What is a Quantum Well Laser?

What are the different types of Quantum Well Lasers? Explain the Advantages and Applications of Quantum Well Lasers?

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- GoPhotonics

Feb 20, 2023

A Quantum well laser is a type of semiconductor laser where the thickness of the active region is very narrow. The thickness of the active region determines the wavelength of the light emitted by the laser. These lasers have active regions of a thickness of about 100 Å. The emission wavelength is typically between 700 nm and 1600 nm for these lasers. The pulse duration of a light pulse from this laser is in the order of a picosecond. Under continuous-wave operation, the quantum well lasers have relatively high output powers, ranging from a few milliwatts to several watts. 

Figure 1: Representation of a Quantum well

A quantum well is a thin-layer semiconductor medium formed by materials with a wider band gap and the particles or carriers (electrons or holes) are confined in this well. The thickness of the quantum well is usually 5 - 20 nm. The particles are confined in one dimension but they are free to move in the other two directions. A quantum well is shown in figure 1. Quantum confinement is an effect in which the dimension of a material is comparable to the de Broglie wavelength of electrons involved and this occurs in the active region of the laser. When the dimension is large, the particles behave as free and the band gap remains at its original energy. But when the dimension is too small, typically in nano-scale, the energy spectrum becomes discrete and the band gap becomes size-dependent. Since the particles decrease, electrons and holes become closer and more energy is required to activate them which results in a blueshift in light emission.

A quantum well laser features a thin active region that gives better optical amplification. The heterojunction is a junction formed between two semiconductors with different bandgaps. If there are two such junctions formed, then they are double heterojunctions. In heterojunction semiconductor lasers, the two semiconductors that create the active region will be having two different refractive indexes. 

Creation of a Quantum Well

A quantum well is typically created by growing thin layers of semiconductor materials using a technique called epitaxy. The semiconductor materials chosen for the quantum well depend on the desired properties and application of the device. Common materials used for quantum wells include gallium arsenide (GaAs) and indium phosphide (InP). The thickness of the quantum well layer is critical for controlling electron and hole confinement and it must be thinner than the surrounding layers to create a potential well. 

The process of epitaxy is employed to cultivate layers of semiconductor materials by depositing them in thin layers onto a substrate such as a wafer of GaAs. The growth can be done using different techniques, such as molecular beam epitaxy or metal-organic chemical vapor deposition. The growth conditions, such as temperature, pressure, and gas flow, must be carefully controlled to ensure the desired layer thickness and quality. Dopants, such as silicon or zinc, can be added to the layers to control the electron and hole concentration. To create a heterostructure with multiple quantum wells, the layer growth process can be repeated several times with different layer thicknesses. Metal contacts can be added to the layers to provide electrical contact to the device.

A quantum well structure is shown in figure 2. Here, semiconductor A (GaAs) is sandwiched between two layers of semiconductor B (AlGaAs). The semiconductors A and B are stacked along the z-direction. To create a quantum well along the z-direction, the band gap of semiconductor A should be smaller than the band gap of semiconductor B. The size of the quantum well is normally in the order of 20 nm or less for quantum property to emerge. As a result, the energy levels of electrons and holes in the well are quantized (discrete energy levels).

Figure 2: Structure and energy band diagram of Quantum well laser

Working of quantum well laser 

When an electric current is applied to the quantum well, electrons and holes are injected into the well. These electrons and holes get confined and trapped within the narrow quantum well as they are forced into discrete energy levels, known as subbands. The population inversion occurs when there are more electrons in the higher energy level than in the lower energy level. The electrons then recombine with holes in the well, releasing energy in the form of photons. These released photons will induce further recombination of electrons and holes by stimulated emission and emit more photons. 

The optical cavity used in a quantum well laser are reflectors, e.g; a Bragg reflector. One of the reflectors is highly reflecting and the other one is partially reflecting. The spontaneously emitted photons in the quantum well bounce back and forth inside the optical cavity-causing stimulated emission and then get amplified. The light in the cavity is emitted out through the low reflective layer.

The structure of the quantum well improves the electron-hole recombination probability that contributes to a low threshold current in quantum well laser. One of the widely used single quantum well lasers is GaAs/AlGaAs.

Multi-Quantum Well (MQW) Laser

Figure 3: Structure of Multi-quantum well laser

A Multi-quantum well laser is created by arranging a single quantum well in a repeated fashion. These lasers have multi-quantum well as the active region. The barrier layers of the quantum wells are sufficiently thick. Since there are multiple wells, the carriers that are not captured in one well can be captured by the next well. They give larger output power and smaller threshold current due to their improved confinement and increase in the density of states. Multi-quantum well lasers are more efficient than quantum well lasers and they can tune the emission wavelength by changing the thickness of quantum wells. Figure 3 shows a schematic of a multi-quantum well laser and Figure 4 shows the energy band diagram of a multi-quantum well laser.

Figure 4: Energy band diagram of a Multi-quantum well laser

At low threshold gain values, the threshold current density increases in multi-quantum well lasers. The transparency current density is a major part of the threshold current density at low threshold gain values. Due to the fact that multi-quantum well structures have many active area components, or quantum wells, compared to single quantum well structures, the transparency current density in multi-quantum well structures is higher. Multi-quantum well structure is superior to single quantum well structure because multi-quantum well lasers require less operating injection current density and optical power to achieve the highest modulation bandwidth. In short, by reducing thermal effects and optical degradation, using multiple-quantum well as the laser active medium makes it easier to attain the high-speed bandwidth limits.

Strained Quantum Well Lasers

A Strained quantum well laser is a kind of quantum well laser that emit more focused beams than any other quantum well lasers, hence they are very efficient. The basic design of a strained quantum well laser consists of a thin layer of semiconductor material, such as gallium arsenide (GaAs), that is sandwiched between two layers of different semiconductor material, such as aluminum gallium arsenide (AlGaAs). The different lattice constants of the materials used create a strain on the thin layer of GaAs causing its energy levels to become quantized. This creates a number of well-defined energy states in the quantum well, which results in a much more efficient light emission than in traditional laser designs.

Their high efficiency is due to the quantization of the energy states in the quantum well allows for more efficient carrier recombination and less energy loss. This increased efficiency leads to higher output power and lower power consumption, which is a major advantage for many applications. They also have the ability to emit light at specific wavelengths. By adjusting the composition and thickness of the various layers in the laser structure, the wavelength of the emitted light can be controlled. This makes strained quantum well lasers ideal for use in optical communication systems, where specific wavelengths are used to transmit information over long distances. Strained quantum well lasers exhibit many other properties such as a very low-threshold current density, and lower linewidth than regular multi-quantum well lasers both under continuous wave operation and under modulation. 

One of the main challenges is the difficulty of manufacturing the complex layered structure required for these lasers. The high precision required for the layer thicknesses and compositions makes manufacturing these lasers challenging and expensive.

Strain-Balanced Quantum Well Lasers

Strain-balanced quantum well lasers (SB-QW lasers) are a type of semiconductor laser that have been developed to address certain limitations of traditional quantum well lasers. These lasers are capable of producing higher output powers while maintaining high levels of efficiency, making them useful for a range of applications in fields such as telecommunications and optical data storage.

The design of a strain-balanced quantum well laser is based on the concept of using layers of semiconductor materials with different lattice constants to balance the strain in the device. In a traditional quantum well laser, the active region (the region where the laser light is generated) is typically composed of a single quantum well layer surrounded by thicker layers of different semiconductor material. However, this can create a significant amount of strain in the active region, which can lead to defects and other performance issues.

In order to solve this issue, strain-balanced quantum well lasers use a more complex design that involves multiple quantum well layers of different materials, with each layer carefully chosen to balance the strain in the device. The result is a device that is more stable and reliable, and is able to produce higher output powers while maintaining high levels of efficiency.

One of the main advantages of strain-balanced quantum well lasers is their ability to produce high output powers. This is due to the fact that the multi-layered design of the device allows for a higher number of active regions, which in turn allows for a higher number of photons to be generated. Also, because the strain in the device is balanced, the active region is less likely to experience defects or other issues that can limit performance.

Strain-balanced quantum well lasers also have a high level of efficiency. This is due to the fact that the design of the device allows for a more efficient conversion of electrical energy into light, which reduces the amount of heat generated and can extend the lifetime of the device. These lasers are an important development in the field of semiconductor lasers, and have a wide range of practical applications in various industries. 

Advantages of Quantum Well Lasers

  • Low threshold current density
  • Excellent temperature feature 
  • High modulation rate 
  • Wavelength adjustability 
  • High speed
  • High output power
  • High quantum efficiency

Applications of Quantum Well Lasers

Quantum well lasers have several applications in material processing due to their unique properties. They can be used for precision cutting and drilling of materials, such as metals, ceramics, and semiconductors. The narrow beam of the laser allows for precise control of the cut or hole size, while the high intensity of the laser can efficiently remove material. They are used for surface treatment of materials, such as cleaning, etching, and texturing. The laser can selectively remove material from the surface to create specific patterns or structures, which can improve the surface properties of the material. They are also used in welding, bonding, 

Quantum well lasers are widely used as single-frequency sources in fiber-optic communications, where they transmit information by converting electrical signals into optical signals. They are also used in optical amplifiers and wavelength-division multiplexing, which allow multiple signals to be transmitted over a single fiber.

These lasers are used in medicine for a variety of applications, including dermatology, ophthalmology, and dentistry. They can be used for precise cutting and ablation of tissue, as well as for photodynamic therapy, which uses light to activate photosensitizing drugs to kill cancer cells.

Quantum well lasers are used in military and defense applications, such as target designation, laser range finding, and laser weapons. The high power and precision of quantum well lasers make them ideal for these applications.

These lasers are also used as pumps for solid-state lasers, in laser printing, and in sensing applications, such as gas detection, temperature sensing, and strain sensing.