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Description
Promotion of carriers to different points in k-space provides the basis for a new family of devices, where the excitation between specific valley states can be used as a switch to perform logic operations, exceeding the limits of conventional electronics for novel technologies.
With advances in stability and control of ultrashort laser pulses, several all-optical approaches to induce valley polarization in materials have been proposed, but still await confirmation through experiments.
One recent proposal using light induced states in hexagonal-BN gained a lot of attention as the mechanism is wavelength independent making it highly versatile.
In this approach a trefoil shaped field is used as the pump, making it fit to the crystal symmetry of h-BN. By rotating this triangular electric field, it is possible to match the boron or nitrogen atoms in the hexagonal lattice of the sample and switch between excitation into either the K or K' valley.
Building such a tailored pump is the first challenge in realizing this method experimentally. In this work we use our 160kHz 3$\mu$m-OPCPA system and build a Mach-Zehnder interferometer like arrangement. This way we can manipulate the polarizations, handedness, intensity-ratio and delay of the 3$ \mu$m beam and its second harmonic in separate arms and shape the resulting bi-circular pump into the trefoil.
To probe the pumped sample a weak 800nm pulse is used to drive low order harmonics inside the h-BN. Valley polarization is imprinted in the helicity of the harmonics and can therefore be measured with an ellipsometry setup.
