Encoding information in an electron’s momentum state (i.e., by which valley in the material’s Brillouin zone the electron occupies) forms the conceptual basis of the burgeoning field of “valleytronics.” In particular, the family of monolayer TMDs such as MoS 2 and WSe 2 has focused attention on valley physics because they provide a facile means of addressing specific valleys in momentum space using light: Owing to strong spin-orbit coupling and lack of crystalline symmetry, monolayer TMDs have valley-specific optical selection rules ( 11– 14), wherein right and left circularly polarized light couples selectively to transitions in the distinct K and K′ valleys of their hexagonal Brillouin zone. The development of novel two-dimensional (2D) materials such as graphene, bilayer graphene, and monolayer transition metal dichalcogenide (TMD) semiconductors has rejuvenated long-standing interests ( 1, 2) in harnessing valley degrees of freedom ( 3– 10).
#INTRINSIC DARK NOISE FREE#
These results provide a viable route toward quantitative measurements of intrinsic valley dynamics, free from any external perturbation, pumping, or excitation. Moreover, the noise signatures validate both the relaxation times and the spectral dependence of conventional (perturbative) pump-probe measurements. This spontaneous “valley noise” reveals narrow Lorentzian line shapes and, therefore, long exponentially-decaying intrinsic valley relaxation. Exploiting their valley-specific optical selection rules, we use optical Faraday rotation to passively detect the thermodynamic fluctuations of valley polarization in a Fermi sea of resident carriers. Here, we demonstrate an entirely noise-based approach for exploring valley dynamics in monolayer transition-metal dichalcogenide semiconductors. With a view toward valley-based (opto)electronic technologies, the intrinsic time scales of valley scattering are therefore of fundamental interest. Together with charge and spin, many novel two-dimensional materials also permit information to be encoded in an electron’s valley degree of freedom-that is, in particular momentum states in the material’s Brillouin zone.
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