Optical frequency combs1,2 establish a rigid phase-coherent link between microwave and optical domains and are emerging as high-precision tools in an increasing number of applications3. Frequency combs with large intermodal spacing are employed in the field of microwave photonics for radiofrequency arbitrary waveform synthesis4,5 and for the generation of terahertz tones of high spectral purity in future wireless communication networks6,7. Here, we demonstrate self-starting harmonic frequency comb generation with a terahertz repetition rate in a quantum cascade laser. The large intermodal spacing caused by the suppression of tens of adjacent cavity modes originates from a parametric contribution to the gain due to temporal modulations of population inversion in the laser8,9. Using multiheterodyne self-detection, the mode spacing of the harmonic comb is shown to be uniform to within 5 × 10−12 parts of the central frequency. This new harmonic comb state extends the range of applications of quantum cascade laser frequency combs10,11,12,13.
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This work was supported by the DARPA SCOUT programme through grant no. W31P4Q-16-1-0002. The authors acknowledge support from the National Science Foundation under award no. ECCS-1614631. Any opinions, findings, conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the Assistant Secretary of Defense for Research and Engineering or of the National Science Foundation. M.P. and D.K. thank J.B. MacArthur for the assembly of RF filters, A.Y. Zhu for sputtering gold on a QCL submount, and N. Rubin for careful reading of the manuscript.
The authors declare no competing financial interests.
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Kazakov, D., Piccardo, M., Wang, Y. et al. Self-starting harmonic frequency comb generation in a quantum cascade laser. Nature Photon 11, 789–792 (2017). https://doi.org/10.1038/s41566-017-0026-y
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