Ultrafast multi-level logic gates with spin-valley coupled polarization anisotropy in monolayer MoS2

Abstract

The inherent valley-contrasting optical selection rules for interband transitions at the K and K' valleys in monolayer MoS2 have attracted extensive interest. Carriers in these two valleys can be selectively excited by circularly polarized optical fields. The comprehensive dynamics of spin valley coupled polarization and polarized exciton are completely resolved in this work. Here, we present a systematic study of the ultrafast dynamics of monolayer MoS2 including spin randomization, exciton dissociation, free carrier relaxation, and electron-hole recombination by helicity- and photon energy-resolved transient spectroscopy. The time constants for these processes are 60 fs, 1 ps, 25 ps, and ~300 ps, respectively. The ultrafast dynamics of spin polarization, valley population, and exciton dissociation provides the desired information about the mechanism of radiationless transitions in various applications of 2D transition metal dichalcogenides. For example, spin valley coupled polarization provides a promising way to build optically selective-driven ultrafast valleytronics at room temperature. Therefore, a full understanding of the ultrafast dynamics in MoS2 is expected to provide important fundamental and technological perspectives.

Figure 1. Schematics of an optically driven ultrafast room-temperature and multi-level logic gate with monolayer MoS₂.

(a) The band diagram of monolayer MoS₂ at the K and K′ valleys. The blue and red colors represent spin-up and spin-down states, respectively. (b) A two-level MoS₂-gate can be written by circularly polarized 2.01 eV (resonant with exciton B) and 1.98 eV (resonant with exciton A) pulses and read by a linearly polarized pulse with a visible broadband spectrum.

Figure 2. Characterization of monolayer MoS₂.

(a) A photoluminescence spectrum, (b) a Raman spectrum and (c) an AFM measurement of monolayer MoS₂. (d) The height profile of MoS₂ gives an average thickness of ~0.72 nm.

Figure 3. Spectra of pump, probe pulses and the stationary absorbance of monolayer MoS₂.

Spectra of 1.89 eV pump pulse (red), 2.01 eV pump pulse (orange), broadband visible probe pulse (gray) and the stationary absorption of monolayer MoS₂ at room temperature (blue).

Figure 4. Transient difference absorbance (ΔA).

ΔA induced by excitation using σ⁺ circularly polarized pump pulse with the photon energy of 1.89 eV and probed by (a) σ⁺ and (b) σ⁻ circularly polarized pulse at 78 K. The black curves are contours of ΔA being zero. (c) Time-resolved ΔA spectra at various time delays between pump and probe pulses. (d) Delay time traces of ΔA at various probe photon energies. In (c) and (d), the red and blue lines represent σ⁺ and σ⁻ probe, respectively. The horizontal green lines show ΔA = 0.

Figure 5. Triple exponential fitting results of the delay time traces of ΔA data at 78 K and the scheme of relaxation processes.

(a), (b) Excited by 1.89 eV and σ⁺ pump pulse. (c) Excited by 2.01 eV and σ⁺ pump pulse. Left column: σ⁺ probe. Right column: σ⁻ probe. Solid circles (red), open squares (blue), open triangles (green), and open circles (gray) represent the components for spin randamization, exciton dissociation, hot carrier relaxation and electron-hole recombinataion, respectively. (a) and (c) ΔA spectra, (b) time constant of each component. Dot lines indicate the estimated values. Inset of (a): time-dependent (in log scale) mean energy of transition band A excited by 1.89 eV and σ⁺ pump pulse at 78 K. The solid squares are the σ⁺ probe and the open squares are the σ⁻ probe. (d) Schematics of the relaxation processes in monolayer MoS₂.

Figure 6. Comparison of the triple exponential fitting results at 78 K and 293 K.

Triple exponential fitting results of the delay time traces of ΔA data excited by 1.89 eV with σ⁺ pump and σ⁺ probe pulse at 293 K (black open squares) and 78 K (red solid circles).