Optical Injection and Detection of Long-Lived Interlayer Excitons in van der Waals Heterostructures

Abstract

Interlayer excitons in semiconducting bilayers separated by insulating hBN layers constitute a promising platform for investigation of strongly correlated bosonic phases. Here, we report an optical method for the generation and characterization of long-lived interlayer excitons. We confirm the presence of tightly bound interlayer excitons by measuring 1s and 2s intralayer excitons in each layer concurrently. Using a pump-probe technique, we find interlayer exciton lifetimes up to 8.8 $μ$s, increasing with the thickness of the hBN. With optical access to long-lived interlayer excitons, our approach provides a new route to explore degenerate Bose--Fermi mixtures of excitons and itinerant electrons with high spatial and temporal resolution.

Figures (4)

• Figure 1: (a) Schematic of the device structure. (b) Normalized reflection spectra ($R/R_0$) of the MoSe2 and WSe2 layers as a function of gate voltage $V_\mu$. Resonances corresponding to the repulsive polaron (RP), attractive polaron (AP), and neutral exciton (X) are indicated. Blue and red dashed lines mark the onset of hole and electron doping, respectively. A weak pump power was present during acquisition ($P_{\rm pump}$ = $\qty{1.7}{\micro\watt}$); its effect on the spectra is negligible. (c) Schematic under pump--conditions. (d) $R/R_0$ at finite pump power ($P_{\rm pump}$ = $\qty{6}{\micro\watt}$). (e) The sum of the peak 1s X (RP) reflection contrasts of the MoSe2 and WSe2 layers, each normalized to its respective reflection contrast in the absence of charges, $\mathcal{Z}_{\mathrm{sum}}$, is shown as a function of $V_\mu$ and $P_\mathrm{pump}$. Distinct charge configurations are labeled as (i, i), (i, h), (e, i), and (h, e), representing combinations of intrinsic (i), hole (h), and electron (e) doping in the Mo and W layers, respectively. The dotted black contour marks the region where the normalized X (RP) reflection contrast in both layers exceeds 0.7, indicating approximate charge neutrality; note, however, that small but finite doping persists near the boundary of the (i, i) region. The dotted light grey contour outlines the region with opposite doping in the two layers, where the normalized X (RP) reflection contrast in each layer falls below 0.7. Arrows indicate the positions of vertical and horizontal linecuts. Data in panels (b)--(e) were acquired using Device 1.
• Figure 2: (a) Schematic of the 1s, 2s intralayer excitons and interlayer exciton (IX). (b) Differential reflection spectra ($\mathrm{d} (R/R_0) / \mathrm{d} E$) plotted as a function of gate voltage $V_\mu$, in the energy range of the Rydberg excitons. Data are shown for pump powers of $P_{\text{pump}} = \qty{1.7}{\micro\watt}$ (upper panel) and $P_{\text{pump}} = \qty{6}{\micro\watt}$ (lower panel). MoSe$_2$ and WSe$_2$ 2s Rydberg excitons are labeled (see Fig. S7 for discussion on 3s Rydberg excitons). (c) Colormap of WSe2 2s contrast as a function of $V_\mu$ and $P_\mathrm{pump}$. The dashed black and grey contour lines delineate different charge configurations, defined as in Fig. \ref{['fig:1']}e. The dashed pink contour marks the boundary where the WSe2 2s contrast falls below 0.055 ($20\%$) in the region where the two layers are oppositely doped. (d,e) Log-scale spectral linecuts at distinct doping configurations (see diamond markers in panel (c)). The blue and red curves correspond to only hole doping in WSe2 and only electron doping in MoSe2, respectively, with the opposite layer remaining charge neutral. The black curve shows simultaneous doping of both layers, with individual charge carrier densities matching those in the red and blue cases. (f) Differential reflection spectra ($\mathrm{d} (R/R_0) / \mathrm{d} E$) plotted across the Rydberg exciton resonances. The green curve, measured at charge neutrality (green diamond), shows the bare Rydberg excitons. The 2s resonance disappears when only one layer is doped (red and blue curves), but remains visible without a significant spectral shift when both layers are doped simultaneously (black curve). Data in panels (b)--(f) were acquired using Device 1.
• Figure 3: Charge carrier densities extracted from the optical spectra measured in Device 1 as a function of the gate voltage, $V_\mu$, and pump power, $P_{\rm pump}$: (a) electron density, $n_e$, in the MoSe$_2$ layer; (b) hole density, $n_h$, in the WSe$_2$ layer; (c) minimum of $n_e$ and $n_h$ (min$(n_e, n_h)$); (d) excess charge $n_e - n_h$. Dotted contour lines denote the same boundaries depicted in Figs. \ref{['fig:1']}e and \ref{['fig:2']}c.
• Figure 4: Time evolution of the minimum of the electron and hole densities, $\min(n_e,n_h)$, as a function of time delay between the pump and probe pulses (see inset). Data points from Device 1 (D1, monolayer hBN spacer) are shown as black filled circles, while those from Device 2 (D2, 3–5‑layer hBN spacer) are shown as dark red open circles. The dashed lines mark the density at which $\min(n_e,n_h)$ has decayed to $1/e$ of its initial value; the corresponding delay times are $\qty{1.4}{\micro\second}$ for Device 1 and $\qty{8.8}{\micro\second}$ for Device 2.