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

兆高斯（MG）磁场中的太赫兹（THz）磁谱是研究新型半导体材料的一种创新方法。最近，通过太赫兹量子级联激光器（QCL）的发展和场强超过1 MG（100 T）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学，以确定材料中的载流子有效质量与较低的mobility.In这个项目中，我们将展示一个太赫兹磁谱仪下MG字段使用紧凑的激光器，如太赫兹QCL作为远红外辐射源，它可以安装在接近磁铁达到MG字段操作。将开发针对该应用的优化QCL。作为证明磁谱学，特别是回旋共振谱学的适用性的原理证明，该技术将被应用于研究一组具有化学计量和非化学计量组成的MnSi样品中的有效电子质量。我们将使用回旋共振吸收测量直接探测MnSi的有效电子质量，其中当探测辐射的频率与回旋加速器频率匹配时发生吸收。可观测性条件，即，电子在散射事件之前完成一个回旋加速器轨道的要求，结合预期的大有效电子质量和MnSi中的低电子迁移率，导致激发波长在THz区域，而磁场强度进入MG范围。目前，利用微磁场实现太赫兹回旋共振光谱的实验还没有成功的实现，而在微磁场中进行太赫兹回旋共振光谱实验，需要单次激发的太赫兹QCL具有恒定的输出功率和超过20微秒的发射频率。同时，峰值功率应超过10 mW。已经存在的QCL是一个起点，以适应他们在脉冲磁场中的太赫兹光谱范围内的传输实验。必须测试这些激光器的发射特性，以确定它们在产生超过1 MG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微秒的脉冲长度位于THz QCL的脉冲操作的当前使用的小于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.