Hybrid laser pulses allow for very fast control of spin and valley currents

(Nanowerk News) The flow of matter, from large-scale flows of water to microscopic flows of electric charges, is fundamental to the infrastructure of modern society. Scientists are constantly looking for ways to control quantum aspects of matter, such as electron spin and valley states, to make breakthroughs in energy efficiency, data storage capacity and processing speed.

Researchers at the Max Born Institute in Berlin recently devised a method for inducing and controlling spin and valley currents over very fast timeframes using specially designed laser pulses. This discovery offers a new perspective in the search for the next generation of information technology.

Ultrafast laser control of the fundamental quantum properties of matter is a critical challenge that must be overcome to develop future information technologies beyond the semiconductor electronics that define our current era. Electron spin and valley indices, emergent properties of two-dimensional materials related to the momentum of quasiparticles, are two promising quantum properties in this regard. Both spintronics and valleytronics offer many potential advantages over classical electronics in terms of data manipulation speed and energy efficiency.

While spin excitation suffers from a loss of dynamic character due to orbit-induced spin precession, the valley wavefunction represents a more stable “data bit” that is threatened only by interval scattering, a feature that can be controlled by sample quality. Valleytronics thus presents a potentially powerful platform to move beyond classic electronics. Circularly polarized laser pulse (ac) versus hencomb laser pulse (df). The vector potential of a circularly polarized light wave applied to the two-dimensional Tungsten Diselenide material produces no residual current after the laser pulse (a,b), only attracts the valley charge state, (c), where the Bloch velocity of all excited states cancels out based on the excitation of the charge which is positioned at the center of the valley (top of Brillouin zone shown by red line). In contrast, the hencomb pulse, in which a circularly polarized light wave is augmented by a symmetrically broken linearly polarized THz component produces an apparent residual current (d,e), which is generated when the symmetry breaking shifts the excited charge from the valley center (f), leading to non-cancellation of current over the distribution of excited charges. While a THz pulse by itself does not cause excitation, in combination with circularly polarized light, the THz polarization vector and amplitude are the key parameters that completely control the final current state. (Image: FVB)

Creating and controlling trough and loop currents will be very important for future valleytronics or spintronics technologies, besides the quantum excitations that encode data bits. Although significant attention has been paid to fine-tuning the shape of light on ultrafast timescales to selectively excite valley quasiparticles, the creation and precise control of valley currents and spin currents – essential for future valleytronics technologies – remains outside the scope of ultrafast light control.

In a study published in Science Advances (“THz induces giant spins and valley currents”), researchers from the Max Born Institute in Berlin demonstrate how a hybrid laser pulse combining two types of polarization allows complete control over the current induced by an ultrafast laser beam.

Controlling charge states with circularly polarized light is well established, derived from the spin-valley locking of transition metal dichalcogenides, resulting from a valley-selective response to circularly polarized light. However, circularly polarized light does not create valley currents, because the Bloch velocity cancels out and there are no net valley currents. To achieve complete control over light-induced valley currents, both in magnitude and direction, researchers must move beyond the spin-valley locking paradigm of circularly polarized light. Schematic illustration of the action of a hencomb pulse: (i) half a THz light cycle drives the intraband motion delivery state far from slit edge to slit edge; (ii) optical components generate states across the slit; (iii) finally the second half cycle of the THz light restores the state to its original momentum. The overall action of the hencomb pulse is to excite the charge at a finite trough momentum determined by the THz pulse polarization vector. (Image: FVB)

Creating an excited-valley state that produces clean valleys and loop currents involves breaking local valley degeneracy. The most effective way to do this is via a linearly polarized single-cycle pulse of comparable duration to a circularly polarized pulse, which falls within the “THZ Window” from 1 THz to 50 THz. The researchers demonstrated that the resulting light shape generates a substantial residual current due to the non-cancellation of the Bloch velocity of the excited apparent momentum.

The physical process underlying this pulse is illustrated in three stages: the half-cycle of the THz component of the pulse drives intraband motion, sending state away from slit edge to slit edge; the circularly polarized component then excites this charge across the gap; finally, the second half cycle of the THz component returns the charge to its original momentum. The polarization vector of the THz light component is the main control parameter of the pulse, with the polarization direction and amplitude determining the direction and amplitude of the light-induced current. In this way, these pulses offer a new route to ultrafast valleytronics and spintronics by providing direct light control over the charge and current state of two-dimensional materials active in the valley.