Quantum Dot Lasers

Quantum Dots

        Quantum dots (QDs) are small conductive regions in a semiconductor containing a variable number of charge carriers( from one to thousand) that occupy well—defined discrete quantum states. They have typical dimensions between nanometers to a few microns. When the space, at any side around a material shrinks to 100A, quantization of the energy levels at the reduced side will occur. In quantum dots electrons are confined in all directions to a volume in space with dimensions on the order of their de Broglie wavelength. Therefore they have no kinetic energy and as a result, they occupy spectrally sharp energy levels like those found in atoms.

 Introduction

        The past decade has seen a tremendous amount of research in the fabrication of semiconductor structures, which was stimulated  by the drive towards increasing miniaturization and performance of solid-state devices. One major step in these developments has been the development of low dimensional devices.

Bandwidth  Limits

        Before discussing in detail how the dynamics of QDs affect the performance of QD devices, in particular directly modulated lasers, it is important to mention briefly what generally limits the bandwidth of semiconductor lasers and the typical methodology for analyzing semiconductor laser performance. Typically high-speed lasers are analyzed using a three-rate-equation model, in which the number of photons, carriers in the active region, and carriers in the core are modeled in three distinct equations.

Fabrication Of Dots

        The unique advantages of QD structures can be realized only if the dots are as uniform as possible in shape and size. Conventional semiconductor-processing techniques that are based on lithography and etching face inherent problems such as limited resolution, and the introduction of surface defects during production. As a result, several research groups have started working on the direct synthesis of quantum nanostructures either by combining epitaxial growth techniques (MBE or MOCVD) with photolithography.

Quantum Dot Vcsels

        Much of the present focus on quantum dots is driven by the promise of inexpensive lasers and detectors for the telecommunications wavelength, utilizing the zero- dispersion window of an optical fiber. There has been an additional incentive to develop lasers grown on GaAs substrates, for easy integration of optical devices with the relatively mature GaAs electronic device technology, moving towards the development of high- speed optical communication systems.

Abstract

Quantum Dots(QDs) are small conductive regions in a semiconductor, containing a number of charge carriers(from one to thousand) that occupy well defined discrete quantum states. They have typical dimensions between nanometers to a few microns .When the space at any side, around a material shrinks to 100Å, quantisation of the energy levels at the reduced side will occur. In quantum dots electrons are confined in all directions to a volume in space with dimensions on the order of their de Broglie wave length, ie,  they have no kinetic energy and as a result they occupy spectrally sharp energy levels like those found in atoms.

 Conclusion

                Though quantum dot lasers show immense potential for superior device performances, there are still some significant problems associated with the control of emission wavelengths reproducibility of the dots, high-temperature reliability and long- term stability of the dots. The current challenge is to match and surpass the performance of quantum well lasers. There is still need for the development of a quantum dot structure lasing around 1.55 micrometer, which is a principal wavelength in fiber optic communications. This would give QD lasers a chance to move into applications such as ultrafast optical data transfer. A key aspect of quantum-dot production challenge will be to improve our control over the dot distribution produced in the self-assembly process.