We have examined ultrafast carrier dynamics and light amplification in ZnO nanowires following subpicosecond
excitation at room temperature. We performed time- and wavelength-resolved pump-probe transmission and gain
measurements on a 'forest' of 100- to 500-nm thick and 20-μm long nanowires, epitaxially grown on a sapphire wafer.
Measurements were done using 267-nm pump pulses for direct, but inhomogeneous excitation, and 800-nm pulses to
achieve homogeneous excitation via three-photon absorption.
At the highest fluences, both for 267-nm and 800-nm pump pulses, a degenerate electron-hole plasma (EHP) is generated
with carrier densities of 1025 m-3 or higher. We observed strong amplification of the probe, accompanied by a rapid decay
(~ 1.5 ps) of the charge carriers. Below ~ 1025 m-3, the EHP becomes non-degenerate and the decay much slower.
A dip in the pump-probe signal was observed, caused by ionization of probe exciton-polaritons by the pump. This effect
allows for a measurement of the exciton-polariton dispersion relation and enhanced light-matter interaction in ZnO
We use the internal picosecond strain pulses to control the electron energy in a semiconductor quantum well. For
generating the strain pulse a 100 nm thick metal transducer was hit by intense laser pulse and a strain pulse with duration
~10 ps and amplitude up to 0.1% was injected into a GaAs substrate. This strain pulse travels strongly directed through
the crystal towards the quantum well generating at each momentary position a "nano-earthquake". When the quantum
well is hit by this "earth quake", the exciton resonance is shifted on a value up to 10 meV on a ps time scale.
We demonstrate ultrafast shifting of a photonic stop band driven by a photoinduced phase transition in vanadium dioxide (VO2) forming a three-dimensional photonic crystal. An ultrashort 120-fs laser pulse induces a phase transition in VO2 filling the pores of an artificial silica opal, thus changing the effective dielectric constant of the opal. Consequently, the spectral position of the photonic stop band blue-shifts producing large changes in the reflectivity. The observed switching of the photonic crystal is faster that 350 fs. The demonstrated properties of opal-VO2 composite are relevant for potential applications in all-optical switches, optical memories, low-threshold lasers, and optical computers.
In this paper we analyze linear and ultrafast non-linear properties of a three-dimensional photonic crystal composed of close-packed SiO2/Au/SiO2 core-shell colloidal particles. Strong coupling between incident light and surface plasmon of spherical gold microcavities appears as sharp features in observed reflectivity spectra in the visible. In a single layer of gold-shell particles, a highly directional diffraction pattern was observed with hexagonal symmetry. The non-linear dynamics of the reflectivity has been studied by femtosecond white-light pump-probe experiments. Abrupt changes limited by the instrumental time resolution, were observed in time-resolved reflection spectra while the signal recovers in about 10 ps. Ultrafast changes in reflectivity reach values as high as 20%. The results are compared with theory.
We report both temperature and excitation density dependent, four wave mixing measurements on Si-D stretch vibrations in deuterated amorphous silicon thin films. Utilising the infrared output of a free electron laser (FEL), we have made transient grating measurements of the temperature dependent anharmonic decay rate of Si-D stretch vibrations in deuterated amorphous silicon. Unlike Si-H vibrations, it is round that the excited deuterium mode relaxed with a single exponential decay rate into collective modes of the host, bypassing the local bending modes. Vibrational photon echo measurements suggest that phase coherence is lost via elastic phonon scattering with excitation (but not temperature) dependent contributions from non-equilibrium phonons. The degradation of p-i-n solar cells with identical intrinsic absorber layers (to those used for the time domain experiments) under prolonged light soaking treatments show that α-Si:D has a superior resistance to light induced defect creation.
Ultrashort strain pulses are a promising tool for the analysis and
manipulation of condensed matter, thin films, and nanostructures.
We present a new and unconventional way to generate coherent
longitudinal acoustic wavepackets of high amplitude in the THz
frequency range using the nonlinear development of picosecond
strain pulses in a crystal. Our work [PRL 89, 285504 (2002)]
demonstrated breakup of an initial wavepacket into a train of
ultrashort strain solitons, using position-dependent Brillouin
scattering. We extend in this paper the interpretation of the
Brillouin scattering data in terms of optical Bragg reflections
off the moving soliton train, using the analogy with an N-slit
diffraction grating. Finally, we show that these short pulses can
excite an electronic two-level system at THz resonance frequency,
allowing for coherent amplification and even the development of a
We describe a mechanism, which links the long-range potential fluctuations induced by charged defects to the low frequency resistance noise widely known as 1/f noise. This mechanism is amenable to the first principles microscopic calculation of the noise spectrum, which includes the absolute noise intensity. We have performed such a calculation for the thin films of hydrogenated amorphous silicon (a-Si:H) under the condition that current flows perpendicular to the plane of the films, and found a very good agreement between the resulting theoretical spectra and the spectra obtained in our own experiments. The mechanism described is quite general. It should be present in a broad class of systems containing poorly screened charged defects.
For a given defect, the rate of charge fluctuations depends on the activation barrier, which an electron should overcome in order to escape from that defect. This rate, in turn, depends on the local random potential. Therefore, our study also introduces a new experimental method of characterizing the random potential landscapes in the vicinity of deep defects.