Ultrafast Thermalization Dynamics Research Articles

Nonlinear Photoluminescence in Gold Thin Films

Promising applications in photonics are driven by the ability to fabricate crystal-quality metal thin films of controlled thickness down to a few nanometers. In particular, these materials exhibit a highly nonlinear response to optical fields owing to the induced ultrafast electron dynamics, which is however poorly understood on such mesoscopic length scales. Here, we reveal a new mechanism that controls the nonlinear optical response of thin metallic films, dominated by ultrafast electronic heat transport when the thickness is sufficiently small. By experimentally and theoretically studying electronic transport in such materials, we explain the observed temporal evolution of photoluminescence in two-pulse correlation measurements that we report for crystalline gold flakes. Incorporating a first-principles description of the electronic band structure, we model electronic transport and find that ultrafast thermal dynamics plays a pivotal role in determining the strength and time-dependent characteristics of the nonlinear photoluminescence signal, which is largely influenced by the distribution of hot electrons and holes, subject to diffusion across the film as well as relaxation to lattice modes. Our findings introduce conceptually novel elements ruling the nonlinear optical response of nanoscale materials, while suggesting additional ways to control and leverage hot carrier distributions in metallic films.

Ultrafast thermalization dynamics in Au/Ni film excited by femtosecond laser double-pulse vortex beam

We theoretically investigated ultrafast thermalization dynamics of spatio-temporal evolutions of temperature fields in typical Au/Ni film excited by femtosecond laser double-pulse vortex beam. It is proposed that the enhanced energy deposition in the layered Au/Ni film can be unprecedentedly facilitated via applying femtosecond laser double-pulse vortex beam, preferentially giving rise to vortical thermalization for the phonon system of bottom Ni layer on picosecond timescale. It can be explained as the intensified dynamics of double-pulse vortex beam interacting with the excited intermediate state of Au/Ni film and the following energy competitive processes of the electron thermal diffusion and the electron-phonon coupling in respective layers. Moreover, the electron-phonon coupling period of Au/Ni film with respect to the crucial laser parameters of pulse separation and vortex beam fluence are explored in details. The results can be basically helpful for understanding the ultrafast dynamics of vortical thermalization for advancing the potential applications of compound-cavity fabrication, stimulated emission depletion (STED) imaging and ultrahigh time-resolved spectroscopy, etc.

Analysis of the thermalization dynamics of two-layer thin films irradiated by femtosecond laser

In this work, ultrafast thermalization dynamics was examined for a variety of two layer-thin films (Au/Si, Au/Ni, Au/W, Au/Al and Au/Pb). Non-equilibrium energy transport under laser irradiation was formulated for the electron and lattice sub-systems of the thin films. A significant reduction in the temperature of the electron and the lattice of the gold surface was observed especially for Au/Si and Au/Ni thin films due to their large G values. Next, the effects of laser power intensity and laser heating duration on the temperature distributions were examined for Au/Ni two-layer thin film. It was found that, as the laser intensity increased, the maximum electron temperature increased dramatically; on the other hand, as the pulse heating duration increased, the electron temperature gradually decreased. It was then concluded that thermal damage threshold of the gold surface can be improved by depositing gold layer on a substrate material with high electron-phonon coupling factor. Hence the thermal failure of thin films used in optical components of ultrafast laser systems or micro/nano electro mechanical systems can be prevented.

Ultrafast thermal dynamics of nano-ripples formation via laser double pulses excitation

The ultrafast thermal dynamics of nano-ripples formation on gold film via ultrafast laser double pulses excitation is theoretically investigated by numerical simulations. The non-equilibrium thermal modulations with respect to the electron and phonon energy transfers within gold film is proposed for predicting the nano-ripples formation. It is revealed that the nano-ripples contrast on gold film surface can be well controlled via tuning the pulse energy ratio, pulse separation and pulse exchange of ultrafast laser double-pulse. It is attributed to the tunable energy transfer routes between the electron thermal diffusion and the electron–phonon coupling via tuning double pulses parameters. The study provides theoretical basis for producing high-contrast ripples for a wide range application in the fields such as high-absorptive solar cells, surface friction devices and super-hydrophobic surface.

Tuning near-field enhancements on an off-resonance nanorod dimer via temporally shaped femtosecond laser

We theoretically investigated ultrafast thermal dynamics tuning of near-field enhancements on an off-resonance gold nanorod dimer via temporally shaped femtosecond (fs) laser double pulses. The nonequilibrium thermal excitation is self-consistently coupled into a near-field scattering model for exploring the ultrafast near-field enhancement effects. It is revealed that the near electric-field localized within the gold nanorod dimer can be largely promoted via optimizing the temporal separation and the pulse energy ratio of temporally shaped femtosecond laser double pulses. The results are explained as thermal dynamics manipulation of plasmon resonances in the nanorod dimer via tailoring temporally shaped femtosecond laser. This study provides basic understanding for tuning near-field properties on poorly fabricated metallic nano-structures via temporally shaped femtosecond laser, which can find potential applications in the fields such as fs super-resolution near-field imaging, near-field optical tweezers, and fs photothermal therapy.

Ultrafast thermalization dynamics in two-layer metal films excited by temporally shaped femtosecond laser

We theoretically investigated ultrafast thermalization dynamics in two-layer Au coated metal films excited by temporally shaped femtosecond laser. It is revealed that the thermalization processes at surface and interface of the two-layer metal films can be alternatively promoted via tailoring pulse separation and pulse energy ratio of temporally shaped double pulses. The results are explained as competitive electron–phonon coupling dynamics in the two-layer metal films, which can be well manipulated by temporally shaped femtosecond laser. This study provides basic understanding for optimizing laser thermal damage in layered metal films via tailoring femtosecond laser for a wide range of applications in the fields such as laser resonant cavity, photoelectric equipments, MEMS and biochips.

Ultrafast dynamics of laser thermal excitation in gold film triggered by temporally shaped double pulses

We present the theoretical studies on ultrafast thermal excitation dynamics of gold film irradiated by temporally shaped femtosecond laser double pulses. It is revealed that the gold film thermalization efficiency can be greatly promoted by optimizing the pulse energy ratio, pulse separation and pulse exchange of temporally shaped femtosecond double pulses. The phenomena are explained as the enhanced laser energy coupling into the electron subsystem due to the interplay of pulse-to-pulse energy coupling. This study should be helpful for basic understanding of the ultrafast thermal dynamics for optimizing the gold film excitation process via shaping femtosecond laser for the potential applications such as thermal plastic deformation effects, laser ablation and laser excitation spectrum.

Ultrafast thermalization dynamics of gold-coated fused silica irradiated by a femtosecond laser

The ultrafast thermalization dynamics in gold-coated fused silica irradiated by femtosecond laser are numerically investigated by Finite Element Method (FEM). The temporally sequential thermal relaxation model, which jointly considers the two temperature relaxation and the thermal diffusion relaxation dynamics is proposed for explaining the thermalization characteristics in gold-coated fused silica system. It is revealed that the gold layer thickness of 0.6 μm is the critical thickness for influencing the temperature distribution across gold-coated fused silica system. In addition, the effect of gold layer thickness on thermalization dynamics of gold-coated fused silica system is examined in detail.