多层MoS_(2)激子形成过程中多体效应的超快光谱响应研究

Ultrafast Spectral Response of Many-Body Effects During Exciton Formation in Multilayer MoS_(2)

采用飞秒瞬态吸收光谱技术研究了多层MoS_(2)在K谷最低激子形成过程中的多体效应对激子共振峰的影响及其内在的多体相互作用机制。经飞秒激光激发后,样品的两个激子峰都表现出早期的瞬间红移和后续先快速后迟缓的蓝移过程。早期的红移幅度随激发功率的提高而增大,但随样品温度的升高而减小。后续最低激子形成后的蓝移幅度随激发功率和样品温度的提高而减小。讨论了引起峰值红移的带隙重整化效应和引起蓝移的带填充效应在激子形成过程中的竞争关系,并且采用指数拟合估计了带隙重整化效应和带填充效应的持续时间随激光功率和样品温度的变化趋势。结果显示,引起共振峰红移的带隙重整化效应对激光功率和样品温度的依赖不明显,而引起共振峰蓝移的带填充效应随着激光功率和样品温度的提高而明显弱化,并且表现出明显的延迟效应。研究结果为动态调控二维材料光电响应性能提供了参考。

Objective Two-dimensional(2D)transition metal dichalcogenides(TMDs)have attracted significant attention due to their uniquely controllable properties.Their strong excitonic effects dominate the optical and electronic responses in 2D-TMDs.Coulomb interactions among carriers in the exciton formation process give rise to many interesting many-body effects,which affect the exciton resonance energy and subsequent exciton decay.Despite extensive efforts,a detailed discussion of many-body effects during the exciton formation process is still absent.Typically,narrow-band laser excitation was used in previous studies to inject free electron-hole pairs with energy greater than the lowest exciton resonance energy.In contrast,broadband excitation is helpful to exploit the strong exciton absorption and improve the time-resolution in transient experiments.How do multibody effects induced by broadband excitation influence the exciton resonance and exciton formation when high-density carriers are injected?The answer is essential for the understanding of the exciton properties and is helpful to design and control the photoelectric response of 2D-TMDs.Here we report the use of transient absorption spectroscopy with 10 fs time-resolution to address the ultrafast spectral response of many-body effects in the formation of the lowest exciton at the K valley in five-layer MoS_(2).Methods Multilayer MoS_(2) samples were prepared by chemical vapor deposition.In the transient absorption spectrum intensity(△A)measurements,an 800nm pulse(1 kHz,35 fs)output from an amplified Ti:sapphire laser system was first spectrally broadened using self-phase modulation via a hollow core fiber flled with noble gas,and the pulse duration was compressed with a set of chirped mirrors.Then,the 800 nm pulse was used to construct the degenerate pump-probe system[Fig.1(a)].The pulse energies and their polarizations were adjusted by a combination of a half-wave plate and a polarizer.The △A signal was obtained by measuring the differential absorbance change △A=-lg(1+△T/T),where△T/T=(Ton-Toff)/Toff,Ton and Toff are the intensities of the transmitted probe light passing through the sample with and without pump excitation,respectively.The time resolution for this apparatus is~10 fs,determined by an autocorrelation trace between the broadband pump and probe pulses[Fig.1(b)].During this experiment,pump light was polarized vertically compared with the probe light.The focal spot diameters of the pump and probe beams are~400 and 200μm,respectively.The Raman spectral measurement indicates that the sample has five layers[Fig.2(b)].Fluence dependence studies confirm that excitation in our experiments occurs in the linear regime.Results and Discussions The broadband laser excitation results in two ground state bleachings(GSB)due to transitions of excitons A and B[Fig.2(a)]in ΔA spectra[Figs.3(a)and 3(b)].The time evolutions of both signals appear to be biphasic,and their global exponential fitting results indicate that the two processes are due to carrier thermalization and cooling,respectively,during the relaxation before the formation of the lowest exciton phase[Fig.3(c)].With carrier relaxation,the intense many-body effects lead to an obvious temporal evolution of exciton resonance,which can be discerned from the first moment trace.The moment traces of two excitons exhibit a similar initial fast red-shift,followed by a slow blue-shift(Figs.4 and 5).The initial red-shift takes place within 50 fs,and the blue-shift is almost finished before~2 ps.Therefore,the exciton resonance positions at 50 fs and 2 ps were extracted as a function of pump power[Figs.4(c)and 4(d)]and sample temperature[Figs.5(c)and 5(d)].The red-shift amplitude increases with increasing pump power and decreases with increasing temperature.For the blue-shift,it decreases with both pump power and sample temperature.The initial red-shift is likely due to the reduction of the repulsive Coulomb interaction between the charges of the same signs,which results in band gap renormalization(BGR).In the following slow process,incomplete compensation of BGR and band-filling effects produce the blue-shift of exciton resonance.The temporal evolution of exciton resonance is further fitted using a bi-exponential function[Figs.4(e),4(f),5(e)and 5(f)].The results demonstrate that the duration of BGR effect is nearly constant with increasing pump power,whereas,that of the band-filling effect is appreciably prolonged with increasing laser power and sample temperature.This may be due to the hot-phonon effect during the band filling process.Conclusions Femtosecond transient absorption spectroscopy was used to study the optical response of the multibody effect among carriers during the lowest exciton formation process at the K-valley in multilayer MoS_(2).Many-body effects on exciton resonance occur along with carrier relaxation.The resonance positions of two excitons demonstrate an early instantaneous red-shift and a subsequent slow blue-shift.The red-shift amplitude increases with increasing pump power and decreases with increasing sample temperature.The amplitude of the red-shift reflects the modulation of BGR effect among early free carriers.The blue-shift amplitude after the lowest exciton formation is reduced with increasing laser power and sample temperature.The duration of the BGR effect is almost insensitive to laser power and sample temperature,whereas,that of the band-filling effect is prolonged.Beyond the fundamental interests in physics,the observation of temporal evolution of exciton resonance is helpful to develop effective methods to dynamically control the photoelectric responses of 2D-TMDs.