Texas A&M University, Department of Biomedical Engineering, 337 Zachry Engineering Center, 3120 TAMU, College Station, Texas 77843, USA.
J Biomed Opt. 2009 Sep-Oct;14(5):054041. doi: 10.1117/1.3253388.
Two-photon-excited fluorescence and second-harmonic generation are characterized as a function of laser pulse duration as short as sub-10-fs. A comparative study is performed where pulse duration is varied by introducing dispersion, as reported previously, and by tailoring pulse spectral width and minimizing its time-bandwidth product (transform-limited pulses). Experimental data and calculations show that characterizing a two-photon signal with the two schemes to vary pulse duration measures different phenomena. Two-photon signal characterization using dispersion-broadened pulses measures only the effect of chirp on the pulse two-photon-excitation spectrum and is independent of molecular response. Transform-limited pulses are used to measure the dependence of two-photon signal generation on pulse duration. Calculations show that deviation from the 1T(p) relationship would be expected as the transform-limited pulse spectral width approaches the molecular two-photon absorption linewidth and exhibits asymptotic behavior for pulse spectral widths 10 times greater than the absorption linewidth. Experimental measurements are consistent with the predicted behavior. The impact of using ultrashort laser pulses on the performance characteristics of nonlinear optical microscopy is discussed.
双光子激发荧光和二次谐波产生的特性可作为激光脉冲持续时间的函数，最短可达亚 10 飞秒。本文进行了一项对比研究，通过引入先前报道的色散来改变脉冲持续时间，通过调整脉冲光谱宽度并最小化其时间带宽积（变换极限脉冲）来改变脉冲持续时间。实验数据和计算表明，用两种方案来改变脉冲持续时间来表征双光子信号测量的是不同的现象。使用展宽脉冲的色散来表征双光子信号仅测量啁啾对脉冲双光子激发光谱的影响，而与分子响应无关。使用变换极限脉冲来测量双光子信号产生对脉冲持续时间的依赖关系。计算表明，当变换极限脉冲光谱宽度接近分子双光子吸收线宽时，预计会偏离 1T(p)关系，并表现出渐近行为，对于光谱宽度是吸收线宽的 10 倍。实验测量与预测的行为一致。讨论了使用超短激光脉冲对非线性光学显微镜性能特性的影响。
