Time-Domain Photothermal AFM Spectroscopy via Femtosecond Pulse Shaping

Abstract

A time-domain version of photothermal microscopy using an atomic force microscope (AFM) is reported, which we call Fourier transform photothermal (FTPT) spectroscopy, where the delay between two laser pulses is varied and the Fourier transform is computed. An acousto-optic modulator-based pulse shaper sets the delay and phases of the pulses shot-to-shot at 100 kHz, enabling background subtraction and data collection in the rotating frame. The pulse shaper is also used to flatten the pulse spectrum, thereby eliminating the need for normalization by the laser spectrum. We demonstrate the method on 6,13-bis­(triisopropylsilylethynyl)­pentacene (TIPS-Pn) microcrystals and Mn-phthalocyanine islands, confirming subdiffraction spatial resolution, and providing new spectroscopic insights likely linked to structural defects in the crystals.

Toward the Next Frontiers of Vibrational Bioimaging

“…On the other hand, implementations of broadband MIP and force-detected AFM+IR are of increasing interest. The combination of AFM+IR with broadband laser and Fourier-transformed schemes has recently opened doors for ultrafast IR spectro-microscopy at the nanoscale. , These techniques are expected to not only increase the imaging speed and multiplexity but also uncover structural and temporal biological information, such as protein conformation and dynamics which are unachievable with linear spectroscopic methods.…”

Atomic Force Microscopy-Based Nanoscale Infrared Techniques for Catalysis

“…Xu et al demonstrated Fourier-transform AFM-IR microscopy with PFIR, introducing a promising alternative nanospectroscopy method . Zanni et al developed Fourier transform photothermal spectroscopy (FTPT) operated in the UV–vis region, providing a helpful protocol for IR-FTPT design . Notably, since the signal is normally collected in comparison between different tip–sample interactions, the theoretical limit for spectral record speed could not exceed the oscillation frequency of the AFM cantilever.…”

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

“…The typical way to generate a spectrum from a broadband light source is through time-domain detection with an interferogram followed by Fourier transform. As broadband visible radiation from a supercontinuum has been demonstrated to generate time-domain photothermal signals detectable by AFM in contact mode, can we incorporate broadband IR radiation for AFM-IR microscopy for spectroscopic information?…”