Mode-locking of THz quantum cascade lasers

太赫兹量子级联激光器的锁模

• 批准号: EP/D025532/1
• 负责人: David Ritchie
• 金额: $ 38.96万
• 依托单位: University of Cambridge
• 依托单位国家: 英国
• 项目类别: Research Grant
• 财政年份: 2006
• 资助国家: 英国
• 起止时间: 2006 至 无数据
• 项目状态: 已结题
• 来源: https://gtr.ukri.org/projects?ref=EP%2FD025532%2F1
• 关键词: Mode; locking; THz; quantum; cascade

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.

在可见光和无线电波之间，有一个特定的电磁波系列，称为Terahertz波。在THZ波的各种来源中，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，可以设计QCLs以在不同频率下发射电磁辐射。最近，已证明它们以5到2 THz之间的频率发射辐射。 QCL是非常紧凑的来源，其尺寸为毫米的一部分，并且能够发出非常高功率的THZ波，比任何其他可移植的THZ辐射来源都高。由于这些原因，它们很可能成为未来几年中最广泛的THZ辐射来源。在这个项目中，我们要证明THZ QCL可以以一种非常特殊的方式工作，称为模式锁定。当在此制度中操作时，激光器会发出一系列非常强大且极为短的脉冲系列。脉冲的重复速率，也称为往返频率等于光子在激光腔中来回传播所需的时间。半导体激光器可以通过在往返频率上调节偏置电压的幅度来强制锁定模式。该制度称为活动模式锁定。有时，它们可以自发从正常运行转到模式的操作。在这种情况下，据说它们是在被动模式锁定的制度中运行的，这可能是由于激光的类型而出现的许多不同原因。在过去的几年中，科学家有一些迹象表明QCL可能会在模式锁定（被动和主动）的状态下运作，但是他们尚未能够证明这一点。实际上，区分模式锁定操作的唯一方法，以及在往返频率下输出功率的强大调制是测量输出脉冲的时间持续时间。进行此测量有各种技术，但是QCL在频率（中红外和远红外）发出，其中大多数这些方法都很难实施。在这个项目中，我们建议利用一种通常用于检测现代THZ系统中THZ辐射的脉冲的技术，并将其应用于THZ QCL。该技术称为光电传输，使用可见模式锁定激光器产生的脉冲来探测来自THZ QCL的脉冲。它非常强大且用途广泛，可以完全重建THZ脉冲的时间形状。它以前从未曾经被授予过QCL，并且会给我们机会在没有模棱两可的情况下找到这些设备可以在模式锁定的状态下操作的条件。此外，通过了解基本的物理过程，我们还将能够修改QCL的设计以优化模式锁定操作，以生产超短而超高的功率THZ脉冲。这种类型的脉冲对于许多具有巨大潜力的应用非常有用。特别是它们将用于在机场的行李扫描，用于检测炸药和非金属武器的应用中。实际上，与X射线相似，THZ波可以通过许多日常材料（例如皮革和大多数布料）看到。但是，与X射线不同，它们可用于识别和区分爆炸物与其他无害物质（例如奶酪或肉类）。为了能够渗透厚厚的层，例如形成普通行李的层，它们必须足够强大。通过实现模式锁定的QCL，我们期望产生具有峰值功率的THZ脉冲，其峰值的100至1000倍用当前THZ系统产生的脉冲。

项目成果

Photo-luminescence study of heterogeneous terahertz quantum cascade lasers

异质太赫兹量子级联激光器的光致发光研究

• DOI: 10.1063/1.3603035
• 发表时间: 2011
• 期刊: Journal of Applied Physics
• 影响因子: 3.2
• 作者: Freeman J

13 GHz direct modulation of terahertz quantum cascade lasers

• DOI: 10.1063/1.2790827
• 发表时间: 2007-10-01
• 期刊: APPLIED PHYSICS LETTERS
• 影响因子: 4
• 作者: Barbieri, Stefano;Maineult, Wilfried;Ritchie, David A.
• 通讯作者: Ritchie, David A.

Terahertz transfer onto a telecom optical carrier

• DOI: 10.1038/nphoton.2007.94
• 发表时间: 2007-07-01
• 期刊: NATURE PHOTONICS
• 影响因子: 35
• 作者: Dhillon, Sukhdeep S.;Sirtori, Carlo;Ritchie, David A.
• 通讯作者: Ritchie, David A.

Dual wavelength emission from a terahertz quantum cascade laser

太赫兹量子级联激光器的双波长发射

• DOI: 10.1063/1.3304783
• 发表时间: 2010
• 期刊: Applied Physics Letters
• 影响因子: 4
• 作者: Freeman J

Improved wall plug efficiency of a 1.9THz quantum cascade laser by an automated design approach

• DOI: 10.1063/1.3030881
• 发表时间: 2008-11
• 期刊: Applied Physics Letters
• 影响因子: 4
• 作者: J. Freeman;Owen J. Marshall;H. Beere;D. Ritchie