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QCL emits a surprising second beam

12 Jan 2009

Princeton researchers have discovered an entirely new mechanism of light emission from quantum cascade lasers.

Attempts to decrease the threshold current and improve the performance of a quantum cascade laser (QCL) have led Kale Franz and colleagues to stumble upon a surprising result. Instead of emitting just one beam, the team's QCL overcomes conventional losses to emit a second beam. The discovery, say researchers, could be applied to enhance the performance of other lasers (Nature photonics doi:10.1038/nphoton.2008.250).

"We've discovered a way to almost completely rid ourselves of one of the primary loss mechanisms that limit laser performance," Kale Franz, a researcher at Princeton University, told optics.org. "Although we are not there yet, the discovery could ultimately lead to better lasers."

The Princeton device operates on most of the basic principles of a standard QCL, but with one unique difference. Instead of using electron transitions from the second to the first state in the quantum well for lasing, Franz and colleagues used the third to second state transition. Unexpectedly, the team found that lasing occurred via the second to first state transition in parallel with the intended third to second state transition.

"When you inject an electron into the device, it can emit a photon for either the third to second state transition, or for the second to first state transition," explained Franz. "We observed two distinct optical transitions, one at 9.5 µm and one at 8.2 µm."

Conventionally, lasers have a certain population of electrons that exist in the lower laser state as well as the upper laser state. Due to these lower laser state electrons, any light generated by the laser gets absorbed back into the upper laser state, in effect robbing the laser of that photon.

Since the emission and re-absorption wavelengths of the Princeton QCL are different, the process is de-coupled and the loss is reduced. This results in lower threshold, more power and higher lasing efficiencies.

"The discovery represents a fundamental shift in how we think about charge carrier dynamics in coupled quantum well systems," explained Franz. "Normally, a substantial electron population has to accumulate in a certain energy state before lasing occurs. This discovery shows that electrons are now useful even when they are in highly non-equilibrium or non-quasi-equilibrium states."

Franz and colleagues are currently debating the best way to optimize their QCL to take advantage of the new lasing mechanism.

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