Coexisting frequency combs spaced by an octave in a monolithic quantum cascade laser

Area of Science:

• Optics and Photonics
• Quantum Electronics
• Spectroscopy

Background:

• Quantum cascade lasers (QCLs) are key for compact mid-infrared and terahertz frequency comb generation.
• Developing dual-frequency comb sources is crucial for advanced spectroscopic techniques.

Purpose of the Study:

• To demonstrate a heterogeneous terahertz quantum cascade laser (TQCL) operating as a double frequency comb source.
• To investigate the coherence properties and potential for broad spectral coverage of this novel device.

Main Methods:

• Fabrication of a heterogeneous TQCL with two octave-spaced active regions based on a four-quantum well design.
• Characterization of simultaneous comb operation using RF beatnote spectroscopy.
• Investigation of free-running coherence properties via external detection and self-mixing.

Main Results:

• The device achieved simultaneous operation of two active regions, emitting 2.3 THz (300 GHz bandwidth) and 4.6 THz (270 GHz bandwidth) combs.
• A combined peak power of 3 mW was recorded at 15 K in pulsed mode.
• Narrow RF beatnotes (∼15 kHz) confirmed simultaneous dual-comb operation, with coherence investigated through beatnote spectroscopy.

Conclusions:

• The engineered TQCL successfully functions as a compact double frequency comb source.
• The device serves as a test bench for waveguide internal self-referencing techniques.
• Comb operation in a highly dispersive region highlights potential for broad spectral coverage using gain bandwidth engineering.