Mode-locked THz QC-VECSELs

这项研究解决了制造太赫兹半导体激光源的挑战，该激光源发射频率在2到5 THz之间（即波长在60到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，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，解决了如何同时产生高功率THz光和高光束质量的挑战，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调整这种激光器架构，以创建所谓的“锁模”太赫兹激光器。在这种激光器中，激光器将以规则的间隔发射一系列极短的高强度光脉冲，而不是发射稳定的连续光。虽然锁模激光器在可见光和红外波长中无处不在，但它们已被证明非常难以实现太赫兹激光器。工作将集中在几种方法上，以适应垂直外腔表面发射激光器的方法，包括哄骗激光器一次发射许多波长，控制所有这些不同波长在激光腔内的传播速度，并在激光腔中引入快速“可饱和吸收体”开关，以促使这些不同的波长锁定行进，使它们都相长干涉以产生短脉冲。如果成功的话，这项研究将产生一种新的太赫兹源，用于天体物理学，大气科学，生物和医学科学，安全检查，非法材料检测，燃烧科学，古物，废物分类，下一代无线通信和非破坏性评估等领域。作为该项目的一部分，该研究将培训研究生和本科生，并将通过参加有针对性的研究项目课程，支持招募和保留代表性不足的少数民族从事工程工作。技术支持本提案的研究目标是开发主动和被动锁模太赫兹量子级联激光器，这些激光器基于超表面垂直外腔表面发射技术发射皮秒脉冲，激光器（VECSEL）概念。两个主要的计划将被调查：主动锁模，其中的增益超颖表面调制通过射频（RF）电注入在往返频率，和被动/混合锁模，其中使用快速饱和吸收器与可选的RF损耗调制。VECSEL的使能组件是由加载有激光增益材料的亚波长天线耦合微腔组成的反射阵列超颖表面;这创建了用作开放腔中的一个反射镜的有源放大镜。所提出的工作的智力价值在于使用超表面VECSEL架构来研究锁模太赫兹激光器。这样的配置给出了工程化超颖表面以实现宽带增益和色散补偿的可能性;此外，外部腔允许功率组合用于高功率输出，沿着腔重复率的控制。另外，外腔将允许快速可饱和吸收体的集成，其可以被单独优化以具有快速恢复时间，而量子级联活性材料被优化以具有长增益恢复时间。更广泛的影响在几个层面上得到解决，包括本科生和研究生的研究经验，结果的传播，技术进步，并推广到代表性不足的少数民族。该奖项反映了NSF的法定使命，并被认为是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的支持。

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.