Abstract ........................................................ i
1 Introduction ................................................. 1
1.1 General introduction .................................... 1
1.2 Coherent light sources in the mid-infrared .............. 3
1.2.1 CO2 laser and other gas lasers ................... 4
1.2.2 Lead salt diode lasers ........................... 5
1.2.3 Antimonide-based semiconductor lasers ............ 7
1.2.4 Sources based on optical parametric generation ... 9
1.3 Quantum cascade lasers ................................. 12
1.3.1 Fundamentals .................................... 12
1.3.2 State of the art ................................ 20
1.3.3 Single-mode devices ............................. 22
2 Theory ...................................................... 29
2.1 Quantum cascade lasers ................................. 29
2.1.1 Electronic states in heterostructures ........... 29
2.1.2 Intersubband relaxation times ................... 32
2.1.3 Spontaneous emission and gain between
subbands ........................................ 33
2.1.4 Intersubband transition linewidth ............... 34
2.1.5 Rate equations .................................. 35
2.2 Grating-coupled external cavity lasers ................. 37
2.2.1 Modeling of the Littrow configuration .......... 38
2.3 Tuning properties ...................................... 42
2.3.1 Coarse tuning range ............................. 42
2.3.2 Fine tuning behavior ............................ 42
2.4 Anti-reflection coating ................................ 44
3 Device fabrication and characterization ..................... 49
3.1 Epitaxial growth ....................................... 49
3.2 Sample processing ...................................... 50
3.3 Electrical and optical characterization ................ 52
3.4 Anti-reflection coatings ............................... 53
4 Pulsed external cavity quantum cascade lasers emitting
near 10 μm .................................................. 59
4.1 Introduction ........................................... 59
4.2 Active region design ................................... 60
4.2.1 The bound-to-continuum design ................... 60
4.2.2 Calculation of gain and spontaneous emission
spectra ......................................... 62
4.2.3 Measured spontaneous emission spectra ........... 64
4.3 Fabry-Perot laser performance .......................... 65
4.4 External cavity laser performance ...................... 66
4.5 Time-resolved spectra .................................. 69
4.5.1 Experimental setup .............................. 70
4.5.2 Results ......................................... 70
4.5.3 Dynamical model ................................. 70
5 Continuous-wave external cavity quantum cascade laser
emitting near μm ............................................ 75
5.1 Introduction ........................................... 75
5.2 Active region design ................................... 76
5.3 Chip fabrication and performance ....................... 77
5.4 Setup for continuous-wave operation .................... 79
5.5 Laser performance ...................................... 79
5.5.1 Threshold current ............................... 79
5.5.2 Coarse tuning range and spectral purity ......... 80
5.5.3 Output power .................................... 80
5.5.4 Fine tuning ..................................... 81
5.6 Laser linewidth ........................................ 82
5.6.1 Theoretical model ............................... 83
5.6.2 Experimental determination of an upper limit
by heterodyne mixing ............................ 85
5.7 High-resolution spectroscopic applications ............. 88
6 External cavity quantum cascade lasers based on
heterogeneous cascade active regions ........................ 93
6.1 Introduction ........................................... 93
6.2 Structure design ....................................... 95
6.2.1 Active region ................................... 95
6.2.2 Waveguide ....................................... 96
6.2.3 Spontaneous emission spectra .................... 97
6.3 Rate equation model .................................... 98
6.4 Fabrication and characterization of the gain
elements .............................................. 102
6.5 Performance of external cavity lasers ................. 105
6.6 Continuous-wave heterogeneous cascade devices ......... 107
7 Conclusions and Outlook .................................... 109
7.1 Summary of the results ................................ 109
7.2 Conclusions ........................................... 1ll
7.3 Outlook ............................................... 111
Acknowledgments ............................................... 113
Published work ................................................ 115
Conference presentations ...................................... 117
Bibliography .................................................. 119
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