title: Two-color, two-photon, and excited-state absorption microscopy.
abstract: We develop a new approach in imaging nonfluorescent species with two-color two-photon and excited state absorption microscopy. If one of two synchronized mode-locked pulse trains at different colors is intensity modulated, the modulation transfers to the other pulse train when nonlinear absorption takes places in the medium. We can easily measure 10(-6) absorption changes caused by either two-photon absorption or excited-state absorption with a RF lock-in amplifier. Sepia melanin is studied in detail as a model system. Spectroscopy studies on the instantaneous two-photon absorption (TPA) and the relatively long-lived excited-state absorption (ESA) of melanin are carried out in solution, and imaging capability is demonstrated in B16 cells. It is found that sepia melanin exhibits two distinct excited states with different lifetimes (one at 3 ps, one lasting hundreds of nanoseconds) when pumped at 775 nm. Its characteristic TPA/ESA enables us to image its distribution in cell samples with high resolution comparable to two-photon fluorescence microscopy (TPFM). This new technique could potentially provide valuable information in diagnosing melanoma.
authors.affiliation: Princeton University, Department of Chemistry, Princeton, New Jersey 08544, USA.
authors.forename: Dan
authors.initials: D
authors.lastname: Fu
authors.forename: Tong
authors.initials: T
authors.lastname: Ye
authors.forename: Thomas E
authors.initials: TE
authors.lastname: Matthews
authors.forename: Gunay
authors.initials: G
authors.lastname: Yurtsever
authors.forename: Warren S
authors.initials: WS
authors.lastname: Warren
mesh_terms: D000818:Animals; D045744:Cell Line, Tumor; D004867:Equipment Design; D019544:Equipment Failure Analysis; D007089:Image Enhancement / Q000295:instrumentation / Q000379:methods; D008543:Melanins / Q000378:metabolism*; D008545:Melanoma / Q000378:metabolism* / Q000473:pathology*; D051379:Mice; D018613:Microscopy, Confocal / Q000295:instrumentation* / Q000379:methods*; D036641:Microscopy, Fluorescence, Multiphoton / Q000295:instrumentation* / Q000379:methods*; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity
chemical_list: D008543:Melanins