A quantum well refers to a nanoscale structure typically found in semiconductor materials. It is formed by sandwiching a thin layer of a low-bandgap material (the quantum well) between two layers of a higher-bandgap material (the barrier layers). The difference in bandgaps between the materials creates a potential well within the quantum well layer, confining the motion of electrons and holes in the vertical direction.
The properties of quantum wells make them suitable for lasers due to the following reasons:
Energy confinement: The quantum well structure confines the motion of charge carriers (electrons and holes) in the quantum well layer. This confinement restricts their energy levels to discrete states, creating a quantized energy spectrum. The energy separation between these states can be engineered by adjusting the width and composition of the quantum well, allowing for precise control of the emitted light wavelength.
Size-dependent bandgap: Quantum wells exhibit a size-dependent bandgap due to quantum confinement effects. As the thickness of the quantum well layer decreases, the energy levels become discrete, and the bandgap increases. By varying the width of the quantum well, it is possible to achieve specific energy levels and corresponding emission wavelengths, including those within the visible spectrum.
High carrier density and radiative recombination: The confinement of charge carriers within the quantum well layer increases the carrier density, enhancing the probability of radiative recombination. Radiative recombination is the process where electrons and holes combine, releasing photons of specific energy (wavelength) as light. The high carrier density in quantum wells promotes efficient radiative recombination, leading to intense and coherent light emission.
Gain enhancement: Quantum wells can provide gain through stimulated emission. When a suitable electrical or optical pumping mechanism is applied, more electrons and holes can be excited to higher energy levels within the quantum well. As these carriers recombine, they stimulate the emission of additional photons that are coherent with the original emitted photons. This amplification effect can lead to population inversion and the creation of an optical gain medium, which is crucial for laser operation.
Tunability: The emission wavelength of a quantum well laser can be tuned by adjusting the width of the quantum well layer. By carefully controlling the dimensions and composition of the quantum well, it is possible to cover a wide range of wavelengths, making quantum well lasers versatile for various applications.
Due to these properties, quantum wells have been extensively used in the development of semiconductor lasers, including visible lasers for displays, laser diodes for optical communication, and infrared lasers for sensing and medical applications.