key clarity technologies i - quantum cascade lasers national and kapodistrian university of athens...
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Key CLARITY technologiesI - Quantum Cascade Lasers
National and Kapodistrian University of AthensDepartment of Informatics and Telecommunications Photonics Technology Laboratory
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.
bandgap
CB
VB
Introduction - Bipolar 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.
Introduction - Intersubband lasers
CB
Quantum Cascade Laser
Light from quantum jumps between subbandsEmission wavelength controlled by thickness: (4 to 160m)
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)
Diode Laser
Light from electron-hole (e-h) recombinationEmission 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)
bandgap
CB
VB layer thickness
CB
Introduction - Bipolar lasers vs QCLs
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 QWL. 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, 1156-1158, 1985.
1986: First observation of sequential resonant tunneling in superlattice QWF. Capasso, K. Mohammed, and A. Y. Cho, “Sequential resonant tunneling through a multiquantum well superlattice”, Applied Physics Letters, 48, 478-480, 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.
….
Milestones
Basic principles – Unipolarity
Initial and final states have the same curvaturethe 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
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
Basic principles – Cascaded geometry
Basic principles – Practical structure
Engineering issues
Steps towards a QCL
Quantum design of optical transitionsBand structure Engineering
Building blocks
Single QWCoupled QWsSuperlattice
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
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
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
QCL performance advantages
Wavelength agility- 3.5 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
QCL performance highlights
Room temperature cw operationvery 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
QCL challenges
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 spectrumis expected
Actual RIN reduction should be identified bycorrelation with the emitted power
Within CLARITY alternative IL techniques are usedin order to approach sub-shot noise operation
QCL noise-reduction with injection locking
10 20 30 40 50 60 70 80 90 100 110-3.1
-3.0
-2.9
-2.8
-2.7
-2.6
Pha
se (
rad)
Time (ns)
Master laser Slave laser (locked)
0.1 1 10
-180
-170
-160
-150
-140
-130
-120
RIN
(dB
/Hz)
Frequency (GHz)
Free running Locked