All-Optical Attosecond Technology

提案摘要：一阿秒是一秒的百万分之一的百万分之一。为了让这么小的数字更真实，一阿秒之于一秒就像一秒之于宇宙的年龄一样。这听起来很不可思议，今天我们可以制造出持续不到100阿秒的闪光，在接下来的五年里，我预测我们将达到25阿秒。想象一下，在这么快的时间尺度上观察这个世界是多么的不同。我们可以“定格”任何围绕原子或分子运行的价电子。为了理解阿秒科学将能够解决的问题的重要性，回想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)通过分子“视紫红质”中的电子激发和被激发电子的快速松弛，我们看到了世界。(2)太阳能电池通过快速有效的电子传递发挥作用。(3)在某些材料（称为“相关材料”）中，两个或两个以上的分子电子同步响应。这些都是阿秒科学将帮助我们理解的问题的例子。该提案涉及三个子主题：•测量强烈的光如何驱动小分子中非常快速的电子激发，•开发一种强大的新技术来跟踪分子或固体中不同电子激发之间的能量交换，•将阿秒技术从原子分子气体（它们被开发的地方）转移到固体（技术应用的地方）。该提案侧重于“全光学方法”，因为“全光学”允许简单灵活的测量。事实上，许多测量只有在“全光学”的情况下才可行。一个类比可能有助于不懂科学的读者理解底层技术。他/她可能会想到软木塞和水波。在这个类比中，软木的反应就像一个电子。为了类比水波，请考虑光波。当波经过波峰和波谷时，软木（或电子）会随之摆动。现在想象一下，电子是原子的一部分。打个比方，想想岸边拴在岩石上的软木塞。如果一个强波来了，电子就会从原子中被拉出来。在这个类比中，绳子可能会断裂。一旦获得自由，电子就会在波中移动，但它会再次与母离子碰撞——当波退去时，软木塞就会撞到岩石上。就像在软木塞-岩石碰撞中我们可能会听到电子-离子碰撞时的噪音一个阿秒脉冲产生了。通过这个类比，你不会惊讶地发现，阿秒脉冲可以通过产生脉冲的光波来塑造和控制。例如，对于这种“全光”控制，我们可以添加不同波长的光或以稍微不同方向传播的波来控制或测量。最后，让我评论一下最后一个目标。到目前为止，气体已经被用于阿秒技术。然而，如果我们能将这项技术转移到固体，那么我们就可以用阿秒科学来研究固态电子产品——尤其是相关材料的电子产品——在这个新的时间尺度上的反应，我们将有一种可用于制造“实用阿秒设备”的材料。

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”.