Hyperbolic Dispersion Arising from Anisotropic Excitons in Two-Dimensional Perovskites

Area of Science:

• Materials Science
• Condensed Matter Physics
• Optics

Background:

• Negative real part of relative permittivity (ϵ^{'} < 0) is crucial for strong light-matter interactions, typically achieved via free electrons or optical phonons.
• Exciton-induced negative permittivity in materials remains less explored.
• Two-dimensional hybrid perovskites (2DHPs) offer a unique platform with alternating inorganic and organic sublattices.

Purpose of the Study:

• To investigate the fundamental optical properties of low-member 2DHPs (N=1, N=2).
• To explore the potential of excitons in 2DHPs to achieve negative permittivity.
• To identify novel dispersion phenomena in 2DHPs.

Main Methods:

• Development of a dielectric-coating based technique for optical property measurement.
• Characterization of optical properties of 2DHPs with N=1 and N=2.
• Analysis of excitonic contributions to permittivity and dispersion.

Main Results:

• Low-member 2DHPs exhibit negative permittivity (ϵ^{'} < 0) due to high exciton binding energy and oscillator strength.
• Hyperbolic dispersion, where permittivity changes sign with direction, is observed in the visible range.
• This natural hyperbolic dispersion arises from anisotropic excitonic behavior in 2DHPs.

Conclusions:

• 2DHPs can achieve negative permittivity and hyperbolic dispersion through excitonic effects.
• These findings open new avenues for light-matter interaction studies and photonic device applications.
• Similar excitonic phenomena may be present in other van der Waals solids.