The Marksborn Institute in Germany uses a hybrid of super-fast lasers to control huge currents

Ultrafine laser control of the fundamental quantum degrees of freedom of matter represents an outstanding fundamental challenge to be met in building the information technologies of the future beyond the semiconductor electronics that define our present age. Two of the most promising quantum degrees of freedom in this regard are the spin of electrons and the "valley index," an emerging degree of freedom for two-dimensional materials related to the momentum of quasiparticles. Both spintronics and glutenonics have many potential advantages over traditional electronics in terms of data processing speed and energy efficiency. However, while spin excitation suffers a loss of dynamic properties due to spin precession due to spin orbits, the valley wave function represents a "data bit" whose stability is only threatened by interval scattering, which is a controllable sample mass feature. As a result, Valleytronics offers a potentially powerful platform for moving beyond classic electronics.

In addition to quantum excitation of encoded data bits, the core of any future valley or spintronics technology lies in the control and creation of valley and spin currents. However, despite continued attention to the task of customizing light forms on ultrafast time scales to selectively excite valley quasi particles, the precise generation and control of valley and spin currents -- essential for any future valley electronics technology -- remains beyond the realm of ultrafast optical control. In a recent study published in Science Advances, a team of researchers from the Max Born Institute in Berlin showed how a hybrid laser pulse combining two polarization types is able to fully control the ultrafine laser induced current.

Control of charge states by circularly polarized light is well established, and the famous transition metal disulfide "spin valley locking" originates from a valley selective response to circularly polarized light. This can be seen as caused by the selection rule involving the magnetic quantum number of d orbitals containing the edge states of the gap. Although the circularly polarized light excites the valley charge, for each quasi momentum excited in the valley, the corresponding -K valley is also excited: the Bloch velocity thus cancels out and there is no clear valley current.

Thus, full control of photoinduced valley currents, their magnitude and direction requires going beyond the spin valley locking paradigm of circularly polarized light. Therefore, the generation of valley excited states that do lead to clear valleys and spin currents must involve breaking local k valleys and degradation of -k valleys. Because the laser vector potential is directly coupled to the crystal quasi momentum, k -> k -- A (t)/c, the most efficient method is through linearly polarized monocyclic pulses, whose duration is the same as that of circularly polarized pulses: such pulses would obviously be within the "terahertz window" of 1 THz to 50 THz. The honeycomb light generates a large residual current (i.e. persists after the laser pulse). This is due to the fact that the Bloch velocity of the excited quasi momentum is not cancelled because the distribution of the excited charge is now being shifted away from the highly symmetric K point by the polarization vector of the terahertz pulse.

Source: Laser Net