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

摘要标题基于芯片的大功率表面发射太赫兹源摘要非技术性：太赫兹（THz）辐射在~1-5 THz的频率范围内具有独特的性质，主要应用于成像，传感，光谱，通信和医疗诊断。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以进行分析。在过去的几十年里，这推动了对高功率THz光产生的深入研究。最近展示的基于中红外量子级联激光器（QCL）内部非线性产生的THz源被证明是唯一的室温半导体光源，其在室温下在1-5 THz范围内提供mW级功率。这种类型的源的主要挑战是产生高达数十毫瓦的更高的THz功率，同时具有大多数实际应用所需的良好光束质量。提出了一种从整个谐振腔中提取太赫兹光的方法来研制高功率输出的芯片型太赫兹光源。这种简单易用和紧凑的源将是一种使技术，这将使更广泛的科学界容易获得太赫兹光谱/成像。技术：拟议的研究的目标是展示一个室温，单片太赫兹源，连续波操作的亚毫瓦级输出功率和脉冲模式操作的几十毫瓦级。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体的有限外耦合孔径尺寸而具有有限的外耦合效率（~6-10%）。该方案采用基于太赫兹衍射光栅的表面发射技术，在半绝缘InP衬底上实现了大功率、高效率的太赫兹QCL源。与传统的分布反馈（DFB）光栅通过提供光反馈与引导激光模式相互作用不同，所提出的太赫兹衍射光栅简单地将入射的切伦科夫发射锥反射到离散方向。通过优化光栅结构，可以实现整个腔体的面发射，衍射效率高达90%。这将极大地提高THz外耦合效率和功率。与体积庞大且昂贵的基于THz源的气体激光器相比，所提出的THz QCL源提供了单片解决方案，一旦开发出来，就有可能使用现有的半导体激光器制造基础设施进行成本效益高的大规模生产。这一项目是一个很好的例子，说明采用多学科方法来规避现有的技术限制。固态物理学、材料科学、非线性光学和激光物理学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，它们一起用于将多个功能元件联合收割机组合成一个单一的、紧凑的、高功率的设备。

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.