1. Key CLARITY technologies I - Quantum Cascade Lasers
National and Kapodistrian University of Athens
Department of Informatics and Telecommunications
Photonics Technology Laboratory
Key CLARITY technologies I - Quantum Cascade Lasers

2. Introduction - Bipolar lasers
In usual laser diodes, transitions occur between different electronic bands of the semiconductor crystal (inter-band transitions).
A photon is emitted when an electron jumps from a semiconductor's conduction band (CB) to a hole in the valence band (VB).
Once an electron has been neutralized by a hole it can emit no more photons.
The wavelength of the photon is determined by the semiconductor bandgap and it is usually in the near infrared region. CB
bandgap VB

3. Introduction - Intersubband lasers
The Quantum Cascade Laser (QCL) is a semiconductor laser involving only one type of carriers. It is based on two fundamental quantum phenomena:
- the quantum confinement
- the tunneling
In the QCL the laser transitions do not occur between different electronic bands (CB- VB) but on intersubband transitions of a semiconductor structure.
An electron injected into the gain region undergoes a first transition between the upper two sublevels of a quantum well and a photon is emitted.
Then the electron relaxes to the lowest sublevel by a non-radiative transition, before tunneling into the upper level of the next quantum well.
The whole process is repeated over a large number of cascaded periods. CB

4. Introduction - Bipolar lasers vs QCLs
Diode Laser
Light from electron-hole (e-h) recombination Emission wavelength controlled by bandgap
Wide gain spectrum due to broad thermal distribution of e, h
One photon per injected e-h pair above threshold
Gain limited by band-structure (absorption coefficient)
Quantum Cascade Laser
Light from quantum jumps between subbands
Emission wavelength controlled by thickness: (4 to 160m)
Narrow gain spectrum due to same curvature of the initial and final states
No threshold for population inversion: gain form the first flowing electron.
Gain limited by electron density in the excited state (i.e. by maximum current one can inject) and cavity losses
Large gain: above threshold N photons per injected electron are generated (N: number of cascaded stages)
bandgap CB VB
layer thickness CB

5. Milestones
1971: First proposal for use of inter-subband transition (Ioffe Inst.) Kazarinov, R.F; Suris, R.A., "Possibility of amplification of electromagnetic waves in a semiconductor with a superlattice“, Soviet Physics - Semiconductors 5, 707–709, 1971. ….
1985: First observation of intersubband absorption in superlattice QW L. C. West and S. J. Eglash, “First observation of an extremely large‐dipole infrared transition within the conduction band of a GaAs quantum well”, Applied Physics Letters, 46, , 1985.
1986: First observation of sequential resonant tunneling in superlattice QW F. Capasso, K. Mohammed, and A. Y. Cho, “Sequential resonant tunneling through a multiquantum well superlattice”, Applied Physics Letters, 48, , 1986.
1994: First realization of QCL in InGaAs/AlInAs/InP pulsed operation, cryogenic conditions (Bell Labs) J. Faist, F. Capasso, D. L. Sivco, C. Sirtori, A. L. Hutchinson, and A. Y. Cho, “Quantum cascade laser,” Science, vol. 264, pp. 553–556, 1994.

6. Basic principles – Unipolarity
Initial and final states have the same curvature the joint density of state is very sharp and typical of atomic transitions
Laser emission from E3-E2 transition (photons)
Phonon emission from E2-E1 transition (crystal vibrations)
E2-E1 transition is fast: it is made resonant with the optical phonon energy
Emission of photons occurs at the same wavelength, thus provides large gain
Gain is limited by the population inversion

7. Basic principles – Cascaded geometry
Electron re-cycling due to cascaded structure:
Each injected electron generates N photons (N is the number of stages)
Potential to decrease the population inversion in each stage
Reduced electron-electron scattering and thus of distribution broadening

8. Basic principles – Practical structure

9. Engineering issues Steps towards a QCL
Quantum design of optical transitions
Band structure Engineering
Building blocks
Single QW
Coupled QWs
Superlattice
Engineering band structure and optical transitions
Because of quantum confinement, the spacing between the subbands depends on the width of the well, and increases as the well size is decreased.
This way, the emission wavelength depends on the layer thicknesses and not on the bandgap of the constituent materials.
Electron lifetime engineering is necessary to fulfill the population inversion condition: τ32 > τ21

10. Operation – Emission wavelengths
Emission wavelength does not depend on the material system
Development of lasers with different wavelengths using the same base semiconductors:
- from 3.5 to 24 µm InGaAs and AlInAs grown on InP
- far-infrared lasers based on the GaAs/AlGaAs material system
Shortest emission wavelength: 2.9 μm from InAs/AlSb

11. QCL performance advantages
The same semiconductor material can be used to manufacture lasers operating across the whole mid-infrared (and potentially even farther in the Far-Infrared) range.
It is based on a cascade of identical stages (typically 20-50), allowing one electron to emit many photons, emitting more optical power.
It is intrisically more robust (no interface recombination).
Since the dominant non-radiative recombination mechanism is optical phonon emission and not Auger effect (as it is the case in narrow-gap materials), it allows intrinsically higher operating temperature. As of now, it is still the only mid-infrared semiconductor laser operating at and above room temperature.
Potential for very high speed modulation: - absence of relaxation oscillations due to fast non-radiative relaxation rates - bandwidth determined by the photon lifetime in the cavity, - hence no advantage, rates up to 10 GHz
Delta-like joint density of states: - symmetric gain curve - zero refractive index change at the gain peak - low alpha (LEF) parameter - no frequency modulation with direct modulation - low linewidth

12. QCL performance highlights
Wavelength agility to 24 μm (AlInAs/GaInAs), 60 to 160 μm (AlGaAs/GaAs) - Multi-wavelength and ultrabroadband operation
High optical power at room temperature: > 1 W pulsed, 0.6 W cw
Narrow linewidth: < 100 kHz; stabilized < 10 kHz
Ultra-fast operation: - Gain switching (50 ps) - Modelocking (3-5 ps)
Applications: trace gas analysis, combustion & medical diagnostics, environmental monitoring, military and law enforcement
Reliability, reproducibility, long-term stability
Industrial Research and Commercialization: Hamamatsu, Thales, Pranalytica, Alpes Lasers, Maxion, Laser Components, Nanoplus, Cascade Technologies, Q-MACS Fraunhofer Institute, PSI, Aerodyne

13. QCL challenges
Room temperature cw operation very high threshold power densities that generate strong self-heating of the devices
Tunable over a broader range
Development of QCL at telecom wavelengths
Increase output power
Mode locking of QCLs for sub-ps generation
QCLs based on valence-band intersubband transitions in SiGe/Si quantum wells
Challenges within CLARITY project - low noise QCLs - sub-shot noise generation - proposed solution: injection locking

14. QCL noise-reduction with injection locking
Investigation of low noise operation using injection locking (IL)
Slave laser locks on the injected master laser
Noise performance is evaluated by the Relative Intensity Noise (RIN)
Strong suppression of the slave laser RIN spectrum is expected
Actual RIN reduction should be identified by correlation with the emitted power
Within CLARITY alternative IL techniques are used in order to approach sub-shot noise operation