Nonlinear spectroscopy and microscopy techniques are ubiquitous in a wide range of applications across physics and biology. However, these usually rely on high-powered pulsed laser systems. A promising alternative is to exploit entangled two-photon absorption (ETPA), which can lead to tens of orders of magnitude lower incident fluxes than in conventional two-photon absorption schemes. However, the role of different entangled degrees of freedom in ETPA was unclear following recent experimental studies, when compared to earlier theoretical works. Here, we first demonstrate a linear dependence of the ETPA rate with the photon-pair rate, a clear signature of ETPA, and estimate the values for the concentration-dependent ETPA cross section for Rhodamine 6G. We then investigate the signature of energy-time entanglement and polarization dependence in the ETPA fluorescence rate and demonstrate a strong dependence of the signal on the interphoton delay that reflects the coherence time of the entangled two-photon wave packet.

中文翻译：

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能量时间纠缠的双光子分子吸收

非线性光谱学和显微技术在物理学和生物学的广泛应用中无处不在。但是，这些通常依赖于高功率脉冲激光系统。一种有前途的替代方法是利用纠缠双光子吸收（ETPA），与传统的双光子吸收方案相比，它可以使入射通量降低数十个数量级。但是，与早期的理论研究相比，最近的实验研究还不清楚不同的纠缠度在ETPA中的作用。在这里，我们首先证明ETPA速率与光子对速率的线性相关性，ETPA的清晰特征，并估算若丹明6G的浓度依赖性ETPA横截面值。