Diffraction-grating coupled surface emitting Terahertz quantum cascade laser source for high power, room temperature continuous wave operation

摘要基于titchip的高功率表面发射太赫兹光源摘要非技术：在~1 ~ 5太赫兹（THz）频率范围内的太赫兹（THz）辐射具有独特的特性，在成像、传感、光谱学、通信和医疗诊断等方面具有广泛的应用。这些应用中的大多数需要足够的太赫兹光功率密度，以便光可以穿过材料并能够被检测到以进行分析。在过去的几十年里，这推动了对产生高功率太赫兹光的激烈研究。最近演示的基于中红外量子级联激光器（QCL）内部非线性产生的太赫兹光源被证明是唯一在室温下在1-5太赫兹范围内提供毫瓦级功率的室温半导体光源。这种类型的源的主要挑战是产生高达几十毫瓦的高太赫兹功率，并且具有良好的光束质量，这是大多数实际应用所需要的。提出了一种通过从整个腔体中提取太赫兹光来开发高功率输出的芯片太赫兹源的新方法。这种易于使用和紧凑的光源将是一种使能技术，可以为更广泛的科学界轻松访问太赫兹光谱/成像。技术：提出的研究目标是演示一个室温、单片太赫兹源，在连续波工作时具有亚毫瓦级输出功率，在脉冲模式工作时具有数十毫瓦级输出功率。在之前的演示中，基于中红外QCL内部非线性产生的边缘发射太赫兹源由于整个腔的有限的离耦孔径大小而具有有限的离耦效率（~6-10%）。本项目的方法是使用基于半绝缘InP衬底中定义的太赫兹衍射光栅的表面发射方案，以实现高功率和高效的太赫兹解耦。与传统分布反馈光栅（DFB）通过提供光反馈与引导激光模式相互作用不同，本文提出的太赫兹衍射光栅只是将入射切伦科夫发射锥衍射到离散方向上。通过优化光栅结构，可以实现整个腔的表面发射，衍射效率高达90%。这将大大提高太赫兹解耦效率和功率。与体积庞大、价格昂贵的基于太赫兹光源的气体激光器相比，所提出的太赫兹QCL源提供了一种单片解决方案，一旦开发出来，就有可能利用现有的半导体激光器制造基础设施进行经济高效的大规模生产。这个项目是利用多学科方法来规避现有技术限制的一个很好的例子。固态物理、材料科学、非线性光学和激光物理都是研究计划的主要组成部分，它们都得到了NSF的普遍支持，在这种情况下，它们一起被用来将多个功能元素组合成一个单一的、紧凑的、高功率的器件。

Abstract TitleChip-based, High-Power Surface Emitting Terahertz SourceAbstractNon-technical:The terahertz (THz) radiation in the frequency range of ~1-5 THz has mainly unique properties and applications in imaging, sensing, spectroscopy, communication, and medical diagnosis. Most of these applications requires a sufficient THz light power density so that the light can pass through materials and able to be detected for analysis. This has fueled intense studies on the generation of high power THz light in the past decades. The recent demonstrated THz source based on nonlinear generation inside a mid-IR quantum cascade laser (QCL) is proved to be the only room temperature semiconductor light source that delivers mW-level power at room temperature in the 1-5 THz range. The main challenge for this type of sources is to generate higher THz power up to tens of milliwatts with good beam quality which is required by most of the real-world application. A novel approach is proposed to develop a chip-based THz source with high power output by extracting THz light from the entire cavity. This simple to use and compact source will be an enabling technology which will allow easy access to THz spectroscopy/ imaging for the broader scientific community.Technical:The objective of the proposed research is to demonstrate a room temperature, monolithic THz source with sub-milliwatt level output power in continuous wave operation and tens of milliwatt level in pulsed mode operation. In the previous demonstrations, edge-emitting THz sources based on nonlinear generation inside a mid-IR QCL have limited outcoupling efficiency (~6-10%) due to the limited outcoupling aperture size regarding to the entire cavity. The approach in this proposed project is to use a surface-emission scheme based on THz diffraction grating defined in the semi-insulating InP substrate of a epi-down bonded THz QCL source for high-power and efficient THz outcoupling. Unlike the traditional distributed-feedback (DFB) grating which interacts with the guiding laser modes by providing the optical feedback, the proposed THz diffraction grating simply diffract the incident Cerenkov emission cone into the discrete directions. With an optimized grating structure, surface emission from the entire cavity with diffraction efficiency up to 90% is achievable. This will drastically increase the THz outcoupling efficiency and power. In contrast to a THz sources based gas lasers, which is bulky and expensive, the proposed THz QCL source offers a monolithic solution that, once developed, has potential for cost-effective mass production using the existing semiconductor laser fabrication infrastructure. This project is an excellent example of a multidisciplinary approach used to circumvent existing technological limitations. Solid state physics, material science, nonlinear optics, and laser physics are all major components of the research plan, which are all supported generally by NSF, and are used, in this case, together to combine multiple functional elements into a single, compact, high power device.