Fourier-Transform Atomic Force Microscope-Based Photothermal Infrared Spectroscopy with Broadband Source

The mechanical detection of photothermal ex-pansion from infrared (IR) absorption with an atomic force microscope (AFM) bypasses Abbe's diffraction limit, forming the chemical imaging technique of AFM-IR. Here, we develop a Fourier transform AFM-IR technique with peak force infrared microscopy and broadband femtosecond IR pulses. A Michelson interferometer creates a pair of IR pulses with controlled time delays to generate photothermal signals transduced by AFM to form an interferogram. A Fourier transform is performed to recover IR absorption spectra. We demonstrate the Fourier transform AFM-IR microscopy on a polymer blend and hexagonal boron nitride. An intriguing observation is the vertical asymmetry of the interferogram, which suggests the presence of multiphoton absorption processes under the tip-enhancement and femtosecond IR lasers. Our method demonstrates the feasibility of time -domain detection of the AFM-IR signal in the mid-IR regime and paves the way toward multiphoton vibrational spectroscopy at the nanoscale below the diffraction limit.

Automated summary

This study develops a Fourier transform AFM-IR technique with peak force infrared microscopy and broadband femtosecond IR pulses, enabling the mechanical detection of photothermal expansion caused by infrared absorption. The method bypasses Abbe's diffraction limit and allows for chemical imaging of materials. The intriguing observation of vertical asymmetry in the interferogram suggests the presence of multiphoton absorption processes.