Fig. 1 Experimentally observed time-dependent shift of plasma frequency in a thin ZnO layer. Left: 3D graph of the absorption change as a function of the sampling frequency and the delay time between excitation and sampling light pulses. Right: concept of a transient difference spectrum. The cold plasma (blue) shows an absorption line at the plasma frequency of the cold electron gas. The excitation light pulse heats the plasma, which leads to a red shift of the plasmon resonance (red). In the time-resolved experiments, the so-called difference spectrum is measured, that is, the absorption line of the hot plasma minus that of the cold plasma (black).

A fundamental and interesting question is the following: Can one manipulate plasma oscillations with light, say change their frequency? This could switch the electrical and optical properties on very short time scales, an ideal instrument for modern optoelectronics. In the most recent issue of the journal Physical Review Letters [115, 147401 (2015)], a research team from the Max Born Institute and the Humboldt University in Berlin has demonstrated a new concept that allows ultrafast switching of plasmon in the semiconductor ZnO ( Movie). In their experiments, the scientists studied plasma oscillations in a 100 nanometer thick, crystalline ZnO layer containing a high density of approximately 10²⁰ free electrons per cubic centimeter. An infrared light pulse of 150 fs duration (1 fs = 10^-15 s) excited the plasma oscillations. Their frequency was measured by the infrared absorption of the plasma by means of a weaker sampling pulse time-resolved. From the shift of the absorption line, the instantaneous frequency of the plasma oscillations was determined (Figure 1). The experiments show a significant redshift, i. Reduction of the plasma frequency. However, the 20% reduction stops only 400 fs, after which the system returns to the original plasma frequency. Throughout the period of the experiment, the electron density remains unchanged.