The research program on THz photonics by Karl Unterrainer (KU) is driven by the idea to close the THz gap between the high frequency electronics and photonics. The existing gap between 0.1 THz and 100 THz (more than 3 orders of the electromagnetic spectrum) is only poorly covered by technology. This range is characterized, however, by a lot of fundamental resonances (phonon resonances, plasmon resonance, impurity transitions) in solids and by a magnitude of vibrational and rotational resonances. This opens a wide field of possible applications from faster signal processing and communications, to sensor applications, including the identification of atmospheric pollutants and use in food quality-control, atmospheric and astrophysical remote sensing, and imaging (including medical imaging) with unique contrast mechanisms. The KU group targets the above goal twofold:
The first approach is based on electrical driven semiconductor devices built from semiconductor nano structures. Their properties can be designed by the one-dimensional growth of different materials. As a consequence quantized states are formed giving the opportunity to design new optoelectronic devices. For the THz range Bloch oscillations in superlattices and intersubband transitions in quantum wells are investigated. The group has investigated collective effects on the intersubband emission and long wavelength cavities. Based on the very successful concept of the mid-infrared quantum cascade laser also a THz quantum cascade laser was realized. Recent results show that the properties of THz-QCLs are dramatically improved in a magnetic field. The group has developed THz QCL microcavities and photonic band gap structures. The better understanding of carrier relaxation allowed the development of low threshold THz QCLs.
The second approach is the THz generation by ultrashort laser pulses. The KU group reported the first THz time-domain measurements of electron intersubband oscillations in quantum wells, detected the phase from the reflection at Bragg mirror. The study of cavity enhanced effects led to the development of a Ti:Sapphire oscillator with intracavity THz generation. The KU group investigates the dynamical properties of quantum cascade lasers (QCL). Improved detectors using band structure engineered self assembled semiconductor quantum dots were demonstrated. Recent work deals with the further improvement of THz technology and its application as well as with the study of carrier and spin relaxation in semiconductor quantum dots. The spectral coverage using different techniques (plasma oscillations, photoconductive switches and degenerate frequency mixing) extends from 0.3 THz up to almost 70 THz. The KU group developed a new technique to combine THz time-domain spectroscopy with THz QCLs which allows for the first time to study stimulated emission in lasers phase-resolved.
The development of THz science and technology relies to a large extent on the availability of novel nano materials. Such nanostructures are of key importance for many future devices with higher efficiency, and improved performance. A key input thereby is that nano-devices can exhibit fundamentally different electronic, optical, magnetic and mechanical properties as compared to conventional devices. Research of the group is placed right at this crossing between classical devices and novel concepts using quantum effects in nanostructures. The development of THz technology will only be possible by the increased knowledge about nanostructures. Understanding the THz interaction of nanostructures and how to control it will become essential to continue technological progress.