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.

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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

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rad)

Time (ns)

Master laser Slave laser (locked)

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RIN

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Frequency (GHz)

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