Spectral behavior of a terahertz quantum-cascade laser

Area of Science:

• Physics
• Quantum Optics
• Terahertz Technology

Background:

• Terahertz (THz) quantum cascade lasers (QCLs) are crucial for spectroscopy and imaging.
• Understanding their spectral characteristics, including linewidth and tuning, is essential for advanced applications.
• Pulsed and continuous-wave (cw) operation modes present unique spectral behaviors.

Purpose of the Study:

• To comprehensively characterize the spectral behavior of two THz QCLs.
• To investigate linewidth and tuning rates under varying temperature and injection current.
• To elucidate the mechanisms behind observed frequency tuning and linewidth limitations.

Main Methods:

• Utilized a heterodyne technique combining two THz QCLs.
• Employed a whisker contact Schottky diode mixer for frequency beatnote detection.
• Analyzed beatnote signals in both time and frequency domains.

Main Results:

• Measured effective laser linewidth and tuning rates as a function of temperature and injection current.
• Demonstrated that injection current tuning effects cannot be solely attributed to thermal mechanisms.
• Quantified intrapulse frequency tuning, limiting the effective linewidth to approximately 5 MHz.

Conclusions:

• The spectral dynamics of THz QCLs are complex, influenced by both thermal and non-thermal effects.
• Injection current plays a critical role in frequency tuning beyond simple temperature dependencies.
• Intrapulse frequency modulation is a key factor limiting the achievable linewidth in these devices.