All-Optical Attosecond Technology

提案摘要：1阿秒等于百万分之一秒。为了让这样一个小数字变得更加真实，一阿秒相当于一秒，就像一秒相当于宇宙的年龄。尽管看起来不可思议，但今天我们可以制造出持续时间不到100阿秒的闪光，而在接下来的五年里，我预测我们将达到25阿秒。想象一下，当我们在这么快的时间尺度上观察世界时，世界会变得多么不同。我们可以“定格”任何围绕原子或分子运行的价电子。 为了认识到阿秒科学能够解决的问题的重要性，请回忆一下，电子形成了将所有分子和固体结合在一起的键。因此，电子为化学反应提供动力。考虑几个例子：(1)通过“视紫红质”分子中的电子激发和被激发电子的非常快速的松弛，我们看到了这个世界。(2)太阳能电池通过快速高效的电子传递来发挥作用。(3)在某些材料(称为“相关材料”)中，分子中的两个或多个电子同步响应。这些都是阿秒科学将帮助我们理解的问题的例子。 这项建议涉及三个分主题： ·测量强光如何在小分子中驱动非常快的电子激发， ·开发一种强大的新技术，以追踪分子或固体中不同电子激发之间的能量交换方式， ·将阿秒技术从原子分子气体(它们的发源地)转移到固体(技术应用所在的地方)。 该建议侧重于“全光学方法”，因为“全光学”允许简单而灵活的测量。事实上，许多测量只有在“全光学”的情况下才是实用的。 一个类比可能会帮助非科学读者理解潜在的技术。他/她可能会想到软木塞和水波。在这个类比中，软木塞的反应就像电子一样。要比喻水波，请想一想光波。当波穿过波峰和波谷时，软木塞(或电子)会相应地上下摆动。现在设想电子是原子的一部分。打个比方，想一想拴在岸边岩石上的软木塞。如果一个强波来了，电子就可以从原子中被拉出来。打个比方，系绳可能会断掉。一旦自由，电子就会在波中移动，但它可能会与其母离子再次碰撞--随着波的退去，软木会撞到岩石上。就像在软木-岩石碰撞中，我们可能会听到电子-离子碰撞中的噪音，一个阿秒脉冲就会产生。 通过这个类比，你将不会惊讶地了解到阿秒脉冲可以通过产生脉冲的光波来整形和控制。例如，对于这种“全光学”控制，我们可以添加不同波长的光或沿略有不同方向传播的波进行控制或测量。 让我以评论最后一个目标来结束。到目前为止，气体已经被用于阿秒技术。然而，如果我们可以将技术转移到固体上，那么我们就可以利用阿秒科学来研究固态电子学--特别是相关材料的电子学--如何在这个新的时间尺度上做出反应，我们就会有一种材料，可以用来制造“实用的阿秒设备”。

Summary of the Proposal: An attosecond is one millionth of one millionth of one millionth of a second. To make such a small number more real, one attosecond is to one second as a second is to the age of the universe. As incredible as it may seem, today we can make light flashes that last less than 100 attoseconds and over the next five years, I predict that we will reach 25 attoseconds. Imagine how different the world seems when it is observed on this fast time-scale. We could “freeze-frame” any valence electron orbiting atoms or molecules. To appreciate the importance of the issues that attosecond science will be able to address recall that electrons form the bonds that hold all molecules and solids together. Therefore, electrons power chemical reactions. Consider a few examples: (1) Through electron excitation in the molecule “rhodopsin” and the very rapid relaxation of the excited electron, we see the world. (2) Solar cells function by fast and efficient electron transport. (3) In some materials (called “correlated materials) two or more of the molecule’s electrons respond synchronously. These are examples of the issues that attosecond science will help us understand. This proposal involves three sub-themes: • Measuring how intense light drives very rapid electron excitation in small molecules, • Developing a powerful new technique to follow how energy is exchanged between different electronic excitations in molecules or solids, • Transferring attosecond technology from atomic molecular gases (where they were developed) to solids (where technological applications lie). The proposal focusses on “all-optical methods” because “all-optical” allows simple and flexible measurement. In fact, many measurements will only be practical if they are “all-optical”. An analogy might help a non-scientific reader understand the underlying technology. He/she might think of a cork and a water wave. The cork responds like an electron in this analogy. For the analogy of the water wave, please think of a light wave. As the wave goes through peaks and valleys, the cork (or the electron) bobs in response. Now imagine that the electron is a part of an atom. For an analogy, think of the cork tethered to a rock near the shore. If a strong wave comes, the electron can be pulled from the atom. In the analogy, the tether can break. Once free, the electron moves in the wave, but it can re-collide with its parent ion -- the cork hits the rock as the wave recedes. Just like in the cork-rock collision we might hear a noise in the electron-ion collision an attosecond pulse is born. With this analogy you will not be surprised to learn that attosecond pulses can be shaped and controlled through the light wave that creates the pulse. For this “all-optical” control we might, for example, add a different wavelength light or a wave travelling in a slightly different direction for control or measurement. Let me conclude by commenting on the last aim. So far, gases have been used for attosecond technology. However, if we could transfer the technology to solids, then we could use attosecond science to study how solid-state electronics – especially the electronics of correlated materials -- responds on this new time scale and we would have a material that can be used to create “practical attosecond devices”.