Researchers Make Progress on Super-Cascade Laser

Achievement date: 
2015
Outcome/accomplishment: 

Researchers affiliated with the NSF-funded Engineering Research Center (ERC) on Mid-Infrared Technologies for Health and the Environment (MIRTHE), which is headquartered at Princeton University, have made progress in developing a super-cascade laser. This is a new type of laser concept that has the potential of being broadly tunable over the biochemical fingerprinting region—a previous “missing link.” See https://www.ee.princeton.edu/events/quantum-cascade-super-cascade-laser

Impact/benefits: 

Many applications require a broadly tunable laser source to detect molecules with broad absorption features. In particular, covering the region of biochemical fingerprints opens the door to markets such as non-invasive biomedical diagnostics, food safety, and stand-off detection in turbid media.

Explanation/Background: 

The researchers' work addressed single gain medium with optical gain spanning 3.3 μm to 12.5 μm and free-running laser operation covering 6.2 μm to 12.5 μm. These results more than triple the record emission bandwidth in the mid-infrared region, enable active laser spanning emission, cover the entire biochemical fingerprinting region, and potentially allow spectroscopy on biological samples, such as breast carcinoma cells (top figure).

The quest for a single laser source with ultra-broad spectral emission has led to a number of advanced yet incomplete approaches that trade bandwidth for performance or simplicity in rather restrictive ways. Here, the MIRTHE team exploited the quantum cascade laser's unique flexibility to demonstrate that the energy subbands within the conduction band of quantum wells can be carefully designed to provide population inversion to multiple independent excited-state transitions (bottom figure) 

Broadband spectral emission is thought to be counteractive to laser action. Lasers are designed to maximize population inversion and oscillator strength, leading inevitably to transitions with spectrally narrow line-widths.