Reversible switching of quantum cascade laser-modes using a pH-responsive polymeric cladding as transducer

Area of Science:

• Optoelectronics
• Materials Science
• Chemical Sensing

Background:

• Quantum cascade lasers (QCLs) are semiconductor lasers with emission wavelength determined by device structure.
• Modulating QCL emission wavelength post-fabrication is challenging but desirable for applications like multi-color light sources and sensors.
• Halochromic materials change color or optical properties in response to pH variations.

Purpose of the Study:

• To demonstrate a novel method for reversible wavelength switching in QCLs.
• To utilize a halochromic polymer cladding for dynamic emission control.
• To explore the potential of such lasers as multi-color light sources and atmospheric pH sensors.

Main Methods:

• Coating an air-waveguide QCL ridge with a thin layer of polyacrylic acid (PAA).
• Introducing wavelength-dependent optical losses via the PAA cladding.
• Exposing the PAA cladding to acidic or alkaline vapors to alter its absorption spectrum and induce wavelength shifts.

Main Results:

• Successfully achieved reversible switching of the QCL emission wavelength.
• Observed a spectral shift of 7 cm⁻¹ upon exposure to acidic or alkaline vapors.
• Demonstrated that the wavelength shift is directly correlated with the pH-induced changes in the polymer's absorption spectrum.

Conclusions:

• The developed halochromic cladding provides a viable method for tuning QCL emission wavelength.
• The reversible nature of the polymer allows for dynamic control over laser output.
• These tunable QCLs show promise as versatile multi-color light sources and sensitive atmospheric pH detectors.