Development of a cryogen-free terahertz time-domain magnetooptical measurement system

We have developed a terahertz time-domain magnetooptical measurement system for investigating the carrier transport of semiconductors at low temperatures and high magnetic fields. To cool a superconducting magnet and a sample, Gifford-McMahon-type refrigerators were employed to avoid the use of liquid cryogen. However, the vibration of the refrigerator is a critical problem because it induces oscillation noise in measured waveforms of terahertz pulses. In order to overcome this problem, a synchronous data sampling method has been developed utilizing the sound of the refrigerator as a trigger for the waveform acquisition. This sampling method succeeded in eliminating the oscillation noise completely. Our measurement system enables us to obtain the magnetooptical spectra in the frequency range from 0.2 to 1.5 THz at temperatures between 5 and 300 K in magnetic fields of up to +/- 10 T. As an example, the measurement of the cyclotron resonances of a two-dimensional electron gas in a GaAs/AlGaAs heterostructure is demonstrated.