It’s always been a challenge to generate terahertz (THz) waves, the span of electromagnetic spectrum between millimeter and optical wavelengths and generally defined as wavelengths from 1 to 0.1 mm or 300 to 3,000 GHz. While there are vacuum-tube and solid-state devices and assemblies that can do so, these bring many challenges and limitations.
Now, a team at MIT in close collaboration with one at nearby Harvard University has developed a laser-pumped, gas-based THz source that can be fine-tuned over 37 lines spanning 0.251 to 0.955 THz. Each line features kilohertz linewidths and power greater than 1 mW.
The underlying physics is obviously extremely sophisticated, based on what’s called rotational population inversions that are optically pumped by a quantum cascade laser (QCL). Also fascinating is that the gas they used for a variety of reasons was nitrous oxide, better known as laughing gas.
Their implementation overcame some previously assumed theoretical limitations about cavity size as well as molecular interaction and collision. This analysis led to the development of a new mathematical theory to describe the behavior of a gas in a molecular gas laser cavity. Their new model tracked thousands of relevant vibrational and rotational states among millions of groups of molecules within a single cavity. New numerical analysis and computational techniques were used to reduce the large problem and make it solvable on a laptop computer. It then analyzed how those molecules would react to incoming infrared light, depending on their position and direction within the cavity.
整个项目都使用现成的标准电光组件，包括由哈佛集团开发的定制可调QCL（也由标准零件构建）。该QCL提供了将MIT组理论预测的能力，以适当地刺激笔的谐振腔（常规腔约为1000倍）。然后，研究人员浏览了气体文库，以鉴定已知会以某种方式旋转红外光并选择一氧化二氮的人。
尽管原则，设计和实施非常先进，但一些数字总结了情况。在安排中(Fig. 1)，从广泛可调的QCL中调整了IR光，以泵送“旋转（RO）振动”过渡并创建旋转种群反转。来自QCL的光被90％-10％的光束分离器（BS）偏转，并通过燃气电池传输。因此，可以通过最大程度地减少使用光电二极管（PD）测量的传输强度来调节QCL与振动过渡的重合。
1. Although complicated and sophisticated, the experimental arrangement at MIT was built with standard parts and used a tunable quantum cascade laser custom built at Harvard. (Source: MIT)
Light is coupled into the laser cavity through a ZnSe window at Brewster’s angle and through a pinhole coupler in the cavity. A vacuum pump, pressure sensor, and gas reservoir are used to set the pressure in both the laser cavity and gas cell.
从QPML的针孔发出的辐射是用轴心抛物线镜（OAP）收集的，通过Teflon透镜聚焦，并通过功率计，检测器或接收器测量，该接收器使用频率的局部振荡器（LO）与信号混合以产生通过频谱分析仪测量的中间频率（如果）。QCL的泵功率用氟化钙底物上的电网偏振器变化。激光腔通过在翻译阶段移动铜镜，将激光腔与激光频率调节。图2shows the construction and tunability, and图3shows the tuning range.
2. Shown is a photograph of the experimental setup depicting the different components of the laser cavity, the path of the QCL beam (red), and how the output light (blue) is collected (A); and a plot of the measured relative emission frequency of the THz gas laser as a function of the back-mirror position for JL= 10, (νTHZ≈ 0.299 THz) (B). (Source: MIT)
3.示意图揭示了给定泵送过渡的THZ激光器的调谐范围。随着气体腔的反向摩洛尔翻译，谐振器的自由光谱范围进行了修改，从而导致腔模式的变化。反过来，这导致激光的发射频率在分子增益曲线的频率范围内移动。（来源：麻省理工学院）
Full details are in their paper with a deceptively short and simple title “可调的紧凑型Terahertz气体激光器” published inAAAS科学;发布了解锁版本以及广泛的补充材料这里。（请注意，尽管本文的作者经常将“简单”一词用于理论基础和物理实现，但这根本不是那样）。这项研究得到了美国陆军研究办公室和国家科学基金会的部分支持。
用…胶带测量X射线？
超越了频谱的THZ部分，墨西哥墨西哥大学（墨西哥城）的研究人员精确测量了标准粘合剂胶带剥离而导致的X射线光谱。这是由于互动效应而发生的，这是一种机械过程，涉及摩擦，破裂，脱皮或变形材料。尽管这种奇怪的效果和X射线“源”已知数十年，但由于“堆积”，排放非常简短，准确地测量。这是多个X射线脉冲在时间上重叠的时候，因此看起来像是单个脉冲，但能量更大。
4.情节显示能源与x射线ph值的数量otons observed for the triboluminescence effect when peeling adhesive tape. The solid angle Ω subtended by the detector was just 5 × 10-6（ω定义为ω= a/r2，其中A是检测器窗口的裸露区域，R是距离源的距离）。（资料来源：墨西哥大学NacionalAutónomadeMéxico通过“激光焦点世界”）
Using an innovative solid-state detector and observation arrangement in a partial vacuum chamber setup, they found maximum individual photon energy to be 30 keV (about half of that in previous studies) with an overall peak at 2.44 keV（图4），这是先前研究中报告的一半。辐射源的平均功率为18.5 NW。细节在他们的论文中“The true spectrum of tribo-generated X-rays from peeling tape,” published in应用物理学Letters. Just don’t scare people by telling them, however accurate it may be, that even using cellophane (such as “Scotch”) tape will generate X-rays!
其他资源：
MIT, “研究人员与笑气产生Terahertz激光器”
Harvard University, “新的激光打开了电磁光谱的大型，未充分利用的区域”
