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

太赫兹（THz）频率下的磁波谱在兆赫磁场（MG）中是一种研究新型半导体材料的创新方法。最近，通过太赫兹量子级联激光器（qcl）的发展和场强超过1mg （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汇集这两个领域的顶尖科学家。该技术的实现将开辟回旋共振光谱法测定低迁移率材料中载流子有效质量的新途径。在这个项目中，我们将展示一个在MG场下工作的太赫兹磁谱仪，使用紧凑的激光器，如太赫兹qcl作为远红外辐射源，它可以安装在接近MG场的磁铁附近。将为此应用程序开发优化的qcl。作为证明磁谱，特别是回旋共振谱的适用性的原理证明，该技术将应用于研究一组具有化学计量成分和非化学计量成分的MnSi样品的有效电子质量。我们将使用回旋加速器共振吸收测量直接探测MnSi的有效电子质量，当探测辐射的频率与回旋加速器频率匹配时，就会发生吸收。可观测性条件，即电子在散射事件发生前完成一个回旋轨道的要求，加上MnSi中期望的大有效电子质量和低电子迁移率，导致激发波长在太赫兹区，而磁场强度进入MG范围。在太赫兹范围内使用MG场的回旋共振光谱迄今尚未得到证实。对于MG磁场下的实验，需要输出功率恒定、发射频率在20微秒左右的单次太赫兹量子激光器。同时，峰值功率应超过10mw。现有的量子激光器是将其用于脉冲磁场中太赫兹光谱范围内的传输实验的一个起点。必须对这些激光器的发射特性进行测试，以确定它们如何在产生1mg以上磁场的异常条件下运行。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。所要求的20微秒脉冲长度介于太赫兹量子激光器的脉冲操作目前使用的小于1微秒的典型脉冲长度和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.