http://thzlabs.tuwien.ac.at/index.php/research/novel-materials
THz Time Domain Spectroscopy
Terahertz time domain spectroscopy (THz TDS) is a powerful spectroscopic technique that allows the time-resolved measurement of light-matter interaction with broadband and powerful THz pulses. Contrary to other spectroscopies, amplitude and phase information are directly retrieved in a single scan, making THz TDS a powerful tool for studying absorption and gain dynamics in semiconductor heterostructures. The time information allows further to retrieve depth information as is used in time-of-flight tomographic imaging.
THz TDS allows us to study and characterize intersubband dynamics of quantum cascade lasers with optical transitions in the THz frequency range (THz QCLs). These lasers are compact sources of coherent THz radiation and consist of a semiconductor gain medium confined in a plasmonic waveguide. The main motivation for our experiments is to get information about internal parameters, such as gain and loss dynamics, all with the aim of improving the performance of THz QCLs.
(D. Bachmann, J. Darmo)
By scaling the incident THz field amplitudes to several tens of kV/cm, the pulses can be used to actively manipulate and control the intersubband excitations, giving rise to non-equilibrium states of matter. From the dynamics following the ultrafast excitation pulse, considerable insight into the microscopic properties of the sample can be gained. We use various techniques to generate these intense, broadband THz pulses, such as four-wave mixing in plasma filaments, optical rectification in nonlinear crystals and large area photoconductive switches. In conjunction with THz metamaterials, the available field strength can reach values in the MV/cm regime, paving the road to THz spectroscopy under extreme conditions.
(D. Dietze, J. Darmo)
THz Quantum Cascade Lasers
Quantum cascade lasers (QCLs) are novel semiconductor devices. Unlike ordinary band gap lasers known from everyday life (laser pointers, Blu-ray/DVD/CD drives), they are using a radically new concept. The energy levels in a quantum cascade laser are formed by quantum mechanically bound states in an artificially grown semiconductor nanostructure. Therefore they are also known as intersubband lasers.
To push the current limits of terahertz quantum cascade lasers we employ novel semiconductor material systems and compensate for imperfections on the atomic level. We have demonstrated performance records with novel InP-based material combinations (InGaAs/GaAsSb) and developed symmetric active regions which are a perfect tool to study growth-related problems such as interface roughness asymmetry and dopant migration.
(C. Deutsch)
Quantum cascade lasers are very promising sources of coherent terahertz radiation for future applications. We considerable improved the device performance by increasing the active region thickness of the laser using a stacking process. The improved optical properties combined with the increased light generation results in a high power THz laser with a record high pulsed output power of almost 1 W.
(M. Brandstetter)
Carrier relaxation has a significant influence on the efficiency of quantum-well-based optoelectronic devices. In terahertz quantum cascade lasers, for example, non-radiative scattering processes are quickly depopulating the upper laser level, reducing the optical gain and limiting the maximum operating temperature. Low-dimensional nanostructures, like semiconductor nanowires, introduce an additional quantum mechanical confinement in the plane of a quantum well structure. We study the prospects and technological challenges of realizing such an additional quantization to enhance the lifetimes of excited states.
(M. Krall)
Novel Materials
The terahertz and mid-infrared spectral region is most suited for the study of novel optical materials and phenomena. We study, for example, THz polaritonics with metamaterials and quantized transitions in semiconductors, light-matter interaction of optical nano-antennas and semiconductor quantum dots and novel two-dimensional materials like graphene or MoS2.
Our group investigates graphene and related materials, such as atomically thin transition metal dichalcogenides, for applications in electronics and optoelectronics. The aim of our work is to advance state-of-the-art of nanodevice technology and provide physical insights in carrier dynamics, energy level schemes, optical response, and many body effects in these materials. (Nanoscale Electronics and Optoelectronics Group homepage)
Typical RF antennas are used to impedance-match a sub-wavelength oscillator, such as an RF circuit, to free space. In much the same way we aim to improve the light-matter interaction of our sub-wavelength devices, InAs quantum dots, by attaching them them to nano-antennas. By tuning the nano-antennas' resonance frequency to the optical frequency of the quantum dot we aim to increase the interband and intersubband optical relaxation rates.
(G. Lilley)
Metamaterials (MMs) are artificial optical materials that consist of regular arrays of subwavelength sized metallic resonators. They constitute an exciting possibility for the efficient coupling of free-space radiation to a wide variety of quantum systems. The combination of such metasurfaces with quantized transitions in semiconductors leads to a whole new range of applications. Examples include large-area surface-emitting quantum-cascade lasers and photodetectors, coherent terahertz amplifiers, and active THz modulators.
(D. Dietze, J. Karaman, J. Darmo)
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