Far-infrared magneto-spectroscopy of novel semiconductor materials in megagauss magnetic fields using quantum-cascade lasers
Final Report Abstract
The goal of the project jointly carried out by groups at Laboratoire National des Champs Magnétiques Intenses (LNCMI) and Paul-Drude-Institut für Festkörperelektronik (PDI) concentrated on the development of an innovative magneto-spectrometer at terahertz (THz) frequencies in megagauss (MG) magnetic fields. The implementation of this technique will open up cyclotron resonance spectroscopy to determine the carrier effective masses in materials with small mobilities and large effective masses. Optimized quantum-cascade lasers (QCLs) that can be installed in the proximity to magnets reaching MG fields were developed. The QCLs are based on GaAs/AlAs heterostructures with the so-called hybrid design. For the experiments in MG magnetic fields, single-shot THz QCLs with a constant output power of at least 10 mW and a stable emission frequency over about 20 µs are required. While the feasibility of such long pulses was known at the beginning of the project, the required frequency and power stability over such long pulses at these high-power levels was proven. For the atmospheric window at 3.43 THz, even continuous-wave operation with an output power above 10 mW could be demonstrated. However, these lasers could not be operated inside the MG setup due to a limited cooling capacity. Therefore, we focused on a 3.43-THz QCL with 5 mW output power over 20 µs long pulses, for which the required stability has been shown. As a proof of concept, a cyclotron resonance spectrometer compatible with semidestructive MG magnetic fields was developed. The QCL was mounted 150 mm above the sample. The THz radiation was guided by hollow THz waveguides with a special design, which allows for operation in the MG environment with its huge values of the temporal derivative of the magnetic field. For the detection of the THz radiation, optimized Ge:Ga detectors with a bandwidth of 1.5 MHz have been used. The performance of the spectrometer was demonstrated by a non-destructive experiment using a GaAs/(Al,Ga)As multiple-quantum well system. Unfortunately, the effective electron mass in a set of MnSi samples with both stoichiometric and nonstoichiometric composition has not yet been studied, since the collaboration with institutions in the Russian Federation has been discontinued due to the war in the Ukraine.
