Picosecond Dynamics of Colloidal Gold Nanoparticles

Abstract

Colloidal gold nanoparticles with an average radius of 15 nm have a surface plasmon absorption band at 530 nm. Excitation by laser pulses of 450 fs duration, and wavelength of 600 or 380 nm “bleached” the plasmon band and produced a transient absorption at the wings of the “bleach” spectrum. The transient absorption was found to have a similar temporal behavior at different wavelengths. Analysis of their temporal behavior showed two time constants: 2.5 ps, and a slower component of >50 ps. Laser excitation close to the plasmon band at 600 nm leads to the formation of “hot” non-Fermi electronic distribution within the colloidal particles. Transient absorption from these “hot” electrons led to different absorptions from that of the plasmon absorption of “cold” electrons. The “hot” electrons relax via electron−phonon coupling in 2.5 ps, and the phonon−phonon relaxation of the lattice occurs in >50 ps. At 380 nm excitation, the amplitude of the blue wing becomes smaller, and the slow component becomes longer, which could be due to possible excitation of the d-band electrons. These results are discussed in terms of Mie theory and a two-temperature model (TTM), and their consequences on the optical absorption spectrum.

Laser-Induced Shape Changes of Colloidal Gold Nanorods Using Femtosecond and Nanosecond Laser Pulses

“…The measured electron-phonon relaxation times depend on the laser pump power 2,3,7 and are on the order of a few picoseconds (1-4 ps). [1][2][3][4][5][6][7][8][9][10][11][12][13] The results obtained for the nanoparticles furthermore compare well with the electronphonon coupling constant measured for bulk gold 28 using similar time-resolved laser techniques. These experiments have mainly been carried out in the low excitation limit in which the temperature of the nanoparticle lattice is raised only by a few tens of degrees (strongly depending on the particle size and laser pump power).…”

“…In the case of a femtosecond laser pulse the absorption of photons by the electrons (100 fs [1][2][3][4][5][6][7][8][9][10][11][12] ), electron-phonon relaxation (heating of the lattice < 10 ps [1][2][3][4][5][6][7][8][9][10][11][12] ), melting (30-35 ps 29 ), and phonon-phonon relaxation (cooling of the lattice > 100 ps 4,7,9 ) are well separated in time. Indeed, they can be thought of as sequential processes.…”

Surface Plasmon Resonance Tunability in Au−VO2 Thermochromic Nano-composites

“…According to Mie theory and even more advanced theoretical models such as the effective medium model, smaller is the metallic nano-particles, blue is the shift of the plasmon frequency. This is evidenced clearly in some elegant experiments using nano or/and femtosec regimes of laser induced size reduction of noble metal particles (52)(53)(54). Figure 4 reports the experimental variation of the nano-gold surface plasmon wavelength versus temperature.…”

Microscale Heat Transfer Transduced by Surface Plasmon Resonant Gold Nanoparticles

“…External heat flux in the system, Q ext , is nearly proportional to linear thermal driving force, with a heat transfer coefficient, h, as the proportionality constant: (9) In this case the energy balance in Eq 5 simplifies to: (10) Introducing a dimensionless driving force temperature, θ, scaled using the maximum system temperature, T max , (11) and a sample system time constant τ s (12) which are substituted into eq 10 and rearranged to yield (13) When laser irradiation ceases, Q I +Q o =0 and the system cools, reducing eq 13 to (14) Eq 14 may be solved using the initial condition θ = 1 at t = 0 to give (15) During laser irradiation, Q I +Q o is finite and system temperature rises to a maximum value when external heat flux given by eq 9 equals heat input via laser transduction given by eq 6: (16) Substituting eq 16 into eq 13 gives (17) Eq 17 can be solved using the initial condition θ = 0 at t = 0 to give (18) …”

“…Suspended nanoparticles irradiated at resonant frequencies dissipate heat to the matrix in a cascade of events. 15,16 Resonant irradiation induces a non-thermal electron distribution that thermalizes in ~500 femtoseconds via elastic electron-electron scattering to an equilibrium Fermi electron distribution that corresponds to a higher electron temperature. 17 The Fermi distribution or 'electron gas'.…”