Solid State Cavity QED with Artificial Atoms: Toward Next Generation Integrated THz Emitters

根据量子力学，围绕原子核运行的电子只能占据离散的轨道，对应于明确定义的能级。当原子发射或吸收光时，电子在两个不同的轨道之间跳跃，它们的能量差决定了发射或吸收光的频率。从普通灯到实验室激光器，光源都是根据这个原理工作的，它们的材料被选择在所需光的频率上进行转换。当原子处于强电场中时，情况变得更加复杂，因为电场会改变现有的轨道或将它们分裂成多个轨道，从而产生新的可能的跃迁。其中一些跃迁对于制造发射所谓太赫兹范围的设备非常有用，太赫兹范围是指频率介于无线电波和红外线之间的光。今天我们缺乏实用的太赫兹光源。这确实是不幸的，因为这种辐射的有用特性。太赫兹辐射在一定程度上穿过纸张、织物甚至生物组织，然而，与X射线相反，它对人类是安全的，因此它可以应用于许多领域，从医学成像到安全扫描仪。虽然如上所述，强电场下原子中的一些跃迁确实位于太赫兹域，但不可能利用它们来实现太赫兹源，要么是因为自然发生的原子的轨道形状使得电子不可能在它们之间跳跃，要么是因为所涉及的过程只有在多个电子之间相互作用时才有可能。这是相当困难的，因为不同原子上的电子相距很远，它们几乎看不见彼此。我的提议的核心思想是用人造原子代替真实的原子，在人造原子中，电子不是绕着原子核旋转，而是被困在一个被称为量子阱的纳米陷阱中，这是一种由不同材料片制成的三明治，每个片的宽度只有几个原子层。这将电子限制在两片“面包”之间。这些人造原子的有趣之处在于，一方面，我们可以通过适当地设计井的形状和大小来改变电子轨道的形状，另一方面，由于许多电子存在于同一个井中，它们之间的相互作用比原子系统中强得多。因此，在强电场中使用人造原子，就有可能利用新的转变来实现廉价、高效和可调谐的太赫兹源。光与物质在量子层面的相互作用是我职业生涯中大部分时间都在研究的领域，这是一个令人着迷的领域。它不仅帮助我们彻底改变了对世界的理解，因为解释吸收和发射光谱的努力是导致量子理论发展的驱动力之一，而且它深刻地改变了我们的日常生活。如今，激光和光电子技术无处不在：在我们的计算机中，在我们的医疗设备中，在我们的通信基础设施中。目前的项目旨在加深我们对人造原子光学跃迁背后的基本物理学的理解，并为新一代太赫兹发射器铺平道路，这些发射器可能在几十年内找到许多拯救生命的应用，从机场扫描仪到医学成像。

According to quantum mechanics, electrons orbiting around the nucleus of an atom can occupy only discrete orbitals, corresponding to well defined energy levels. When the atom emits or absorbs light, electrons jump between two different orbitals, whose energy difference gives the frequency of the emitted or absorbed light.Light emitters, from common lamps to laboratory lasers, work on this principle and the material they are made of is chosen to have transitions at the frequency of the desired light. When the atoms are in presence of a strong electric field, the situation becomes more complex, as the field modifies the existing orbitals or splits them into multiple ones, giving rise to new possible transitions.Some of these transitions would be very useful to produce devices emitting in the so-called terahertz range, that is light whose frequency lays between the radio waves and the infrared. Today we lack practical sources of terahertz light. This is really unfortunate because of the useful properties of such radiation. Terahertz radiation passes through paper, fabric, and even biological tissues to a limited extent, yet, contrary to X rays, it is safe for humans and it can thus be applied in a number of fields, from medical imaging to security scanners.While, as explained above, some transitions in atoms under strong electric fields do lay in the terahertz domain, it is not possible to harness them to realize terahertz sources, either because the very shape of orbitals in naturally occurring atoms makes it impossible for an electron to jump between them, or because the involved process becomes possible only if multiple electrons interact between them. This is rather difficult as electrons on different atoms are so far apart that they almost do not see each other.The idea at the heart of my proposal is to use instead of real atoms, artificial ones, in which the electrons, instead of orbiting around nuclei, are trapped in a nanometric trap called a quantum well, a sort of sandwich made of slices of different materials, each the width of few atomic layers. This confines the electrons between the two "bread" slices. The interest of these artificial atoms is that, on one side, we can modify the shape of the electronic orbitals by properly engineering the form and the size of the well and, on the other side, as many electrons are present inside the same well, interaction between them is much stronger than in atomic systems. Using artificial atoms in presence of strong electric fields, it is thus possible to harness new transitions to realize cheap, efficient and tunable terahertz sources.The interaction of light and matter at the quantum level, the domain in which I worked during most of my career, is a fascinating field. Not only has it helped to revolutionize our understanding of the world, as the effort to explain absorption and emission spectra was one of the driving forces that led to the development of quantum theory, but it has deeply modified our everyday life. Lasers and optoelectronic technologies are today everywhere: in our computers, in our medical devices, in our communication infrastructure. This present project aims to both deepen our understanding of the fundamental physics behind optical transitions in artificial atoms and pave the way to a new generation of terahertz emitters that in a few decades could find their ways to a number of life-saving applications, from airport scanners to medical imaging.