Microwave technology applied to terahertz quantum cascade lasers

At Terahertz (THz) frequencies metals are still excellent materials to guide and confine electromagnetic radiation with relatively low losses. Therefore the concepts developed in the microwave range to design efficient waveguides and resonators can be successfully transferred up to this frequency region. A successful example of such "technology transfer" is the so-called metal-metal resonator, effectively used as a waveguide for THz Quantum Cascade Lasers (QCLs). This type of resonator is essentially a downscaled version of a microstrip waveguide, widely used at microwave frequencies. In this work we report on microwave impedance measurements of metal-metal ridge-waveguide THz QCLs. Experimental data, recorded at 4K in the 100MHz-55GHz range, are well reproduced by distributed-parameter transmission-line simulations, showing that the modulation cutoff is limited by the propagation losses that increase for higher microwave frequencies, yielding a 3dB modulation bandwidth of ~70GHz for a 1mm-long ridge. By using a shunt-stub matching we demonstrate amplitude modulation of a 2.3THz QCL up to 24GHz. In the last part of this work we discuss the experimental evidence of a feedback-coupling between the intracavity THz field and the microwave field generated by the beating of the Fabry-Perot longitudinal modes above the lasing threshold.

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