Modeling Quantum Cascade Lasers : The Challenge of Infra-Red Devices

Sammanfattning: We live in a time of rapid scientific and technological advancement. People liv- ing 100 years ago could never dream of inventions like those having completely changed our way of life, and our perception of the world; computers, mobile phones, the Internet, space travel, unraveling the mysteries of the early universe and distant galaxies, and our insight into the microscopic world of quantum phe- nomena. Today we are at the dawn of an era of nanotechnology, with computers components being only tens of nanometers in size, and nano-devices making their entrance into wide industrial use.One such nanoscopic device is the Quantum Cascade Laser (QCL). Like all as lasers, it emits electro-magnetic waves, which essentially is light. In fact, our eyes can only detect a very narrow range of wavelengths in the electro-magnetic spectrum, which stretches from very long radio waves, followed by microwaves, terahertz and infrared waves, through the wavelengths of visible light, up to ultra-violet UV-A and UV-B radiation (giving us a good tan), and finally X- rays and gamma-rays from radio-active decay and cosmic radiation. In the case of, e. g. conventional laser pointers, the light wave has wavelengths of 500 (blue light) up to 700 (red light) nanometers and is visible to our eyes.What is special about the QCL is that it does not emit visible light, but light in the terahertz and infrared (IR) regions, and it does so using the same technology as normal light emitting diode (LED) lights. These regions are both interesting for applications in spectroscopy, i. e. the detection of chemical substances by looking at light going through them. For example, QCLs can be used to detect very small quantities of explosive materials, diagnose exhaled air in patients, monitor green house gases and pollution in the atmosphere, and examining the contents of far away stars. But, like IR cameras, they can also be used to make images of that which our eyes cannot see; with terahertz light we can see through clothes and thin materials, which could be used to screen patients in the emergency room without having to remove their clothes, or at the airport to see if they are carrying a weapon.In this work, I have simulated QCLs using a complicated theoretical model, which in detail accounts for the motion of the electrons inside the device, and their interaction with their surroundings. By improving this model, we are actually able to reproduce real QCL behavior remarkably well. This can help to improve future QCLs, and to understand the basic physical mechanisms underlying their operation. This to me is what physics is all about: To seek to understand nature and make use of this knowledge to help making life better.