Control of the Exciton Radiative Lifetime in van der Waals Heterostructures

Optical properties of atomically thin transition metal dichalcogenides are controlled by robust excitons characterized by a very large oscillator strengths. Encapsulation of monolayers such as ${\mathrm{MoSe}}_2$ in hexagonal boron nitride (hBN) yields narrow optical transitions approaching the homogenous exciton linewidth. We demonstrate that the exciton radiative rate in these van der Waals heterostructures can be tailored by a simple change of the hBN encapsulation layer thickness as a consequence of the Purcell effect. The time-resolved photoluminescence measurements show that the neutral exciton spontaneous emission time can be tuned by one order of magnitude depending on the thickness of the surrounding hBN layers. The inhibition of the radiative recombination can yield spontaneous emission time up to 10 ps. These results are in very good agreement with the calculated recombination rate in the weak exciton-photon coupling regime. The analysis shows that we are also able to observe a sizable enhancement of the exciton radiative decay rate. Understanding the role of these electrodynamical effects allows us to elucidate the complex dynamics of relaxation and recombination for both neutral and charged excitons.

Figure 1

(a) Schematics of the investigated ${\mathrm{MoSe}}_2$ monolayer embedded in hexagonal boron nitride. (b) Schematics of the cross section and optical microscope image of the van der Waals heterostructure $\mathrm{hBN}/\mathrm{ML}$ ${\mathrm{MoS}}_2/\mathrm{hBN}$ (sample III), where the same monolayer is embedded in a cavitylike structure characterized by different bottom hBN layer thickness $d$. (c) Optical intensity map calculated at the ${\mathrm{MoSe}}_2$ monolayer location as a function of both the emission wavelength and the bottom hBN layer thickness $d$. The horizontal white dotted line corresponds to the neutral exciton emission wavelength ($\sim 756\mathrm{nm}$). (d) cw photoluminescence spectrum of sample II ($d=273\mathrm{nm}$) showing the emission of both the neutral ($X$) and charged ($T$) exciton, $T=7\mathrm{K}$.

Figure 2

(a) Left: normalized photoluminescence intensity (log scale) of the neutral exciton $X$ as a function of time for sample I ($d=180\mathrm{nm}$) and sample II ($d=273\mathrm{nm}$); the full lines correspond to the biexponential fits (see text). The instrument response is obtained by detecting the backscattered laser pulse (wavelength 712 nm) on the sample surface, see the dotted line labeled Laser; Right: enlargement of the rise time (linear scale). (b) Calculated (full line) and measured (symbols) neutral exciton radiative lifetime as a function of the hBN bottom layer thickness $d$. The red dashed curve is the calculated intensity of the electromagnetic field in our structure [same calculation as in Fig. 1]. Inset: normalized time-resolved photoluminescence intensity in sample III for three different hBN bottom layer thicknesses. (c) Normalized cw PL intensity of the neutral exciton in sample I and sample II clearly showing different linewidths. Because the energy of the PL peak slightly depends on the sample and the samples’ position by a few meV, the origin of the energy axis is taken at the PL peak. Inset: PL linewidth (FWHM) for 10 different positions in samples I and II.

Figure 3

(a) Normalized photoluminescence intensity of the charged exciton ($T$) as a function of time for different bottom hBN layer thicknesses $d$. The full lines correspond to monoexponential fits of the decay time ${\tau }_T$. (b) Measured (symbols) and fitted (full line) of neutral ($X$) and charged ($T$) exciton dynamics for encapsulated ${\mathrm{MoSe}}_2$ monolayer with a bottom hBN layer thickness $d=273\mathrm{nm}$ (sample II). Inset: schematics of the two-levels model used to describe both neutral ($X$) and charged ($T$) exciton dynamics (see text).