Summer 2004: Quantum Cascade Lasers
Princeton University, Princeton NJ
My first experience as a full-time researcher was a multifaceted one. I began by learning the basics of tiny quantum cascade lasers (or QCLs) and researching the pros and plausibility of using Group IV semiconductors (specifically Silicon and Silicon-Germanium, or SiGe) as materials for making them. We had a simple sample produced, which I helped mount and tested for light emission. I designed a new sample holder to accommodate the bi-directional light emission made possible by new lasers like this one. In an unrelated project, I used a laser design simulation to design a new ‘excited-state’ QCL and optimize its theoretical performance.
Quantum Cascade Lasers Explained
Semiconductor materials like silicon have certain ‘energy states’ that an electron inside it can maintain. When an electron falls from a high energy state to a lower one, it releases energy in the form of light or heat. Different materials have different energy states, so if we layer two semiconductors on top of each other over and over we get a set of ‘quantum wells,’ so called because a plot of energy versus distance looks like a set of wells (high-low-high-low-high, etc.). The material with the higher energy state creates a series of ‘barriers’ between the wells.
If we but an electrical bias across the material (like a battery), we essentially add energy to one side, putting the wells on a slant. This makes our set of wells into more of a staircase that electrons can tumble down. The energy states, however, do not slant but rather get pushed towards one another and interact in complex ways. If we design the thickness of our wells and barriers just right, electrons tumble down our staircase until they reach one big step, at which point every electron that falls emits a single wavelength of light. If enough electrons make this fall, the light emitted is intense enough to make a laser. Quantum cascade lasers emit mid-infra-red light that is absorbed by certain gases, making QCLs useful for trace gas sensing applications (among other things).
The motivation driving Group IV lasers is multifaceted: they are cheaper, they are easier to fabricate, and they integrate directly with electronics, bringing super-fast “optical computing” closer to reality. However, SiGe lasers are difficult to realize because the available energy states are more complicated and less obliging than those of the more common “Group III” QCLs. Also, a difference between the crystal size of Si and that of SiGe puts strain our design and limits the structures we can make.
While current Group III lasers use the lowest energy levels available for their electron staircase, an “excited-state” QCL uses higher energy levels. Such lasers would have the capacity for more efficient output than current QCLs, but their design would have to be much more complex. I discovered this firsthand when I designed the first excited-state QCL for use in the Gmachl lab.
The SiGe laser did emit light and, although its performance is still far below that of current QCLs, the potential advantages have merited further research. The excited-state QCL has undergone much research and is the subject of a paper published in Applied Physics Letters in 2007, of which I am a co-author.
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