Non-linear terahertz spectroscopy of quantum materials

太赫兹(THz)频率范围介于发达的电子学和光学区之间，迄今尚未得到充分利用，特别是在非线性现象、相干控制和非平衡系统等领域。这些领域在可见光和微波区域提供了如此多令人着迷的物理学，但在太赫兹区域几乎没有人触及过。相比之下，有大量的激发系统和材料在这个能量范围内具有基本跃迁或共振：固体原子、低维电子系统、超导体中的约瑟夫森等离子体波、晶体材料和生物分子中的振动模式。虽然近年来利用线性THz光谱来研究这些材料的光谱吸收越来越有兴趣，但更令人兴奋的前景是利用THz辐射的强脉冲来相干控制量子系统的状态。迄今为止，对THz非线性现象的开发还很少，这主要是由于设计和制造高性能THz源和探测器的困难。近年来，人们进行了大量的研究工作来消除所谓的“太赫兹缺口”，现在有几种产生和检测太赫兹辐射的方案，其中最成功的可以说是太赫兹时间域光谱学(TDS)。这些系统被广泛用于光谱学，并已在各种系统中得到应用，包括研究半导体中的电子动力学；确定分子晶体中的振动模式；爆炸物检测；生物分子识别；生物医学成像；以及超材料研究。然而，太赫兹地区在非线性光谱和光学控制领域仍然落后，在这些领域，光与物质的相互作用超出了弱吸收，电磁辐射被用来扰动样品。为了控制和准确地做到这一点，需要太赫兹辐射的强窄带脉冲。目前，提供这种类型辐射的唯一可用的太赫兹源是自由电子激光器(FEL)，它既很强，而且可以在从太赫兹到紫外线的非常宽的频率范围内进行调谐。然而，这些系统的主要缺点是它们的大小、成本和支持它们所需的基础设施。尽管如此，FEL在许多科学研究领域都有很高的需求，如超导研究、石墨烯科学、物理化学和生物学。此外，对自由电子激光或合适的替代品的需求可能会增长，因为有人提议将强烈的太赫兹辐射用于控制酶催化的反应和生物分子的非线性光谱。该项目将开发高功率、高精度的太赫兹源，以询问一系列活跃在太赫兹范围内的量子材料。该项目将利用EPSRC资助的以电子和电气工程为基础的新的放大激光设施，以产生带宽适合所研究材料的高场太赫兹脉冲。目标和目标：从具有一定带宽范围的放大激光脉冲中产生高场和高峰值功率的太赫兹脉冲。一些材料需要100 GHz以下的带宽，这需要新的技术。在太赫兹范围内执行窄跃迁量子材料的线性光谱。这些材料包括半导体中的杂质等固体原子材料，以及量子点和量子井等电子受限半导体纳米结构。利用高场太赫兹脉冲对上述材料进行非线性光谱和相干操纵。

The terahertz (THz) frequency range, lying between the well-developed electronics and optical regions, has been under-exploited to date, particularly in the areas of nonlinear phenomena, coherent control and non-equilibrium systems. These fields, which have provided so much fascinating physics in the visible and microwave regimes, have barely been touched upon in the THz region. By contrast, there are a wealth of exciting systems and materials that possess fundamental transitions or resonances at this energy range: atoms-in-solids, low-dimensional electron systems, Josephson plasma waves in superconductors, vibrational modes in crystalline materials and bio-molecules. While there has been increasing interest in recent years of using linear THz spectroscopy to study the spectral absorption of these materials, the more exciting prospect is making use of intense pulses of THz radiation to coherently control the state of a quantum system The lack of exploitation to-date of THz nonlinear phenomena principally arises from the difficulty in designing and fabricating high-performance THz sources and detectors. In recent years there has been an intense research effort to close the so-called 'THz gap' and there are now several schemes for the generation and detection of THz radiation, arguably the most successful of which has been THz time-domain spectroscopy (TDS). These systems are widely used for spectroscopy and have found application in a diverse range of systems, including investigations of electron dynamics in semiconductors; determining vibrational modes in molecular crystals; explosives detection; biomolecule identification; biomedical imaging; and metamaterial studies.The THz region, however, still lags in the areas of non-linear spectroscopy and optical control, where the light-matter interaction goes beyond weak absorption and electromagnetic radiation is used to perturb the sample. To do this controllably and accurately, intense narrowband pulses of THz radiation are required. Currently, the only available THz source for providing this type of radiation is the free-electron laser (FEL) which are both intense and can be tuned over a very wide frequency range, from THz to UV. The major drawback of these systems, however, is their large size, cost, and the infrastructure required to support them. Despite this, FELs are in high demand in many areas of scientific research, such as studies of superconductivity, graphene science, physical chemistry and biology. In addition, the demand for FELs, or a suitable alternative, is likely to grow as there are proposals for intense THz radiation to be used for control of enzyme catalysed reactions and for non-linear spectroscopy of biological molecules.The project will develop high-power, high-precision terahertz sources to interrogate a range of quantum materials that are active in the terahertz range. The project will make use of the new EPSRC-funded amplified laser facility based in the electronic and electrical engineering, to generate high-field terahertz pulses with bandwidths tailored to the material under investigation. Aims and objectives:Generation of high-field and high-peak-power terahertz pulses from amplified laser pulses with a range of bandwidths. Bandwidths below 100 GHz will be required for some materials and novel techniques will be required for these.Perform linear spectroscopy of quantum materials with narrow transitions in the terahertz range. These include atom-in-solid materials such as impurities in semiconductors, and electronic confined semiconductor nanostructures such as quantum dots and quantum wells.Non-linear spectroscopy and coherent manipulation of the above materials using high-field terahertz pulses.