1 Introduction .................................................. 5
1.1 The terahertz range ...................................... 5
1.2 Terahertz applications ................................... 6
1.2.1 Astronomy ......................................... 6
1.2.2 Spectroscopy ...................................... 7
1.2.3 Imaging ........................................... 7
1.3 Terahertz sources ........................................ 8
1.3.1 Electronic sources ............................... 11
1.3.2 Optical sources .................................. 12
1.4 Bridging the gap ........................................ 15
2 Theory ....................................................... 19
2.1 The QCL, a unipolar intersubband laser .................. 19
2.2 QCL modeling ............................................ 23
2.2.1 Energy states in heterostructures ................ 23
2.2.2 Spontaneous and stimulated emission .............. 25
2.2.3 Gain in QCLs ..................................... 26
2.2.4 Rate equations ................................... 28
2.2.5 Resonant tunneling ............................... 32
2.3 Transport models ........................................ 35
2.4 Scattering .............................................. 36
2.4.1 Optical phonon ................................... 37
2.4.2 Acoustical phonon ................................ 39
2.4.3 Interface roughness .............................. 40
2.4.4 Alloy disorder ................................... 42
2.4.5 Ionized impurity ................................. 43
2.4.6 Electron-electron ................................ 44
2.4.7 Screening models ................................. 45
3 Experimental methods ......................................... 53
3.1 Sample fabrication ...................................... 53
3.1.1 Growth ........................................... 53
3.1.2 Double metal ridge lasers ........................ 55
3.1.3 LC laser ......................................... 56
3.2 Characterization ........................................ 59
3.2.1 Spectral characterization ........................ 59
3.2.2 Laser power characterization ..................... 60
3.2.3 LC laser power characterization .................. 61
3.2.4 Reflection measurement ........................... 63
4 Low frequency terahertz QCLs ................................. 65
4.1 Terahertz active region design .......................... 65
4.1.1 Active regions below 2 THz ....................... 69
4.2 Design of low frequency terahertz QCLs .................. 71
4.2.1 Challenges below 2 THz ........................... 71
4.2.2 Optical losses ................................... 72
4.2.3 Low frequency bandstructure ...................... 75
4.2.4 Lifetimes ........................................ 78
4.2.5 Terahertz waveguides ............................. 80
4.3 QCL emitting from 1.6 to 1.8 THz ........................ 85
4.4 QCLs emitting from 1.2 to 1.6 THz ....................... 90
4.4.1 Bandstructure scaling ............................ 90
4.4.2 Laser characterization ........................... 92
4.5 Comparison of lasers from 2.1 - 1.2 THz ................. 97
4.5.1 Frequency coverage ............................... 98
4.5.2 Emitted power .................................... 98
4.5.3 Temperature performance ......................... 101
4.5.4 Waveguide losses ................................ 104
4.5.5 Lifetimes ....................................... 105
4.5.6 Injection efficiency ............................ 106
4.6 Application of a 1.5 THz QCL ........................... 109
4.7 Conclusions ............................................ 110
5 Magneto-transport of low frequency QCLs ..................... 113
5.1 Magneto-transport ...................................... 113
5.1.1 The Hamiltonian ................................. 114
5.1.2 Transport in the magnetic field ................. 116
5.1.3 Elastic current ................................. 120
5.1.4 Injection efficiency of low frequency QCLs ...... 121
5.1.5 Comparison of the 1.5 and 1.3 THz laser ......... 124
5.2 Towards a 1 THz laser .................................. 124
5.2.1 Scaling to 1.1 THz .............................. 124
5.2.2 Scaling to 1.0 THz .............................. 127
5.2.3 4 well laser at 1.2 THz ......................... 129
5.3 Conclusions ............................................ 132
6 The lumped circuit Laser .................................... 133
6.1 Introduction ........................................... 133
6.1.1 Microcavity lasers .............................. 133
6.1.2 Metamaterials ................................... 136
6.2 LC resonator ........................................... 138
6.2.1 From the ideal to the real device ............... 138
6.2.2 Simple model of the LC resonator ................ 139
6.2.3 Fullwave simulation ............................. 141
6.2.4 Microwave model ................................. 148
6.2.5 Comparison of the 3 models ...................... 154
6.2.6 Measurements .................................... 156
6.3 LC laser ............................................... 158
6.3.1 Device fabrication .............................. 158
6.3.2 Optical characterization ........................ 159
6.3.3 Emission of circular LC lasers .................. 168
6.4 Purcell effect in the LC laser ......................... 171
6.4.1 Light-matter interaction in the weak coupling
regime .......................................... 172
6.4.2 Purcell factor in the LC resonator .............. 177
6.4.3 QCL rate equations in a microcavity ............. 178
6.4.4 Comparison with experiment ...................... 182
6.5 Outlook ................................................ 189
6.5.1 Engineering of the LC resonator ................. 189
6.5.2 Lasers .......................................... 194
6.5.3 Strong coupling ................................. 196
6.5.4 Detectors ....................................... 198
6.5.5 Terahertz emitter ............................... 199
7 Conclusions and perspectives ................................ 203
Appendix ....................................................... 207
A Processing .................................................. 207
A.l Acid based etch solutions .............................. 207
A.2 Resists ................................................ 208
A.3 Plasma processing ...................................... 209
A.3.1 Nitride deposition .............................. 209
A.3.2 Nitride etching ................................. 209
A.3.3 GaAs etching .................................... 210
B Microstrip-lines ............................................ 211
B.l Propagation constant ................................... 211
B.2 Open Ends .............................................. 213
С Microcavity Rate equations .................................. 214
C.l Solution of the rate equations ......................... 214
C.2 Approximation .......................................... 216
C.3 Purcell factor fit in the threshold region ............. 216
Bibliography ................................................... 219
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