[Paper Review] NOVEL SCHEMES FOR ULTRAFAST MANIPULATION OF QUANTUM MATERIALS

Paolo Franceschini, Luca Carletti+13 more|arXiv (Cornell University)|Sep 28, 2020

Perovskite Materials and Applications46 references19 citations

TL;DR

This study demonstrates ultrafast, geometry-tuned control of Fano resonances in CsPbBr3 perovskite nanoparticles by combining size engineering with sub-picosecond photo-injection of free carriers. When nanoparticle size is tuned to achieve a Fano dip, photoexcitation induces a reversed transmittivity modulation—opposite to that in larger or bulk systems—enabling all-optical switching with sub-wavelength spatial control via interplay between excitonic states and Mie resonances.

ABSTRACT

The full control of the fundamental photophysics of nanosystems at frequencies as high as few THz is key for tunable and ultrafast nano-photonic devices and metamaterials. Here we combine geometrical and ultrafast control of the optical properties of halide perovskite nanoparticles, which constitute a prominent platform for nanophotonics. The pulsed photoinjection of free carriers across the semiconducting gap leads to a sub-picosecond modification of the far-field electromagnetic properties that is fully controlled by the geometry of the system. When the nanoparticle size is tuned so as to achieve the overlap between the narrowband excitons and the geometry-controlled Mie resonances, the ultrafast modulation of the transmittivity is completely reversed with respect to what is usually observed in nanoparticles with different sizes, in bulk systems and in thin films. The interplay between chemical, geometrical and ultrafast tuning offers an additional control parameter with impact on nano-antennas and ultrafast optical switches.

Motivation & Objective

• To achieve sub-picosecond modulation of optical properties in halide perovskite nanoparticles.
• To explore the interplay between geometric control (Mie resonances) and ultrafast carrier dynamics.
• To reverse the typical photo-induced transmittivity response in nanoparticles by tuning Fano resonance lineshape.
• To enable new designs for ultrafast optical switches and tunable metamaterials using nanoscale control.

Proposed method

• Used time- and energy-resolved pump-probe spectroscopy to measure differential transmission (δT/T) in 150 nm and 300 nm CsPbBr3 nanoparticles.
• Engineered nanoparticle size to tune the overlap between narrowband excitons and geometry-controlled Mie resonances.
• Applied finite element simulations to model the non-equilibrium charge distribution and coupling to cavity modes.
• Employed Mie theory to calculate extinction, scattering, and absorption cross-sections for spherical nanoparticles.
• Used Kramers-Krönig relations to extract refractive index changes from measured δT/T spectra.
• Incorporated carrier-dependent bandgap renormalization and band filling via the Nilsson approximation and effective mass model.

Experimental results

Research questions

• RQ1 How does nanoparticle size influence the Fano resonance lineshape in halide perovskite nanoparticles?
• RQ2 What is the ultrafast optical response of CsPbBr3 nanoparticles when the Fano resonance is inverted (i.e., a dip instead of a peak)?
• RQ3 Can the photo-induced change in optical constants be reversed by tuning the geometry of the nanoparticle?
• RQ4 How do Mie resonances and excitonic states interplay to control the far-field electromagnetic response on sub-picosecond timescales?
• RQ5 What is the role of bandgap renormalization and band filling in determining the sign and magnitude of the transient transmittivity change?

Key findings

• For 300 nm nanoparticles, the Fano resonance exhibits a dip (q = −0.41 ± 0.01), leading to a reversed photo-induced transmittivity response compared to 150 nm NPs.
• In 300 nm NPs, photoexcitation causes a decrease in transmittivity (negative δT/T) at 2.4 eV, opposite to the increase seen in 150 nm NPs and bulk systems.
• The transient response in 300 nm NPs is dominated by bandgap renormalization and band filling, with δT/T reaching −8 × 10⁻³ at 200 fs.
• Finite element simulations confirm that the reversal is due to the interplay between excitonic states and Mie modes, with electric dipole (ED) and magnetic dipole (MD) modes being the dominant contributors.
• Multipole decomposition shows that photoexcitation suppresses scattering at the Fano dip, consistent with reduced optical cross-sections.
• The model accounts for high-energy interband transitions via a frequency-independent refractive index shift δn₀, improving agreement with experimental δT/T amplitudes.