Terahertz cyclotron resonance spectroscopy of an AlGaN/GaN heterostructure using a high-field pulsed magnet and an asynchronous optical sampling technique

Recently, compact pulsed magnets have been developed, which can provide the high magnetic fields (up to 30 T) that were previously only accessible at national-scale facilities on a table-top in a laboratory environment.9 There have also been several reports of cyclotron resonance spectrometers being developed that combine pulsed magnetic fields with broadband laser-based terahertz radiation sources.10–12 Laser-based terahertz spectroscopy is a time-domain technique that typically employs slow scanning mechanical delay stages to acquire the terahertz waveforms through combining time-delayed near-infrared and terahertz pulses in a nonlinear medium or photoconductive antenna. The key challenge in utilizing these sources with pulsed magnetic fields has been in developing a detection scheme that can measure the terahertz waveforms within the several millisecond duration of the magnetic field.
Approaches to achieving this have included both replacing the slow mechanical delay stage with a fast rotating mirror10 and using two lasers synchronized with an electronically controlled optical sampling (ECOPS) technique.12 The ECOPS scheme was the most suitable method for use with short duration magnetic field pulses as Noe II et al.12 showed that it could be used to record four terahertz waveforms during an approximately 14 ms magnetic field pulse.
In this letter, we report on the characterization of the 2DEG in an AlGaN/GaN heterostructure at a temperature of 77 K. The measurements were carried out using a laboratory-based cyclotron resonance spectrometer that incorporated a high-field pulsed magnet and an asynchronous optical sampling (ASOPS) terahertz detection scheme. The use of an ASOPS scheme allowed around 100 terahertz waveforms to be recorded over the approximately 14 ms magnetic field pulse. This enabled the determination of the effective mass, sheet density, and mobility for electrons in the 2DEG as 0.228 ± 0.002m0, 8.0 × 1012 cm−2, and 9000 cm2 V−1 s−1, respectively. Prior to studying the AlGaN/GaN heterostructure, the capabilities of the spectrometer were determined using a high-mobility AlGaAs/GaAs heterostructure. The spectrometer was shown to be able to measure a 2DEG with a sheet density greater than 2.6 × 1011 cm−2 and a mobility of up to 1.8 × 105 cm2 V−1 s−1.
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