Photothermally Probing Vibrational Excited-State Absorption with Nanoscale Spatial Resolution through Frequency-Domain Pump- Probe Peak Force Infrared Microscopy

The diffraction limit binds the spatial resolution of optical spectroscopy to a finite fraction of the light wavelength. Traditional far-field pump/probe spectroscopy that detects the excited-state absorption (ESA) is not an exception. In this work, we present a new spectroscopic route to measure ESA based on the mechanical detection of the photothermal responses with the peak force infrared (PFIR) microscopy. We probe the vibrational ESA through the difference of the photothermal effects between temporal overlap and offset of two frequency-tunable infrared pulses. Two-dimensional PFIR spectra are collected on the carbonyl of a polymer with ESA responses. Also, we spatially map the ESA response of a structured polymer. The spatial resolution of the pump-probe PFIR microscopy is not bound by the diffraction limit that restraints to a finite fraction of the wavelength. Further, implementation of the detection paradigm of pump-probe microscopy will provide access to the highly desired two-dimensional infrared nanoscopy.