This transmission electron microscope image illustrates a cross-sectional view of a semiconductor wafer used for fabricating a quantum cascade laser. The stripes in the photograph are alternating layers of InGaAs and InAlAs. The width of smallest stripe is as small as few angstroms. This material is meticulously designed this way in order to produce mid-infrared light.
In conventional semiconductor lasers, the material band gap regulates the emission wavelength. Therefore, in order to achieve a range of wavelengths, different materials have to be used. Moreover, materials used to traditionally produce mid-infrared light are not reliable. With band gap engineering, well-studied and established III-V semiconductor can be used to design structure intelligently as shown in the picture so that the light amplification is due to intersubband transitions rather than energy band gap. Now, traditional hurdles for producing mid-infrared light can be overcome and III-V semiconductor materialsí advantages in easy processing and patterning can be harnessed.