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

摘要基于芯片的高功率表面发射太赫兹源摘要非技术：太赫兹辐射在~1-5太赫兹的频率范围内主要具有独特的性质和在成像、传感、光谱、通信和医疗诊断方面的应用。这些应用中的大多数都需要足够的THz光功率密度，以便光可以穿过材料并能够被检测到进行分析。在过去的几十年里，这推动了对产生高功率太赫兹光的密集研究。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一一种在室温下在1-5太赫兹范围内提供毫瓦级功率的室温半导体光源。这种类型的光源的主要挑战是产生高达数十毫瓦的高太赫兹功率和良好的光束质量，这是大多数现实世界应用所要求的。提出了一种新的方法，通过从整个腔体中提取太赫兹光来开发基于芯片的高功率输出的太赫兹光源。这种简单易用且紧凑的光源将是一项使更广泛的科学界能够轻松获得太赫兹光谱/成像的技术。技术：拟议研究的目标是展示一种室温的单片太赫兹光源，在连续波操作中具有亚毫瓦级别的输出功率，在脉冲模式操作中具有数十毫瓦级别的输出功率。在前面的演示中，基于中红外准分子激光器内部非线性产生的边缘发射THz源，由于整个腔体的有限外耦合孔径尺寸，其外耦合效率有限(~6-10%)。提出的方案是在外延键合THzQCL源的半绝缘InP衬底上定义一种基于THz衍射栅的表面发射方案，以实现高功率和高效率的THz输出。与传统的分布反馈(DFB)光栅通过提供光反馈来与导模相互作用不同，所提出的THz衍射光栅只需将入射切伦科夫发射锥衍射到离散方向。通过优化的光栅结构，可以实现整个腔体的表面发射，衍射效率高达90%。这将极大地提高太赫兹的外耦合效率和功率。与体积庞大且价格昂贵的基于太赫兹光源的气体激光器相比，建议的太赫兹QCL光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具有成本效益的大规模生产。该项目是采用多学科方法绕过现有技术限制的一个很好的例子。固体物理、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，所有这些都得到了NSF的普遍支持，在这种情况下，这些都被用来将多种功能元件组合到一个单一、紧凑、高功率的设备中。

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.