Spatially Resolved Two-Dimensional Infrared Spectroscopy via Wide-Field Microscopy

Area of Science:

• Spectroscopy
• Microscopy
• Chemical Imaging

Background:

• Traditional infrared (IR) microscopy methods like FTIR have limitations in spatial resolution and contrast for complex samples.
• Two-dimensional infrared (2D IR) spectroscopy provides rich molecular information but has been challenging to implement in a wide-field imaging format.
• Advanced vibrational spectroscopy techniques are needed for detailed analysis of heterogeneous materials and biological systems.

Purpose of the Study:

• To report the development and demonstration of the first wide-field microscope for measuring two-dimensional infrared (2D IR) spectroscopic images.
• To achieve hyperspectral imaging with diffraction-limited spatial resolution by collecting over 16,000 2D IR spectra concurrently.
• To explore the advantages of 2D IR microscopy over FTIR for molecular discrimination and noise reduction.

Main Methods:

• Utilized a new focal plane array detector and mid-infrared (mid-IR) pulse shaping for concurrent spectral data acquisition.
• Employed a dual acousto-optic modulator pulse shaper to scan femtosecond pulse pairs, collecting both frequency axes in the time domain.
• Demonstrated the technique by imaging polystyrene beads with metal carbonyls and a USAF test target.

Main Results:

• Achieved diffraction-limited spatial resolution in the generated 2D IR spectroscopic images.
• Observed enhanced contrast compared to FTIR microscopy due to the nonlinear scaling of the 2D IR signal with the absorptivity coefficient.
• Demonstrated improved molecular discrimination and noise elimination using off-diagonal peaks arising from vibrational anharmonicities.

Conclusions:

• The developed 2D IR microscope provides a powerful new tool for analyzing complex and heterogeneous samples with high molecular specificity.
• This technique enables the extraction of information on vibrational lifetimes, molecular couplings, and transition dipole orientations for contrast generation.
• Potential applications include studying protein folding, amyloid proteins in tissues, and structural dynamics in mid-IR material devices.