Broadband terahertz metasurface lasers

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。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)需要具有高输出功率和极佳光束质量的紧凑型太赫兹辐射源。例如，在天体物理、大气科学、生物和医学科学、安全筛查、非法材料检测和无损评估等领域的应用都可以受益于太赫兹源。太赫兹量子级联激光器是这一光谱范围内最有希望的辐射源之一，但要使其更加实用，仍需克服几个障碍。在过去的研究中，该团队展示了一种新的激光架构，它解决了如何在获得高功率和高光束质量的同时获得高功率的挑战，被称为THz量子级联垂直外腔表面发射激光器。下一个挑战是让这种架构在“宽带”配置下工作，即如何在不牺牲功率或光束质量的情况下，制造出能发射多种波长的激光器。因此，这项研究的统一主题是开发一套方法，使太赫兹激光能够发射(A)可在大范围内调谐的单一波长的光，或(B)同时在大范围内发射多个波长的光。这项工作的大部分工作都集中在设计一个关键的使能部件：所谓的“主动变形表面”，它可以被认为是一个人造镜子，它不仅反射光线，而且还放大光线。作为该项目的一部分，这项研究将培养研究生和本科生，并将通过参与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。这项计划的研究目标是开发基于放大反射阵列亚表面的宽带太赫兹量子级联激光器，并以可扩展的高功率和优良的光束质量运行。在这一总体主题中，我们提出了两个主要的推动：(A)开发可广泛调谐的单模THz外腔激光器，(B)开发宽带多模THz激光器和频率梳。垂直外腔表面发射激光器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反射阵列亚表面；这产生了一个有源放大镜，在开放的腔中充当一个反射镜。提出的工作中的智力优势体现在开发新型的反射阵列亚表面，这种表面在很大的分数带宽上提供增益。第二个创新之处在于新的激光腔体结构，这是通过变形表面设计的灵活性而实现的。这种方法的优势在于：(A)VECSEL腔结构能够在衍射受限光束中支持可扩展的高功率，以及(B)在空间域和谱域中局部设计亚表面的幅度、相位和偏振响应的灵活性。这对光谱学和多光谱成像的影响可能很大，因为之前许多可调谐单模THz量子级联激光器和频率梳的演示要么输出功率有限，要么光束模式较差，或者两者兼而有之。更广泛的影响是在几个层面上解决的，包括本科生和研究生的研究经验、成果的传播和技术进步。具体而言，将通过为招收和留住人数不足的少数族裔城市管理工程新生而设计的课程制定研究项目，开展外联活动。

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.