Ultrafast Coherent Control of Valley Polarization in a 2D Semiconductor

Today's information processing technology relies on electronics, with transistor switches reaching speeds as high as 800 GHz yet appearing to approach their limits. The next disruptive step in increasing speed of information processing should come from driving electronic response in two-dimensional materials with ultrafast controlled lightwaves. This so-called lightwave electronics aims to use ultrashort pulses of light to switch electric currents and can potentially operate at nearly PHz rates. Lightwave valleytronics targets a new degree of freedom for information processing offered by excitons in two-dimensional materials with broken inversion symmetry: the valley pseudospin. The valley pseudospin is associated with the occupation of energy degenerate, but distinct valleys K and K’.<br/>The optical selection rule, which couples the valleys with circularly polarized light, provides important implications for the development of valleytronics-based devices[1]. However, to this day, the short valley lifetime (i.e. few picoseconds) has prevented any practical implementation of valleytronics. Recently, the realization of a translating-wedge based identical pulses encoding system (TWINS) enabled the generation of phase-locked collinear pulses with a delay controlled on a sub-fs scale[2]. Exploiting this technology, we experimentally prove a new all-optical coherent ultrafast protocol to manipulate the valley polarization in two-dimensional semiconductors[3]. In our experiment, a couple of delayed phase-locked ultrashort laser pulses with perpendicular polarization enables us to induce a positive or negative valley polarization in a WS2 monolayer. Our findings show that by making sub-cycle adjustments to the delay, we can continuously switch from exciting one valley to exciting the other.<br/>Then, exploiting four phase-locked pulses, we realize an ultrafast switch of the valley polarization at room temperature: by carefully controlling the delays between the excitation pulses we quench and amplify the valley polarization on a sub-100 fs temporal scale. Our measurements also allow us to extract the excitonic dephasing time. Our experimental findings are validated by theoretical simulations calculated from first principles.<br/>Our results demonstrate the possibility to control the valley polarization in two-dimensional semiconductors on a ultrafast timescale, opening a new route for ultrafast information processing with low-power few-cycle light pulses available today. We also provide a novel approach to investigate the properties of these materials.<br/><br/>[1] Mak, K. F., He, K., Shan, J. & Heinz, T. F. Control of valley polarization in monolayer MoS2 by optical helicity. Nature Nanotechnology 7, 494–498 (2012).<br/>[2] Brida, D., Manzoni, C. & Cerullo, G. Phase-locked pulses for two-dimensional spectroscopy by a birefringent delay line. Optics Letters 37, 3027 (2012).<br/>[3] Silva, R. E. F., Ivanov, M. & Jiménez-Galán, Á. All-optical valley switch and clock of electronic dephasing. Optics Express 30, 30347 (2022).<br/>[4] Gucci, F <i>et al., in preparation</i>