Only recently, a novel type of intensity autocorrelator in the time-domain has been reported for the Terahertz frequency range. The technique is based on fast electro-optic sampling in a double beam configuration and its temporal resolution is ultra-fast, as short as only few hundreds of femtoseconds. In particular, the self-referencing character of the technique is suitable for any type of source, including free-running sources. These unique characteristics enable therefore the investigation of the output profile of Terhertz Quantum Cascade Laser based Frequency Combs, with typical roundtrip times of few tens of picoseconds. The output dynamics of such devices have been investigated theoretically by Maxwell-Bloch equations and experimentally using Shifted Wave Interference Fourier Transform Spectroscopy. In this work, we present the results of the direct measurement of intensity autocorrelations of a Terahertz comb around 2.5 THz, when operated in the comb and high-noise regime, with radio-frequency beatnotes of 800 Hz and few MHz, respectively. We find the laser to be both amplitude- and frequency-modulated in both regimes, with a modulation ratio of the intensity of roughly 90 percent.The technique might come to use in future for the measurement of free-running pulses at Terahertz frequencies with high temporal resolution.
Ileana-Cristina Benea-Chelmus, Markus Rösch, Giacomo Scalari, Mattias Beck, and Jérôme Faist, "Electric field and intensity correlations of a terahertz comb based on fast electro-optic sampling (Conference Presentation)," Proc. SPIE 10111, Quantum Sensing and Nano Electronics and Photonics XIV, 101110E (Presented at SPIE OPTO: January 29, 2017; Published: 28 April 2017); https://doi.org/10.1117/12.2253171.5393916908001.
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