Synopsis: Quiet cascade

In groundbreaking low-noise laser experiments, the linewidth limit of quantum cascade lasers is confirmed to be much lower than predicted by standard laser models.
Synopsis figure
Illustration: S. Bartalini et al., Phys. Rev. Lett. (2010)

Quantum cascade lasers are semiconductor lasers that emit in the infrared range. Unlike typical semiconductor lasers, where lasing is a result of recombination of electron-hole pairs across the band gap, quantum cascade lasers operate through intersubband transitions in a stack of quantum wells.

Until recently, it was thought that the fundamental frequency noise of all semiconductor lasers, including quantum cascade lasers, could be described through the Schawlow-Townes formula. This formula contains a systematic prefactor, the Henry linewidth enhancement factor, which accounts for refractive index variations from electron density fluctuations. While this formula describes line broadening in semiconductor lasers well, theory has predicted and experiments have shown that the Henry linewidth factor for quantum cascade lasers is nearly negligible. Hitherto the predicted noise level limits for quantum cascade lasers have been outside of our experimental reach. Recently, though, the search for the correct interpretation of the Schawlow-Townes formula for quantum cascade lasers has been invigorated by theoretical predictions that the fundamental linewidth is much lower than that of interband semiconductor lasers.

In results published in Physical Review Letters, Saverio Bartalini and co-workers at the Istituto Nazionale di Ottica (INO-CNR/LENS) in Florence and at the Second University of Naples, both in Italy, have shown that the linewidth limit for quantum cascade lasers is far below the limit set by the Schawlow-Townes formula when taking into account the spontaneous emission only. Through intricate experiments that required the building of a laser driver with unprecedentedly low noise levels, they showed that the Henry linewidth enhancement factor is negligible. In addition, they found that the linewidth depends on an effective coupling of the spontaneous emission with the lasing mode, and is reduced by the presence of nonradiative relaxation processes. – Daniel Ucko


Features

More Features »

Announcements

More Announcements »

Subject Areas

Optics

Previous Synopsis

Quantum Information

A smoother quantum measurement

Read More »

Next Synopsis

Particles and Fields

Loopy precision

Read More »

Related Articles

Focus: 3D Images 10 Times Faster
Interdisciplinary Physics

Focus: 3D Images 10 Times Faster

3D x-ray phase-contrast images take as little as one-tenth the usual time to acquire using a technique that halves the number of required “photos.” Read More »

Viewpoint: Photonic Hat Trick
Optics

Viewpoint: Photonic Hat Trick

Two independent groups have provided the first experimental demonstration of genuine three-photon interference. Read More »

Synopsis: A Neat Way to Slow Down Light
Optics

Synopsis: A Neat Way to Slow Down Light

A new technique slows down light in a crystal by simply shining a laser on it and varying an applied voltage. Read More »

More Articles