Shanu Mohd, Acharyya Jitendra Nath, Sankar Muniappan, Vijaya Prakash G
Nanophotonics Lab., Department of Physics, Indian Institute of Technology Delhi, Hauz Khas, New Delhi 110016, India.
Department of Chemistry, Indian Institute of Technology Roorkee, Roorkee 247667, India.
Inorg Chem. 2023 Aug 14;62(32):12895-12904. doi: 10.1021/acs.inorgchem.3c01585. Epub 2023 Jul 31.
Optical nonlinearities of discrete absorption energy levels of one of the typical heterocyclic aromatic molecules, free-base porphyrins, have been probed over a broad spectral region (400-1600 nm) utilizing intense femtosecond pulses. A wide range of strong one- and multiphoton-induced nonlinear absorptions of both the blue-end Soret (B) band (a → b) and red-end orbital mixing split quasi-allowed Q-bands (Q(0,0; 0,1), Q(0,0; 0,1), a → e) are critically probed and reported. During the resonant excitation within B- (400 nm) and Q-bands (600-750 nm), the nonlinear absorption has become predominant by the saturation of absorption (SA) of the one-photon absorption (1PA) process due to ground-state bleaching. At nonresonant wavelengths, it is dominated by the reverse saturation of absorption (RSA), involving various nonlinear processes of two-, three-, and four-photon (2PA, 3PA, and 4PA) absorptions, either to B- or Q-bands (1100-1600 nm). The laser intensity-dependent nonresonant (2PA, 800 nm) excitations for the prominent B-band show a distinct cross-over from SA to RSA, contributed by the excited-state absorption (ESA) utilizing a three-photon induced (3PA) process, whereas resonant (1PA, 400 nm) excitation reveals a systematic strong SA process. Both wavelength- and intensity-dependent nonlinear refractive index studies exhibit positive electronic Kerr-based self-focusing effects, with prominent contributions of nonlinear absorption and higher-order effects. The spectrally discrete, highly intense laser probing of individual energy bands and the consequent variety of nonlinearities can be broadly generalized for many free-base porphyrins and metalloporphyrins. The present studies provide a strong foundation and new insight into the broad categories of macrocycles, such as porphyrins and phthalocyanines, for myriad applications in nonlinear optics and bio/optophotonics.
利用强飞秒脉冲，在宽光谱区域（400 - 1600 nm）对典型杂环芳香分子之一的游离碱卟啉的离散吸收能级的光学非线性进行了探测。对蓝端Soret（B）带（a → b）和红端轨道混合分裂准允许Q带（Q(0,0; 0,1)、Q(0,0; 0,1)、a → e）的广泛强单光子和多光子诱导非线性吸收进行了严格探测并报道。在B带（400 nm）和Q带（600 - 750 nm）内的共振激发过程中，由于基态漂白，非线性吸收因单光子吸收（1PA）过程的吸收饱和（SA）而占主导。在非共振波长下，它由吸收的反向饱和（RSA）主导，涉及双光子、三光子和四光子（2PA、3PA和4PA）吸收的各种非线性过程，无论是到B带还是Q带（1100 - 1600 nm）。对于突出的B带，与激光强度相关的非共振（2PA，800 nm）激发显示出从SA到RSA的明显转变，这是由利用三光子诱导（3PA）过程的激发态吸收（ESA）引起的，而共振（1PA，400 nm）激发则显示出系统的强SA过程。波长和强度相关的非线性折射率研究均显示出基于电子克尔效应的正自聚焦效应，非线性吸收和高阶效应贡献显著。对各个能带进行的光谱离散、高强度激光探测以及由此产生的各种非线性效应，可广泛推广到许多游离碱卟啉和金属卟啉。本研究为卟啉和酞菁等大环化合物在非线性光学和生物/光光子学中的众多应用提供了坚实基础和新见解。
