Laser absorption spectroscopy has emerged as one of the most important applications for semiconductor quantum cascade (QC) lasers, particularly in the mid-infrared spectral regime where many gases of interest exhibit fundamental absorption features. Laser absorption spectrometers have the potential to match or exceed the sensitivity of electrochemical sensors presently used aboard the International Space Station, and laser-based detectors are capable of long-duration maintenance-free operation without consumable components.

To fabricate buried heterostructure distributed-feedback (DFB) lasers, the QC laser wafer must be grown using precise molecular beam epitaxy or chemical vapor deposition techniques to form the nanometer-scale quantum well structures that make up the QC gain region. Following grating fabrication using lithography and chemical etching, the corrugated semiconductor surface must be thoroughly cleaned to enable an additional epitaxial growth step, as even a fraction of one monolayer of contamination will impede epitaxy. Epitaxial growth requires capital-intensive high-vacuum reactor systems. Eliminating this second growth step greatly reduces the overall process requirements and cost of DFB QC laser fabrication.

Fabrication of index-coupled DFB QC lasers with low-loss, spin-on dielectric materials enables high efficiency while maintaining lower-cost processing. Continuous-wave (CW) operation of QC lasers at room temperature was demonstrated using a single growth process followed by etched grating fabrication with polyimide infilling. Once the initial wafer growth is performed with molecular beam epitaxy, the remaining processing steps can be carried out in a standard semiconductor processing facility. Sufficient interaction between the optical mode and DFB grating is achieved by deep anisotropic etching and infilling with high refractive-index contrast material. Gratings are etched into the QC upper cladding layer using a chlorine/ hydrogen/methane plasma etching process, and the high-aspect-ratio grating openings are filled with spin-on polyimide. This novel DFB QC laser geometry avoids optical loss by making use of etched index-coupled gratings infilled with non-absorbing polymer, and electrical access is maintained since the gratings do not span the full width of the laser ridge.

This work was done by Ryan M. Briggs, Clifford F. Frez, and Siamak Forouhar of Caltech for NASA’s Jet Propulsion Laboratory.

In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to:

Innovative Technology Assets Management
JPL
Mail Stop 321-123
4800 Oak Grove Drive
Pasadena, CA 91109-8099
E-mail: This email address is being protected from spambots. You need JavaScript enabled to view it.

Refer to NPO-49037.


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Semiconductor Quantum Cascade Lasers Fabricated Without Epitaxial Regrowth

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This article first appeared in the November, 2015 issue of Photonics Tech Briefs Magazine.

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