Portable THz cyclotron resonance spectrometer

In this paper we present a very compact THz cyclotron resonance (CR) spectrometer based on far IR quantum cascade lasers emitting at 11.4mkm and ~15mkm.
The CR spectrometer is schematically shown on the Figure 1. It incorporates an interchangeable GaAs/AlGaAs quantum cascade laser placed below the bottom edge of the magnet, where the absolute value of magnetic field is as low as 15 T when the maximum field of 70T is reached in the centre. Emitted light is collected by ZnSe microlens, guided through the stainless steel waveguide, and focused on the sample installed in the centre of the magnetic field coil. Transmitted light is then focused on the Si doped boron blocked impurity band (Si:B BIB) photodetector placed above the coil to minimize the value of magnetic field acting on the detector. The waveguide is purged with helium to avoid air condensation. The QCL source is driven by short current pulses 1mks long with amplitude ~2 Amps remaining constant during magnetic field shot. An appropriate repetition rate of the order of 3-4 kHz was chosen to avoid thermal degradation of the intensity. We record 4 signals during the magnetic field pulse: voltage drop across the laser, current through the laser, response of the photodetector, and, finally, magnetic field value. We use fast 100 MHz acquisition card with an onboard memory sufficient for 1s full speed acquisition. Especially designed software then removes useless points, corresponding to the absence of laser emission or regions of instability due to transient processes in the long power supply lines. Possible distortions due to QCL heating and modulation of its intensity by magnetic fields are avoided by normalisation of the signal with the sample installed by the signal without sample.