Outline

Quantum Dots (QD) Confinement Effect

Quantum Dot Lasers (QDL) Historical Evolution Predicted Advantages Basic Characteristics Application Requirements

Q. Dot Lasers vs. Q. Well Lasers Comparison of different types of QDLs Bottlenecks Breakthroughs Future Directions Conclusion

Quantum Dots (QD)

Semiconductor nanostructures Size: ~2-10 nm or ~10-50 atoms

in diameter Unique tunability

Confinement of motion can be created by: Electrostatic potential

e.g. in e.g. doping, strain, impurities, external electrodes

the presence of an interface between different semiconductor materials e.g. in the case of self-assembled QDs

the presence of the semiconductor surface e.g. in the case of a semiconductor nanocrystal

or by a combination of these

Quantum Confinement Effect

E = Eq1 + Eq2 + Eq3, Eqn = h2(qqnπ/dn)2 / 2mc

Quantization of density of states: (a) bulk (b) quantum well (c) quantum wire (d) QD

QD Lasers – Historical Evolution

QDL – Predicted Advantages

Wavelength of light determined by the energy levels not by bandgap energy: improved performance & increased flexibility to adjust the wavelength

Maximum material gain and differential gain Small volume:

low power high frequency operation large modulation bandwidth small dynamic chirp small linewidth enhancement factor low threshold current

Superior temperature stability of I threshold

I threshold (T) = I threshold (T ref).exp ((T-(T ref))/ (T 0)) High T 0 decoupling electron-phonon interaction by increasing the

intersubband separation. Undiminished room-temperature performance without external thermal

stabilization

Suppressed diffusion of non-equilibrium carriers Reduced leakage

QDL – Basic characteristics

An active medium to create population inversion by pumping mechanism: photons at some site

stimulate emission at other sites while traveling

Two reflectors: to reflect the light in

phase multipass amplification

Components of a laser

An energy pump source electric power supply

QDL – Basic characteristics

An ideal QDL consists of a 3D-array of dots with equal size and shape

Surrounded by a higher band-gap material confines the injected carriers.

Embedded in an optical waveguide Consists lower and upper cladding layers (n-doped

and p-doped shields)

QDL – Application Requirements

Same energy level Size, shape and alloy composition of QDs close to

identical Inhomogeneous broadening eliminated real

concentration of energy states obtainedHigh density of interacting QDs

Macroscopic physical parameter light outputReduction of non-radiative centers

Nanostructures made by high-energy beam patterning cannot be used since damage is incurred

Electrical control Electric field applied can change physical properties

of QDs Carriers can be injected to create light emission

Q. Dot Laser vs. Q. Well Laser

In order for QD lasers compete with QW lasers: A large array of QDs since their active volume is

small An array with a narrow size distribution has to be

produced to reduce inhomogeneous broadening Array has to be without defects

may degrade the optical emission by providing alternate nonradiative defect channels

The phonon bottleneck created by confinement limits the number of states that are efficiently coupled by phonons due to energy conservation Limits the relaxation of excited carriers into lasing

states Causes degradation of stimulated emission Other mechanisms can be used to suppress that

bottleneck effect (e.g. Auger interactions)

Q. Dot Laser vs. Q. Well Laser

Comparison of efficiency: QWL vs. QDL

Comparison

High speed quantum dot lasers

Advantages

Directly Modulated Quantum Dot Lasers

•Datacom application•Rate of 10Gb/s

Mode-Locked Quantum Dot Lasers

•Short optical pulses•Narrow spectral width•Broad gain spectrum

InP Based Quantum Dot Lasers

•Low emission wavelength•Wide temperature range•Used for data transmission

Comparison

High power Quantum Dot lasers

Advantages

QD lasers for Coolerless Pump Sources

•Size reduced quantum dot

Single Mode Tapered Lasers

•Small wave length shift•Temperature insensitivity

Bottlenecks

First, the lack of uniformity.Second, Quantum Dots density is insufficient.Third, the lack of good coupling between QD

and QD.

The early models were based on the assumptions:

Only one confined electron level and hole level Infinite barriers Equilibrium carrier distribution Lattice matched heterostructures

Breakthroughs

Temperature dependence of light-current characteristics

Modulation waveform at 10Gbps at 20°C and 70 °C with no current adjustment

Future Directions

Widening parameters range

Further controlling the position and dot size

Decouple the carrier capture from the escape procedure

Combination of QD lasers and QW lasers

Reduce inhomogeneous linewidth broadening

Surface Preparation Technology

Allowing the injection of cooled carriers

Raised gain at the fundamental transition energy

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Conclusion

During the previous decade, there was an intensive interest on the development of quantum dot lasers. The unique properties of quantum dots allow QD lasers obtain several excellent properties and performances compared to traditional lasers and even QW lasers.

Although bottlenecks block the way of realizing quantum dot lasers to commercial markets, breakthroughs in the aspects of material and other properties will still keep the research area active in a few years. According to the market demand and higher requirements of applications, future research directions are figured out and needed to be realized soon.

Thank you!