Abstract-Dynamics of ultra-broadband terahertz quantum cascade lasers for comb operation.

Hua Li, Pierre Laffaille, Djamal Gacemi, Marc Apfel, Carlo Sirtori, Jeremie Leonardon, Giorgio Santarelli, Markus Rösch, Giacomo Scalari, Mattias Beck, Jerome Faist, Wolfgang Hänsel, Ronald Holzwarth, Stefano Barbieri

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We present an experimental investigation of the multimode dynamics and the coherence of terahertz quantum cascade lasers emitting over a spectral bandwidth of ~1THz. The devices are studied in free-running and under direct RF modulation. Depending on the pump current we observe different regimes of operation, where RF spectra displaying single and multiple narrow beat-note signals alternate with spectra showing a single beat-note characterized by an intense phase-noise, extending over a bandwidth up to a few GHz. We investigate the relation between this phase-noise and the dynamics of the THz modes through the electro-optic sampling of the laser emission. We find that when the phase-noise is large, the laser operates in an unstable regime where the lasing modes are incoherent. Under RF modulation of the laser current such instability can be suppressed and the modes coherence recovered, while, simultaneously, generating a strong broadening of the THz emission spectrum.

Abstract-Coupled-cavity terahertz quantum cascade lasers for single mode operation

H. Li^1,a), J. M. Manceau^1,b), A. Andronico¹, V. Jagtap¹, C. Sirtori¹, L. H. Li²,E. H. Linfield², A. G. ² and S. Barbieri^1,c)

 HIDE AFFILIATIONS
1 Laboratoire Matériaux et Phénomènes Quantiques, Université Paris Diderot and CNRS, UMR 7162, 10 rue A. Domont et L. Duquet, 75205 Paris, France
2 School of Electronic and Electrical Engineering, University of Leeds, Leeds LS2 9JT, United Kingdom
a) Electronic mail: hua.li@univ-paris-diderot.fr
b) Present address: Institut d'Electronique Fondamentale, Université Paris Sud and CNRS, UMR 8622, 91405 Orsay, France
c) Electronic mail: stefano.barbieri@univ-paris-diderot.fr
Appl. Phys. Lett. 104, 241102 (2014); http://dx.doi.org/10.1063/1.4884056
http://scitation.aip.org/content/aip/journal/apl/104/24/10.1063/1.4884056
We demonstrate the operation of coupled-cavity terahertz frequency quantum-cascade lasers composed of two sub-cavities separated by an air gap realized by optical lithography and dryetching. This geometry allows stable, single mode operation with typical side mode suppression ratios in the 30–40 dB range. We employ a transfer matrix method to model the mode selection mechanism. The obtained results are in good agreement with the measurements and allow prediction of the operating frequency.

Abstract-Continuous-wave coherent imaging with terahertz quantum cascade lasers using electro-optic harmonic sampling

M. Ravaro¹, V. Jagtap¹, G. Santarelli², C. Sirtori¹, L. H. Li³, S. P. Khanna³, E. H. Linfield³, and S. Barbieri¹

^{http://apl.aip.org/resource/1/applab/v102/i9/p091107_s1?isAuthorized=no}

¹Laboratoire Matériaux et Phénomènes Quantiques (MPQ) and CNRS, UMR 7162, Université Paris-Diderot, 10, rue A. Domont et L. Duquet, 75205 Paris, France
²Laboratoire Photonique, Numérique et Nanosciences (LP2N) and CNRS, UMR 5298, Université de Bordeaux 1, Institut d'Optique, 351 cours de la Libération, 33405 Talence, France
³School of Electronic and Electrical Engineering, University of Leeds, Leeds LS2 9JT, United Kingdom                        

We demonstrate a coherent imaging system based on a terahertz (THz) frequency quantum cascade laser (QCL) phase-locked to a near-infrared fs-laser comb. The phase locking enables coherent electro-optic sampling of the continuous-wave radiation emitted by the QCL through the generation of a heterodyne beat-note signal. We use this beat-note signal to demonstrate raster scan coherent imaging using a QCL emitting at 2.5 THz. At this frequency the detection noise floor of our system is of 3 pW/Hz and the long-term phase stability is <3°/h, limited by the mechanical stability of the apparatus.