We rigorously apply the sum rules to the sum-over-states expression to calculate the fundamental limits of the dispersion of the two-photon absorption cross-section. A comparison of the theory with the data suggests that the truncated sum rules in the three-level model give a reasonable fundamental limit. Our ansatz that the two photon absorption cross-section near the limit must have only three dominant statesis is supported by a rigorous analytical calculation that the resonant term gets smaller as more states are added. We also find that the contributions of the non-explicitly resonant terms can not be neglected when analyzing real molecules with many excited states, even near resonance. However, puzzling as it may be, extrapolating an off-resonant result to resonance using only the resonant term of the three-level model is shown to be consistent with the exact result. In addition, the off-resonant approximation is shown to scale logarithmically when compared with the full three-level model. This scaling can be used to simplify the analysis of measurements. We find that existing molecules are still far from the fundamental limit; so, there is room for improvement. But, reaching the fundamental limit would require precise control of the energy-level spacing, independently of the transition dipole moments -- a task that does not appear possible using today's synthetic approaches. So, we present alternative methods that can still lead to substantial improvements which only require the control of the transition moment to the first excited state. While it is best to normalize measured two photon absorption cross-sections to the fundamental limits when comparing molecules, we show that simply dividing by the square of the number of electrons per molecule yields a good metric for comparison.
Conceptual Insights into the Interaction of the Phase of the Electric Field and Molecular Spectroscopy to Further Understand Molecular Isomerization and Solute-Solvent Interactions
Absorption cross-section spectroscopy of single strong coupling system between plasmon and molecular exciton resonance using single silver nanoparticle dimer generating surface enhanced resonant Raman scattering
Laser Pulse Diagnostics of Ultrafast <8 fs Pulses Through Two-Photon Absorption Fluorescence In Liquid Media -- The Role of GVD and Third-Order Dispersion
Magnetically Induced Transparency-Absorption and Normal-Anomalous Dispersion Characteristics of ${}^{87}\text{Rb}$ Medium or Any J-Type Configuration Atomic Vapors Subject to a Vector Magnetic Field and a Weak Resonant Pump
Alikhanyan National Laboratory (Yerevan Physics Institute)（阿利哈尼扬国家实验室（耶烈万物理研究所））；Institute for Physical Research, Armenian National Academy of Sciences（亚美尼亚国家科学院物理研究所）
Measurement of the two-photon absorption cross-section of liquid argon with a time projection chamber
本文报道了在时间投影室（LAr TPC）探测器中激光诱导266 nm液态氩的多光子电离。由激光束产生的电子信号是用于校准和监测下一代大质量LAr TPC的强大工具。我们设计和测试的探测器使我们能够以前所未有的准确度和精度测量LAr的双光子吸收截面：sigma_ex=（1.24 \pm 0.10 stat \pm 0.30 syst）10^{-56} cm ^4 s {-1}。
Kramers-Kronig causality in integrated photonics: The spectral tension between ultraviolet transition and mid-infrared absorption
Southern University of Science and Technology（南方科技大学）；International Quantum Academy（国际量子科学院）；Shenzhen Futian SUSTech Institute for Quantum Technology and Engineering（深圳福田南科大量子技术与工程研究院）；College of Physics and Optoelectronic Engineering, Shenzhen University（深圳大学物理与光电工程学院）；Institute of Advanced Photonics Technology, School of Information Engineering, Guangdong University of Technology（广东工业大学信息工程学院先进光子技术研究所）
PRISA: a simple software for determining refractive index, extinction co-efficient, dispersion energy, band gap, and thickness of semiconductor and dielectric thin films
