Quantum tunneling high-speed nano-excitonic modulator

Abstract

High-speed electrical control of nano-optoelectronic properties in two-dimensional semiconductors is a building block for the development of excitonic devices, allowing the seamless integration of nano-electronics and -photonics. Here, we demonstrate a high-speed electrical modulation of nanoscale exciton behaviors in a MoS₂ monolayer at room temperature through a quantum tunneling nanoplasmonic cavity. Electrical control of tunneling electrons between Au tip and MoS₂ monolayer facilitates the dynamic switching of neutral exciton- and trion-dominant states at the nanoscale. Through tip-induced spectroscopic analysis, we locally characterize the modified recombination dynamics, resulting in a significant change in the photoluminescence quantum yield. Furthermore, by obtaining a time-resolved second-order correlation function, we demonstrate that this electrically-driven nanoscale exciton-trion interconversion achieves a modulation frequency of up to 8 MHz. Our approach provides a versatile platform for dynamically manipulating nano-optoelectronic properties in the form of transformable excitonic quasiparticles, including valley polarization, recombination, and transport dynamics.

Fig. 1. Quantum tunneling nanoplasmonic cavity and high-speed exciton-trion interconversion.

a Illustration of high-speed trion-to-exciton (left) and exciton-to-trion (right) conversions within the QNC. Excitation light is projected with wave vector $\overrightarrow{k}$ and its electric field ${\overrightarrow{E}}_{\mathrm{exc}}$. b Band diagram of the MoS₂ ML in contact with the plasmonic Au tip under positive (top), zero (middle), and negative (bottom) V_tip. c Schematic of the QNC combined with the autocorrelation measurement setup. Abbreviations: neutral-density filter (ND), half-wave plate (λ/2), beam splitter (BS), objective lens (OL), tuning fork (TF), long-pass filter (LF), multimode fiber (MF), avalanche photodiode (APD), and function generator (FG), Lead zirconate titanate (PZT), quantum tunneling nanoplasmonic cavity(QNC), monolayer (ML), photoluminescence (PL), quantum yield (QY), time-correlated single-photon counting (TCSPC), neutral exciton (X₀), trion (X-), electron (e-), hole (h+).

Fig. 2. Spatial distribution of optical field and electric potential in the quantum tunneling nanoplasmonic cavity.

a Distribution of the optical field intensity ${∣E_{\mathrm{z}}∣}^2$ without (left) and with (right) the HfO₂ layer, when the tip-HfO₂ distance is 2 nm. b ${∣E_{\mathrm{z}}∣}^2$ distribution in a xy-plane for different tip-HfO₂ distances. The tip-sample distance is denoted as d. The z-position of cross-sectional view is fixed along the white dashed line (MoS₂ ML) in (a). c Distribution of the electric potential without (left) and with (right) HfO₂ layer when d = 2 nm, upon applying the DC bias on the tip. d Profile of optical field intensity ${∣E_{\mathrm{z}}∣}^2$ (black dashed line) and electric potential (blue filled region), derived from white dashed lines in (a) and (c).

Fig. 3. Nanoscale exciton-trion interconversion through optical and electrical control.

a Contour plot of PL spectra of the MoS₂ ML as a function of the tip-sample distance. b Distance-dependent change of X₀ (red) and X- (blue) emission intensities, derived from (a). c PL spectra of the MoS₂ ML as a function of the $V_{\mathrm{tip}}^{\mathrm{DC}}$, when the tip-sample distance is shorter than 3 nm (quantum tunneling regime). d Lorentz fitted PL spectra of the MoS₂ ML when the $V_{\mathrm{tip}}^{\mathrm{DC}}$ is +10 V (top), 0 V (middle), and −10 V (bottom) on the Au tip. Black dashed lines in (a) and (c) indicate the energies of X₀ and X-.

Fig. 4. Autocorrelation measurement of the high-speed electrical modulation of excitons and trions.

a Illustration depicting modified exciton behaviors in the MoS₂ ML with a different polarity of modulation amplitude. Left and right panels indicate the transition to X- dominant and X₀ dominant states, respectively. b Measured coincidence of the electrically modulated PL intensity for different modulation frequencies, with the fixed amplitude of +5 V (X₀ modulation) and the excitation power of  ~150 μW. c High-speed electrical modulation of X- conversion (−5 V, blue) and X₀ conversion (+5 V, red) with the excitation power of  ~3.5 mW. τ and $V_{\mathrm{tip}}^{\mathrm{AC}}$ represent the period of excitonic modulation and the AC bias on the Au tip, respectively.