Soliton Formation through Self-Induced Transparency in Semiconductor Microcavities.

光脉冲通过光学材料显示出许多有趣和有用的效果，特别是当脉冲非常明亮和持续时间非常短时。通常情况下，由于绕射和色散，脉冲会在空间和时间上传播。然而，当脉冲非常明亮时，非线性效应可以准确地抵消这种扩散，并且光脉冲的传播不会改变任何形状：孤子或光弹。当光被限制在一个较小的腔中时，更容易形成稳定的孤子，而腔孤子现在引起了人们的极大兴趣。它们可以作为存储和处理数据用于光学存储或光学计算的一种有用的方式。另一种类型的效应(相干传播)是当脉冲的持续时间与材料的寿命(其性质可以改变的时间尺度)相比非常短时出现的。最引人注目的例子之一是自感透明(SIT)，即正常情况下吸收光的材料对明亮的短持续时间光脉冲变得透明。这就允许了另一种类型的光弹或“静坐孤子”。在过去的几年里，利用近似理论预测了腔-坐孤子的有趣性质，并可能应用于产生超短脉冲序列(孤子锁模)。然而，它们的存在还没有得到实验证明，也没有更准确的理论来证实。最近，我们在理查德·费曼的电磁场原子模型的基础上，发展了一种新的、精确的非线性相干脉冲传输理论。该理论预测了可以在实验上探测到的腔坐孤子的特征。这是赠款申请的主要重点。更广泛地说，这一新理论将是探索极端非线性体系中新的物理效应的有用工具。半导体，如砷化镓，与光相互作用强烈，可以形成高质量的光腔。然而，它的寿命很短，一些理论家认为相干传播效应将是微弱的。然而，已经在实验上观察到了自诱导透明效应。我们将使用不同的理论方法，结合实验测量，来理解这种差异，并建立半导体SIT的条件。实验工作的重点将是演示半导体腔中的坐腔孤子。将使用几种方法，包括测量脉冲传播和光散射来检测光学栅格的形成。这些研究将利用基于钛掺杂蓝宝石的新型超短脉冲激光放大器所提供的明亮的超短光脉冲。如果半导体晶体非常小，它们将形成具有长材料寿命的量子点(QD)，是产生相干传播效应的良好起点。2002年从理论上描述了半导体量子点的自诱导透明性，并在2003年进行了实验验证：要理解这个热门话题，还有很多工作要做。我们将把我们的理论扩展到处理量子点，并对相干传播进行实验测量。半导体晶体将在谢菲尔德的国家中心生长，腔将在谢菲尔德和萨里的微细加工中产生。我们在法国的合作者是相干非线性实验方面的专家，我们在美国的合作者是电磁场中原子精确处理的先驱。该项目利用萨里高级技术研究所最近安装的先进激光和超级计算机，正是该研究所成立时要做的项目类型：利用先进的实验和理论技术研究新的基本现象，并将其应用于信息技术和通信。

The passage of light pulses through an optical material shows many interesting and useful effects, especially when the pulse is very bright and very short in duration. Normally, the pulse will spread out in space and time as a result of diffraction and dispersion. However when the pulse is very bright, nonlinear effects can exactly cancel this spreading, and the light pulse propagates without any change in shape: a 'soliton' or 'light bullet'. It is easier to form stable solitons when the light is confined to a small cavity, and 'cavity solitons' are now attracting a lot of interest. They could be useful as a way of storing and manipulating data for optical storage or optical computing.Another type of effect (coherent propagation) is seen when the pulse is very short in duration compared to the material's lifetime (the timescale on which its properties can change). One of the most striking examples is self-induced transparency (SIT), in which a material which normally absorbs light becomes transparent to a bright, short-duration light pulse. This allows another class of light bullet or 'SIT soliton'. During the past few years, interesting properties of cavity-SIT solitons have been predicted using approximate theories, with possible applications in the generation of ultrashort pulse trains (soliton mode-locking). However, their existence has not yet been demonstrated experimentally nor confirmed by a more accurate theory. We have recently developed a new, accurate, theory of nonlinear coherent pulse propagation, based on Richard Feynman's model of atoms in an electromagnetic field. The theory predicts signatures of cavity-SIT solitons which can be detected experimentally. This is the main focus of the grant application. More generally, the new theory will be a useful tool for exploring new physical effects in the extreme nonlinear regime.Semiconductors such as gallium arsenide interact strongly with light, and can form high-quality optical cavities. However the lifetime is very short, and some theorists have stated that coherent propagation effects will be weak. Nevertheless, self-induced transparency effects have been observed experimentally. Our different theoretical method, together with experimental measurements, will be used to understand this discrepancy and to establish the conditions for SIT in semiconductors. The emphasis of the experimental work will be the demonstration of cavity-SIT solitons in semiconductor cavities. Several approaches will be used, including measurements of pulse propagation and light scattering to detect formation of optical gratings. These investigations will exploit the bright, ultrashort light pulses available from the new breed of ultrashort pulse laser amplifiers based on titanium-doped sapphire.If semiconductor crytals are made very small, they form 'quantum dots' (QDs) which have long material lifetimes and are a good starting point for coherent propagation effects. Self-induced transparency in semiconductor QDs was described theoretically in 2002 and demonstrated experimentally in 2003: much work remains to be done to understand this 'hot topic'. We will extend our theory to treat quantum dots, as well as make experimental measurements of coherent propagation.The semiconductor crystals will be grown at the National Centre at Sheffield, and the cavities will be produced by micro-fabrication at both Sheffield and Surrey. Our collaborator in France is an expert in coherent nonlinear experiments, and our US collaborator is the pioneer of exact treatment of atoms in electromagnetic fields. The project makes use of advanced lasers and supercomputers recently installed at the Advanced Technology Institute at Surrey, and is exactly the type of project which the Institute was set up to do: investigating new fundamental phenomena using advanced experimental and theoretical techniques, with applications in information technology and communications.