Mode-locked THz QC-VECSELs

这项研究解决了制造太赫兹半导体激光光源的挑战，该光源发射频率在2至5太赫兹(即波长在60至150微米)之间的电磁波。太赫兹频率范围是电磁频谱中相对不发达的部分，它位于红外和微波之间。在过去的研究中，该团队展示了一种新的激光架构，该架构解决了如何在高光束质量的情况下同时产生高功率THz光的挑战，称为THz量子级联垂直外腔表面发射激光器。这项研究的目标是如何使这种激光器结构适应于创建所谓的锁模太赫兹激光器。在这样的激光器中，激光器不是发射稳定的连续光，而是以规则的间隔发射一系列极短的高强度光脉冲。虽然锁模激光器在可见光和红外波段普遍存在，但事实证明，在太赫兹激光器上很难实现锁模激光器。工作重点将集中在几种方法上，以适应垂直外腔表面发射激光方法，包括诱骗激光同时发射多种波长，控制所有这些不同波长在激光腔内的传播速度，以及在激光腔中引入快速的“可饱和吸收”开关以鼓励这些不同波长同步传播，以便它们都建设性地干涉以产生短脉冲。如果成功，这项研究将产生一种新的太赫兹源，用于天体物理、大气科学、生物和医学、安全检查、非法材料探测、燃烧科学、古董、废物分类、下一代无线通信和无损评估等领域。作为该项目的一部分，这项研究将培训研究生和本科生，并将通过参加一个有针对性的研究项目课程，支持招募和保留代表不足的少数族裔到工程学。技术说明这项建议的研究目标是开发基于亚表面垂直外腔表面发射激光器(VECSEL)概念的主动和被动锁模太赫兹量子级联激光器，发射皮秒脉冲。将研究两种主要方案：主动锁模和被动/混合锁模，在主动锁模中，通过在往返频率处注入射频(RF)电注入来调制增益亚表面；在被动/混合锁模中，使用快速可饱和吸收体和可选的射频损耗调制。VECSEL的使能部件是由加载了激光增益材料的亚波长天线耦合微腔组成的Reflectarray亚表面；这创造了一个有源放大镜，在开放的腔中充当一个反射镜。这项工作的学术价值在于使用亚表面VECSEL结构来研究锁模太赫兹激光器。这样的结构使得可以设计亚表面来实现宽带增益和色散补偿；此外，外腔允许功率合成以获得高功率输出，同时控制腔的重复频率。此外，外腔将允许集成快速可饱和吸收体，它可以单独优化以具有快速恢复时间，而量子级联有源材料则经过优化以具有长增益恢复时间。在几个层面上解决了更广泛的影响，包括本科生和研究生的研究经验、成果的传播、技术进步以及对代表性不足的少数群体的推广。这一奖项反映了NSF的法定使命，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，被认为是值得支持的。

This research addresses the challenge of making terahertz semiconductor laser sources that emit electromagnetic waves with frequencies between 2 and 5 THz (i.e. wavelengths between 60 and 150 microns). The terahertz frequency range is relatively underdeveloped part of the electromagnetic spectrum, which resides between the infrared and microwave. In past research, this team demonstrated a new laser architecture that addressed the challenge of how to generate high power THz light simultaneously with high beam quality, known as the THz quantum-cascade vertical-external-cavity surface-emitting-laser. The goal of this research is how to adapt this laser architecture to create a so-called “mode-locked” terahertz laser. In such a laser, instead of emitting steady continuous light, the laser would emit a series of extremely short high-intensity light pulses at regular intervals. While mode-locked lasers are ubiquitous in the visible and infrared wavelengths, they have proven very difficult to implement for terahertz lasers. Work will focus on several approaches to adapt the vertical-external-cavity surface-emitting-laser approach, including coaxing the laser to emit many wavelengths at once, controlling the speed of propagation of all of these various wavelengths within the laser cavity, and introducing a fast “saturable absorber” switch into the laser cavity to encourage these various wavelengths to travel in lock-step so that they all constructively interfere to create a short pulse. If successful, this research would result in a new terahertz source for applications in the fields of astrophysics, atmospheric science, biological and medical sciences, security screening, illicit material detection, combustion science, antiquities, waste-sorting, next-generation wireless communications, and non-destructive evaluation. As a part of the project, the research will train graduate and undergraduate students, and will support recruitment and retention of underrepresented minorities to engineering through participation in a targeted research project course.Technical descriptionThe research goal of this proposal is the development of active and passive mode-locked terahertz quantum-cascade lasers that emit picosecond pulses based upon the metasurface vertical-external-cavity surface-emitting-laser (VECSEL) concept. Two primary schemes will be investigated: active mode-locking in which the gain metasurface is modulated via radiofrequency (RF) electrical injection at the round trip frequency, and passive/hybrid mode-locking where a fast saturable absorber is used with optional RF loss modulation. The enabling component of the VECSEL is a reflectarray metasurface made up of sub-wavelength antenna-coupled microcavities loaded with laser gain material; this creates an active amplifying mirror which serves as one mirror in an open cavity. The intellectual merit in the proposed work lies in the use of the metasurface VECSEL architecture to investigate mode-locked terahertz lasers. Such a configuration gives the possible of engineering the metasurface to achieve broadband gain and dispersion compensation; furthermore the external cavity allows power combining for high-power output along with control of the cavity repetition rate. Additionally, the external cavity will allow integration of a fast saturable absorber, which can be separately optimized to have a fast recovery time while the quantum-cascade active material is optimized to have a long gain recovery time. The broader impacts are addressed at several levels including undergraduate and graduate research experiences, dissemination of results, technology advancement, and outreach to underrepresented minorities. Outreach will specifically occur through development of research projects for a course designed for the recruitment and retention of underrepresented minority first-year engineering students.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.