Far-infrared magneto-spectroscopy of novelsemiconductor materials in megagauss magnetic fields using quantum-cascade lasers

兆高斯(MG)磁场中太赫兹(THz)频率的磁谱是研究新型半导体材料的一种创新方法。最近，由于太赫兹量子级联激光器(QCL)的发展和场强大于1 MG(100T)的脉冲磁场的实现，这种方法已经成为可能。该项目的主要目标是将这两个领域的领先科学家聚集在一起。这项技术的实施将开启回旋共振光谱测量低迁移率材料中载流子有效质量的大门。在这个项目中，我们将展示一台工作在MG场下的THz磁谱仪，它使用THzQCL等紧凑型激光器作为远红外辐射源，可以安装在到达MG场的磁体附近。将为该应用程序开发优化的QCL。为了证明磁光谱学，特别是回旋共振光谱学的适用性，我们将应用这一技术来研究一组具有化学计量和非化学计量成分的MNSI样品中的有效电子质量。我们将使用回旋共振吸收测量来直接探测MNSI的有效电子质量，当探测辐射的频率与回旋频率匹配时，吸收发生。可观测性条件，即要求电子在散射事件之前完成一个回旋轨道，再加上MNSI中预期的大的有效电子质量和低的电子迁移率，导致激发波长在太赫兹区域，而磁场强度进入MG范围。太赫兹回旋共振光谱目前还没有在MG磁场中得到证实，要在MG磁场中进行实验，需要输出功率和发射频率在20微秒以上的单次太赫兹QCL。同时，峰值功率应超过10 mW。现有的QCL是使其适用于在脉冲磁场中太赫兹光谱范围内传输实验的起点。必须对这些激光器的发射特性进行测试，以确定它们在产生高于1毫克的磁场期间如何在不寻常的条件下工作。必须优化器件的输出功率，以在磁场脉冲的长度期间产生足够大的信噪比。所需的20微秒的脉冲长度介于当前用于脉冲操作的小于1微秒的典型脉冲长度和用于太赫兹QCL的连续波操作的无限长脉冲之间。虽然我们已经证明了这样的脉冲长度是可行的，但对于这些相当长的脉冲，所需的频率和功率稳定性仍有待证明。

Magneto-spectroscopy at terahertz (THz) frequencies in megagauss (MG) magnetic fields is an innovative approach for the investigation of novel semiconducting materials. Recently, this approach has become possible through both, the development of THz quantum-cascade lasers (QCLs) and the realization of pulsed magnetic fields with field strengths above 1 MG (100 T). The principal target of the project is to bring together leading scientists in these two fields. The implementation of this technique would open up cyclotron resonance spectroscopy to determine the carrier effective masses in materials with lower mobilities.In this project, we will demonstrate a THz magneto-spectrometer operating under MG fields using compact lasers such as THz QCLs as the source of the far-infrared radiation, which can be installed in proximity to magnets reaching MG fields. Optimized QCLs for this application will be developed. As a proof of principle to demonstrate the applicability of magneto-spectroscopy, in particular of cyclotron resonance spectroscopy, this technique will be applied to study the effective electron mass in a set of MnSi samples with both, stoichiometric and non-stoichiometric composition.We will directly probe the effective electron mass of MnSi using cyclotron resonance absorption measurements, where the absorption occurs when the frequency of the probing radiation matches the cyclotron frequency. The observability condition, i.e., the requirement that the electron completes one cyclotron orbit before a scattering event, in combination with the expected large effective electron mass and the low electron mobility in MnSi leads to an excitation wavelength in the THz region, while the magnetic field strengths enter the MG range. Cyclotron resonance spectroscopy in the THz range using MG fields has not been demonstrated so far.For the experiments in MG magnetic fields, single-shot THz QCLs with a constant output power and emission frequency over about 20 microseconds are required. At the same time, the peak powers should exceed 10 mW. Already existing QCLs are a starting point to adapt them for transmission experiments in the THz spectral range in pulsed magnetic fields. The emission properties of these lasers have to be tested to determine how they can be operated under the unusual conditions during the generation of magnetic fields above 1 MG. The output power of the devices has to be optimized to generate a sufficiently large signal-to-noise ratio during the length of the magnetic field pulse. The required pulse length of 20 microseconds lies between the currently used typical pulse length of less than 1 microsecond for pulsed operation and infinitely long pulses for continuous-wave operation of THz QCLs. While we have already shown that such a pulse length is feasible, the required frequency and power stability have still to be demonstrated for these rather long pulses.