You ZhouZhou Lab · Materials Science & Engineering · University of Maryland

Research

Quantum phases of electrons and excitons, and how light gets at them

Two-dimensional semiconductors are the cleanest place we know to make electrons interact strongly and then look at what they do. The group's work runs along three coupled lines: creating correlated phases in van der Waals heterostructures, probing them with excitons, and engineering the light–matter coupling that controls both.

Theme 1

Creating correlated phases

Wigner crystals, moiré insulators, and interlayer excitons

What phases form when Coulomb energy wins over kinetic energy, and how far can we push them?

The large effective masses and weak screening of transition-metal dichalcogenide monolayers make electron–electron interactions dominate at densities and temperatures that are accessible in a dilution-free cryostat. We use this to realize phases that were predicted decades ago and to invent new ones. Interlayer Coulomb interactions in a MoSe2/hBN/MoSe2 heterostructure lock electrons in the two layers into a bilayer Wigner crystal whose formation and quantum melting we map optically. Moiré superlattices in WS2/WSe2 and twisted WSe2 give excitonic Mott insulators and quantum anomalous Hall states. And a single hBN spacer between two MoSe2 layers keeps interlayer excitons tightly bound and bright while a gate tunes their energy and drives them across the device, which is the starting point for electron–hole crystals and exciton condensates.

Bilayer Wigner crystal layer 1layer 2
Two interlocking electron lattices: the bilayer Wigner crystal of the 2021 Nature paper.

Theme 2

Excitons as many-body probes

Polaron spectroscopy of correlated electrons

Can a single exciton tell us how a crystal of electrons moves?

An exciton placed in an electron sea is dressed by its surroundings and becomes a polaron. In a Fermi liquid this gives the familiar attractive and repulsive polaron branches; in a Wigner crystal the exciton drags a lattice distortion with it and becomes a Wigner polaron. Because the dressing depends on the state of the electrons, the exciton's optical spectrum becomes a quantitative probe of many-body physics that is otherwise nearly invisible: the stiffness and melting of an electron crystal, the emergence of quantum anomalous Hall states in a moiré lattice, and the onset of a Mott insulator seen through how far excitons diffuse. Several of these measurements are joint work with Mohammad Hafezi's group at Maryland and with theory collaborators at ETH Zurich, Heidelberg, and Harvard.

Wigner polaron
A Wigner polaron: the exciton (red) and the distortion it induces in the surrounding electron crystal.

Theme 3

Engineering light–matter interaction

Atomically thin mirrors, cavities, nonlinearity, and lattices

How much of a photonic structure can be built from the semiconductor itself, and what does that let us do to excitons?

Excitons in clean monolayer MoSe2 couple to light so efficiently that a single layer reflects like a mirror, with a reflectance that a gate voltage switches. Building on this we have tuned exciton radiative lifetimes mechanically, assembled a cavity from two monolayer mirrors that supports chiral flat bands, and shown that Fermi-polaron resonances in gated WSe2 produce an optical nonlinearity strong enough to switch transmission at low light levels. The newest direction imposes potentials on excitons far below the diffraction limit: a plasmonic metasurface converts laser light into a sub-wavelength optical lattice, and the domain walls of twisted hexagonal boron nitride trap excitons electrostatically in one-dimensional channels. Together these tools let us shape the same excitons we use as probes.

Two monolayer excitonic mirrors forming an optical cavity
An optical cavity built from two monolayer mirrors (Sci. Adv. 2024).

How we work

Materials, devices, and optics under one roof.

Van der Waals assembly

Dry-transfer stacking of exfoliated MoSe2, WSe2, MoTe2, graphene, and hBN into dual-gated heterostructures, including twisted and moiré stacks with ten or more layers.

Low-temperature optical spectroscopy

Reflectance, photoluminescence, and polaron spectroscopy in closed-cycle cryostats with magnetic fields; spatially resolved diffusion imaging; nonlinear and power-dependent measurements down to low photon numbers.

Nanophotonics and nanofabrication

Plasmonic metasurfaces and gratings, electrostatic superlattices, and gate-defined traps that shape the potential landscape of excitons on scales below the wavelength of light.

Thin-film synthesis

Pulsed laser deposition of correlated oxides (VO2, SmNiO3, LaNiO3) and ionic-liquid gating, the group's longer-running line on electrically driven metal–insulator transitions and their devices.

Theory and facilities partners

Close collaboration with theory groups (Hafezi and Galitski at Maryland; Imamoglu at ETH Zurich; Schmidt at Heidelberg; Demler and Esterlis) and with the Quantum Material Press at Brookhaven National Laboratory.

Training

Students learn synthesis, nanofabrication, cryogenic optics, electronics, and data analysis on the same project; undergraduates and high-school interns work alongside the Ph.D. students and postdocs every summer.

Support

Current and recent awards.

  • 2026–31QLCI: Institute for Robust Quantum Simulation (RQS), Phase II. National Science Foundation; M. Hafezi (PI), Y. Zhou (Senior Personnel).
  • 2025–28Sub-wavelength optical lattices in two-dimensional materials. Army Research Office; Y. Zhou (PI), M. Hafezi (co-PI).
  • 2025–27Data Driven Engineering Research (DataDrivER). Army Research Office; D. Woodbury (PI), Y. Zhou (Project Lead).
  • 2025–27ExpandQISE: Exchange interaction between persistent spin helix and defect spin qubits for quantum information. National Science Foundation; Y. Ren (PI), Y. Zhou (co-PI).
  • 2024–25Vacuum enhancing giant optical nonlinearity of Fermi polarons. DARPA; Y. Zhou (PI).
  • 2023–28Probing and controlling novel electronic and magnetic ordering in electron–hole Wigner crystals. Department of Energy, Early Career Award; Y. Zhou (PI).
  • 2023–25Nano-patterning for enhancing superconductivity. DARPA; M. Hafezi (PI), V. Galitski and Y. Zhou (co-PIs).
  • 2022–27CAREER: Crystallizing electrons in coupled atomically thin semiconductors. National Science Foundation; Y. Zhou (PI).

Also supported by the Ralph E. Powe Junior Faculty Enhancement Award (ORAU, 2023) and Army Research Office and NSF supplements for undergraduate and high-school research internships.