How Nonlocal Effects Alter Emission Spectra From Quantum Wells

Even the smallest electron orbit within a gallium arsenide quantum well, created by a magnetic field of a few Tesla, spans tens of nanometers, revealing an inherent “nonlocal” response to electromagnetic excitation. Researchers at Queen’s University and Technische Universität Berlin demonstrate that these nonlocal effects, stemming from the spatial dispersion of Landau levels, modify electromagnetic responses at distances up to hundreds of nanometers, a surprisingly large range for quantum phenomena. From a microscopic theory of the nonlocal susceptibility, they derive the Green’s function, the central quantity governing all electromagnetic interactions, and evaluate Purcell factors, Lamb shifts, and emission spectra. This work details how near-field gradients can brighten locally dipole-forbidden transitions at multiples of the cyclotron frequency, suggesting enhanced light emissions through these quantum well interactions. The findings are relevant to nanostructured terahertz architectures and align with recent experiments utilizing Landau level polaritons.

Landau Levels and Cyclotron Orbits in 2DEGs

Their work demonstrates that the influence of these Landau levels extends far beyond the immediate vicinity of an electron, modifying electromagnetic responses at distances reaching hundreds of nanometers. This nonlocal behavior stems from the discrete nature of Landau levels, quantized cyclotron orbits arising from a strong perpendicular magnetic field. Researchers at Queen’s University and Technische Universität Berlin derive the Green’s function, a central quantity governing electromagnetic interactions, to model these effects, moving beyond simplified, localized approximations. This allowed them to evaluate Purcell factors, Lamb shifts, and emission spectra from a quantum emitter positioned near the two-dimensional electron gas. The results reveal a breakdown of local Landau level theory, with nonlocal Purcell factors that exceed local predictions by up to two orders of magnitude at multiples of the cyclotron frequency, for separations up to several hundred nanometers.

This enhancement is particularly noticeable at distances where calculations traditionally assume a local response. The study unveils a phenomenon where locally dipole-forbidden transitions are brightened because the spatial dispersion of the Landau level response relaxes the usual selection rules governing light emission. These findings have implications for the design of advanced nanostructured terahertz architectures, where these nonlocal effects are relevant given the typical length scales involved. As the researchers explain, “Our results show that a local material model becomes inadequate if the electric field varies over the spatial extent of the occupied cyclotron orbits, which is typical with nanostructured THz architectures.” This detailed understanding of Landau level nonlocality promises to refine the development of future quantum devices operating in the terahertz regime.

Nonlocal Susceptibility and the Green’s Function Derivation

The pursuit of increasingly miniaturized terahertz architectures demands a re-evaluation of how materials respond to electromagnetic fields at the nanoscale. Conventional models often treat materials as responding locally to excitation, but recent work demonstrates this simplification breaks down when features approach the scale of fundamental quantum properties. Researchers at Queen’s University and Technische Universität Berlin are focusing on “nonlocal” effects, where the response at one point is influenced by the material’s behavior over distances of tens to hundreds of nanometers.

The study demonstrates that current models falter when electric fields vary over distances comparable to the electron’s cyclotron orbit. This effect is distinct from nonlinear optical phenomena. The implications extend beyond fundamental physics, providing a crucial theoretical foundation for interpreting recent experimental observations and enabling the exploitation of these nonlocal effects in future terahertz technologies.

Researchers at Queen’s University and Technische Universität Berlin are investigating nanoscale light-matter interactions, and recent work details a surprising nuance in how terahertz radiation interacts with electrons confined within gallium arsenide quantum wells. This breakdown of conventional theory has significant implications for the design of advanced nanostructured terahertz devices. Calculations typically assume a local response, but the researchers demonstrate this assumption fails when considering the inherent spatial extent of Landau levels. The team derive a microscopic Green’s function to accurately model these effects, moving beyond simplified, localized approximations. The consequences of this nonlocality are substantial; the researchers find nonlocal Purcell factors that exceed the local predictions by up to two orders of magnitude at separations up to several hundred nanometers, and these effects are particularly relevant to ultrastrong-coupling regimes where off-resonant transitions are significant.

The ability to manipulate light at the nanoscale is driving advances in terahertz technologies, and recent theoretical work reveals a surprising mechanism for enhancing light emission from quantum wells. This inherent “nonlocality” fundamentally alters how the quantum well interacts with light. Researchers at Queen’s University and Technische Universität Berlin have shown that these effects result in nonlocal Purcell factors that exceed the local predictions by up to two orders of magnitude at separations up to several hundred nanometers and at multiples of the cyclotron frequency. The team derive the Green’s function, the central quantity governing all electromagnetic interactions, to accurately model these effects. Calculations assume a local response, where electrons react to the electromagnetic field at their location, leading to effects stemming from spatial dispersion of the Landau level response.

The assumption that quantum effects are confined to the nanoscale often overlooks their surprisingly long reach. This challenges conventional models used in terahertz device design and opens new avenues for manipulating light-matter interactions. Researchers at Queen’s University and Technische Universität Berlin detailed a microscopic theory of nonlocal susceptibility and derive the Green’s function to model electromagnetic interactions. Crucially, the study highlights that the spatial extent of electron orbits, even the smallest, defined by the magnetic length, is substantial. The implications are particularly striking for dipole-forbidden transitions, with nonlocal Purcell factors that exceed the local predictions by up to two orders of magnitude at separations up to several hundred nanometers. This effect is linear in its response, unlike nonlinear optical phenomena. This finding is especially relevant to the design of advanced terahertz architectures where nanoscale fabrication introduces electric field variations below the wavelength, demanding a more nuanced understanding of nonlocal effects.