Phase-sensitive lock-in detection for high-contrast mid-infrared photothermal imaging with sub-diffraction limited...

Area of Science:

• Mid-infrared spectroscopy
• Photothermal imaging
• Super-resolution microscopy

Background:

• Mid-infrared photothermal imaging is generally limited by thermal blurring and heat transport.
• Heterogeneous samples with similar absorption to their embedding medium pose challenges for standard amplitude-based imaging.
• Existing methods struggle to achieve high contrast and resolution for complex samples in the mid-infrared.

Purpose of the Study:

• To demonstrate, for the first time, imaging of the phase output of a lock-in amplifier in mid-infrared photothermal vibrational microscopy.
• To combine phase imaging with nonlinear demodulation for enhanced resolution and contrast.
• To achieve sub-diffraction limited spatial resolution in label-free mid-infrared imaging.

Main Methods:

• Utilized a lock-in amplifier to acquire the phase signal from mid-infrared photothermal excitation.
• Investigated the relative contribution of the out-of-phase lock-in signal to extract heat transport information.
• Combined relative diffusive phase imaging with nonlinear demodulation at the second harmonic.

Main Results:

• Achieved high-contrast differentiation of heterogeneous samples by imaging the photothermal phase signal.
• Mapped sample inhomogeneities in thermal diffusion properties with high sensitivity and sub-diffraction limited resolution.
• Detected 1-μm melamine beads with a 1.3-μm spatial resolution (FWHM), exceeding the probe diffraction limit by 2.5x.

Conclusions:

• Imaging the phase signal from a lock-in amplifier provides complementary information to amplitude imaging for heterogeneous samples.
• The developed technique enables mapping of thermal diffusion properties with high sensitivity and resolution.
• This approach paves the way for super-resolution, label-free imaging in the mid-infrared spectral range.