Precision molecular spectroscopy with a phase-locked terahertz quantum-cascade laser.

Area of Science:

• Terahertz (THz) spectroscopy
• Molecular physics
• Laser spectroscopy

Background:

• Quantum cascade lasers (QCLs) are crucial for THz applications.
• Precise frequency control of QCLs is essential for high-resolution spectroscopy.
• Existing THz spectroscopy methods can be complex and require cryogenic cooling.

Purpose of the Study:

• To demonstrate precision molecular spectroscopy using a phase-locked QCL.
• To achieve precise frequency control of a QCL across its tuning range.
• To measure methanol transitions with high accuracy in a compact, cryogen-free setup.

Main Methods:

• Utilized a phase-locked quantum cascade laser (QCL) operating at 3.4 THz.
• Employed a Schottky diode harmonic mixer to generate an intermediate frequency (IF) signal for phase-locking.
• Referenced the microwave signal to a rubidium frequency standard for precise frequency control.

Main Results:

• Successfully phase-locked the QCL over its entire frequency tuning range.
• Identified and precisely quantified ten methanol transitions between 3443.5 and 3447.6 GHz.
• Achieved high precision in determining transition frequencies and broadening parameters.

Conclusions:

• Demonstrated a compact and cryogen-free method for precision THz molecular spectroscopy.
• The phase-locking technique provides accurate frequency control for QCLs.
• This advancement has significant implications for molecular spectroscopy and THz applications.