Quantum Dot Lasers


Sunlight contains light of varied colors. What determines the color of the light is the length of the waves, or wavelength. The locations of the wave peaks and valleys are referred to as phase. Lasers emit light waves that are in phase and have a uniform wavelength, allowing high-intensity light to travel in a straight line over long distances.

Light is emitted from an atom when an electron traveling around its nucleus transits from one orbit to another low-energy orbit. The wavelength of the light is determined by the difference in energy between the two orbits. However, as the orbits of an atom are determined by nature, for atoms of the same type every electron will emit light of the same wavelength.

In ordinary semiconductors, electrons of varied energy move about freely and thus emit light wavelengths that are not uniform. Moreover, the movement of the electrons increases as temperature rises, further widening the range of wavelengths of emitted light. This results in fewer electrons contributing to the generation of laser light, and an increase in threshold current.

When electrons are confined in small semiconductor boxes roughly equal in size to the electrons' wavelength, i.e., when they are confined in quantum dots, the wave properties become more marked, and the energy of the electrons takes on discrete values. If a large number of quantum dots having energy levels that resonate with the laser wavelength can be fabricated, then electrons can be converted to laser light very efficiently. As the electrons are confined in three dimensions, the electrons' state remains unchanged even if the temperature rises, with the result that threshold current is not temperature-dependent. The use of quantum dots could therefore enable the ideal semiconductor laser.