Research towards materials and devices determined by dimensions and structural control in the nanometer range is a large effort with many resources dedicated worldwide. This interest is driven, on one hand, by basic science, because quantum phenomena determine the properties and pave the road towards new functions. On the other hand, rapid development of industrial technology demands materials and devices with more functionality, higher switching speeds, and higher integration. These requirements led to the development of fabrication techniques on the atomic scale which open the way towards the realization of revolutionary concepts in solid state science and technology.

The region of the electromagnetic spectrum between visible (infrared) photonics and high frequency electronics is called the terahertz range. It can be defined from 0.3 to 30 THz (1 mm – 10 µm wavelength) and is a frontier research area. 

We search for novel THz sources, investigate THz light-matter interaction and novel two-dimensional photonic devices. Solid state quantum devices allow the realization of quantum optical concepts, e.g. for lasers and photon emitters. Quantum size effects in nanostructures generate new optical functionalities. Genuine quantum devices such as quantum cascade lasers can be realized. Quantum cascade lasers are based on transitions between quantized energy levels in semiconductor heterostructures. Depending on the design of the layer thickness, the transition energy and the emission wavelength can be freely adjusted. We study THz sources with new cavity designs (photonic crystal cavities), coupling between cavities, and strong cavity – intersubband coupling.

We use femtosecond laser pulses to generate few cycle THz pulses using elementary excitation in solids with nonlinear frequency mixing. The development of more intensive femtosecond laser pulses is a requirement for the generation of high-intensity THz transients. With these high-intensity pulses we will be able to enter the strong THz field physics. A unique aspect of our approach is a new phase-resolved THz time domain method. This allows to study all strong field effects phase resolved, i.e. we can control the phase of the driving pulses as well as detect the phase of the response. From these experiments we will gain new insights into the fundamentals of nonlinear interaction and into the dephasing and scattering processes of nanostructures. The latter will be very useful for the understanding of intersubband THz sources. In particular, for the investigated cavities, strong confinement is expected, which increases the coupling between THz radiation and intersubband transitions. The long term goal is to be able to combine the advantages of THz emission from intersubband transitions with the phase-control of few-cycle THz pulses.

Furthermore, we investigate graphene and related materials, such as atomically thin transition metal dichalcogenides, for applications in electronics and optoelectronics. These materials are crystalline and thus of high material quality. The aim of our work is to advance state-of-the-art nano-device technology and provide physical insights in carrier dynamics, energy level schemes, optical responses, and many-body effects in these materials.