I was attracted to the idea of a Quantum Cascade Laser (QC Laser) after I had invented a detector, which is a device that converts light into an electrical current. This is the opposite flow of energy from a semi-conductor laser that uses a current to produce laser light. In my detector, electrons flowing down an energy staircase created an extra electron at each step leading to an electron avalanche. I wondered whether we could do this in reverse with an injection of an electron to produce a cascade of photons as it tumbles down an energy staircase. The QC Laser concept eventually emerged from intense brainstorming within my Bell Labs team, which included brilliant young scientists such as Jerome Faist and Carlo Sirtori, and it became a reality thanks to that marvelous crystal growth technique, Molecular Beam Epitaxy, pioneered by my colleague Al Cho, which allows one to create artificial materials with man-made properties by spray-painting atoms on a surface one layer at a time.
As soon as we had a working QC Laser that could operate at room temperatures, I decided we had to share it with as a many potential users in the field. We gave QC Lasers away, at no charge, even though we were ahead of the rest of the field by at least five years. My reasoning was simple; by sharing our devices we could very quickly find and develop practical applications for our invention.
Today, QC Lasers are flown through the atmosphere to measure environmental pollution and the impact on climate change. They are also employed as ultrasensitive detectors of trace gases and vapors down to the subpart per billion level for applications in homeland security, such as the detection of explosives, for combustion diagnostics and industrial process control and for medical diagnostics, such as breath analysis. Commercialization is in full swing with 20 companies ranging from start up to billion dollar companies making and selling QC lasers. Under DARPA funding, my team at Harvard, in collaboration with Pranalytica Inc., has developed powerful continuous wave QC lasers for one of the most important Department of Defense applications, infrared countermeausures, which protect aircraft against the insidious threat of incoming shoulder fired missiles.