MODULADOR EN CONFIGURACION TRANSVERSAL

1.2.1 Dynamics of Non-radiative Electron Relaxation

There has been great interest in the non-radiative processes of electronic relaxation in noble metal nanoparticles since these govern the damping and dephasing of the plasmon resonance (including surface contribution to these effects which may be important in nanosized particles).2 These processes also constitute the absorption part of the plasmon resonance. The dynamics of hot electrons is important also from the point of view of photoelectrochemical processes in noble metal nanoparticles, potentially useful for applications like solar energy conversion.69

The dynamics of non-radiative electron relaxation processes has been studied by femtosecond time-resolved laser techniques. Femtosecond pulses can be used to create a non-equilibrium excitation of the metal electrons following which they relax via non- radiative processes.2,7,70 The femtosecond pump-probe transient absorption spectroscopy technique has become very useful in following the dynamics of the relaxation processes in plasmonic nanoparticles.70 As per this technique, a femtosecond pulse (pump) excites the electrons via inter-band transitions (at a wavelength of 400 nm in the case of gold) to empty states above the Fermi level. The non-equilibrium/non-Fermi electronic

timescale of 500 fs) to create a “hot” Fermi electron distribution.2 The creation of this broadened Fermi distribution results in the broadening of the plasmon resonance, which is manifested as a transient bleach of the plasmon absorption. As the electrons subsequently cool to equilibrium, the plasmon resonance absorption is recovered. A second femtosecond pulse overlapping with the surface plasmon absorption maximum of the nanoparticles is used as a probe to follow the recovery of the plasmon resonance absorption (or decay of the transient bleach) with sub-picosecond resolution, thus yielding the kinetic trace of the hot electron relaxation (see Figure 1.8).70 The transient bleaching of the plasmon resonance followed by its ultrafast recovery is potentially useful in ultrafast optical switching.

Pump-probe studies from our group2,7,70-73 and others74 have established that the electron cooling following thermalization involves a initial fast decay component, which is attributed to the exchange of the hot electron energy with the nanoparticle lattice through electron-phonon scattering. This takes place on a timescale of ~1 ps in the case of gold.2,7,70 Following this, the hot lattice cools by transferring energy to the medium via phonon-phonon coupling processes, on a timescale of 100 ps, corresponding to a slower component of the decay.2,7,70 _{Thus, the light absorbed by the electrons is converted into}

heat within the nanoparticle, subsequently leading to the heating of the local medium surrounding the nanoparticle.

It must be noted that the electron-phonon relaxation time is laser pump energy dependent and it increases with increasing pump energy due to the linear increase in the electronic heat capacity with increase in hot electron temperature.2,70 _{The consequently}

which is generally dependent on the thermal properties of the medium.73,75,76 It is therefore necessary to carry out electron-phonon relaxation measurements at low laser fluence and extrapolate their value to zero fluence in order to get measurements that can be compared. One must also ensure that different samples being compared have the same optical density at the excitation/pump wavelength.

Figure 1.8: Transient absorption spectra of 15 nm gold nanospheres recorded at different delay times following their excitation at 400 nm with 100 femtosecond laser pulses. The steady-state UV-vis absorption spectrum of the colloidal gold solution is also shown for comparison. The inset shows the decay of the transient bleach when the particles are monitored at the plasmon absorption maximum at 520 nm. Fitting of the decay curve gives electron-phonon and phonon-phonon relaxation times of 3.1 and 90 ps, respectively. (Reprinted with permission from S. Link, M. A. El-Sayed, J. Phys.Chem. B 1999, 103, 8410. Copyright 1999 American Chemical Society)70

The electron-phonon relaxation in gold nanoparticles is found to be similar to that in the bulk and has no dependence on the particle size in the size range from 2-100 nm and the shape (nanorods of aspect ratio 2 to 5).7,72 Thus, there seems to be negligible surface contribution to the electron-phonon relaxation in gold. Size dependence of the

electron-phonon relaxation has however been observed in tin,77 gallium,78 and copper nanoparticles.79 This difference has been proposed by Hodak et al. to depend on the metal electron density relative to its atomic mass.80 For heavy metals like gold, the electron- surface collisions are not sufficient to displace the heavy nuclei, as a result of which surface phonons are not very efficiently excited and thus they do not contribute significantly to the electron-phonon relaxation. Metals with lighter nuclei (e.g. silver) or more than one valence electron (e.g. copper) are expected to exhibit more efficient surface phonon excitation in small-size nanoparticles.80 This was found to be valid for copper nanoparticles by Darugar et al. from our group.79

1.2.2 Photothermal Applications of Noble Metal Nanoparticles

The strong light absorption followed by rapid photothermal conversion of gold nanoparticles has been utilized in a number of applications including laser nanostructuring,40,81 generating nanomotion,82 and for biomedicine.30-32,83-87 The photothermal energy can be channeled effectively and selectively for a desired application by controlling the rate of laser energy deposition relative to the rate at which the heat is dissipated. When the rate of the energy deposition is faster than the heat dissipation away from the nanoparticle, the nonradiative processes result in the rapid heating of the nanoparticle, which may result in:

i) Excitation of coherent lattice phonon vibrations42,43,88 in the nanoparticle potentially useful in nanophotonic modulation

ii) Melting (on the timescale of 30 ps)81 or reshaping of the nanoparticles40

iii) Nanoparticle atomic ablation89 _{and even projectile ejection of the particle with jet-like}

At lower rates of laser energy deposition, the photothermal heating of the medium surrounding the nanoparticle can be achieved. In such a case, the metal nanoparticles serve essentially as “light-activated nanoscopic heaters” useful for biomedical applications especially the selective laser photothermolysis of cancer cells.30-32,83,84 _Gold

nanoparticles conjugated to antibodies can be selectively targeted to diseased cells without significant binding to healthy cells.90 Irradiation of the cancer cells selectively labeled with the nanoparticles, with a laser of frequency overlapping with the plasmon absorption maximum of the nanoparticles results in selective heating and destruction of cancer cells, at much lower laser powers than those required to destroy healthy cells to which the nanoparticles do not bind specifically. Gold nanoparticles (10-50 nm size range) offer 6 or more orders higher absorption coefficients compared to conventional dyes.13 Therefore, much lower laser energies can be used to achieve cell destruction, making therapy minimally invasive.

While the use of visible light resonant gold nanospheres can be useful for external skin/surface cancer treatments, for tumors within bodily tissue, it becomes necessary to use NIR light in the biological window.34 One method demonstrated by the El-Sayed group involves the use of gold nanorods of aspect ratio 3.9, which have a longitudinal plasmon band around 800 nm overlapping with a NIR Ti:sapphire laser.32 NIR laser irradiation of cancer cells labeled with gold nanorod/anti-EGFR conjugates resulted in selective destruction of the cancer cells. NIR imaging/therapy has also been achieved with NIR-resonant gold nanoshells and nanocages. Recently, Huang et al. demonstrated a novel way to achieve NIR therapy using gold nanospheres by utilizing their two-photon absorption of 800-nm Ti:sapphire laser light.91