Terahertz Magneto-Spectroscopy

Terahertz time-domain spectroscopy offers unique possibilities for characterizing samples in high magnetic fields. Properties of interest include the conductivity, scattering time, sheet carrier concentration and electron mobility in semiconductors, in order to identify materials suitable for the design of fast transistors. The same parameters also play an important role in the optimization of high-temperature superconductors.
Cyclotron resonance spectroscopy is an established experimental technique to measure the effective masses of charge carriers (a parameter which, in turn, serves as an input to calculate some of the characteristics above). However, for heavy carrier masses, cyclotron frequencies are small, and extremely high magnetic fields – several 10 Tesla – are needed to uncover the sought-after properties. Such high field strengths are usually achieved with pulsed magnets, where the discharge of a large capacitor bank gives rise to a short, intense current pulse in a carefully designed coil, which in turn generates a magnet pulse with a duration of no more than a few milliseconds.
The short duration of the magnetic field calls for very fast terahertz measurements. Given that several traces are to be recorded in the presence of the magnetic field, very short terahertz acquisition times are mandatory – ideally, less than a millisecond. One of the most promising fast terahertz techniques is dubbed ECOPS (electronically-controlled optical sampling), a concept invented and patented by TOPTICA Photonics1. In an ECOPS setup, the terahertz transmitter and receiver are driven by two separate fs-lasers. The pulse trains of the two lasers are synchronized, and with the help of a piezo actuator, the repetition rate of one of the lasers is modulated, periodically accelerating and retarding its pulses with respect to those of the second laser. The net effect is similar to a conventional, mechanical delay stage, yet the modulation frequency and hence, measurement speed is approximately 100 times faster. TOPTICA’s ECOPS-based platform TeraFlash smart acquires 1600 complete terahertz pulse traces per second2. Compared to ASOPS (asynchronous optical sampling, another rapid terahertz technique based on synchronized fs-lasers), ECOPS achieves a significantly higher dynamic range at the same measurement speed, or vice versa, a drastically (~50x) higher measurement speed for the same dynamic range3.

Terahertz magneto-spectroscopy using a table-top magnet. Terahertz pulses transmitted through a 100 nm graphite layer on kapton film. Each of the vertical stripes represents a complete pulse trace. The green curve shows the temporal evolution of the magnetic field.
Representative terahertz pulses with and without the magnetic field, corresponding to the “blue” and “red” pulses in the left-hand figure. Adapted from ref. 4.

The suitability of ECOPS for terahertz magneto-spectroscopy was first demonstrated by researchers from TOPTICA and the Laboratoire National des Champs Magnétiques Intenses (Toulouse, France), employing a table-top pulsed magnet with a peak field strength of 3 T. Using a thin graphite sample, they showed that the amplitude of the transmitted pulses changed significantly in the presence of the magnetic field, an effect attributed to a polarization change of the terahertz signal passing through the graphite layer4.
Recently, a team of scientists from, amongst others, Los Alamos National Laboratory, Johns Hopkins University and Brookhaven National Laboratory (USA) used one of TOPTICA’s ECOPS systems to investigate the high-temperature superconductor La2-xSrxCuO4 in pulsed magnetic fields up to 31 T. Their work, now accessible on arXiv5, presents the first direct measurement of the cyclotron resonance in this type of superconductor. The authors found the cyclotron mass of the holes (p-type charge carriers) to be almost five times that of the electron mass. Since the halfwidth of the magnet pulse was only 1.5 ms, this measurement only became possible thanks to the high measurement speed of TOPTICA’s TeraFlash smart system.

1 F. Tauser, C. Rausch, J.H. Posthumus, F. Lison: Electronically controlled optical sampling using 100 MHz repetition rate fiber lasers; Proc. Int. Soc. Opt. Photonics (2008), 6881, 68810O.
2 M. Yahyapour, A. Jahn, K. Dutzi, T. Puppe, P. Leisching, B. Schmauss, N. Vieweg, A. Deninger: Fastest thickness measurements with a terahertz time-domain system based on electronically controlled optical sampling; Appl. Sci. 9 (2019) 1283; doi:10.3390/app9071283.
3 Y. Kim, D.-S Yee: High-speed terahertz time-domain spectroscopy based on electronically controlled optical sampling; Opt. Lett. 35 (2010) 3715.
4 M. Yahyapour, N. Vieweg, T. Puppe, A. Deninger, O. Drachenko, J. Léotin: Terahertz time-domain magneto-spectroscopy using electronically controlled optical sampling; Proc. 41st International Conference on Infrared, Millimeter, and Terahertz waves (2016).
5 K.W. Post, A. Legros, D.G. Rickel, J. Singleton, R.D.McDonald, X. He, I. Božović, X. Xu, X. Shi, N.P. Armitage, S.A. Crooker: Observation of cyclotron resonance and measurement of the hole mass in optimally-doped La2-xSrxCuO4; arXiv:2006.09131v1 (2020).