Coherent instabilities and mode locking in QC lasers

(collaboration with F. Capasso and F. Kaertner groups)

 

Photoexcited or electrically injected non-equilibrium carriers in semiconductor quantum wells have sub-picosecond lifetimes and even shorter decoherence times. This makes observation of these phenomena very difficult task. On the other hand, one can utilize ultrafast dynamics and inertia-less nonlinearity of quantum-confined carriers in order to achieve generation of ultrashort pulses in quantum well semiconductor lasers. We have recently shown that saturable nonlinearity of QC lasers can trigger the instability of their CW operation through the mechanism that has been predicted by Risken Nummedal, Graham, and Haken four decades ago [1,2] but turned out to be very difficult to observe in other kinds of lasers. We found that QC lasers with their ultrafast relaxation times, large dipole moment of laser transition, and strong nonlinearity of the active core are uniquely suited for observations of RNGH instability and associated generation of a broad phase-modulated comb of longitudinal modes [3,4]. We presented the compelling evidence for the elusive RNGH instability and parametric Rabi oscillations in laser spectra (Fig. 1). Moreover, multimode spectra of QC lasers pave the way to generation of ultrashort pulses through mode-locking techniques. First QC laser generating 3-ps pulses through active mode locking has been recently reported in our paper [5].

 

Fig. 1. Optical spectra vs pumping current obtained in cw at 300 K with a buried heterostructure lasers emitting at 8.47 mm. (b) Spectral splitting and twice the Rabi frequency vs square root of output power collected from a single laser facet. The different quantities reported on the graph were deduced from the experimental data shown in (a). The red line is a least-square linear fit of the data.

 

[1] H. Risken and K. Nummedal, J. Appl. Phys. 39, 4662 (1968).

[2] P. Graham and H. Haken, Z. Phys. 213, 420 (1968).

[3] C. Y. Wang, L. Diehl, A. Gordon, C. Jirauschek, F. X. Kartner, A. Belyanin, D. Bour, S. Corzine, G. Haoer, M. Troccoli, J. Faist, and F. Capasso, Coherent Instabilities in a Semiconductor Laser with Fast Gain Recovery, Phys. Rev. A (Rapid Communications), 75, 031802 (2007). 

[4] A. Gordon, C.Y. Wang, L. Diehl, F. X. Kärtner, A. Belyanin, D. Bour, S. Corzine, G. Höfler, H. C. Liu, H. Schneider, T. Maier, M. Troccoli, J. Faist, and F. Capasso, Multimode regimes in quantum cascade lasers: From coherent instabilities to spatial hole burning, Phys. Rev. A, 77, 053804 (2008).

[5] 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, 12929 (2009).