Erzeugung von Terahertzstrahlung durch nichtlinear-optische Mischprozesse von kohärenter Mittelinfrarotstrahlung in Halbleiter-Wellenleiterstrukturen
Final Report Abstract
The aim of this project was the direct generation of coherent continuous wave (cw) THz radiation by difference frequency generation (DFG) in semiconductor waveguides. The DFG process is a nonlinear process in which two beams of frequencies ω1 and ω2 are superimposed in a medium with second order susceptibility χ(2)ijk leading to the generation of a wave with frequency ω3 = ω1-ω2. In our case the frequencies of the signal and pump wavelength (ω1, ω2) are in the mid infrared (MIR) spectrum whereas the idler wave (ω3) is in the THz range. The main challenges of this approach are • The realization and optimization of a reliable two-wavelength source in the MIR • Optimization of the nonlinear process, in particular design and simulation of the structures to achieve phase matching and good conversion efficiency • Detection and characterisation of the generated THz radiation. During the first year we realized that for a single project partner and in the framework of the granted resources it will not be possible to achieve these ambitious goals. For this purpose one needs e.g. a European research initiative where laser sources and the nonlinear structures are realized and freely accessible to us. Therefore we decided to keep the philosophy of the project but to change directions in the sense that we advance the components and instrumentalization that are necessary for the project but also switch to conventional photomixing on photoconductive antennas. The experience gathered in this area, especially the design of the two-wavelength source, can then be transferred directly to the MIR. The main achieved results of this project are: 1) We analyzed several semiconductor waveguide designs numerically and calculated the propagation constants of the waves to determine phase matching conditions. Furthermore, the mode profiles and the effective area were calculated. The considered structures are basesd on rectangular waveguide shapes that provide guiding in both lateral dimensions. Depending on the choice and composition of the semiconductor materials phasematching can be achieved for the entire mid infrared spectrum covered by QCLs (4-11µm). Coherence lengths in the range of tens of mm are feasable for all structures. The structures can be used for DFG as long as the length of the device is longer than the coherence length. This tuning range depends on the design of the device and can reach several hundred GHz. The calculated conversion efficiency of the considered devices is estimated to be in the range of 1µW/W2. 2) The line width of the THz radiation depends on the line width of the MIR radiation. We therefore built and validated a set-up to measure the line width of the QCL which is expected to be in the range of some 100 kHz. The applied method is a modified self homodyne set up that works without long delay lines and is therefore suitable for measurements in the MIR. 3) The last area of work covers the realization of a reliable two-wavelength source in the near infrared that is afterwards utilized for photomixing on photoconductive antennas. The source is a laser diode with a double external cavity. That means that both modes are generated in the same laser. This has the advantage of an intrinsically perfect overlap of the modes. Furthermore, fluctuations in the difference frequency are reduced because both modes experience the same fluctuations. The difference frequency can be tuned between 250GHz and several THz. The amplitudes of the modes and the difference frequency are stable over time periods longer than one hour. The NIR radiation is then used for photo mixing and the generated THz radiation is characterized. The THz radiation shows a quadratic dependence on the applied bias voltage and NIR optical power. The radiation is linearly polarized and the frequency response of the antenna shows that THz generation up to a frequency of 2 THz is possible. The THz spectrum was measured with a FTIR spectrometer and shows single mode operation.
Publications
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Generation of terahertz waves by difference frequency generation of mid-infrared radiation., 4-th International Conference on Advanced Optoelectronics and Lasers (CAOL'2008), Alushta, Crimea, Ukraine together with International Workshop "THz radiation: Basic Research and Applications" (TERA'08), Alushta, Crimea, Ukraine
D. Blömer and W. Elsäßer
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Generation of Tunable Continuous-Wave THz Radiation using a Two Colour External Cavity Diode Laser. Winter Topicals 2010, Majorca, Spain
D. Blömer, V. Montanaro, S. Berning and W. Elsäßer
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Generation of tunable continuous-wave THz radiation via photo mixing using a dual wavelength external cavity diode laser. 456.WE-Heraeus-Seminar on: THz Radiation, Generation, Detection and Applications, 2010, Bad Honnef, Germany
D. Blömer, V. Montanaro, S. Berning and W. Elsäßer
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Untersuchungen zur Rückkopplungsinterferometrie mit Quantenkaskadenlasern. Ph. D. Thesis 2011, TU Darmstadt
J. v. Staden
