Mode-locking of THz quantum cascade lasers

在可见光和无线电波之间，有一个特殊的电磁波家族，称为太赫兹波。在各种太赫兹波源中，最有前途的是量子级联激光器（QCL）。与标准二极管激光器不同，QCL可以被设计成发射不同频率的电磁辐射。最近，它们已经被证明可以发射频率在5和2 THz之间的THz辐射。QCL是非常紧凑的源，具有毫米量级的尺寸，并且能够发射非常高功率的THz波，高于任何其他便携式THz辐射源。由于这些原因，它们很可能在未来几年成为最广泛的THz辐射源。在这个项目中，我们想证明THz QCL可以以一种非常特殊的方式起作用，称为锁模。当在这种状态下工作时，激光器会发出一系列非常强大和极短的脉冲。脉冲的重复频率，也称为往返频率，等于光子在激光腔中来回移动所需的时间。半导体激光器可以通过精确地在往返频率处调制偏置电压的幅度来强制进入锁模。这种机制被称为主动锁模。有时它们可以自发地从正常工作切换到锁模工作。在这种情况下，它们被认为是在被动锁模的状态下工作，这可能由于许多不同的原因而发生，这取决于激光器的类型。在过去的几年里，科学家们已经有一些迹象表明QCL可能在锁模状态下工作（被动和主动），但是他们还没有能够证明这一点。事实上，区分锁模工作和在往返频率下输出功率的强调制的唯一方法是测量输出脉冲的持续时间。有各种技术来执行这种测量，但是QCL发射的频率（中红外和远红外），其中大多数方法都很难实现。在这个项目中，我们建议利用一种技术，通常用于检测脉冲的太赫兹辐射在现代太赫兹系统中，并将其应用到太赫兹QCL。这种技术被称为光电导光学门控，它使用可见光锁模激光器产生的脉冲来探测来自THz QCL的脉冲。它是非常强大和通用的，将允许THz脉冲的时间形状的完整重建。它从来没有被应用到QCL之前，并将给我们一个机会，找到，没有歧义，在什么条件下，这些设备可以在锁模制度。此外，通过了解基本的物理过程，我们也将能够修改QCL的设计，以优化锁模操作，用于产生超短，超高功率的THz脉冲。这种类型的脉冲对于THz波具有巨大潜力的许多应用将是非常有用的。特别是，它们将用于机场行李扫描等应用，以检测爆炸物和非金属武器。事实上，与X射线类似，太赫兹波可以穿透许多日常材料，如皮革和大多数类型的布料。然而，与X射线不同的是，它们可以用来识别和区分爆炸物与其他无害物质，如奶酪或肉类。为了能够穿透厚层，例如形成普通行李的那些，它们必须足够强大。通过实现锁模QCL，我们期望产生峰值功率为当前THz系统产生的峰值功率的100至1000倍的THz脉冲。

Between the visible light and radio waves, there is a particular family of electromagnetic waves, known as Terahertz waves. Among the various sources of THz waves, one of the most promising is the Quantum Cascade Laser (QCL). Unlike standard diode lasers, QCLs can be designed to emit electromagnetic radiation at different frequencies. Recently, they have been demonstrated to emit THz radiation with frequencies between 5 and 2 THz. QCLs are very compact sources, with dimensions of the order of a fraction of a millimeter, and are capable of emitting THz waves of very high power, higher than any other portable source of THz radiation. For these reasons they are likely to become the most widespread source of THz radiation in the years to come. In this project we want to demonstrate that THz QCLs can function in a very special way called mode-locking . When operated in this regime, lasers emit a regular series of very powerful and extremely short pulses. The repetition rate of the pulses, also called round-trip frequency is equal to one over the time needed for a photon to travel back and forth across the laser cavity. Semiconductor lasers can be forced into mode-locking by modulating the amplitude of the bias voltage at exactly the round trip frequency. This regime is called active mode-locking . Sometimes they can switch spontaneously from normal to mode-locked operation. In this case they are said to operate in a regime of passive mode-locking , which can occur for many different reasons, depending on the type of laser. During the last few years scientists have had some indications that QCLs may operate in a regime of mode-locking (passive and active), however they have not yet been able to prove it. In fact, the only way to distinguish between mode-locked operation, and what could be just a strong modulation of the output power at the round-trip frequency, is to measure the time duration of the output pulses. There are various techniques to perform this measurement, however QCLs emit at frequencies (mid-infrared and far-infrared) where most of these methods are very difficult to implement. In this project we propose to exploit a technique which is normally used to detect pulses of THz radiation in modern THz systems, and to apply this to THz QCLs. The technique, called photoconductive optical gating , uses the pulses generated by a visible mode-locked laser for probing those coming from the THz QCL. It is extremely powerful and versatile and will allow for the complete reconstruction of the temporal shape of the THz pulse. It has never beenapplied before to QCLs, and will give us the chance to find, without ambiguity, under which conditions these devices can be operated in a regime of mode-locking. Moreover, by understanding the fundamental physical processes, we will also be able to modify the design of QCLs to optimize mode-locking operation, for the production of ultra-short, ultra-high power THz pulses. Such type of pulses would be extremely useful for many applications where THz waves have a great potential. In particular they would be used in applications such as luggage scanning in airports, for the detection of explosives and non-metallic weapons. In fact, similarly to X-rays, THz waves can see through many everyday materials such as leather and most types of cloth. However, unlike X-rays, they can be used to recognize and distinguish an explosive from other harmless substances such as cheese or meat. In order to be able to penetrate thick layers, such as those forming ordinary luggage, they must be sufficiently powerful. By realizing mode-locked QCLs, we expect to produce THz pulses with peak powers in the order of 100 to 1000 times those generated with current THz systems.