Reduction of Droop for Antimonide-based Mid-Infrared Lasers

自第一台激光器问世以来的60年里，半导体激光器的效率已经有了很大的提高，包括集成了许多级，其中光是在重复的步骤而不是在单个界面上产生的，通过级联配置增加了输出功率。即使有了这些进步，这些设备也没有在理论上的理想状态下运行。该项目的目标是确定导致电流不被转换为激光的损耗机制，并将这些知识应用于改进未来级联半导体激光器的设计。这将增加输出功率和效率，特别是对于在光谱的中红外部分中发射的设备，该波长范围对于环境监测，医学和国土安全中的应用很重要，包括化学传感和红外对抗。该方法不仅包括电流注入，光激发和光谱学，而且还将二维石墨烯片与包含周期表第三和第五列元素的三维半导体合金集成，以创建同时光学透明且导电和导热的顶部接触。提高效率有可能大大降低输入功率要求和运营成本，同时提高便携性。这项工作中提出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职业，并将通过让女性和少数民族学生接触本科生和研究生水平的前沿研究，为留住他们做出贡献。在锑基异质结构中采用II型带对准的带间级联激光器最近在3-6微米波长，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，存在效率的"下降"，这降低了在连续波（cw）或单模操作中可获得的最大功率。高温下输出功率受限的物理原因尚不清楚。确定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以上的最大输出功率。这将提高器件的效率，使得高于阈值的注入载流子将有助于激光输出，而不会因自发发射或非辐射复合机制而损失，因此大大降低了输入功率要求和操作成本。接触将被优化以收集自发发射，并且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，所有这些都通过分裂脊制造和/或透明石墨烯接触的集成来增强。石墨烯的高光学、电学和热传导性不仅有助于数据收集，而且将通过石墨烯-半导体界面的研究更广泛地推进光电器件的开发。该项目的结果将提供一个新的理解所需的机制，以实现钉扎的载流子密度，并最终提高这些激光器的高温输出功率。这一贡献是重要的，因为它将使激光器件的有源区和包层区的重新设计，以增加最大连续波输出功率，并导致更有效的高温操作。因此，锑化物基半导体激光器将有可能成为国土安全、环境监测和医疗应用中的中红外应用技术，如用于儿童哮喘早期检测的呼吸分析。该奖项反映了NSF的法定使命，并被认为值得通过使用基金会的智力价值和更广泛的影响审查标准进行评估来支持。

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.