Modelocking in terahertz-frequency quantum cascade lasers

太赫兹频率范围位于电磁波谱的微波和中红外区域之间，但由于难以制造方便的源和探测器，长期以来一直未被开发；太赫兹辐射的频率太高，不能由移动电话中使用的电子技术产生，但频率太低，不能由CD播放机激光等光学技术产生。然而，在过去的二十年里，由于创新的光源、探测器和成像系统的发展，该领域取得了显著的增长。由于这些发展，太赫兹仪器现在正应用于制药和汽车工业，以及半导体器件的高分辨率故障隔离和集成电路的3D成像。一个值得注意的发展是量子级联激光器，它包括由数千层半导体材料形成的一系列量子阱，每层都控制在原子层的厚度。这些设备现在被世界各地的研究小组用作高功率太赫兹波的紧凑来源。然而，科学界面临的一个挑战是开发能够产生超短（皮秒持续时间）太赫兹辐射脉冲的量子级联激光器。利用激光模型锁定在光谱的可见和红外区域实现了超短激光脉冲的产生，这导致了广泛的新的测量技术和技术，包括时间分辨测量、超高速通信、高精度计量和光谱学。这种在太赫兹范围内工作的激光器的发展将为计量学、相干成像和断层扫描、材料分析和分子光谱学开辟新的机会。该博士项目旨在研究高功率太赫兹脉冲如何与这些激光器中的量子阱相互作用，并利用这一认识开发一种可以产生超短太赫兹脉冲的模型锁定量子级联激光装置。

The terahertz frequency range sits between the microwave and mid-infrared regions of the electromagnetic spectrum, but has long resisted exploitation owing to difficulties in fabricating convenient sources and detectors; terahertz radiation is too high in frequency to be generated by the electronic techniques used in mobile telephones, but too low in frequency to be produced by the optical techniques exploited in, for example, CD player lasers. Nevertheless, the last twenty years have witnessed a remarkable growth in the field owing to the development of innovative sources, detectors, and imaging systems. Due to these developments terahertz instrumentation is now finding application in the pharmaceutical and automotive industries, and in high-resolution fault isolation in semiconductor devices and 3D imaging of integrated circuits.One notable development has been the quantum cascade laser, which comprises of a series of quantum wells formed by thousands of layers of semiconductor material, each controlled to atomic-layer thickness. These devices are now used by research groups around the world as a compact source of high-power terahertz waves. Yet one challenge to the scientific community has been to develop quantum cascade lasers that can generate ultrashort (picosecond-duration) pulses of terahertz radiation. The generation of ultrashort laser pulses has been achieved in the visible and infrared regions of the spectrum using laser modelocking, which has led to a wide range of new measurement techniques and technologies including time-resolved measurements, ultra-high speed communications, and high-precision metrology and spectroscopy. The development of such a laser operating in the terahertz range would open up new opportunities for metrology, coherent imaging and tomography, materials analysis and molecular spectroscopy.This PhD project aims to study how high-power terahertz pulses interact with the quantum wells in these lasers, and to use this understanding to develop a modelocked quantum cascade laser device that can generate ultrashort terahertz pulses.