Directional High Power THz Quantum Cascade Lasers

Terahertz (THz) radiation provides several unique features, which are particularly interesting for future applications in various scientific, industrial and biomedical disciplines. The unique properties of molecules at THz frequencies are making this spectral region very attractive for a large variety of applications. The chemical fingerprint caused by rotational and vibrational transitions of molecules results in a simple identification scheme. Substances can be investigated by optical absorption measurements in a contact-free way. The extraction of samples and cumbersome chemical analysis becomes redundant. One possible application is environmental monitoring. As THz radiation is not ionizing, medical examination can be performed in-vivo. Imaging and quality control systems often exploit another material property at THz frequencies: a large class of matter is opaque in the visible but transparent in the THz regime. Polymers and plastics are prominent examples. THz imaging could enable to resolve their inner structure with high spatial resolution. Quantum Cascade Lasers (QCLs) are the only compact high power sources of coherent THz radiation. They are based on optical transitions between quantized states of semiconductor nanostructures. The transition energy can be designed freely to cover new spectral regions. However, todays QCLs are still lacking of output power, beam quality, efficiency and room temperature operation. The main objective of this proposal is to exploit the fascinating potential of the quantum mechanical design of the optical functionality provided by these structures: One project part will concentrate on the improvement of the quantum cascade laser active region employing non- standard material systems. Based on our experience from previous projects, improved design concepts will be evaluated with numerical methods and tested experimentally, with the goal of high power laser emission. The second project part will evaluate several waveguide concepts theoretically and experimentally to efficiently couple out the generated light from the laser cavity. Commonly used waveguides for high power emission will be further improved by advanced fabrication techniques. Moreover, novel resonator concepts will be investigated, such as random laser cavities. These techniques provide efficient extraction of the optical power from the laser cavity additionally to a low-divergent output beam, which is highly desirable for future applications at these wavelengths. In the final stage of this project, the newly developed active regions will be combined with the advanced laser cavities in order to realize a high power THz QCL with broadband emission spectrum and low output beam divergence. This, in fact, will be a significant step towards implementing THz QCLs in future applications, where high power THz radiation is required, such as real-time imaging, or remote sensing.

Terahertz (THz) radiation provides several unique features, which are particularly interesting for future applications in various scientific, industrial and biomedical disciplines. Quantum Cascade Lasers (QCLs) are the only chip-size high power sources of coherent THz radiation. They are based on optical transitions between quantized states of semiconductor nanostructures. The transition energy can be designed freely to cover new spectral regions. However, todays QCLs are still lacking of output power, beam quality, efficiency and room temperature operation. Within this project we have significantly improved the performance of QCLs by introducing a new quantum mechanical design which used very tall but thin (about one atomic layer) barriers. This approach led to an increase of the operating temperature to ~ 200 K. This allowed us for the first time to use thermoelectric cooling instead of using liquid helium or liquid nitrogen. This is an important breakthrough for the application in (bio)chemical sensing or imaging systems. Furthermore, this active region allowed us also to realize a THz frequency comb with 30 equidistant laser modes by using a ringshaped laser cavity. A unique advantage of THz QCLs is their scalability. By varying the thickness of the wells and barriers, it is possible to engineer the energy levels and to obtain a laser transition for a userdefined emission wavelength. The active region of conventional THz QCLs consists of a periodic arrangement of several unit cells with an identical design. While these single unit cell lasers provide high gain at the designed wavelength, the emission frequency range is limited by the gain bandwidth of the chosen unit cell design. To overcome this limitation, it is possible to design active regions consisting of different unit cell designs and stack them into a single active region. By using this concept we realized a heterogeneous THz QCL consisting of five different sub-stacks which showed ultrabroadband emission covering a frequency range of 2.6 THz. We investigated the use of so-called random laser cavities as a broadband, coherent light source in the terahertz range. A control scheme was developed to reshape the emission spectra of the lasers by applying an optical field that restructures the permittivity of the active medium. Furthermore, a spatial light modulator was combined with an optimization procedure to transform an initially multi-mode THz random laser into a tunable single-mode source. This control scheme provides new degrees of freedom that can be used to create broadly tunable sources with potential applications in self-referenced spectroscopy. This allows to predict spatial modulation patterns for desired laser spectra in real-time, eliminating the need for lengthy and costly simulation and optimization iterations.