Van der Waals heterostructures, formed by the precise stacking of two-dimensional materials, exhibit unique optical properties that are not present in their individual constituent layers. The weak interlayer interactions, governed by van der Waals forces, allow for the engineering of optoelectronic behavior through controlled layer arrangement, twist angles, and interlayer spacing. These heterostructures enable phenomena such as interlayer excitons, efficient energy transfer pathways, and enhanced light-matter interactions, making them a rich platform for studying fundamental optical processes and developing advanced photonic devices.
One of the most significant optical features of van der Waals heterostructures is the formation of interlayer excitons, where electrons and holes are spatially separated across different layers. In a MoS2-WSe2 heterostructure, for example, the type-II band alignment causes electrons to localize in the MoS2 layer while holes reside in the WSe2 layer. The resulting interlayer excitons exhibit lifetimes orders of magnitude longer than intralayer excitons, reaching several nanoseconds at low temperatures due to reduced electron-hole overlap. The energy of these excitons can be tuned by adjusting the interlayer distance or applying external electric fields, with shifts exceeding 100 meV reported under moderate gate voltages. The twist angle between layers further modifies the excitonic properties, introducing moiré potentials that lead to spatially varying emission patterns observable in photoluminescence mapping.
Energy transfer processes in van der Waals heterostructures occur with high efficiency due to the atomic-scale proximity of layers. Förster resonance energy transfer (FRET) and Dexter transfer mechanisms dominate depending on the material system and interlayer separation. In graphene-quantum dot heterostructures, FRET efficiencies above 80% have been measured, enabled by the strong dipole coupling between emitters and the graphene layer. Transition metal dichalcogenide heterostructures exhibit even more complex energy transfer pathways, where charge transfer can compete with exciton transfer depending on the band alignment. Ultrafast spectroscopy reveals these processes occurring on timescales shorter than 1 ps, with the interlayer coupling strength directly influencing the transfer rates.
Plasmonic effects emerge when van der Waals heterostructures incorporate metallic layers or doped semiconductors. Graphene-insulator-graphene stacks support interlayer plasmon modes with frequencies tunable from terahertz to mid-infrared ranges through electrostatic gating. The plasmon dispersion relations in these systems show strong dependence on the dielectric environment, with hexagonal boron nitride (hBN) encapsulated structures exhibiting reduced losses compared to air-exposed interfaces. In heterostructures combining plasmonic materials with emitters, such as graphene-MoS2 stacks, plasmon-exciton coupling leads to Purcell enhancement factors exceeding 10, significantly modifying the radiative decay rates.
The absorption characteristics of van der Waals heterostructures display unique features arising from interlayer interactions. While individual 2D materials typically show step-like absorption edges, heterostructures exhibit additional peaks and shifts due to hybridized states. In MoS2-WS2 bilayers, the absorption spectrum shows signatures of both layer-hybridized excitons and interlayer charge transfer states. The absorption strength can be modulated by the relative orientation of layers, with aligned heterostructures showing pronounced excitonic resonances compared to twisted configurations. The optical conductivity of graphene-based heterostructures demonstrates interlayer screening effects that modify the universal absorption from 2.3% per layer in isolated graphene to lower values in stacked geometries.
Emission properties undergo dramatic changes in van der Waals heterostructures compared to isolated layers. Photoluminescence quenching occurs in many systems due to efficient charge or energy transfer, with quenching efficiencies reaching 99% in graphene-coupled TMDCs. However, carefully designed heterostructures can instead enhance emission through dielectric screening or plasmonic effects. In hBN-encapsulated WSe2-MoSe2 heterostructures, emission yields improve by over an order of magnitude compared to bare layers due to reduced defect scattering and modified exciton-phonon coupling. The emission spectra also show new peaks corresponding to interlayer excitons, with their relative intensities controllable via the stacking sequence.
Nonlinear optical responses become significantly enhanced in van der Waals heterostructures due to the combined effects of strong light-matter interactions and symmetry breaking at interfaces. Second harmonic generation (SHG) in twisted TMDC bilayers shows complex polarization dependence governed by the moiré superlattice symmetry. Third harmonic generation (THG) efficiency in graphene-hBN heterostructures exceeds that of either material alone, with measured susceptibility values approaching 10^-7 esu. The nonlinear response can be electrically tuned over wide ranges, with modulation depths exceeding 90% demonstrated in gated graphene-oxide heterostructures.
Photoluminescence mapping techniques provide spatial resolution of the optical properties across van der Waals heterostructures. Confocal microscopy with sub-micron resolution reveals spatial variations in interlayer exciton emission correlated with local strain and moiré patterns. Hyperspectral imaging captures the energy distribution of excitonic states across different regions of a heterostructure, showing energy variations up to 50 meV across domains of a few micrometers. Time-resolved photoluminescence maps further add temporal dimension to these measurements, exposing nanosecond-scale variations in exciton lifetimes across the sample.
Ultrafast spectroscopy methods are essential for probing the dynamics of energy and charge transfer in van der Waals heterostructures. Pump-probe measurements with femtosecond resolution have identified multiple timescales for interlayer processes, from sub-100 fs charge transfer to picosecond-scale energy relaxation. Transient absorption spectroscopy reveals the formation and decay of interlayer excitons, with their distinct signatures appearing at energies below those of intralayer excitons. Two-dimensional electronic spectroscopy provides even more detailed information about coupling between states, resolving the coherent interactions between layers that occur on femtosecond timescales.
The optical properties of van der Waals heterostructures are highly sensitive to environmental conditions and sample preparation. Encapsulation with hBN layers preserves the intrinsic optical characteristics by preventing surface contamination and oxidation, leading to narrow photoluminescence linewidths below 1 meV in some systems. Temperature-dependent measurements show the evolution of interlayer excitons, with their binding energies typically ranging from 50-300 meV depending on the material combination. Applied strain modifies the optical response through changes in band structure and interlayer coupling, with shifts of several meV per percent strain commonly observed.
Technological applications of these optical properties are emerging in several directions. Heterostructures designed for light harvesting benefit from the broad spectral absorption and efficient charge separation enabled by interlayer processes. Optical modulators exploit the strong electro-absorption effects in gated heterostructures, achieving modulation speeds into the gigahertz range. Quantum light sources utilize the deterministic placement of emitters in heterostructure geometries to produce single photons with high purity and indistinguishability. The ability to combine materials with complementary optical properties in a single stack continues to drive innovation in photonic device concepts beyond what is possible with individual 2D materials.
The study of optical properties in van der Waals heterostructures remains an active area of research with many open questions. The precise control of interlayer coupling, the understanding of many-body effects in moiré systems, and the development of scalable fabrication methods all present challenges that must be addressed to fully exploit these material systems. As characterization techniques improve and theoretical models become more sophisticated, new phenomena and applications will undoubtedly continue to emerge from these artificially stacked atomic assemblies.