Alphafactor of a Terahertz Quantum Cascade Laser

1
Alpha-factor of a Terahertz Quantum Cascade Laser
Richard Green, Lukas Mahler, Ji-Hua Xu and
Alessandro Tredicucci NEST- INFM and Scuola
Normale Superiore, Pisa Guido Giuliani,
Università di Pavia Harvey Beere, David
Ritchie Cavendish Laboratory, University of
Cambridge
2
The THz gap
100
10
Output Power ( Watts )
Impatt
Photonics
1
III-V's
QC Laser
HG
100m
Gunn
10m
Electronics
SLED
1m
RTD
100µ
THz QCs!
Lead salts
RTD array
10µ
1µ
1000
100
10
1
.1
.01
Frequency ( Terahertz )
Photo-mixer
3
THz applications

• Why do we want THz lasers?
• Potential applications
• Chemical sensing
• Gases (eg pollution monitoring)
• Solids (eg explosives, pharmaceuticals)
• Potentially chemical selective imaging
• Local oscillators for astronomy
• Medical diagnosis and imaging
• Free space comms?

4
Quantum cascade lasers

• Unipolar semiconductor lasers
• Quantum emission is between subbands in quantum
  wells
• ? Energy determined by layer thicknesses, not
  bandgap
• Cascade many radiative transitions for each
  electron

5
Quantum cascade lasers
First demonstrated in mid-IR (Faist et al, 1994)
THz QCL (since 2002)
Many different relaxation paths

• Best semiconductor laser source in MIR (4-20 µm)
• RT, cw operation
• Maximum average powers gt 100 mW
• Wavelengths 3-20 µm

• Cryogenic operation (Tmax 167?)
• Maximum average powers 100mW
• Wavelengths 60- 200 µm

6
LEF0?
Laser transition between two parallel
subbands ?symmetric gain curve

• However
• Non-parabolicity
• Change in populations with current
• Cross-absorption

von Staden et al Opt. Lett. 31 2574
7
Experiment

• Single mode emission
• Threshold lt 400 mA
• Plasmon waveguide

8
LEF- preliminary results

• See change in feedback parameter with aperture
• No evidence for zero LEF
• LEF 0.5

9
LEF ? 0?
LEF 0 prediction based on symmetric
differential gain curve Cross absorption from gs
will reduce with higher injection
10
Gain curve
Define diff. gain in terms of I, not
ncarrier Increasing current reduces losses ?
asymmetric diff. gain
11
Conclusions

• Measure LEF of a THz QCL
• LEF0.5
• Due to cross- absorption effects

r.green_at_sns.it
12
Device requirements

• Single mode emission
• Threshold lt 400 mA
• Plasmon waveguide