Quantum Dot Laser [QD Laser]

QUANTUM DOT LASER [QD LASER]

Introduction

A quantum dot laser is a semiconductor laser that uses quantum dots as the active laser medium in its light emitting region.

In quantum dot lasers, the confinement is in all three spatial directions.

Conventional laser operation requires the presence of a laser medium containing active atoms with discrete energy levels between which the laser emission transition takes place. It also needs a mechanism for population inversion whereby an upper energy level acquires a population of electrons exceeding that of lower-lying ground-state level.

In a He-Ne laser, the active atoms are mixed with He and in a Nd:YAG solid state laser the active atoms are neodymium ions substituted in Yittrium aluminium garnate crystal.

In quantum dot laser, quantum dots play the role of active atoms

Construction

The Fig. 5.22 shows a quantum-dot near infrared laser diode grown on an n-doped GaAs substrate. The top p-metal layer has a GaAs contact layer. Immediately below it, there are a pair of 2 μm-thick Al0.85 Ga0.15 As cladding or bounding layers that surround a 190-nm-thick waveguide made of Al0.05 Ga0.95 As in between p-metal and n-substrate. The front view of quantum laser diode is shown in Fig. 5.23.

Here, the waveguide plays the role of conducting the emitted light to the exit ports at the edges of the structure.

The waveguide [dark dotted on the figure labeled QD] is a 30-nm thick GaAs region, and centered in this region are 12 monolayers of In_0.5Ga_0.5As quantum dots with a density of 1.5 × 10¹⁰ cm². The details of the waveguide region is drawn below the bottom of Fig. 5.22.

Working

1. The electron and hole recombination causes the emission of laser light.

2. By varying the length Lc [1 to 5 mm] and width W [4 to 60 μm], the laser light with particular wavelength will be emitted.

3. A particular wavelength of 1.32 μm which is in the near-infrared region can be produced for a current setting just above the 4.1 mA threshold value, labelled point 'a', is shown in Fig. 5.24.

4. The faces of the layer were coated with high reflectively material [ZnSe/MgF₂] where the light is reflected back and forth to increase the stimulated emission, and in turn the laser emission is enhanced.

Example (optional)

For example, if L_C= 1.02 mm and W = 9 μm, the laser output power versus the current at room temperature is ploted in Fig. 5.25.

From Fig. 5.25, it is found that, for pulsed operation, the threshold current intensity increases sharply with the temperature above 200 K.

Advantages of QD Lasers

(i) Broad spectrum with a specific wavelength of light emission can be obtained by changing dot size.

(ii) Because of very small active volume, only very less population inversion is necessary for lasing.

(iii) It is less temperature dependence of threshold current density.

(iv) It operates even at high frequency.

(v) Gain is 2-3 times larger than Q-well.

(vi) It is a efficient laser.

Disadvantages

(i) It is very difficult to form high quality dots [Uniform size and Higher density]

(ii) Difficult to manufacture because of nanometer size.

(iii) Reduced efficiency of electron-photon interactions that results from the discrete atomic-like energy structures, an effect that reduces the ability of carriers to enter into the band-edge states and hence reduces. luminescence efficiencies.

Applications

(i) QD lasers are used in medicine [optical coherence tonography].

(ii) QD lasers are used in display technologies [laser TV], Spectroscopy and telecommunications.

(iii) QD lasers are used in optical transmission system and optical LANs.

(iv) At present researchers have developed a new type of pulsed laser that uses quantum dots of emit bursts not of light, but of darkness - an idea that could prove useful for optical communication and rapid chemical analysis