Noncontact photoacoustic lipid imaging by remote sensing on first overtone of the C-H bond

Lipid imaging by conventional photoacoustic microscopy subjects to direct contact sensing with relatively low detection bandwidth and sensitivity,which induces superficial imaging depth and low signalto-noise ratio(SNR)in practical imaging scenarios.Herein,we present a photoacoustic remote sensing microscopy for lipid distribution mapping in bio-tissue,featuring noncontact implementation,broad detection bandwidth,deep penetration depth,and high SNR.A tailored high-energy pulsed laser source with a spectrum centered at 1750 nm is used as the excitation beam,while a cofocused 1550 nm continuous-wave beam is used as the probe signal.The pump wavelength is selected to overlap the first overtone of the C-H bond in response to the intensive absorption of lipid molecules,which introduces a much-enhanced SNR(55 dB)onto photoacoustic remote sensing(PARS)signals.Meanwhile,the optical sensing scheme of the photoacoustic signals provides broadband detection compared to the acoustic transducer and refrains the bio-samples from direct contact operations by eliminating the ultrasonic coupling medium.Taking merits of the high detection sensitivity,deep penetration depth,broadband detection,and high resolution of the PARS system,high-quality tissue scale lipid imaging is demonstrated in a model organism and brain slice.