High-resolution high-contrast mid-infrared imaging of fresh biological samples with ultraviolet-localized photoacoustic microscopy (Conference Presentation)

Abstract

Label-free mid-infrared (MIR) imaging provides rich chemical and structural information of biological tissues without staining. Conventionally, the long MIR wavelength severely limits the lateral resolution owing to optical diffraction; moreover, the strong MIR absorption of water ubiquitous in fresh biological samples results in high background and low contrast. Here, we present a novel approach, called ultraviolet-localized MIR photoacoustic microscopy (ULM-PAM), to achieve high-resolution and water-background–free MIR imaging of fresh biological samples. In our approach, a pulsed MIR laser thermally excites the sample at the focal spot, and a pulsed ultraviolet (UV) laser photoacoustically detects the resulting transient temperature rise owing to the Grüneisen relaxation effect, thereby reporting the intensity of the MIR absorption by the sample. The imaging resolution of our method is determined by the wavelength of the UV laser, which is one order of magnitude shorter than that of the mid-IR laser (2.5 μm to 12 μm). In addition, in the UV region from 200 nm to 230 nm, most important organic molecules in biological tissues, including proteins, lipids and nuclei acids, have strong absorption, while water is transparent. Therefore, our method can achieve high resolution and water-background free IR imaging of fresh biological samples. For cell cultures, our method achieved high-resolution and high-contrast infrared imaging of lipids, proteins. The capability of label-free histology of this method is also demonstrated in thick biological tissues, such as brain slices. Our approach provides convenient high-resolution and high-contrast MIR imaging, which can benefit diagnosis of fresh biological samples.