Quantum-cascade lasers (QCLs) are promising radiation sources for high-resolution spectroscopy with sub-MHz or even kHz spectral resolution for the terahertz (THz) spectral region. However, micro-optical integration becomes crucial when it comes to space applications. Besides the small mass and volume, micro-optical integration is important for the optical performance in such a harsh environment. The reasons are the vibrations of the required cryocooler and other non-stationary mechanical and microphonic disturbances, which act back on the laser by the residual external optical feedback from direct and indirect reflections. Without measures, the result is a linewidth enhancement of typically several MHz and a reduced output power stability, which is both problematic for demanding applications such as heterodyne radiometry. Eliminating the underlying effects requires a micro-optical platform with integrated optical isolator.

The key objective of this project is the development of integrated THz QCLs for high-resolution spectroscopy and future space applications at 3.5 THz and 4.7 THz. These two frequencies are very important for the detection of the OH radical and neutral atomic oxygen, respectively, in atmospheric research as well as in astronomy. The overarching goal is to integrate the QCL chip with the most important optical components in a micro-optical assembly, not larger than a matchbox. This includes the QCL chip, the optics for optical isolation, outcoupling, and beam shaping as well as an optical fiber.

MicroQCL Team:
Prof. Heinz-Wilhelm Hübers, heinz-wilhelm.huebers@dlr.de
Dr. Martin Wienold, martin.wienold@dlr.de

Dr. Katrin Paschke, katrin.paschke@fbh-berlin.de
Alexander Sahm, alexander.sahm@fbh-berlin.de

Dr. Lutz Schrottke, lutz@pdi-berlin.de
Dr. Xiang Lü, lue@pdi-berlin.de

Terahertz  radiation,  whose  frequency  lies  between  those  of  infrared  radiation  and  microwave radiation,  has  a  broad  range  of  applications,  e.g.  in  non-destructive  testing,  medical  imaging, security  screening,  as  well  as  high-bit-rate  wireless  communications.  However,  the  notorious “terahertz gap”, mainly due to the lack of cost-efficient, compact, high-power emitters at around 0.3-3 THz,  has  prevented  the  large-scale  application  of  terahertz  radiation.  As  the  technology continues to advance, the main objective of this project is to help filling the “terahertz gap” by an innovative interdisciplinary power-combing approach for emitters, which is based on two hitherto unconnected advanced technologies.

This  project involves two  terahertz  research  groups: That of Prof. Masahiro Asada  at the Tokyo Institute  of  Technology  in  Japan  and  that  of  Prof.  Hartmut  G.  Roskos  at  Goethe-University Frankfurt am Main in Germany. Prof. Asada’s group develops and optimizes resonant tunneling diodes (RTDs) as compact, chip-based electronic terahertz emitters, and they have become a world-leading laboratory for this type  of terahertz  radiation source. Their RTDs  can  now operate  from tens of GHz up to 1.94 THz, and thus cover a large part of the “terahertz gap”. But the output power of the RTDs remains limited to tens of microwatt, which is not sufficient for most of the envisaged terahertz applications,  and no good strategy has been visible  to substantially increase the  output power. Coming from a different area of research of  terahertz photonics, we now suggest a novel synergistic  approach  to  this  problem.  During  the  last  few  years,  we  have  built  and  studied  one-dimensional photonic crystal cavities with high quality factor in the terahertz frequency range, and demonstrated  strong  light-matter  interaction  by  coupling  the  cavity  photons  with  metamaterial plasmons within the cavity. An example of strong coupling between complementary metamaterials and the cavity photons is shown in the following Figure 1, where Rabi splitting is clearly visible.

It occurred to us that by combining a terahertz cavity with an array of RTDs, one should be able to substantially  increase  the  output  power  of  the  RTDs  via  strong  coupling  leading  to  coherent superradiant  emission  from  the  RTD  arrays.  Funded  by  a  grant  of  DFG  to  start  an  international collaboration,  we  were  able  to  build  the  foundation  for  this  ambitious  endeavor  by  successful numerical simulations and joint experimental work in Japan. On this basis, we now enter into a full project to put into practice the  idea of the combination of a strong-coupling cavity with state-of-the-art RTD arrays to form a powerful and compact photonic terahertz emitter. 

RTD Team:
Prof. Dr. Hartmut Roskos, roskos@physik.uni-frankfurt.de
Dr. Fanqi Meng, fmeng@physik.uni-frankfurt.de

Coming soon