Plasmonic devices and metamaterials have paved the way towards unprecedented optical phenomena, such as improved focusing, electromagnetic cloaking, enhanced spectroscopic sensitivity, or the implementation of novel optical properties like negative or giant refractive indices. Whereas most experimental studies to date investigate the far-field of such structures, gaining a comprehensive understanding of the underlying mechanisms requires monitoring their near-fields. Due to typical structure dimensions on the wavelength to sub-wavelength scale, however, near-field studies with the required spatial resolution are highly challenging and experiments in the long-wavelength regime can be advantageous. Especially, imaging at terahertz (THz) frequencies proved to be immensely powerful for detailed investigations of the near-fields around sub-wavelength metallic structures. Based on coherent emission and detection of single-cycle THz pulses this approach allows measuring all three time-dependent electric fields components with sub-ps temporal and sub-wavelength spatial resolution. In our laboratory we have developed several unique THz near-field imaging tools with which we investigate fundamental building blocks of metamaterials. Most of our findings can be transferred to other spectral regions and help designing new materials with unusual properties and functionalities.
If designed appropriately such sub-wavelength structures can also be used as efficient antennas focusing incident THz radiation to sub-diffraction limited volumina. Within those, the electric or magnetic near-fields can be greatly enhanced. Such high fields open up myriads of high field and nonlinear THz science studies. From a practical viewpoint, field enhancement is of importance due to the present technological limitations in the peak fields obtainable from current THz sources. Simulations and experiments showed that field enhancement up to several tens of thousands is feasible and with our laser system we approach the 10 to 1000 MV/cm electric field strength regime.
Currently, we use such high THz fields for a number of scientific applications:
- When implemented in a conventional transient absorption spectroscopy setup, they allow us to study charge transfer processes within molecular systems in the presence of strong local fields. This is important from a fundamental point of view as it expands current capabilities of so-called Stark spectroscopy with the prospect of measuring transient dielectric properties of molecules in solution. In case the fields are sufficiently strong they may also allow us to switch or even steer charge transfer in molecules and bring us a step closer to THz molecular electronics.
- When such strong fields interact with solids they modify the band diagramm giving rise to new transient phenomena. Currently, we are mostly interested in graphene and related materials.
- Over the past years we started to investigate the use of such strong fields in view of advanced accelerators and free electron lasers. It turns out that individual building blocks of metamaterials or one-dimensional lattices of such building blocks allow for electron and photon diagnostic tools with promise for unprecedented temporal resolution. Furthermore, new types of electromagnetic undulator structures and many more applications can be realized by following such concepts.
For further Information please contact Prof. Thomas Feurer
