ABSTRACT
Nanoacoustic strains are generated in Silicon by chirped femtosecond laser pulses using thin Titanium films as transducers. We investigate the effect that the generating laser pulse chirp has on the amplitude of the induced strains, manifested as Brillouin oscillations observed in degenerate femtosecond pump-probe transient reflectivity measurements. The strain amplitude is larger when negatively chirped pulses are used, which is attributed to the more efficient conversion of laser pulse light into acoustic strain in the Titanium transducer. Our present studies clearly show that the dependence of the Brillouin amplitude and the lattice strain is a non-monotonous function of the laser chirp parameter. An optimum negative laser pulse chirp is found for which the strain amplitude is maximized. A detailed thermomechanical model satisfactorily supports the experimental findings. In such a way, it is possible to suppress or enhance the induced nanoacoustic strain amplitude, thus all-optically controlling it by at least a factor of two.
ABSTRACT
We demonstrate the capability to control the ripple periodicity on polycrystalline ZnO films by applying temporally delayed femtosecond double pulses. It is shown that there is a characteristic pulse separation time for which one can switch from low- to high- spatial-frequency ripple formation. Results are interpreted based on the relation of the characteristic delay time with the electron-phonon relaxation time of the material. Our results indicate that temporal pulse shaping can be advantageously used as a mean to control the periodic nanoripples' formation and thus the outcome of laser assisted nanofabrication process, which is desirable for the applications of nanopatterned transparent semiconductors.
Subject(s)
Algorithms , Lasers , Molecular Imprinting/methods , Oscillometry/methods , Zinc Oxide/chemistry , Zinc Oxide/radiation effectsABSTRACT
We exploit cross-phase modulation by a strong driving pulse onto a weaker probe pulse at a different wavelength to induce the formation of an X wave possessing the typical nondispersive and nondiffractive propagation properties.
ABSTRACT
Temporally shaped, femtosecond laser pulses have been used for controlling the size and the morphology of micron-sized metallic structures obtained by using the Laser Induced Forward Transfer (LIFT) technique. We report the effect of pulse shaping on the size and morphology of the deposited structures of Au, Zn, Cr on a function of the pulse separation time ??t (from 0 to 10 ps) of double pulses of variable intensities generated by using a liquid crystal spatial light modulator (SLM). The observed differences in size and morphology are correlated with the outcome of pump-probe experiments for the study of electron-phonon scattering dynamics and subsequent energy transfer processes to the bulk in the different metals employed. We propose that in metals with weak electron-lattice coupling, the electron ballistic motion and the resulting fast electron scattering at the film surface, as well as the internal electron thermalization process are crucial to the morphology and size of the transferred material. Therefore, temporal shaping within the corresponding time scales of these processes may be used for tailoring the features of the metallic structures obtained by LIFT.
Subject(s)
Metals/chemistry , Metals/radiation effects , Models, Chemical , Surface Properties/radiation effects , Computer Simulation , Energy Transfer , Light , Scattering, RadiationABSTRACT
Transient changes of the complex self-energy of the 5d(z2) surface state on Gd(0001) after intense optical excitation are investigated by femtosecond time-resolved photoemission. We observe an ultrafast (<100 fs) broadening of the linewidth due to e-e scattering followed by a decrease of the binding energy due to thermal expansion of the lattice. In addition, we resolve a periodic breathing of the band structure which originates from a coherent phonon. An amplitude of 1 pm is derived from the binding energy shift upon lattice displacement calculated by density functional theory.
ABSTRACT
Time-resolved photoelectron spectroscopy is employed to study the dynamics of photoexcited electrons in optimally doped Bi{2}Sr{2}CaCu{2}O{8+delta} (Bi-2212). Hot electrons thermalize in less than 50 fs and dissipate their energy on two distinct time scales (110 fs and 2 ps). These are attributed to the generation and subsequent decay of nonequilibrium phonons, respectively. We conclude that 20% of the total lattice modes dominate the coupling strength and estimate the second momentum of the Eliashberg coupling function lambdaOmega{0}{2}=360+/-30 meV{2}. For the typical phonon energy of copper-oxygen bonds (Omega{0} approximately 40-70 meV), this results in an average electron-phonon coupling lambda<0.25.
ABSTRACT
Femtosecond time-resolved photoemission is used to investigate the time evolution of electronic structure in the Mott insulator 1T-TaS2. A collapse of the electronic gap is observed within 100 femtoseconds after optical excitation. The photoemission spectra and the spectral function calculated by dynamical mean field theory show that this insulator-metal transition is driven solely by hot electrons. A coherently excited lattice displacement results in a periodic shift of the spectra lasting for 20 ps without perturbing the insulating phase. This capability to disentangle electronic and phononic excitations opens new directions to study electron correlation in solids.
ABSTRACT
Femtosecond electron and spin dynamics of the Gd(0001) surface are investigated by time-resolved photoemission and second harmonic generation. Upon optical excitation the spin polarization of the surface state is reduced by half while its exchange splitting remains nearly unchanged. Electron-magnon interaction is proposed to facilitate electron-spin-flip scattering among spin-mixed surface and bulk states, which provides a mechanism for ultrafast demagnetization.
