[Paper Review] Asymmetric Rydberg blockade of giant excitons in Cuprous Oxide

Julian Heckötter, Valentin Walther+4 more|arXiv (Cornell University)|Oct 29, 2020

Strong Light-Matter Interactions31 references37 citations

TL;DR

This study demonstrates asymmetric Rydberg blockade in giant excitons within cuprous oxide (Cu2O) using a two-color pump-probe excitation scheme, achieving strong, long-range interactions over distances up to several micrometers. The key result is the observation of universal, power-law correlated many-body states with a distinct blockade radius extending over micrometer-scale ranges, confirmed by spectroscopic signatures of inter-species interactions and spatial correlations.

ABSTRACT

The ability to generate and control strong long-range interactions via highly excited electronic states has been the foundation for recent breakthroughs in a host of areas, from atomic and molecular physics [1, 2] to quantum optics [3, 4] and technology [5-7]. Rydberg excitons provide a promising solid-state realization of such highly excited states, for which record-breaking orbital sizes of up to a micrometer have indeed been observed in cuprous oxide semiconductors [8]. Here, we demonstrate the generation and control of strong exciton interactions in this material by optically producing two distinct quantum states of Rydberg excitons. This makes two-color pump-probe experiments possible that allow for a detailed probing of the interactions. Our experiments reveal the emergence of strong spatial correlations and an inter-state Rydberg blockade that extends over remarkably large distances of several micrometers. The generated many-body states of semiconductor excitons exhibit universal properties that only depend on the shape of the interaction potential and yield clear evidence for its vastly extended-range and power-law character.

Motivation & Objective

• To probe long-range, strongly correlated many-body states of semiconductor excitons in a solid-state platform.
• To overcome the challenge of accessing strong interactions in solid-state systems, where spectral shifts often dominate over spatial correlations.
• To realize and characterize asymmetric Rydberg blockade between two distinct Rydberg exciton states with different principal quantum numbers.
• To demonstrate that the interaction potential between excitons exhibits a universal, long-range power-law character (V ∝ r⁻⁶) over micrometer-scale distances.
• To use one exciton species as a probe to directly observe spatial correlations and blockade effects induced by the other

Proposed method

• Employed a two-color optical excitation scheme using spectrally narrow lasers to create Rydberg excitons in distinct quantum states (n′ = 16 pump, n = 6–20 probe).
• Used a chopper-modulated pump beam and lock-in detection to measure differential probe transmission ∆I, isolating interaction-induced changes in transmission.
• Measured probe absorption spectra across a range of pump powers to map the dependence of interaction effects on exciton density.
• Applied a mean-field theory model to compare with experimental data, revealing its failure to capture strong correlation effects.
• Used a correlation-based theoretical model accounting for excitation blockade and spatial correlations to explain the observed universal spectral shapes.
• Extracted the spatial correlation function g(2)(r) from data, revealing an extended blockade radius on the order of several micrometers.

Experimental results

Research questions

• RQ1 Can strong, long-range interactions between Rydberg excitons in a solid-state material be probed via two-color pump-probe spectroscopy?
• RQ2 What is the spatial extent of the Rydberg blockade in giant excitons of Cu2O, and does it exceed typical atomic-scale ranges?
• RQ3 How do inter-species interactions between excitons in different Rydberg states influence the spectral response of the probe field?
• RQ4 Do the observed interaction signatures exhibit universal scaling behavior independent of microscopic details, as predicted by power-law potentials?
• RQ5 Can the asymmetric blockade regime—where inter-species interactions dominate over intra-species effects—be experimentally realized and characterized?

Key findings

• The Rydberg blockade radius in Cu2O excitons extends over several micrometers, far exceeding typical atomic-scale blockade ranges.
• The interaction potential between excitons exhibits a universal power-law dependence V(r) ∝ r⁻⁶, confirmed by the observed spectral shape and scaling of the differential transmission signal.
• The measured differential probe transmission ∆I increases with pump power, indicating the presence of strong inter-species interactions and spatial correlations.
• A mean-field theory fails to reproduce the observed spectral features, particularly the universal maximum and root at fixed detunings ∆max and ∆0, indicating the dominance of strong many-body correlations.
• The spatial correlation function g(2)(r) between pump and probe excitons shows a pronounced blockade at distances below the blockade radius, confirming the existence of long-range quantum correlations.
• The system exhibits a distinct asymmetric blockade where interactions between different Rydberg states dominate, enabling the use of one species as a sensitive probe for the other.