Aperiodic Lattices for Photonic Engineering of Terahertz Quantum Cascade Lasers

光学波长尺度的光子晶格(即由空间变化的折射率形成的晶格)提供了一种非常强大的机制来定义和修改一系列光学器件中的光子共振特性。尽管到目前为止这种方法非常成功，但它只基于周期晶格(只有一个基本的空间频率，并且只与一种颜色的光相关联)，因此不能控制受限光子的颜色。在这个方案中，我们描述了使用非周期晶格(ALS)来提供显著先进的光谱功能，例如，仅举一个例子，在用户定义的频率下的多色激光。在这个方案中，我们的目标是将AL概念应用于太赫兹(THz)量子级联激光器(QCL)，以实现可调谐。这一建议是非常及时的，因为它利用了THz QCL制造技术的最新重要进展，并将这些技术与对非周期结构特性的新理论见解相结合。特别是，这一提议提出了第一个将AL集成到其光电子结构中的有源光电子学器件。近年来，THz技术(频率：1-10 THz，波长：30-300微米)因其在广泛的商业和安全应用中的重要影响而备受关注，例如在医学、微电子、安全成像、生物毒素检测、农业、气体传感和环境监测以及法医学等领域。THz QCL的发展已经成为这些技术领域蓬勃发展的关键发展。然而，缺乏太赫兹QCL的可调性也是一个主要的限制因素。因此，由这一提议产生的紧凑、相干、可调的THzQCL的展示将成为THz光子学发展的重要推动因素。THzQCL采用了复杂的技术来控制电子的传播，有源区由一种半导体材料的几个原子厚度的重复超晶格组成，并与另一种材料的类似薄的势垒层交错。在这些半导体纳米结构中，能带分为子带和微带，能量间隔在几十到几百毫电子伏特之间，这决定了电子传输，也使新的光学跃迁成为可能。当在材料上施加偏置电压时，这种子带间跃迁的周期性级联就建立起来了。然后通过电注入实现激光所需的布居反转。通过调整量子阱和势垒的特定顺序来形成电子AL，可以随意定制THz QCL的电子和光学特性。另一方面，光子AL以用户定义的方式提供任意的滤光片响应，例如，在单个或多个波长提供高透射率和高分辨率输出。与传统方法相比，光子AL提供的新颖过滤功能与电子AL提供的增益谱一起提供了对单个或多个激光波长的更大控制。因此，在单个THzQCL器件中集成电子和光子ALS的组合方法是该提议的另一个重要方面。这种集成使光子与宿主材料的相互作用时间大大增加，并产生了显著的光电非线性和量子效应。

Optical wavelength-scale photonic lattices (i.e. lattices formed from a spatially-varying refractive index) offer a very powerful mechanism to define and modify the photon resonance characteristics in a range of optical devices. Although highly successful up to now this approach has only been based on periodic lattices (with only a single, underlying spatial frequency, and associated with only a single colour of light) and so does not provide the ability to control the colour of the confined photons. In this proposal, we describe the use of aperiodic lattices (ALs) to provide significantly advanced spectral functionalities, e.g. to give just one example, multi-coloured lasing at user-defined frequencies. In this proposal, we aim to apply AL concepts to a terahertz (THz) quantum cascade laser (QCL) so as to enable tunability. This proposal is highly timely, since it takes advantage of recent important advances in THz QCL fabrication technologies and combines them with new theoretical insights into the properties of aperiodic structures. In particular, this proposal proposes the first ever active photonics device exhibiting an AL integrated into its opto-electronic structure.Recent years has seen THz technology (frequency: 1-10 THz, wavelength: 30-300 micron) the focus of much attention owing to its important impact in a wide range of commercial and security applications, for example, in medicine, microelectronics, security imaging, biotoxin detection, agriculture, gas sensing and environmental monitoring, and forensic science, etc. The development of the THz QCL has been a key development in the burgeoning of these technology areas. However, lack of THz QCL tunability has also acted as a major constraint. Hence, the demonstration of a compact, coherent, tunable THz QCL arising from this proposal will act as a significant enabler in the advance of THz photonics.The THz QCL employs sophisticated techniques for the control of electron propagation, with an active region comprising a repeated superlattice of only a few atoms thick of one semiconductor material, interleaved with similarly thin barrier layers of another material. In these semiconductor nanostructures, the energy bands split into subbands and minibands, with energy separations of several tens to a few hundreds of millielectronvolts, which determine electronic transport and also enable new optical transitions. When a bias voltage is applied across the material, a periodic cascade of such intersubband transitions is established. The population inversion necessary for lasing is then achieved through electrical injection. Adjusting the specific sequence of quantum wells and barriers to form an electronic AL allows both the electronic and optical properties of the THz QCL to be tailored at will. A photonic AL, on the other hand, provides arbitrary filter responses in a user-defined way, for example, to provide high transmission and high-resolution output at single or multiple wavelengths. The novel filtering functionality available from the photonic AL in conjunction with the gain spectrum available from the electronic AL provides far greater control of single or multiple laser wavelengths than with conventional methods. A combined approach to integrate both electronic and photonic ALs within a single THz QCL device is therefore another important aspect of this proposal. Such integration gives photons a strongly enhanced interaction time with the host material, and creates significant opto-electronic nonlinear and quantum effects.