Interferometric Near-field Fano Spectroscopy of Single Halide Perovskite Nanoparticles

Abstract

Semiconducting halide perovskite nanoparticles support Mie-type resonances that confine light on the nanoscale in localized modes with well-defined spatial field profiles yet unknown near-field dynamics. We introduce an interferometric scattering-type near-field microscopy technique to probe the local electric field dynamics at the surface of a single MAPbI₃ nanoparticle. The amplitude and phase of the coherent light scattering from such modes are probed in a broad spectral range and with high spatial resolution. In the spectral domain, we uncover a Fano resonance with a 2π phase jump. In the near-field dynamics, this Fano resonance gives rise to a destructive interference dip after a few femtoseconds. Mie theory suggests that the interference between electric quadrupole and magnetic dipole modes of the particle, with spectra affected by resonant interband absorption of MAPbI₃, lies at the origin of this effect. Our results open up a new approach for probing local near-field dynamics of single nanoparticles.

Keywords: Fano interference; MAPbI3 nanoparticle; near-field scattering-type spectroscopy; time-domain near-field spectroscopy.

Figure 1

(a) Schematic setup of spectral interferometry scanning near-field spectroscopy (SI-SNOM). P-polarized Light from a 6 fs Ti:sapphire laser is focused onto the sample. Phase stable spectral interferograms between the light scattered from the sample and the laser light are recorded with a fast last line scan camera at a rate above the tip modulation frequency f. (b) Exemplary spectral interferogram. (c) Spectrum of the laser in the reference arm. (d) Scanning electron microscope image of single MAPbI₃ particles fabricated by laser ablation.

Figure 2

(a) Topography image of a 3 μm × 3 μm area of a sample with laser-ablated MAPbI₃ nanoparticles on a glass coverslip. (b) Map of the optical near-field signal recorded for broadband, linearly p-polarized excitation of the sample with a 6 fs Ti:sapphire laser and detected using an avalanche photodiode (APD). The image shows the APD signal, demodulated at the third harmonic of the tip oscillation (S_3f). For small particles, the images show a dipole-like field distribution, mapping primarily the z-component of the electric near-field on the nanoparticle surface. Larger particles show an additional ring-like structure near their rim. (c,d) Simulated map of the z-component of the local optical near-field pointing along the taper axis, summed over a spectral range from 650 to 950 nm, at the surface of MAPbI₃ nanoparticles with radii of 50 nm (c) and 250 nm (d), respectively. The simulations reproduce the ring-like field at the rim of larger particles.

Figure 3

Near-field response functions σ_NF(ω) and local near-field dynamics at the surface of a MAPbI₃ nanoparticle reconstructed from spectral interferometry SNOM. The middle panel shows a spatial map of the spectrally integrated local near-field response functions recorded in the vicinity of a particle with a diameter of 500 nm. The left panels show the amplitude (black circles) and spectral phase (red circles) of σ_NF(ω) at positions A-D at the sample surface. At A and B, a reduction in near-field scattering connected with a 2π phase jump is found around 1.72 eV. A Fano model is used to fit the results (solid curves). The right panels display the corresponding reconstructed near-field dynamics E₀(t)⊗r_NF(t) (red lines) at A-D, together with the bandwidth limited electric field profile of the excitation laser E₀(t). The long-lived response at A and B reflects the excitation of the EQ mode while the characteristic dip around 3 fs arises from its Fano-type interference with the short-lived dipole modes. At C and D, the excitation of EQ is weak and the dynamics is dominated by the quasi-instantaneous scattering from the glass substrate.

Figure 4

Phenomenological Lorentzian oscillator model for the response function σ(ω) for a Fano-type resonance. (a) Complex plane representation of the response function of a Lorentzian resonance R(ω) (black), a quasi-continuum B(ω) = |B|exp(i3π/2) (blue), and the resulting Fano-type interference R(ω)+B(ω) (red). (b,c) Amplitude (b) and spectral phase (c) of R(ω) (black), B(ω) (blue) and Fano resonance R(ω) + B(ω) (red). The Fano resonance exhibits a 2π phase jump if Lorentzian and background interfere destructively and the on-resonance amplitude of R(ω) exceeds the background amplitude. (d) Resulting time-domain near-field signal E(t) = E₀(t)⊗r(t), for excitation with a 6 fs Gaussian-shape pulse E₀ (dashed black). The convolutions with the response of the Lorentzian resonance r_R, the background r_B and the Fano interference (r_R + r_B) are depicted in black, blue and red, respectively.

Figure 5

(a) Multipole decomposition of the far-field Mie scattering efficiency of a MAPbI₃ nanosphere with 250 nm radius, optically excited with linearly x-polarized light incident along the z-direction. The blue (red) line depicts the magnetic dipole (quadrupole) mode (MD,MQ); the black (green) line shows the electric dipole (quadrupole) mode (ED,EQ). (b,c) Amplitude and spectral phase of the z-component of the electric near-field generated by the EQ mode at a distance of 1 nm from the surface of a MAPbI₃ nanosphere as a function of energy and horizontal position x. The plot is taken across a line with y = 0, oriented along the incident field direction. (d,e) Amplitude phase of the EQ-mode near-field spectra at different positions close to the rim of the nanoparticle. (f) Amplitude and spectral phase of the z-component of the electric near-field, generated by the interference of the EQ, MD and MQ modes close to the rim of the nanosphere. A 2π-phase jump is observed at 1.51 eV as a result of the destructive interference of EQ and MD mode. The enhanced z-component of the electric field at 1.39 eV is the contribution of the MQ mode.