Not Just Better Resolution: A Detailed Study of the Signal Distribution in Mid-Infrared Optical Photothermal Imaging.

Area of Science:

• Microscopy
• Spectroscopy
• Optical Physics

Background:

• Spatial resolution in imaging systems is typically assessed using the point spread function (PSF).
• The PSF is fundamental in light microscopy but has been underexplored in optical photothermal infrared (O-PTIR) imaging.
• O-PTIR imaging exhibits distinct axial signal characteristics compared to conventional light microscopy.

Purpose of the Study:

• To investigate the radial and axial intensity distributions of O-PTIR systems.
• To characterize the axial point spread function (PSF) of O-PTIR imaging.
• To compare O-PTIR PSF with conventional Raman microscopy.

Main Methods:

• Theoretical modeling and experimental data acquisition.
• Analysis of a commercial Raman microscope and a custom O-PTIR transmission instrument.
• Focus on the axial PSF of the O-PTIR system.

Main Results:

• The axial photothermal signal in O-PTIR is markedly different from conventional light microscope imaging responses.
• Previous studies indicated a two-lobed O-PTIR signal for sub-diffraction limit particles, dependent on laser focus offset.
• This study confirms the presence of a two-lobed signal shape in objects comparable to the probe beam wavelength.

Conclusions:

• The axial PSF of O-PTIR imaging is significantly different from conventional microscopy.
• A two-lobed signal shape is characteristic of O-PTIR, even for objects near the probe beam wavelength.
• Understanding the O-PTIR PSF is crucial for accurate spatial resolution assessment in nanoscale imaging.