applications in information processing will be discussed.

Second, when an intense, few-cycle light pulse impinges on a dielectric or semiconductor material, the electric field will interact nonlinearly with the solid, driving a coherent current. An asymmetry of the ultrashort, carrier-envelope-phase-stable waveform results in a net transfer of charge, which can be measured by macroscopic electrical contacts [5]. We investigate lightwave-driven currents in gallium nitride using an optical parametric chirped pulse amplifier delivering few-cycle laser pulses of nearly twice the duration and repetition rate two orders of magnitude higher than in previous work. Our results are in good agreement with theoretical modelling based on interfering multi-photon transitions, using the exact laser pulse shape. Substantially increasing the repetition rate and relaxing the constraint on the pulse duration marks an important step forward in exploring lightwave-driven electronics.

References:

1. E. Mårsell et.al Nano Lett. 15 (2015) 6601

2. E. Lorek et.al, Optics Express 23 (2015) 31460

3. E. Mårsell et.al, Appl. Phys. Lett. 107 (2015) 201111

4. E. Mårsell et al, Nano Lett. 18 (2018) 907

5. A. Schiffrin, et al, Nature 493 (2013) 70