High-Q resonant cavities for terahertz quantum cascade lasers

Area of Science:

• Terahertz (THz) Science and Technology
• Optical Engineering
• Quantum Cascade Lasers

Background:

• Resonant cavities are crucial for enhancing light-matter interactions and enabling high-resolution spectroscopy.
• Quantum cascade lasers (QCLs) provide a coherent source in the THz range, but their spectral properties can be influenced by external feedback.
• Wire-grid polarizers offer a versatile method for coupling light into and out of optical cavities.

Purpose of the Study:

• To design, realize, and characterize two distinct resonant THz cavity designs.
• To compare experimental resonator parameters with theoretical predictions.
• To investigate the impact of optical feedback from a THz cavity onto a continuous-wave quantum cascade laser (QCL).

Main Methods:

• Fabrication and characterization of two resonant THz cavity designs utilizing wire-grid polarizers.
• Injection of cavities with a continuous-wave quantum cascade laser (QCL) operating at 2.55 THz.
• Measurement of cavity quality factor (Q) and resonant peak widths.
• Analysis of optical feedback effects using a second cavity as a frequency reference.

Main Results:

• Achieved a high quality factor (Q) of approximately 2.5 × 10^5 for the THz cavities.
• Observed resonant peaks with linewidths in the few MHz range, comparable to Doppler-limited THz molecular transitions.
• Demonstrated that cavity linewidths are slightly broader than the free-running QCL emission spectrum.
• Examined the effects of optical feedback from one cavity to the QCL.

Conclusions:

• The developed THz cavities exhibit excellent performance, suitable for high-resolution THz spectroscopy.
• The narrow linewidths achieved are advantageous for resolving fine spectral features in molecular transitions.
• The study provides insights into the interplay between THz cavities and QCLs, crucial for stabilizing laser sources.