title: High-resolution simultaneous three-photon fluorescence and third-harmonic-generation microscopy.
abstract: In recent years, nonlinear laser scanning microscopy has gained much attention due to its unique ability of deep optical sectioning. Based on our previous studies, a 1,200-1,300-nm femtosecond laser can provide superior penetration capability with minimized photodamage possibility. However, with the longer wavelength excitation, three-photon-fluorescence (3PF) would be necessary for efficient use of intrinsic and extrinsic visible fluorophores. The three-photon process can provide much better spatial resolution than two-photon-fluorescence due to the cubic power dependency. On the other hand, third-harmonic-generation (THG), another intrinsic three-photon process, is interface-sensitive and can be used as a general structural imaging modality to show the exact location of cellular membranes. The virtual-transition characteristic of THG prevents any excess energy from releasing in bio-tissues and, thus, THG acts as a truly noninvasive imaging tool. Here we demonstrated the first combined 3PF and THG microscopy, which can provide three-dimensional high-resolution images with both functional molecule specificity and sub-micrometer structural mapping capability. The simultaneously acquired 3PF and THG images based on a 1,230-nm Cr:forsterite femtosecond laser are shown with a Hoechst-labeled hepatic cell sample. Strong 3PF around 450 nm from DNA-bounded Hoechst-33258 can be observed inside each nucleus while THG reveals the location of plasma membranes and other membrane-based organelles such as mitochondria. Considering that the maximum-allowable laser power in common nonlinear laser microscopy is less than 10 mW at 800 nm, it is remarkable that even with a 100-mW 1,230-nm incident power, there is no observable photo damage on the cells, demonstrating the noninvasiveness of this novel microscopy technique.
authors.affiliation: Graduate Institute of Electro-Optical Engineering and Department of Electrical Engineering, National Taiwan University, Taipei 10617, Taiwan.
authors.forename: Shi-Wei
authors.initials: SW
authors.lastname: Chu
authors.forename: Shih-Peng
authors.initials: SP
authors.lastname: Tai
authors.forename: Chia-Lin
authors.initials: CL
authors.lastname: Ho
authors.forename: Chi-Hung
authors.initials: CH
authors.lastname: Lin
authors.forename: Chi-Kuang
authors.initials: CK
authors.lastname: Sun
mesh_terms: D006690:Bisbenzimidazole; D006528:Carcinoma, Hepatocellular; D045744:Cell Line, Tumor; D006801:Humans; D007089:Image Enhancement; D008113:Liver Neoplasms; D018613:Microscopy, Confocal / Q000295:instrumentation / Q000379:methods*; D036641:Microscopy, Fluorescence, Multiphoton / Q000295:instrumentation / Q000379:methods*
chemical_list: D006690:Bisbenzimidazole