Epitaxial Ge_2Sb_2Te_5 probed by single cycle THz pulses of coherent synchrotron radiation

摘要

A THz-probe spectroscopy scheme with laser-induced single cycle pulses of coherent synchrotron radiation is devised and adapted to reveal the dynamic THz transmittance response in epitaxially grown phase change materials upon 800 nm fs-laser excitation. Amorphous (a-) and crystalline (c-) films of the prototypical Ge_2Sb_2Te_5 (GST) alloy are probed with single cycle THz pulses tuned to the spectral range of the highest absorption contrast at 2 THz. After an initial instantaneous sub-picosecond (ps) dynamic THz transmittance drop, the response of a-GST in that range is dominated only by a short recovery time τ_(short) = 2 ps of the excited carriers. On the contrary, the behavior of the c-GST response displays a short decay of 0.85 ps followed by a long one τ_(long) = 90 ps, suggesting that vacancy layers in an ordered c-GST play a role as dissipation channel for photo-induced free carriers.