Linear and nonlinear excitation induced ultrafast absorption dynamics in laser ablated and chemically synthesized gold nanoparticle colloids

Highlights

• Absorption dynamics of gold NPs from laser ablation and chemical synthesis.

• Transient absorption dynamics in the plasmonic region.

• Ultrafast probing of the electron dynamics under intraband and interband excitation.

• Ability of strong ground-state bleach with additional photo-induced absorption.

• Prominent electron-phonon coupling dependence on excitation fluence.

Abstract

Study of excited state electron dynamics on gold nanoparticles produced by femtosecond laser ablation and chemical synthesis routes are conducted through the pump-probe technique under nonlinear (800 nm, two-photon region) and linear (350 nm, one-photon region) excitations. The temporally delayed broadband probe in the plasmonic region signifies the occurrence of dominant ground state bleach and photo-induced absorption. The excited state electrons, specifically those corresponding to below-resonant excitation, undergo relaxation as a brisk tapering curve due to electron-electron coupling followed by a gradual rise attributed to electron-phonon coupling events. The gold colloids show longer duration of electron-phonon process at 350 nm excitation than at 800 nm with an almost linear dependence of decay time with increasing excitation energy. The resonant excitation regime of chemically synthesized sample shows a visible red-shift compared to their laser ablated counterparts. This study proves efficient in better understanding energy dependent electron dynamics at different excitation wavelengths.

Introduction

Ultrafast spectroscopic methods based on pulsed lasers emerged to investigate the evolution of rapid atomic level events and the past few decades have witnessed femtosecond pulsed lasers which have proved an efficient means for the exploration of these exotic molecular and atomic level processes. Femtosecond laser-material interaction focusing certain fundamental insights in ultrafast continuous optical imaging and four-dimensional scanning electron microscopy [1], information regarding breakage of chemical bonds in femtochemistry [2], femtosecond isomerization reactions [3,4], analysis of photosynthesis dynamics [5,6], electron relaxation studies in quantum structures [7], photoacoustic imaging [8], ultrafast molecular reaction dynamics [9], single molecular dynamics [10,11] have been comprehensively studied in the past. Recent works on the interaction of femtosecond light with a single particle yield prominent scientific understanding of the behavior of individual nanoparticles and have added a new dimension in the field of light-matter interaction [[12], [13], [14], [15]].

Plasmonic metal nanoparticles (NPs) have been fascinating materials because of their huge nonlinear optical response and potential in technologically important applications. Their electronic, physical, optical and electrical properties can be tuned depending upon the size of the nanostructures [16]. Gold nanoparticles have been very useful in the diagnosis of breast cancer in women due to its high empathy with specific cancer cells in which these nanoparticles are surface functionalized to design targeted drug delivery system. Mostafa A. El-Sayed group demonstrated the optical imaging-based cancer diagnostics by utilizing gold nanoparticles [17].

Gold nanoparticles (AuNPs) interact with laser light causing an enhancement in external electromagnetic fields due to intense localized surface plasmon resonance (LSPR) excitation, giving rise to strong absorption of excitation light in the visible region. The excited plasmons undergo electromagnetic decay on a femtosecond timescale by transferring their energy onto other photons which get re-emitted (radiative decay) or by energy transfer to excited state electrons (non-radiative decay). The relaxation of these hot electrons with a non-Fermi distribution in AuNPs is found to occur through electron-electron (<100 fs), electron-phonon (1–10 ps) and phonon-phonon (~100 ps) interactions. Structural and morphological geometry of these nanoparticles affect the excited state electron dynamics. Hence, these ultrafast relaxation events in Au nanostructures which are embedded in heterogeneous configurations perceive great attention in applications as plasmonic energy conversion [18], photocatalysis [19], photothermal therapy [20], single molecule detection [21], light harvesting and SERS applications [22].

The pump-probe spectroscopic technique occupies a superior position in dealing with the exploration of these short-lived processes and voluminous research exists to reinforce its importance in the research on ultrafast dynamics in AuNPs [[23], [24], [25], [26]]. The pump-probe study for evolution of electron dynamics exists in literature for the size range of 5–100 nm although excited state dynamics in AuNPs of small sizes (<5 nm) are less studied due to synthesis complexities [27,28]. Photo-excitation of AuNPs with the pump pulse renders the conduction electrons thermalized on a 10–100 fs timescale. The relaxation pathways of these excited electrons occur broadly in three processes: (1) The electron-electron (e-e) coupling where the excited electrons transfer energy to other neighboring electrons within few fs (2) The electron-phonon (e-p) coupling of 1–10 ps wherein the excited state electronic energy induces phonons of the gold lattice (3) The phonon-phonon (p-p) coupling where the lattice phonon thermal energy is dissipated into the surrounding medium in >100 ps timescale [29].

The nanosecond absorption dynamics and mechanism of laser induced- AuNP size reduction has been reported by Yamada et al. [30] and nanosecond electron thermalization dynamics in laser fabricated Au and Ag₅₀Au₅₀ NPs have reported by Krishnakanth et al. [31]. Temer et al. [43] reported the resonant (380 nm) excited picosecond absorption dynamics (~2.5 ps and 50 ps) associated with e-p (~2.5 ps) and p-p (>50 ps) relaxation processes of the chemically synthesized Au NPs. Domantas et al. [40] studied the ultrafast dynamics from 380 nm excitation (290 fs) of colloidal Aluminium NPs and found that e-p relaxation times are equivalent or longer than Au and Ag NPs e-p relaxation. In other works, they [41,42] also demonstrated resonant excited (400–430 nm) ultrafast transient dynamics measurements on chemically synthesized Ag@TiO₂ nanocubes, nanoporous Au thin films and nanowires. Picosecond characteristics of resonant (400 nm) excited transient absorption in chemically synthesized Au and Ag NP colloids have also been previously reported [38,[43], [44], [45]].

In general, many attempts are made in the past to study the ultrafast absorption and relaxation dynamics of plasmonic metal nanostructures, however most of those studies are either confined to one-photon excitation (ω_exe > ω_p) or focused to the metal NPs obtained from a specific fabrication methodology. The novelty of the presented work is to demonstrate the categorical differences of the linear and nonlinear excitation induced absorption processes upon gold nanoparticles obtained from two widely diverse methods. Exclusive information is anticipated in the relative dynamic evolution of absorption under ultrafast resonant and non-resonant excitations, especially the ground state bleaching and consequent transient relaxation of the excited electrons which are governed through the electron-phonon coupling process.

In the present study, we present a systematic and relative study of ultrafast absorption dynamics from both resonant (one-photon, 350 nm) and non-resonant (two-photon, 800 nm) excitations. Energy dependent transient absorption spectra were recorded at interband and intraband excitation with 350 nm and 800 nm respectively to assess the electronic behaviour and kinetics.