Quantum well laser

QUANTUM WELL LASER

The quantum well based lasers are called quantum well laser.

Types or Quantum Well Laser

1. Single-quantum-well Laser (SQWL)

2. Multi-quantum-well Laser (MQWL)

• Single-quantum-well (SQW) corresponding to a single active region and multi-quantum-well corresponding to multiple active regions. (MQW)

• When the bandgap energy of the barrier layer differs from the cladding layer in an MQW device, it is usually referred to as a modified multi-quantum-well laser (MMQWL)

A quantum well laser is shown in fig.5.29 The active laser (GaAs) is in between p-type AlGaAs and n-type AlGaAs. The metal contacts are provided to bias the QWL. The energy band diagram of QWL is shown in (b)

In this laser the current is confined to narrow strip. It will limit the amount of current flowing in the laser, and thus prevent thermal damage to the semiconductor.

This, also limit the laser gain region and in turn the dimension of the laser mode in the lateral direction. So, when the current flows through the layers of the semiconductor into the active region, it produces gain only in a narrow Gaussian-shaped stripe. This is shown in fig. 5.30.

Working

In quantum-well lasers, the active layer is made thinner than 10 nm. Therefore, the electronic and the optical properties are drastically altered due to the reduced dimensionality. Thus, the motion of the free electrons are confined in one direction and free other two mutually perpericular directions.

The electron confinement results in a quantisation in the permitted energy levels.

En = h² n² / 8mL²

Here, h is Planck's constant, m the effective mass of the electrons, n an integer and L, the thickness of the quantum well. The valence band also can have discreate energy levels.

The variation of spacing of the band-gap in semiconductor materials can vary the wavelength of laser emission.

It can be made by using different bandgap semiconductor in active region.

The discrete energy levels associated with quantum-well laser geometry is shown in fig. 5.31.

For quantum-wells, a graph of energy versus density of states does not follow the parabola. Instead, it has a distribution that has steps as shown in fig.5.31. At the level of threshold current, electrons occupy only the place above the first level of energy i.e. E₁.

When transitions take place between the level it gives narrower wavelengths than for the bulk material. This is because the graph of ρ (E) verses E is very narrow.

Here, the electrons isolated over a much narrower range of energy than in normal semiconductors.

This results in higher gain at much lower threshold current. In quantum-well lasers threshold current as low as 0.5 milliamp/cm have been achieved.

The wavelength of the laser can be changed by:

(a) Varying the thickness of the quantum-well.

(b) Varying the composition of the quantum-well. This happens due to the change in the energy band-gap.

Advantages of quantum well lasers

Quantum well lasers have attracted a great deal of attention by their many advantages such as

• Low threshold current density.

• excellent temperature feature

• high modulation rate and

• wavelength adjustability etc.

Disadvantages of quantum well lasers

A disadvantage is that MQW lasers produce a broader linewidth than SQW ones the most important characteristic of a semiconductor junction is the fact that the crystalline structure of the material must be continuous across the junction.

Applications of Quantum well laser

Quantum-well (QW) active semiconductor lasers enjoy widespread commercial use in

• optoelectronic applications ranging from high-power noilar sources for medical therapy

• material processing

• laser printing, and pumps for solid-state laser to lower output power single-mode

• Single-mode fasoll single-frequency for sources telecommunications.