We show that two-photon absorption (TPA) is highly anisotropic in CdSe nanoplatelets, thus promoting them as a new class of directional two-photon absorbers with large cross sections. Comparing two-dimensional k-space spectroscopic measurements of the one-photon and two-photon excitation of an oriented monolayer of platelets, it is revealed that TPA into the continuum is a directional phenomenon. This is in contrast to one-photon absorption. The observed directional TPA is shown to be related to fundamental band anisotropies of zincblende CdSe and the ultrastrong anisotropic confinement. We recover the internal transition dipole distribution and find that this directionality arises from the intrinsic directionality of the underlying Bloch and envelope functions of the states involved. We note that the photoemission from the CdSe platelets is highly anisotropic following either one- or two-photon excitation. Given the directionality and high TPA cross-section of these platelets, they may, for example, find employment as efficient logic AND elements in integrated photonic devices, or directional photon converters.

中文翻译：

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k空间光谱显示CdSe纳米片中的定向双光子吸收

我们表明，CdSe纳米片中的双光子吸收（TPA）具有高度的各向异性，因此促进了它们作为具有大横截面的新型定向双光子吸收剂的发展。比较二维k血小板定向单层的单光子和双光子激发的空间光谱测量表明，TPA进入连续体是一种定向现象。这与单光子吸收相反。观察到的定向TPA与闪锌矿CdSe的基带各向异性和超强各向异性限制有关。我们恢复了内部跃迁偶极子分布，并发现此方向性是由所涉及的状态的基础Bloch和包络函数的固有方向性引起的。我们注意到，CdSe血小板的光发射在一个或两个光子激发之后是高度各向异性的。考虑到这些血小板的方向性和较高的TPA横截面，它们可能例如