We demonstrate that the cavity resonance frequency –the round-trip frequency – of Terahertz quantum cascade lasers can be injection-locked by direct modulation of the bias current using an RF source. Metal-metal and single-plasmon waveguide devices with roundtrip frequencies up to 35GHz have been studied, and show locking ranges above 200MHz. Inside this locking range the laser round-trip frequency is phase-locked, with a phase noise determined by the RF-synthesizer. We find a square-root dependence of the locking range with RF-power in agreement with classical injection-locking theory. These results are discussed in the context of mode-locking operation.

© 2010 Optical Society of America

1. E. A. Avrutin, J. H. Marsh, and E. L. Portnoi, “Monolithic and multi-GigaHertz mode-locked semiconductor lasers: constructions, experiments, models and applications,” IEE Proc. Optoelectron. 147(4), 251–278 (2000). [CrossRef]
2. J. E. Bowers, P. A. Morton, A. Mar, and S. W. Corzine, “Actively mode-locked semiconductor lasers,” IEEE J. Quantum Electron. 25(6), 1426–1439 (1989). [CrossRef]
3. L. A. Jiang, M. E. Grein, E. P. Ippen, C. McNeilage, J. Searls, and H. Yokoyama, “Quantum-limited noise performance of a mode-locked laser diode,” Opt. Lett. 27(1), 49–51 (2002). [CrossRef]
4. D. Y. Kim, D.-S. Seo, and H.-F. Liu, “Observation of very efficeint hybrid mode-locking in InGaAs/InGaAsInP multiple quantum well distributed Bragg reflector laser diode,” Appl. Phys. Lett. 67(21), 3075–3077 (1995). [CrossRef]
5. S. Arahira, and Y. Ogawa, “Synchronous mode-locking in passively mode-locked semiconductor laser diodes using optical short pulses repeated at subharmonics of the cavity round-trip frequency,” IEEE Photon. Technol. Lett. 8(2), 191–193 (1996). [CrossRef]
6. T. Hoshida, H-F. Liu, M. Tsuchiya, Y. Ogawa, T. Kamiya, “Subharmonic Hybrid Mode-Locking of a Monolithic Semiconductor Laser,” IEEE J. Sel. Top Quantum Electron. 2, 514-516 (1996).
7. R. Paiella, Intersubband transitions in quantum structures, (Mc-Graw Hill Nanoscience and Technology, New York, 2006)
8. R. Paiella, R. Martini, F. Capasso, C. Gmachl, H. Y. Hwang, D. L. Sivco, J. N. Ballargeon, A. Y. Cho, E. A. Whyttaker, and H. C. Liu, “High-frequency modulation without the relaxation oscillation resonance in quantum cascade lasers,” Appl. Phys. Lett. 79(16), 2526–2528 (2001). [CrossRef]
9. R. Paiella, F. Capasso, C. Gmachl, H. Y. Hwang, D. L. Sivco, A. L. Hutchinson, A. Y. Cho, and H. C. Liu, “Monolithic active mode-locking of quantum cascade lasers,” Appl. Phys. Lett. 77(2), 169–171 (2000). [CrossRef]
10. C. Y. Wang, L. Kuznetsova, V. M. Gkortsas, L. Diehl, F. X. Kärtner, M. A. Belkin, A. Belyanin, X. Li, D. Ham, H. Schneider, P. Grant, C. Y. Song, S. Haffouz, Z. R. Wasilewski, H. C. Liu, and F. Capasso, “Mode-locked pulses from mid-infrared quantum cascade lasers,” Opt. Express 17(15), 12929–12943 (2009). [CrossRef] [PubMed]
11. S. Kumar, Q. Hu, and J. L. Reno, “186 K operation of terahertz quantum-cascade lasers based on a diagonal design,” Appl. Phys. Lett. 94(13), 131105 (2009). [CrossRef]
12. G. Scalari, C. Walther, M. Fischer, R. Terazzi, H. Beere, D. Ritchie, and J. Faist, “THz and sub-THz quantum cascade lasers,” Laser Photon Rev. 3(1-2), 45–66 (2009). [CrossRef]
13. B. S. Williams, “Terahertz quantum cascade lasers,” Nat. Photonics 1(9), 517–525 (2007). [CrossRef]
14. . W. Maineult, L. Ding, P. Gellie, P. Filloux, C. Sirtori, S. Barbieri, J. F. Lampin, T. Akalin, I. Sagnes, H. E. Beere, and D. A. Ritchie, “Microwave modulation of THz quantum cascade lasers: a transmission?line approach,” Appl. Phys. Lett. 96(2), 021108–021110 (2010). [CrossRef]
15. . S. Barbieri, W. Maineult, S. S. Dhillon, C. Sirtori, J. Alton, N. Breuil, H. E. Beere, and D. A. Ritchie, “13 GHz direct modulation of terahertz quantum cascade lasers,” Appl. Phys. Lett. 91(14), 143510 (2007). [CrossRef]
16. . M. Hajenius, P. Khosropanah, J. N. Hovenier, J. R. Gao, T. M. Klapwijk, S. Barbieri, S. Dhillon, P. Filloux, C. Sirtori, D. A. Ritchie, and H. E. Beere, “Surface plasmon quantum cascade lasers as terahertz local oscillators,” Opt. Lett. 33(4), 312–314 (2008). [CrossRef] [PubMed]
17. . T. Yilmaz, C. M. Depriest, P. J. Delfyett, Jr., A. Braun, and J. Abeles, “Measurement of residual phase noise and longitudinal-mode linewidth in a hybridly mode-locked external linear cavity semiconductor laser,” Opt. Lett. 27(10), 872–874 (2002). [CrossRef]
18. . C. Worrall, J. Alton, M. Houghton, S. Barbieri, H. E. Beere, D. Ritchie, and C. Sirtori, “Continuous wave operation of a superlattice quantum cascade laser emitting at 2 THz,” Opt. Express 14(1), 171–181 (2006). [CrossRef] [PubMed]
19. . The origin of these sidebands is not clear. However they are not relevant for this work.
20. . S. Barbieri, J. Alton, C. Baker, T. Lo, H. Beere, and D. Ritchie, “Imaging with THz quantum cascade lasers using a Schottky diode mixer,” Opt. Express 13(17), 6497–6503 (2005). [CrossRef] [PubMed]
21. . S. Barbieri, W. Maineult, L. Ding, P. Gellie, P. Filloux, C. Sirtori, T. Akalin, J. F. Lampin, I. Sagnes, H. E. Beere, and D. A. Ritchie, “Microwave technology applied to THz quantum cascade lasers,” Proc. SPIE 7608, 76080X (2010). [CrossRef]
22. . A. E. Siegman, Lasers, (University Science Books, Mill Valley, 1986).
23. . Z. Ahmed, L. Zhai, A. J. Lowery, N. Onodera, and R. S. Tucker, “Locking bandwidth of actively mode-locked semiconductor lasers,” IEEE J. Quantum Electron. 29(6), 1714–1721 (1993). [CrossRef]
24. . As mentioned above in the text, the oscillation in the intensity of fRF is produced by the non-ideal directivity of the coupler (Fig. 2(a)).
25. . M. DiDomenico, Jr., “Small-signal analysis of internal (coupling-type) modulation of lasers,” J. Appl. Phys. 35(10), 2870–2876 (1964). [CrossRef]
26. . O. P. McDuff, and S. E. Harris, “Nonlinear theory of internally loss-modulated lasers,” IEEE J. Quantum Electron. 3(3), 101–111 (1967). [CrossRef]
27. . R. Adler, “A study of locking phenomena in oscillators,” Proc. IEEE 61(10), 1380–1385 (1973). [CrossRef]
28. . B. Razavi, “A study of injection-locking and pulling in oscillators,” IEEE J. Sol. State Circuits. 39(9), 1415–1424 (2004). [CrossRef]
29. . S. Kohen, B. S. Williams, and Q. Hu, “Electromagnetic modeling of terahertz quantum cascade laser waveguides and resonators,” Appl. Phys. Lett. 97, 053106–053108 (2005).
30. . S. Barbieri, J. Alton, J. Fowler, H. E. Beere, E. H. Linfield, and D. A. Ritchie, “2.9 THz quantum cascade lasers operating up to 70 K in continuous wave,” Appl. Phys. Lett. 85(10), 1674 (2004). [CrossRef]
31. . M. Hajenius, P. Khosropanah, J. N. Hovenier, J. R. Gao, T. M. Klapwijk, S. Barbieri, S. Dhillon, P. Filloux, C. Sirtori, D. A. Ritchie, and H. E. Beere, “Surface plasmon quantum cascade lasers as terahertz local oscillators,” Opt. Lett. 33(4), 312–314 (2008). [CrossRef] [PubMed]
32. . S. Barbieri, J. Alton, H. E. Beere, E. H. Linfield, D. A. Ritchie, S. Withington, G. Scalari, L. Ajili, and J. Faist, “Heterodyne mixing of two far-infrared quantum cascade lasers by use of a point-contact Schottky diode,” Opt. Lett. 29(14), 1632–1634 (2004). [CrossRef] [PubMed]
33. . A. Barkan, F. K. Tittel, D. M. Mittleman, R. Dengler, P. H. Siegel, G. Scalari, L. Ajili, J. Faist, H. E. Beere, E. H. Linfield, A. G. Davies, and D. A. Ritchie, “Linewidth and tuning characteristics of terahertz quantum cascade lasers,” Opt. Lett. 29(6), 575–577 (2004). [CrossRef] [PubMed]
34. . S. Barbieri, P. Gellie, G. Santarelli, L. Ding, W. Maineult, C. Sirtori, R. Colombelli, H. E. Beere, and D. A. Ritchie, “Phase locking of a 2.7THz quantum cascade laser to a mode-locked Er-fiber laser,” Nat. Photonics 4, 636 - 640 (2010). [CrossRef]
35. . E. E. Orlova, J. N. Hovenier, T. O. Klaassen, I. Ksalynas, A. J. L. Adam, J. R. Gao, T. M. Klapwijk, B. S. Williams, S. Kumar, Q. Hu, and J. L. Reno, “Antenna model for wire-lasers,” Phys. Rev. Lett. 96,173904 ?173906 (2006). [CrossRef] [PubMed]
36. . W. Maineult, P. Gellie, A. Andronico, P. Filloux, G. Leo, C. Sirtori, S. Barbieri, E. Peytavit, T. Akalin, J.-F. Lampin, J. Alton, H. Beere, and D. R. Ritchie, “Double metal quantum cascade lasers with micro-TEM horn antenna,” Appl. Phys. Lett. 93(18), 183508 (2008). [CrossRef]
37. . Compared to Section 2, for the measurements presented in this Section we used a different SA with a higher resolution (1Hz compared to 10Hz).
38. . We tried using longer devices. Unfortunately the cooling power needed to cool the QCLs was beyond the capability of our cryostat.
39. . R. Paiella, F. Capasso, C. Gmachl, D. L. Sivco, J. N. Baillargeon, A. L. Hutchinson, A. Y. Cho, and H. C. Liu, “Self-mode-locking of quantum cascade lasers with giant ultrafast optical nonlinearities,” Science 290(5497), 1739–1742 (2000). [CrossRef] [PubMed]
40. . C. Y. Wang, L. Diehl, A. Gordon, C. Jirauschek, F. X. Kärtner, A. Belyanin, D. Bour, S. Corzine, G. Höfler, M. Troccoli, J. Faist, and F. Capasso, “Coherent instabilities in a semiconductor laser with fast gain recovery,” Phys. Rev. A 75(3), 031802 (2007). [CrossRef]
41. . H. Choi, L. Diehl, Z. K. Wu, M. Giovannini, J. Faist, F. Capasso, and T. B. Norris, “Gain recovery dynamics and photon-driven transport in quantum cascade lasers,” Phys. Rev. Lett. 100(16), 167401 (2008). [CrossRef] [PubMed]
42. . M. Amanti, G. Scalari, R. Terazzi, M. Fischer, M. Beck, J. Faist, A. Rudra, P. Gallo, and E. Kapon, “Bound-to-continuum terahertz quantum cascade laser with a single-quantum-well phonon extraction/injection stage,” N. J. Phys. 11(12), 1–19 (2009). [CrossRef]
43. . C. Walther, G. Scalari, J. Faist, H. Beere, and D. Ritchie, “Low frequency terahertz quantum cascade laser operating from 1.6 to 1.8 THz,” Appl. Phys. Lett. 89(23), 231121 (2006). [CrossRef]
44. . C. Walther, M. Fisher, G. Scalari, R. Terazzi, N. Hoyler, and J. Faist, “Quantum cascade laser operating from 1.2 to 1.6 THz,” Appl. Phys. Lett. 91(13), 131122 (2007). [CrossRef]
45. . S. Ramo, J. R. Whinnery, and T. V. Duzer, Field and waves in communication electronics, 2nd ed. (Wiley, 1984).

Appl. Phys. Lett.

• D. Y. Kim, D.-S. Seo, and H.-F. Liu, “Observation of very efficeint hybrid mode-locking in InGaAs/InGaAsInP multiple quantum well distributed Bragg reflector laser diode,” Appl. Phys. Lett. 67(21), 3075–3077 (1995). [CrossRef]
• R. Paiella, R. Martini, F. Capasso, C. Gmachl, H. Y. Hwang, D. L. Sivco, J. N. Ballargeon, A. Y. Cho, E. A. Whyttaker, and H. C. Liu, “High-frequency modulation without the relaxation oscillation resonance in quantum cascade lasers,” Appl. Phys. Lett. 79(16), 2526–2528 (2001). [CrossRef]
• R. Paiella, F. Capasso, C. Gmachl, H. Y. Hwang, D. L. Sivco, A. L. Hutchinson, A. Y. Cho, and H. C. Liu, “Monolithic active mode-locking of quantum cascade lasers,” Appl. Phys. Lett. 77(2), 169–171 (2000). [CrossRef]
• . W. Maineult, L. Ding, P. Gellie, P. Filloux, C. Sirtori, S. Barbieri, J. F. Lampin, T. Akalin, I. Sagnes, H. E. Beere, and D. A. Ritchie, “Microwave modulation of THz quantum cascade lasers: a transmission?line approach,” Appl. Phys. Lett. 96(2), 021108–021110 (2010). [CrossRef]
• . S. Barbieri, W. Maineult, S. S. Dhillon, C. Sirtori, J. Alton, N. Breuil, H. E. Beere, and D. A. Ritchie, “13 GHz direct modulation of terahertz quantum cascade lasers,” Appl. Phys. Lett. 91(14), 143510 (2007). [CrossRef]
• S. Kumar, Q. Hu, and J. L. Reno, “186 K operation of terahertz quantum-cascade lasers based on a diagonal design,” Appl. Phys. Lett. 94(13), 131105 (2009). [CrossRef]
• . S. Kohen, B. S. Williams, and Q. Hu, “Electromagnetic modeling of terahertz quantum cascade laser waveguides and resonators,” Appl. Phys. Lett. 97, 053106–053108 (2005).
• . S. Barbieri, J. Alton, J. Fowler, H. E. Beere, E. H. Linfield, and D. A. Ritchie, “2.9 THz quantum cascade lasers operating up to 70 K in continuous wave,” Appl. Phys. Lett. 85(10), 1674 (2004). [CrossRef]
• . W. Maineult, P. Gellie, A. Andronico, P. Filloux, G. Leo, C. Sirtori, S. Barbieri, E. Peytavit, T. Akalin, J.-F. Lampin, J. Alton, H. Beere, and D. R. Ritchie, “Double metal quantum cascade lasers with micro-TEM horn antenna,” Appl. Phys. Lett. 93(18), 183508 (2008). [CrossRef]
• . C. Walther, G. Scalari, J. Faist, H. Beere, and D. Ritchie, “Low frequency terahertz quantum cascade laser operating from 1.6 to 1.8 THz,” Appl. Phys. Lett. 89(23), 231121 (2006). [CrossRef]
• . C. Walther, M. Fisher, G. Scalari, R. Terazzi, N. Hoyler, and J. Faist, “Quantum cascade laser operating from 1.2 to 1.6 THz,” Appl. Phys. Lett. 91(13), 131122 (2007). [CrossRef]

IEE Proc. Optoelectron.

• E. A. Avrutin, J. H. Marsh, and E. L. Portnoi, “Monolithic and multi-GigaHertz mode-locked semiconductor lasers: constructions, experiments, models and applications,” IEE Proc. Optoelectron. 147(4), 251–278 (2000). [CrossRef]

IEEE J. Quantum Electron.

• J. E. Bowers, P. A. Morton, A. Mar, and S. W. Corzine, “Actively mode-locked semiconductor lasers,” IEEE J. Quantum Electron. 25(6), 1426–1439 (1989). [CrossRef]
• . Z. Ahmed, L. Zhai, A. J. Lowery, N. Onodera, and R. S. Tucker, “Locking bandwidth of actively mode-locked semiconductor lasers,” IEEE J. Quantum Electron. 29(6), 1714–1721 (1993). [CrossRef]
• . O. P. McDuff, and S. E. Harris, “Nonlinear theory of internally loss-modulated lasers,” IEEE J. Quantum Electron. 3(3), 101–111 (1967). [CrossRef]

IEEE J. Sel. Top Quantum Electron.

• T. Hoshida, H-F. Liu, M. Tsuchiya, Y. Ogawa, T. Kamiya, “Subharmonic Hybrid Mode-Locking of a Monolithic Semiconductor Laser,” IEEE J. Sel. Top Quantum Electron. 2, 514-516 (1996).

IEEE J. Sol. State Circuits.

• . B. Razavi, “A study of injection-locking and pulling in oscillators,” IEEE J. Sol. State Circuits. 39(9), 1415–1424 (2004). [CrossRef]

IEEE Photon. Technol. Lett.

• S. Arahira, and Y. Ogawa, “Synchronous mode-locking in passively mode-locked semiconductor laser diodes using optical short pulses repeated at subharmonics of the cavity round-trip frequency,” IEEE Photon. Technol. Lett. 8(2), 191–193 (1996). [CrossRef]

J. Appl. Phys.

• . M. DiDomenico, Jr., “Small-signal analysis of internal (coupling-type) modulation of lasers,” J. Appl. Phys. 35(10), 2870–2876 (1964). [CrossRef]

Laser Photon Rev.

• G. Scalari, C. Walther, M. Fischer, R. Terazzi, H. Beere, D. Ritchie, and J. Faist, “THz and sub-THz quantum cascade lasers,” Laser Photon Rev. 3(1-2), 45–66 (2009). [CrossRef]

N. J. Phys.

• . M. Amanti, G. Scalari, R. Terazzi, M. Fischer, M. Beck, J. Faist, A. Rudra, P. Gallo, and E. Kapon, “Bound-to-continuum terahertz quantum cascade laser with a single-quantum-well phonon extraction/injection stage,” N. J. Phys. 11(12), 1–19 (2009). [CrossRef]

Nat. Photonics

• B. S. Williams, “Terahertz quantum cascade lasers,” Nat. Photonics 1(9), 517–525 (2007). [CrossRef]
• . S. Barbieri, P. Gellie, G. Santarelli, L. Ding, W. Maineult, C. Sirtori, R. Colombelli, H. E. Beere, and D. A. Ritchie, “Phase locking of a 2.7THz quantum cascade laser to a mode-locked Er-fiber laser,” Nat. Photonics 4, 636 - 640 (2010). [CrossRef]

Opt. Express

• . S. Barbieri, J. Alton, C. Baker, T. Lo, H. Beere, and D. Ritchie, “Imaging with THz quantum cascade lasers using a Schottky diode mixer,” Opt. Express 13(17), 6497–6503 (2005). [CrossRef] [PubMed]
• . C. Worrall, J. Alton, M. Houghton, S. Barbieri, H. E. Beere, D. Ritchie, and C. Sirtori, “Continuous wave operation of a superlattice quantum cascade laser emitting at 2 THz,” Opt. Express 14(1), 171–181 (2006). [CrossRef] [PubMed]
• C. Y. Wang, L. Kuznetsova, V. M. Gkortsas, L. Diehl, F. X. Kärtner, M. A. Belkin, A. Belyanin, X. Li, D. Ham, H. Schneider, P. Grant, C. Y. Song, S. Haffouz, Z. R. Wasilewski, H. C. Liu, and F. Capasso, “Mode-locked pulses from mid-infrared quantum cascade lasers,” Opt. Express 17(15), 12929–12943 (2009). [CrossRef] [PubMed]

Opt. Lett.

• L. A. Jiang, M. E. Grein, E. P. Ippen, C. McNeilage, J. Searls, and H. Yokoyama, “Quantum-limited noise performance of a mode-locked laser diode,” Opt. Lett. 27(1), 49–51 (2002). [CrossRef]
• . M. Hajenius, P. Khosropanah, J. N. Hovenier, J. R. Gao, T. M. Klapwijk, S. Barbieri, S. Dhillon, P. Filloux, C. Sirtori, D. A. Ritchie, and H. E. Beere, “Surface plasmon quantum cascade lasers as terahertz local oscillators,” Opt. Lett. 33(4), 312–314 (2008). [CrossRef] [PubMed]
• . T. Yilmaz, C. M. Depriest, P. J. Delfyett, Jr., A. Braun, and J. Abeles, “Measurement of residual phase noise and longitudinal-mode linewidth in a hybridly mode-locked external linear cavity semiconductor laser,” Opt. Lett. 27(10), 872–874 (2002). [CrossRef]
• . M. Hajenius, P. Khosropanah, J. N. Hovenier, J. R. Gao, T. M. Klapwijk, S. Barbieri, S. Dhillon, P. Filloux, C. Sirtori, D. A. Ritchie, and H. E. Beere, “Surface plasmon quantum cascade lasers as terahertz local oscillators,” Opt. Lett. 33(4), 312–314 (2008). [CrossRef] [PubMed]
• . S. Barbieri, J. Alton, H. E. Beere, E. H. Linfield, D. A. Ritchie, S. Withington, G. Scalari, L. Ajili, and J. Faist, “Heterodyne mixing of two far-infrared quantum cascade lasers by use of a point-contact Schottky diode,” Opt. Lett. 29(14), 1632–1634 (2004). [CrossRef] [PubMed]
• . A. Barkan, F. K. Tittel, D. M. Mittleman, R. Dengler, P. H. Siegel, G. Scalari, L. Ajili, J. Faist, H. E. Beere, E. H. Linfield, A. G. Davies, and D. A. Ritchie, “Linewidth and tuning characteristics of terahertz quantum cascade lasers,” Opt. Lett. 29(6), 575–577 (2004). [CrossRef] [PubMed]

Phys. Rev. A

• . C. Y. Wang, L. Diehl, A. Gordon, C. Jirauschek, F. X. Kärtner, A. Belyanin, D. Bour, S. Corzine, G. Höfler, M. Troccoli, J. Faist, and F. Capasso, “Coherent instabilities in a semiconductor laser with fast gain recovery,” Phys. Rev. A 75(3), 031802 (2007). [CrossRef]

Phys. Rev. Lett.

• . H. Choi, L. Diehl, Z. K. Wu, M. Giovannini, J. Faist, F. Capasso, and T. B. Norris, “Gain recovery dynamics and photon-driven transport in quantum cascade lasers,” Phys. Rev. Lett. 100(16), 167401 (2008). [CrossRef] [PubMed]
• . E. E. Orlova, J. N. Hovenier, T. O. Klaassen, I. Ksalynas, A. J. L. Adam, J. R. Gao, T. M. Klapwijk, B. S. Williams, S. Kumar, Q. Hu, and J. L. Reno, “Antenna model for wire-lasers,” Phys. Rev. Lett. 96,173904 ?173906 (2006). [CrossRef] [PubMed]

Proc. IEEE

• . R. Adler, “A study of locking phenomena in oscillators,” Proc. IEEE 61(10), 1380–1385 (1973). [CrossRef]

Proc. SPIE

• . S. Barbieri, W. Maineult, L. Ding, P. Gellie, P. Filloux, C. Sirtori, T. Akalin, J. F. Lampin, I. Sagnes, H. E. Beere, and D. A. Ritchie, “Microwave technology applied to THz quantum cascade lasers,” Proc. SPIE 7608, 76080X (2010). [CrossRef]

Science

• . R. Paiella, F. Capasso, C. Gmachl, D. L. Sivco, J. N. Baillargeon, A. L. Hutchinson, A. Y. Cho, and H. C. Liu, “Self-mode-locking of quantum cascade lasers with giant ultrafast optical nonlinearities,” Science 290(5497), 1739–1742 (2000). [CrossRef] [PubMed]

Other

• . S. Ramo, J. R. Whinnery, and T. V. Duzer, Field and waves in communication electronics, 2nd ed. (Wiley, 1984).
• . A. E. Siegman, Lasers, (University Science Books, Mill Valley, 1986).
• . As mentioned above in the text, the oscillation in the intensity of fRF is produced by the non-ideal directivity of the coupler (Fig. 2(a)).
• . Compared to Section 2, for the measurements presented in this Section we used a different SA with a higher resolution (1Hz compared to 10Hz).
• . We tried using longer devices. Unfortunately the cooling power needed to cool the QCLs was beyond the capability of our cryostat.
• . The origin of these sidebands is not clear. However they are not relevant for this work.
• R. Paiella, Intersubband transitions in quantum structures, (Mc-Graw Hill Nanoscience and Technology, New York, 2006)

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