Indirect absorption spectroscopy using quantum cascade lasers: mid-infrared refractometry and photothermal...

Area of Science:

• Spectroscopy
• Optical Physics
• Materials Science

Background:

• Vibrational spectra provide crucial molecular information.
• Existing methods for recording spectra can be limited in sensitivity.
• Optical properties, like refractive index, are sensitive to molecular presence.

Purpose of the Study:

• To develop and demonstrate two novel indirect schemes for recording vibrational spectra.
• To achieve high sensitivity, down to sub-monolayer detection.
• To utilize the real part of the refractive index for spectral analysis.

Main Methods:

• Mid-infrared refractometry: Utilizes a quantum cascade laser (QCL) spot imaging to determine total internal reflection angles, enabling absorption line determination via beam profile analysis.
• Photothermal spectroscopy: Employs a tunable QCL to excite molecular monolayer vibrational resonances, causing heating and a refractive index change detected by a visible probe laser.

Main Results:

• Both mid-infrared refractometry and photothermal spectroscopy successfully recorded vibrational spectra.
• The techniques demonstrated sensitivity to the real part of the refractive index.
• Sub-monolayer detection sensitivities were achieved, showcasing high sensitivity.

Conclusions:

• Indirect recording of vibrational spectra via refractive index measurements is feasible and highly sensitive.
• Quantum cascade lasers are effective tools for both excitation and optical measurements in these schemes.
• These methods offer promising alternatives for sensitive molecular analysis and characterization.