Anomalous ultrafast dynamics of hot plasmonic electrons in nanostructures with hot spots

Abstract

The interaction of light and matter in metallic nanosystems is mediated by the collective oscillation of surface electrons, called plasmons. After excitation, plasmons are absorbed by the metal electrons through inter- and intraband transitions, creating a highly non-thermal distribution of electrons. The electron population then decays through electron-electron interactions, creating a hot electron distribution within a few hundred femtoseconds, followed by a further relaxation via electron-phonon scattering on the timescale of a few picoseconds. In the spectral domain, hot plasmonic electrons induce changes to the plasmonic resonance of the nanostructure by modifying the dielectric constant of the metal. Here, we report on the observation of anomalously strong changes to the ultrafast temporal and spectral responses of these excited hot plasmonic electrons in hybrid metal/oxide nanostructures as a result of varying the geometry and composition of the nanostructure and the excitation wavelength. In particular, we show a large ultrafast, pulsewidth-limited contribution to the excited electron decay signal in hybrid nanostructures containing hot spots. The intensity of this contribution correlates with the efficiency of the generation of highly excited surface electrons. Using theoretical models, we attribute this effect to the generation of hot plasmonic electrons from hot spots. We then develop general principles to enhance the generation of energetic electrons through specifically designed plasmonic nanostructures that could be used in applications where hot electron generation is beneficial, such as in solar photocatalysis, photodetectors and nonlinear devices.

Plasmonic hot carrier dynamics in solid-state and chemical systems for energy conversion

“…Ultrafast pump-probe measurements of plasmonic nanostructures use a high-intensity laser pulse to excite a large number of electrons and measure the optical response as a function of time using a delayed probe pulse [77,78]. The typical signal observed in these experiments is an initial fast rise (10-100 fs) attributed to electron-electron scattering that converts fewer high-energy excited carriers into several more lower-energy carriers, followed by a slower decay (100 fs to 1 ps) attributed to electron-phonon scattering.…”

Insight into plasmonic hot-electron transfer and plasmon molecular drive: new dimensions in energy conversion and nanofabrication

“…First, Harutyunyan et al 47 reported anomalous ultrafast dynamics of plasmonic hot electrons in hybrid metal/oxide NSs with hot spots when they varied the geometry and composition of the NS and the excitation wavelength. They showed a large ultrafast contribution to the excited electron decay signal in the hybrid NSs of gold nanodisks with diameters of 100-150 nm on top of a 30-nm-thick continuous gold film separated by a spacer layer (several nanometers thick) containing hot spots.…”

Harvesting the loss: surface plasmon-based hot electron photodetection

“…The TCO-based structures have been demonstrated to produce responsivity up to 70 mA/W under a reverse bias of −3 V in the visible regime [112] (Figure 3F). In terms of detection speed, recent ultrafast studies on hot electron relaxation dynamics [117] have revealed hot electron relaxation times as fast as ~45 fs. Furthermore, the speed was strongly dependent on the local field enhancement and therefore can be controlled by tailoring the plasmonic geometry.…”

“…For hot carrier distribution, although much work has been devoted to providing theoretical predictions of hot carrier distribution in plasmonic nanostructures [30,31,69,71,72,158], more experimental work is needed to validate these models. In addition, a better understanding of the hot carrier relaxation timescale [75,117,[159][160][161] in various materials as well as the transport dynamics in nanostructures would provide more valuable information for designing plasmonic devices with efficient hot carrier transport to the Schottky interface. Lastly, engineering the metal-semiconductor interface on the atomic level could also lead to improved hot electron transfer efficiencies [157,162].…”