Enhancing Ground-State Population and Macroscopic Coherence of Room-Temperature ${\mathrm{WS}}_2$ Polaritons through Engineered Confinement

Exciton polaritons (polaritons herein) in transition-metal dichalcogenide monolayers have attracted significant attention due to their potential for polariton-based optoelectronics. Many of the proposed applications rely on the ability to trap polaritons and to reach macroscopic occupation of their ground energy state. Here, we engineer a trap for room-temperature polaritons in an all-dielectric optical microcavity by locally increasing the interactions between the ${\mathrm{WS}}_2$ excitons and cavity photons. The resulting confinement enhances the population and the first-order coherence of the polaritons in the ground state, with the latter effect related to dramatic suppression of disorder-induced inhomogeneous dephasing. We also demonstrate efficient population transfer into the trap when optically injecting free polaritons outside of its periphery.

Figure 1

(a) Sample design for creating a trap for ${\mathrm{WS}}_2$ polaritons by placing a small monolayer (red dot) on top of a ${\mathrm{WS}}_2/{\mathrm{Ga}}_2{\mathrm{O}}_3$ heterostructure (orange dot), sandwiched between DBR mirrors. (b) Calculated distribution of the light field inside the microcavity. The different materials are represented by their respective dielectric constants $n$. (c) Calculated angle-resolved transmission spectra of the structure at the position of (left) the large monolayer and (right) the small monolayer, marked with the orange and red dots, respectively. The energy difference $\mathrm{\Delta }V$ corresponds to the effective trapping potential at the position of the small monolayer in (a).

Figure 2

(a) Microscope image of the ${\mathrm{WS}}_2/{\mathrm{Ga}}_2{\mathrm{O}}_3/{\mathrm{WS}}_2$ heterostructure on top of a DBR substrate. The large monolayer spans $\sim 20\mu \mathrm{m}$ in $y$ and $>50\mu \mathrm{m}$ in $x$ direction (see Supplemental Material [36]), and the top monolayer is $\sim 4\mu \mathrm{m}$ in both directions. The scale bar is $5\mu \mathrm{m}$. (b) Reflectivity spectra of the (blue) ${\mathrm{WS}}_2/{\mathrm{Ga}}_2{\mathrm{O}}_3$ and (red) ${\mathrm{WS}}_2/{\mathrm{Ga}}_2{\mathrm{O}}_3/{\mathrm{WS}}_2$ heterostructure. (c) PL image of the complete microcavity excited with a large Gaussian laser spot ($\mathrm{FWHM}\approx 30\mu \mathrm{m}$, $\lambda =532\mathrm{nm}$). The scale bar is $5\mu \mathrm{m}$. (d) Spatially resolved PL spectrum at $k_{\parallel }\approx 0$ along the horizontal dashed line in (c). (e) Angle-resolved PL spectrum at $x\approx 30\mu \mathrm{m}$. (f) Polariton mode tomography in the trap ($x\approx 30\mu \mathrm{m}$) at the energies of the (1) ground, (2) first excited, (3) second excited, and (4) continuum state $\sim 15\mathrm{meV}$ above the potential well, marked with the arrows in (e).

Figure 3

(a) Energy and (b) momentum-resolved PL of the (red) free and (black) trapped polaritons. (c),(d) Power-dependent intensities at low energy and bottleneck regions for the (c) free and (d) trapped polaritons extracted from the angle-resolved PL spectra [36]. Black lines are the polariton intensities calculated using the model in [36].

Figure 4

(a) and (b) Spectrally resolved Michelson interference maps of the (a) free and (b) trapped polaritons [36]. The overlays are the normalized interference fringes at the energies marked by the $x$-axis position, with the amplitude corresponding to their visibility and $g^{(1)}(\mathrm{\Delta }x)$ [36]. (c) Temporal first-order coherence $g^{(1)}(\mathrm{\Delta }\tau )$. Solid lines are the fits accounting for inhomogeneous and homogeneous decoherence (see text).

Figure 5

(a) Schematics of the ring-shaped excitation around the small monolayer. (b) Spatially resolved intensity map of the excitation with the outline of the small monolayer. (c) Angle-resolved PL spectrum of the trap area. (d) Spectrally resolved Michelson interference map of the trapped polaritons and (inset) the normalized interference fringes.