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\bibitem{Walther_etal2006} Walther, C., Scalari, G., Faist, J., Beere, H., \& Ritchie, D. (2006). Low frequency terahertz quantum cascade laser operating from 1.6~to~1.8~THz. \textit{Appl. Phys. Lett.}, \textit{89}, 231121(1--3).

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