Discrete Vernier tuning in terahertz quantum cascade lasers using coupled cavities

Area of Science:

• * Physics
• * Electrical Engineering
• * Materials Science

Background:

• * Terahertz quantum cascade lasers (TQCLs) are crucial for various applications, but their frequency tunability is often limited.
• * Existing tuning methods for TQCLs can be complex or lead to undesirable changes in output power.

Purpose of the Study:

• * To demonstrate a novel discrete Vernier frequency tuning technique for TQCLs.
• * To achieve precise and controllable frequency shifts without affecting laser output power.
• * To explore the bandwidth and directionality of frequency tuning using this method.

Main Methods:

• * Fabricated a two-section coupled-cavity TQCL device with a narrow air gap.
• * Utilized focused ion beam milling for precise gap creation post-packaging.
• * Employed differential electrical biasing: short pulses for the lasing section and wider pulses for localized heating in the tuning section.
• * Engineered thermally-induced shifts in longitudinal cavity modes for frequency tuning.

Main Results:

• * Demonstrated discrete Vernier frequency tuning with controllable blue or red shifts.
• * Achieved significant tuning bandwidths of 50 GHz and 85 GHz in different device designs.
• * Observed no corresponding change in emitted power during frequency tuning.
• * Interchanging device sections resulted in red shifts of 20 GHz and 30 GHz.

Conclusions:

• * The demonstrated discrete Vernier tuning method offers superior frequency control for TQCLs compared to standard ridge lasers.
• * This technique provides a robust way to precisely adjust terahertz frequencies without power degradation.
• * The findings pave the way for advanced tunable terahertz sources in spectroscopy, imaging, and communications.