[Paper Review] Ultrafast Generation of Pseudo-magnetic Field for Valley Excitons in WSe2 Monolayers

Jonghwan Kim, Xiaoping Hong+6 more|arXiv (Cornell University)|Jul 9, 2014

2D Materials and Applications21 citations

TL;DR

This study demonstrates ultrafast, all-optical control of valley excitons in monolayer WSe2 using circularly polarized femtosecond laser pulses, inducing a valley-selective optical Stark effect that generates an effective pseudomagnetic field exceeding 170 Tesla. The effect lifts valley degeneracy instantaneously without dissipation, enabling coherent manipulation of valley pseudospin for quantum information applications.

ABSTRACT

A new degree of freedom, the valley pseudospin, emerges in atomically thin two-dimensional transition metal dichalcogenides (MX2) and has attracted great scientific interest. The capability to manipulate the valley pseudospin, in analogy to the control of spin in spintronics, can open up exciting opportunities in valleytronics. Here we demonstrate that an ultrafast and ultrahigh valley pseudomagnetic field can be generated using circularly polarized femtosecond pulses to selectively control the valley degree of freedom in monolayer MX2. Employing ultrafast pump-probe spectroscopy, we observed a pure and valley-selective optical Stark effect in WSe2 monolayers from the non-resonant pump, which instantaneously lift the degeneracy of valley exciton transitions without any dissipation. The strength of the optical Stark effect scales linearly with both the pump intensity and the inverse of pump detuning. An energy splitting more than 10 meV between the K and K_prime valley transitions can be achieved, which corresponds to an effective pseudomagnetic field over 170 Tesla. Our study demonstrates efficient and ultrafast control of the valley excitons with optical light, and opens up the possibility to coherent manipulate the valley polarization for quantum information applications.

Motivation & Objective

• To achieve ultrafast, coherent control of valley pseudospin in two-dimensional transition metal dichalcogenides.
• To demonstrate the generation of an effective pseudomagnetic field via optical excitation without dissipation.
• To explore the feasibility of valleytronics using non-resonant, circularly polarized femtosecond pulses.
• To quantify the strength and dynamics of the valley-selective optical Stark effect in WSe2 monolayers.
• To enable potential applications in quantum information processing through coherent valley polarization control.

Proposed method

• Ultrafast pump-probe spectroscopy was employed to probe the valley exciton dynamics in monolayer WSe2.
• Circularly polarized femtosecond laser pulses were used to selectively excite K and K' valley states.
• The non-resonant pump induced a valley-selective optical Stark shift, lifting degeneracy between K and K' valley transitions.
• The strength of the optical Stark effect was measured as a function of pump intensity and detuning.
• The effective pseudomagnetic field was calculated from the observed energy splitting using the relation ΔE = g*μ_B*B_eff.
• The system's response was analyzed in the absence of thermalization or relaxation effects, ensuring pure optical control.

Experimental results

Research questions

• RQ1 Can ultrafast, all-optical control of valley pseudospin be achieved in monolayer WSe2?
• RQ2 What is the maximum effective pseudomagnetic field strength achievable via optical excitation?
• RQ3 How does the energy splitting between valley excitons scale with pump intensity and detuning?
• RQ4 Can the valley degeneracy be lifted without dissipation or thermalization?
• RQ5 What is the potential for coherent manipulation of valley polarization in a 2D semiconductor?

Key findings

• An energy splitting of over 10 meV was observed between K and K' valley exciton transitions under optimal conditions.
• The observed energy splitting corresponds to an effective pseudomagnetic field exceeding 170 Tesla.
• The optical Stark effect scaled linearly with both pump intensity and the inverse of pump detuning.
• The effect was purely optical and non-dissipative, with no thermalization or relaxation observed.
• The valley-selective control was achieved on a femtosecond timescale, enabling ultrafast manipulation.
• The results demonstrate a viable pathway for coherent, all-optical control of valley pseudospin in 2D materials.