Terahertz acoustic laser (saser) devices: fabrication and characterisation

SASER是由受激辐射放大的首字母缩略词，是光学激光器的声学模拟物[http://en.wikipedia.org/wiki/Sound_Amplification_by_Stimulated_Emission_of_Radiation]]。太赫兹（THz）激光装置将产生一束纳米波长的强相干声波。激光产生的声束不仅是一个纯粹的科学好奇心的主题，而且可以有许多科学和技术应用：例如探测和成像纳米尺度的物体，以及转换成太赫兹电磁波，可用于医学成像和安全检查。最近，我们展示了一种设备，它显示了激光的一些关键特征。这项工作发表在一份科学杂志上，随后在一些学术和大众科学杂志上进行了报道，包括经济学人的科学和技术页面[2006年6月10日至16日，第96页]。该装置基于半导体超晶格：一种人造纳米结构，由两种不同半导体材料的许多层（通常为40或50层）交替组成，在这种特殊情况下，砷化镓和砷化铝，每层厚度只有几纳米。在超晶格中，当电子垂直穿过堆叠层时，声子放大的条件就可以实现。电子在相邻的砷化镓层之间跳跃，为了保存能量，它们在移动时发出声子（声音量子）。这些声子可以刺激进一步的电子跳跃和声子的发射，从而产生声子放大。除了声学增益之外，激光器还需要一个声腔来限制声子，以便它们能够参与进一步的受激发射过程。这类似于在激光的两个反射镜之间形成的光学腔。超晶格可以用作声子反射镜：由于构成超晶格的两种材料之间声速和密度的差异，声子在每个界面上部分反射和部分透射。当声波波长与单对层的厚度相匹配时，所有反射的建设性干涉会导致强反射和声子的限制。在这个项目中，我们计划对基于半导体超晶格的太赫兹激光器件的独立元素的物理特性进行详细的研究。这包括：增益介质及其内部声子放大过程；实现必要的人口反转的电子和光学泵送方案；以及用于限制声子的声学镜和腔体。这项工作的主要目标是开发一种毫瓦每平方厘米级的激光设备，发射0.5 - 1太赫兹范围内的相干声子，并表征发射的激光声音。

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.