National and Kapodistrian University of Athens

Department of Informatics and Telecommunications Photonics Technology Laboratory

Key CLARITY technologies

I - Quantum Cascade Lasers

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

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

Introduction - Bipolar lasers vs QCLs

Diode Laser

CB

bandgap

VB 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

CB

layer thickness

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)

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 1985.

‐

dipole infrared transition within the conduction band of a GaAs quantum well”, Applied Physics Letters, 46, 1156-1158,

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, 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.

….

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 E 3 -E 2 transition (photons) Phonon emission from E 2 -E 1 transition (crystal vibrations) E 2 -E 1 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

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

Basic principles – Practical structure

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:

τ 32 > τ 21

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

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

QCL performance highlights

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 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

QCL noise-reduction with injection locking

Investigation of low noise operation using injection locking (IL) -2.6

Master laser Slave laser (locked) -2.7

Slave laser locks on the injected master laser -2.8

-2.9

-3.0

-3.1

10 20 30 40 50 60 Time (ns) 70 80 Noise performance is evaluated by the Relative Intensity Noise (RIN) 90 100 110 -120 Free running Locked Strong suppression of the slave laser RIN spectrum is expected -130 -140 Actual RIN reduction should be identified by correlation with the emitted power -150 Within CLARITY alternative IL techniques are used in order to approach sub-shot noise operation -160 -170 -180 0.1

1 Frequency (GHz) 10