Collaborative Research: Quantum cascade laser transceivers for terahertz wireless communication

太赫兹是位于微波和红外范围之间的电磁波谱区域，由于缺乏合适的技术来产生和操纵，也被称为“太赫兹间隙”。一方面，用于产生微波的传统电子设备不能在更高频率下操作，而另一方面，诸如太赫兹激光器的光源通常需要低温操作，这是不切实际的。因此，需要新的方法来开发方便的太赫兹源。该项目的目标是展示基于高功率中红外半导体激光器（所谓的量子级联激光器）的新型太赫兹源，该激光器旨在产生由精确等距的太赫兹频率间隔分隔的频率梳。由此产生的太赫兹辐射源将显示出室温操作、窄线宽和宽可调谐性。这对于许多应用，特别是遥感，都是有吸引力的。事实上，从气体到药物、爆炸物和生物分子的数百种化学物质在太赫兹范围内都具有明显的吸收和发射特征。太赫兹传感将允许人们监测臭氧消耗，气候变化和环境污染。它将使我们深入了解银河系内外恒星的形成和衰变。这种太赫兹源在材料研究中也非常有价值，因为物质中的许多基本激发，如等离子体振荡和声波，都在太赫兹中表现出共振。所提出的新器件架构的核心由中红外量子级联激光器组成，该激光器产生纵向模式之间具有太赫兹间隔的光学频率梳，称为谐波频率梳。然而，不是像典型的频率梳那样使用从激光器发射的红外光，在此，利用构成梳的光学模式的腔内拍频来在室温下产生相干太赫兹信号。该项目的重点是展示这种新的太赫兹源的传感应用。这些设备将受益于前所未有的紧凑性，占地面积小于1平方厘米。由于频率梳的性质，它们将产生具有窄线宽（Hz范围内）和高稳定性的太赫兹音调。此外，由于激光器的快速电子动力学，它们将能够在室温下以从微波到太赫兹区域的宽调谐范围工作。通过连接和同步这些设备的阵列，将有可能相干地放大发射功率，并实现太赫兹光束控制，例如光束转向和整形。由于这些独特的功能，拟议的来源将竞争对手，并可能优于其他现有的系统太赫兹sensing.This奖项反映了NSF的法定使命，并已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。

The terahertz is a region of the electromagnetic spectrum lying between microwaves and the infrared range, also known as the "terahertz gap" due to the lack of suitable technologies for its generation and manipulation. On the one hand, conventional electronic devices used to produce microwaves cannot operate at higher frequencies, while on the other hand optical sources such as terahertz lasers typically require cryogenic operation, which is impractical. Thus, novel approaches are needed to develop convenient terahertz sources. The goal of this project is to demonstrate a new class of terahertz sources based on a high-power mid-infrared semiconductor laser (so-called quantum cascade laser) designed to generate a comb of frequencies separated by precisely equidistant terahertz frequency intervals. The resulting terahertz radiation sources will show room temperature operation, narrow linewidth, and wide tunability. These would be attractive for many applications, especially remote sensing. Indeed, hundreds of chemicals from gases to drugs, explosives, and biomolecules have telltale absorption and emission features in the terahertz range. Terahertz sensing would allow one to monitor the ozone depletion, climate change, and environmental pollution. It would give insights into the formation and decay of stars in our galaxy and beyond. Such terahertz sources would also be very valuable in the studies of materials, since many fundamental excitations in matter such as plasma oscillations and sound waves exhibit resonances in the terahertz. The core of the proposed new device architecture consists of a mid-infrared quantum cascade laser generating an optical frequency comb with a terahertz spacing between longitudinal modes, named a harmonic frequency comb. However, instead of using infrared light emitted from the laser as in typical frequency combs, here the intracavity beating of the optical modes constituting the comb is exploited to generate a coherent terahertz signal at room temperature. The focus of this project is to demonstrate such new terahertz sources for sensing applications. These devices will benefit from unprecedented compactness, having a footprint smaller than 1 square centimeter. Thanks to the nature of a frequency comb, they will generate terahertz tones with narrow linewidth (in the Hz range) and high stability. Moreover, they will be able to operate at room temperature with a broad tuning range, from microwaves to the terahertz region, as a result of the fast electron dynamics of the laser. By connecting and synchronizing an array of such devices, it will be possible to coherently scale up the emitted power and enable terahertz beam control, such as beam steering and shaping. Because of these unique features, the proposed sources will rival and potentially outperform other existing systems for terahertz sensing.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.