Nanosecond-Resolution Photothermal Dynamic Imaging via MHz Digitization and Match Filtering

Abstract

Photothermal microscopy has enabled highly sensitive label-free imaging of absorbers, from metallic nanoparticles to chemical bonds. Photothermal signals are conventionally detected via modulation of excitation beam and demodulation of probe beam using lock-in amplifier. While convenient, the wealth of thermal dynamic is not revealed. Here, we present a lock-in free, mid-infrared photothermal dynamic imaging (PDI) system by MHz digitization and match filtering at harmonics of modulation frequency. Thermal-dynamic information is acquired at nanosecond resolution within single pulse excitation. Our method not only increases the imaging speed by two orders of magnitude, but also obtains four-fold enhancement of signal-to-noise ratio over lock-in counterpart, enabling high-throughput metabolism analysis at single-cell level. Moreover, by harnessing the thermal decay difference between water and biomolecules, water background is effectively separated in mid-infrared PDI of living cells. This ability to nondestructively probe chemically specific photothermal dynamics offers a valuable tool to characterize biological and material specimens.

Bond-selective intensity diffraction tomography

“…The emerging mid-infrared photothermal (MIP) microscopy 19 inherits IR absorption spectroscopy's advantages but circumvents conventional IR microspectroscopy's low-resolution and slow speed limit. MIP microscopy provides diffraction-limited resolution at the visible band using a visible probe beam and is compatible with both point-scanning [26][27][28][29] and wide-field [30][31][32][33][34][35] configurations. Yet, existing MIP microscopy suffers from slow volumetric imaging speed and low depth resolution.…”

Roadmap on Label‐Free Super‐Resolution Imaging

“…A recently developed photothermal dynamic imaging system successfully addresses this limitation by digitizing the whole photothermal process. [461] After performing the Fourier transform, all the harmonics is added to enhance the signal-to-noise ratio. In the meantime, all recorded data adds another benefit of time-resolved imaging.…”