The excited-state dynamics of three types of zeaxanthin aggregates are probed with transient absorption
spectroscopy on the femtosecond-to-microsecond timescale. Triplet excited states form via singlet fission in all
three aggregates within several hundred femtoseconds. The transient absorption spectra are consistent with an S2, but not S1, parent state for singlet fission. The quantum yield of triplet states in one of the weakly-coupled
aggregates is at least 60-80% immediately following photoexcitation. The same aggregate has a 10-30% yield of S1
excited states, which have a dominant decay time of ~8 ps. For the strongly-coupled H-aggregate, a new transient
absorption band with maximum 400−420 nm is found. The band is assigned to a triplet state with T1→Tn transition that is strongly exciton-coupled to either the S0→S2 transition of surrounding ground-state chromophores, or a T1→Tn transition of a nearby triplet excited state. The yield of triplet states could be 180% or more in the strongly coupled aggregate, as inferred from the absence of S1 signal. Fast annihilation depletes most of the triplet population in the aggregates on the picosecond timescale, however a measurable fraction persists beyond 1 μs.
Optical sectioning provides three-dimensional (3D) information in biological tissues. However, most imaging techniques implemented with optical sectioning are either slow or deleterious to live tissues. Here, we present a simple design for wide-field multiphoton microscopy, which provides optical sectioning at a reasonable frame rate and with a biocompatible laser dosage. The underlying mechanism of optical sectioning is diffuser-based temporal focusing. Axial resolution comparable to confocal microscopy is theoretically derived and experimentally demonstrated. To achieve a reasonable frame rate without increasing the laser power, a low-repetition-rate ultrafast laser amplifier was used in our setup. A frame rate comparable to that of epifluorescence microscopy was demonstrated in the 3D imaging of fluorescent protein expressed in live epithelial cell clusters. In this report, our design displays the potential to be widely used for video-rate live-tissue and embryo imaging with axial resolution comparable to laser scanning microscopy.
We describe a stand alone CARS module allowing upgrade of a two-photon microscope with CARS modality. The
Stokes beam is generated in a commercially available photonic crystal fiber (PCF) using fraction of the power of
femtosecond excitation laser. The output of the fiber is optimized for broadband CARS at Stokes shifts in 2900cm-1
region. The spectral resolution in CARS signal is 50 cm-1. It is achieved by introducing a bandpass filter in the pump
beam. The timing between the pump and Stokes pulses is preset inside the module and can be varied. We demonstrate
utility of the device on examples of second harmonic, two-photon fluorescence and CARS images of several biological
and non-biological samples. We also present results of studies where we used CARS modality to monitor in real time the
process of fabrication of microstructures by two-photon polymerization.