Terahertz acoustic laser (saser) devices: fabrication and characterisation

SASER是通过辐射的受激发射进行声音放大的首字母缩写，并且是光学激光的声学模拟[http：//en.wikipedia.org/wiki/Sound_Amplification_by_Stimulated_Emission_of_Radiation]。太赫兹（THz）激光装置将产生具有纳米波长的强相干声波束。除了作为纯粹的科学好奇心的主题之外，由saser产生的声束还可以具有许多科学和技术应用：例如，纳米尺度物体的探测和成像，以及转换为THz电磁波，其可以用于医学成像和安全检查。这项工作发表在一份科学杂志上，随后在一些学术和科普杂志上报道，包括《经济学人》的科学和技术版[2006年6月10日至16日，第96页]。该器件基于半导体超晶格：一种人造纳米结构，由许多（通常为40或50）两种不同半导体材料的交替层组成，在这种特殊情况下，砷化镓和砷化铝，每层都有几纳米厚。在超晶格中，当电子垂直穿过叠层时，可以实现声子放大的条件。电子在相邻的砷化镓层之间跳跃，为了保存能量，它们在行进时发出声子（声音的量子）。这些声子可以激发进一步的电子跳跃和声子发射，从而引起声子放大。除了声学增益之外，saser还需要一个声腔来限制声子，以便它们可以参与进一步的受激发射过程。这类似于激光器的两个反射镜之间形成的光学腔。超晶格可以用作声子镜：由于组成超晶格的两种材料之间的声速和密度差异，声子在每个界面处部分反射，部分透射。当声波的波长与一对层的厚度相匹配时，所有反射的相长干涉会导致强烈的反射和声子的限制。在本项目中，我们计划对基于半导体超晶格的THz激射器件的各个元件的物理进行详细的研究。其中包括：增益介质和其中的声子放大过程;用于实现必要的粒子数反转的电和光泵浦方案;以及用于限制声子的声镜和腔。这项工作的主要目标是研制一种毫瓦/平方厘米级的saser装置，发射0.5 - 1太赫兹范围内的相干声子，并对所发射的saser声音进行解调。

SASER is the acronym for Sound Amplification by Stimulated Emission of Radiation and is the acoustic analogue of the optical laser [http://en.wikipedia.org/wiki/Sound_Amplification_by_Stimulated_Emission_of_Radiation]. A terahertz (THz) saser device would produce an intense beam of coherent acoustic waves with nanometre wavelengths. As well as being a subject of pure scientific curiosity, the acoustic beams produced by saser could have a number of scientific and technological applications: e.g. probing and imaging of nanometre scale objects, and conversion to THz electromagnetic waves, which may be used for medical imaging and security screening.Recently, we have demonstrated a device which displays some of the key characteristics expected of a saser. The work was published in a scientific journal and subsequently reported in a number of academic and popular science magazines, including the science and technology pages of The Economist [10-16 June 2006, p96]. The device was based on a semiconductor superlattice: a man-made nanostructure consisting of many (typically 40 or 50) alternating layers of two different semiconductor materials, in this particular case Gallium Arsenide and Aluminium Arsenide, each a few nanometres thick. In the superlattice, the conditions for phonon amplification can be achieved when electrons are made to travel vertically through the stack of layers. The electrons hop between neighbouring Gallium Arsenide layers and, to conserve energy, emit a phonon (quantum of sound) as they go. These phonons can stimulate further electron hops and emission of phonons giving rise to phonon amplification. In addition to acoustic gain, a saser requires an acoustic cavity to confine the phonons so that they are available to take part in further stimulated emission processes. This is analogous to the optical cavity formed between the two mirrors of a laser. Superlattices can be used as phonon mirrors: owing to the differences of the speed of sound and density between the two materials making up the superlattice, phonons are partly reflected and partly transmitted at each interface. Constructive interference of all the reflections, which occurs when the sound wavelength matches the thickness of a single pair of layers, leads to a strong reflection and confinement of the phonons.In this project, we plan to carry out a detailed investigation of the physics of the separate elements of a THz saser device based on semiconductor superlattices. These include: the gain medium and the process of phonon amplification within it; the pumping schemes, both electrical and optical, for achieving the necessary population inversion; and the acoustic mirrors and cavities for confining phonons. The main goal of the work is to develop a milliwatt per square centimetre class saser device emitting coherent phonons in the range 0.5 - 1 THz and to characterise the saser sound emitted.