 Hello and welcome to lecture on Injection Laser Structures. Learning outcomes. By the end of this session, student will be able to illustrate semiconductor injection laser structures. Before starting this topic, try to recall strip geometry of double hetrojunction injection lasers. Injection lasers are available with different four laser structures and those are first type of laser structure is gain guided lasers, second index guided laser, third type is quantum well lasers and the fourth and the last type is quantum dot lasers. We will see each laser structure in detail, gain guided lasers. Injection of multi mode injection lasers with a single or small number of lateral modes is achieved by the use of strip geometry. These devices are often called gain guided lasers. The contradiction of the current flow to the strip is realized in the structure either by implanting the regions outside the strip with protons to make them highly resistive or by oxide or p-n junction isolation. It has an active region of gallium arsenide bounded on the both sides of aluminum gallium arsenide regions. This technique has been widely applied especially for multi-mode laser structures. These multi-mode laser structures used in the shorter wavelength regions. The current is confined by etching a narrow strip in a silicon dioxide film. Two other basic techniques for the fabrication of gain guided laser are illustrated in figure number 1A and 1B which shows the proton isolated strip and p-n junction isolated strip structure respectively. In figure 1A, the resistive region formed by the proton bombardment gives better current confinement than a simple oxide strip and has superior thermal properties due to the absence of silicon dioxide layer. Figure 1B shows p-n junction isolation which involves a selective diffusion through n-type surface region in order to reach the p-type layers. But none of these structures confines all the radiation and current to the strip region and spreading occurs on both sides of the strip. With strip width of 10 micrometer or less, such planar strip laser provide highly efficient coupling into multi-mode fibers. But significantly lower coupling efficiency is achieved into small core diameter single mode fibers. The next type of structure is index-guided lasers. The drawbacks associated with gain guided laser structures were largely overcome through the development of index-guided injection lasers. In some such structures with weak index-guiding, the active region waveguide thickness is varied by growing it over a channel or ridge in the substrate as shown in figure 2. The ridge not only provides the location for weak index-guiding but also acts as a narrow current confining strip. These devices have been fabricated to operate at various wavelengths with a single lateral mode and room temperature threshold current as low as 18 milliampere with output power of 25 milliwatt. More typically, the threshold currents for such weak index-guided structures are in the range of 40 to 60 milliampere. Laser 3A compares the light output current characteristics for a ridge waveguide laser with that of an oxide strip gain-guided device. Alternatively, the application of a uniformly thick planar active waveguide can be achieved through lateral variations in the confinement layer thickness or the refractive index. The inverted rib waveguide device sometimes called planoconvex waveguide is illustrated in figure 3B. Strong index-guiding along the junction plane can provide improved transfer mode control in injection lasers. This can be achieved using a buried heterostructure device in which the active volume is completely buried in a material of wider bandgap and lower refractive index. The structure of buried heterostructure laser is shown in figure number 4. The optical field is well confined in both transverse and lateral direction within these lasers, providing strong index-guiding of the optical mode together with good carrier confinement. The segment of injected current to the active region is obtained through the reverse bias rejunctions of higher bandgap materials. The third type of laser structure is quantum well lasers. In this structure, thin active layer around of 10 nanometer instead of 0.3 micrometer causes drastic changes to the electronic and optical properties in comparison with the conventional double heterojunction injection lasers. Hence, quantum well lasers exhibit an inherent advantage over conventional double heterojunction injection devices in that they allow high gain at low carrier density, thus providing the possibility of significantly lower threshold currents. Quantum well lasers are available in three structures. First structure is single quantum well, second multi-quantum well and third is graded index separate confinement heterostructures. Both single quantum well corresponding to a single active region and multi-quantum well corresponding to a multi-quantum well active layer regions are lasers utilized. The layers separating the active region are called barrier layers. The energy band diagram of active region of single quantum well structure is shown in figure number 5a. Figure 5b shows energy band diagram of multi-quantum well structure. It may be observed in figure number 5c that when bandgap energy of the barrier layer differs from the cladding layer in a multi-quantum well device, it is usually referred as a modified multi-quantum well laser. Layer confinement of optical mode is obtained in multi-quantum layers in comparison with single quantum well lasers resulting in a lower threshold current for these devices. The last type of laser structure is quantum dot lasers. More recently, quantum well lasers have been developed in which the device contains a single discrete atomic structure or so called quantum dot. Quantum dots are small elements that contain a tiny droplet of free electrons forming a quantum well structure. Hence, a quantum dot laser is also referred as dot in a well device. Quantum dot lasers are fabricated using semiconductor crystalline materials and have typical dimensions between nanometer and few microns. The size and shape of these structures and therefore the number of electrons they contain may be precisely controlled such that a quantum dot laser can have anything from single electron to several thousand electrons. Quantum dot lasers do not suffer from thermal broadening and their threshold current is also temperature insensitive. If conventional injection laser diodes is regarded as three-dimensional and quantum well is confined to two dimensions then a quantum dot structure can be considered to be a zero-dimensional. It should be noted that single-dimensional structure forms a quantum wire or dash. The above hierarchy is illustrated in figure number 6 which identifies four different possible structures for the semiconductor laser with their corresponding energy responses with respect to the carrier density shown underneath. The three-dimensional structure of the conventional injection laser is displayed in the figure number 6A and single quantum well structure exhibits two dimensions that is length and height where the corresponding energy representation is shown in figure number 6B by a staircase response. However, when this structure is reduced to one dimension that is the length only. It displays a sharp rise and an exponential fall in the carrier density. Since this one-dimensional quantum well structure is confined to only the device length then in general it appears as a long wire and hence it is known as a quantum wire which is shown in figure number 6. The zero-dimensional that is single point structure is shown in figure number 6D however corresponds to a single dot which results in an impulse response for the variation in the charge carrier density with increasing number of carriers. These are the references. Thank you.