Broadband terahertz metasurface lasers

这项研究解决了制造太赫兹半导体激光源的挑战，该源发射频率在2到5太赫兹之间（即波长在60到150微米之间）的电磁波。紧凑的太赫兹辐射源需要具有高输出功率和优异的光束质量，用于一系列光谱学和多光谱成像应用（即不同的太赫兹波长/颜色提供额外的图像对比度）。例如，天体物理学、大气科学、生物和医学、安全检查、非法材料探测和非破坏性评估等领域的应用都可以从太赫兹源中受益。太赫兹量子级联激光器是该光谱范围内最有前途的辐射源之一，但要使其更加实用，还必须克服几个障碍。在过去的研究中，该团队展示了一种新的激光架构，该架构解决了如何同时获得高功率和高光束质量的挑战，称为太赫兹量子级联垂直外腔表面发射激光器。下一个挑战是使这种结构在“宽带”配置下工作，即如何使激光器在不牺牲功率或光束质量的情况下在宽波长范围内发射。因此，这项研究的统一主题是开发一套方法，使太赫兹激光器能够发射(a)可以在宽范围内调谐的单一波长的光，或者(b)在宽范围内同时发射多个波长的光。这项工作主要集中在设计一个关键的使能组件：所谓的“有源超表面”，它可以被认为是一个人造镜子，不仅反射光，而且放大它。作为该项目的一部分，该研究将培训研究生和本科生，并将通过参与有针对性的研究项目课程，支持招募和留住代表性不足的少数族裔。本提案的研究目标是开发基于放大反射超表面的宽带太赫兹量子级联激光器，该激光器具有可扩展的高功率和优异的光束质量。在这个总体主题中，我们提出了两个主要的重点：(a)发展广泛可调谐的单模太赫兹外腔激光器，(b)发展宽带多模太赫兹激光器和频率梳。垂直外腔面发射激光器的使能元件是由载有激光增益材料的亚波长天线耦合微腔组成的反射超表面；这创造了一个主动放大镜，作为一个开放腔镜。在提出的工作的智力价值是目前在发展新的反射超表面，提供增益超过一个大分数带宽。第二个创新在于新的激光腔结构，这是由于超表面设计的灵活性而新实现的。这种方法的强大之处在于(a) VECSEL腔结构在衍射受限光束中支持可扩展的高功率的能力，以及(b)在空间和光谱域局部设计超表面的振幅、相位和极化响应的灵活性。对光谱学和多光谱成像的影响可能很大，因为许多先前的可调谐单模太赫兹量子级联激光器和频率梳的演示要么输出功率有限，要么光束模式差，要么两者兼有。更广泛的影响涉及几个层面，包括本科生和研究生的研究经验，成果的传播和技术进步。具体地说，外联活动将通过为一门课程制定研究项目来进行，该课程是为招募和保留代表性不足的少数民族大学工程新生而设计的。

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). Compact sources of terahertz radiation that operate with both high output power and excellent beam quality are needed for a range of spectroscopy and multi-spectral imaging applications (i.e. where different terahertz wavelengths/colors provide additional image contrast). For example, applications in the fields of astrophysics, atmospheric science, biological and medical sciences, security screening, illicit material detection, and non-destructive evaluation all can benefit from terahertz sources. Terahertz quantum-cascade lasers are one of the most promising sources of radiation in this spectral range, however several hurdles must still be overcome to make them more practical. In past research, this team has demonstrated a new laser architecture that addresses the challenge of how to get high power simultaneously with high beam quality, known as the THz quantum-cascade vertical-external-cavity surface-emitting-laser. The next challenge is to make this architecture work in a "broadband" configuration, i.e. how to make lasers that emit across a wide range of wavelengths, without sacrificing power or beam quality. Hence, the unifying theme of this research is to develop a set of approaches that allow terahertz lasers to emit either (a) a single wavelength of light that can be tuned across a wide range, or (b) many wavelengths of light simultaneously across a wide range. Much of this work focuses upon design of a key enabling component: the so-called "active metasurface", which can be considered an artificial mirror, which not only reflects the light, but amplifies it as well. 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.The research goal of this proposal is the development of broadband terahertz quantum-cascade lasers based upon amplifying reflectarray metasurfaces, and that operate both with scalable high power and excellent beam quality. Within this overarching theme, we propose two primary thrusts: (a) the development of widely-tunable single-mode THz external cavity lasers, (b) the development of broadband multi-mode THz lasers and frequency combs. The enabling component of the vertical-external-cavity surface-emitting-laser 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 is present in the development of novel reflectarray metasurfaces that provide gain over a large fractional bandwidths. A secondary innovation lies in the novel laser cavity configurations that are newly enabled by the flexibility of metasurface design. The power of this approach lies in (a) the ability of the VECSEL cavity configuration to support scalable high powers in a diffraction limited beam, and (b) the flexibility to locally engineer the amplitude, phase, and polarization response of the metasurface in both the spatial and spectral domain. The impact for spectroscopy and multi-spectral imaging is potentially large, since many previous demonstrations of tunable single-mode THz quantum-cascade lasers and frequency combs are either limited in output power, have a poor beam pattern, or both. The broader impacts are addressed at several levels including undergraduate and graduate research experiences, dissemination of results, and technology advancement. Outreach will specifically occur through the development of research projects for a course designed for the recruitment and retention of underrepresented minority URM engineering freshmen.