Reduction of Droop for Antimonide-based Mid-Infrared Lasers

自第一台激光器问世以来的60年里，在提高半导体激光器的效率方面取得了长足的进步，包括集成了多个阶段，其中光是以重复的步骤产生，而不是在单一的界面上产生，通过级联结构使输出功率成倍增加。即使有了这些进步，这些设备还没有按照它们的理论理想运行。该项目的目标是确定导致电流不能转化为激光的损耗机制，并将这一知识应用于改进未来级联半导体激光器的设计。这将提高输出功率和效率，特别是对于在光谱的中红外部分发射的设备，这是一个对环境监测、医学和国土安全应用非常重要的波长范围，包括化学传感和红外对抗。该方法不仅包括电流注入、光学激发和光谱分析，还包括将二维石墨烯薄片与包含元素周期表第三和第五列元素的三维半导体合金相集成，以创建同时具有光学透明和电和热传导的顶部接触。提高效率有可能大幅降低输入功率要求和运营成本，同时提高便携性。这项工作中建议的课堂和实验室活动，包括制作视频和引人入胜的课程，将吸引K-12学生从事电气和计算机工程方面的职业，并将通过使女性和少数族裔学生接触到本科和研究生水平的尖端研究来帮助留住他们。在基于锑的异质结构中使用第二类能带排列的带间级联激光器最近在3-6微米波长取得了成功，这是化学传感、红外对抗和自由空间光通信等应用的重要光谱范围。然而，当这些激光器被驱动到阈值以上时，效率会“下降”，从而降低了在连续波(CW)或单模操作中可获得的最大功率。高温下输出功率受到限制的物理原因尚不清楚。识别防止载流子钉扎的基本机制将允许创建新的波函数工程方法，以增加这些激光器在室温或高于室温时的最大输出功率。这将提高器件的效率，从而使阈值以上的注入载流子将有助于激光输出，而不会因自发辐射或非辐射复合机制而丢失，从而显著降低输入功率需求和运营成本。触点将被优化以收集自发辐射，限制行为的物理机制将通过光-电流、光-光、光谱、电流-电压和泵浦-探测测量来量化，所有这些都通过分离脊形制造和/或透明石墨烯触点的集成而得到增强。石墨烯的高光学、电学和热导率不仅有助于数据的收集，而且将通过对石墨烯-半导体界面的研究推动更广泛的光电子器件的发展。该项目的结果将使人们对实现载流子密度钉扎并最终增加这些激光器的高温输出功率所需的机制有一个新的理解。这一贡献是重要的，因为它将使激光器件的有源区和包覆区重新设计，以增加最大连续输出功率，并导致更有效的高温操作。因此，基于锑的半导体激光器将有可能成为一种使能技术，用于国土安全、环境监测和医疗应用中的中红外应用，例如用于早期发现儿童哮喘的呼吸分析。外展活动包括为K-12学生和教师以及新生工程师开发视频和课程。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

In the past 60 years since the demonstration of the first laser, great strides have been made in improving the efficiency of semiconductor lasers, including the integration of numerous stages where light is generated at repeated steps rather than at a single interface, multiplying the output power possible through a cascade configuration. Even with these advances, these devices do not yet operate at their theoretical ideal. The goal of this project is to identify loss mechanisms that result in current not being converted to laser light and to apply that knowledge to improving the design of future cascade semiconductor lasers. This would increase output power and efficiency, particularly for devices emitting in the mid-infrared portion of the spectrum, a range of wavelengths important for applications in environmental monitoring, medicine, and homeland security, including chemical sensing and infrared countermeasures. The approach will include not only current injection, optical excitation, and spectroscopy, but also the integration of two-dimensional sheets of graphene with three-dimensional semiconductor alloys containing elements from the third and fifth columns of the periodic table to create a top contact that is simultaneously optically transparent and both electrically and thermally conductive. Improving efficiency has the potential to substantially reduce input power requirements and operating costs while increasing portability. Classroom and laboratory activities proposed in this work, including creation of videos and engaging lessons, will attract K-12 students to careers in electrical and computer engineering and will contribute to retention of female and minority students by exposing them to cutting edge research at the undergraduate and graduate level.Interband cascade lasers employing type-II band alignment in antimonide-based heterostructures have demonstrated recent success at 3-6 micrometer wavelengths, an important spectral range for applications such as chemical sensing, infrared countermeasures, and free-space optical communications. However, there is a “droop” in efficiency when these lasers are driven above threshold that reduces the maximum power obtainable in continuous wave (cw) or single mode operation. The physical cause for the limitations on output power at high temperatures is not understood. Identifying the fundamental mechanisms that prevent carrier pinning will permit creation of new wave function engineering approaches to increase the maximum output power of these lasers at or above room temperature. This will increase the efficiency of devices so that injected carriers above threshold will contribute to laser output and not be lost to spontaneous emission or non-radiative recombination mechanisms, and consequently substantially reduce input power requirements and operating costs. Contacts will be optimized to collect spontaneous emission, and the physical mechanisms of the limiting behavior will be quantified through light-current, light-light, spectral, current-voltage, and pump-probe measurements, all enhanced by the integration of split-ridge fabrication and/or transparent graphene contacts. The high optical, electrical, and thermal conductivity of graphene will not only aid in the collection of data but will advance optoelectronic device development more broadly through the study of the graphene-semiconductor interface. The results of this project will provide a new understanding of the mechanism required to achieve pinning of the carrier densities and ultimately to increase the high-temperature output power in these lasers. This contribution is significant because it will enable the redesign of the active and cladding regions of laser devices in order to increase the maximum cw output power and result in more efficient high temperature operation. Thus, antimonide-based semiconductor lasers would have the potential to be an enabling technology for mid-infrared applications in homeland security, environmental monitoring, and medical applications such as breath analysis for early detection of asthma in children. Outreach activities include video and curriculum development for K-12 students and teachers as well as for freshman engineers.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.