22 research outputs found
Probing ultrafast symmetry breaking in photo-stimulated matter
The nature of a phase transition is inherently connected to the changes in
the crystalline symmtry, which is typically probed by elastic or inelastic
scattering with neutrons, electrons or photons. When such a phase transition is
stimulated by light or other sudden perturbations the solid evolves along a
non-equilibrium pathway of which the underlying physics is poorly understood.
Here we use picosecond Raman scattering to study the photo-induced ultrafast
dynamics in Peierls distorted Antimony. We find evidence for an ultrafast
non-thermal reversible structural phase transition. Most surprisingly, we find
evidence that this transition evolves toward a lower symmetry, in contrast to
the commonly accepted rhombohedral-to-simple cubic transition path. Our study
demonstrates the feasibility of ultrafast Raman scattering symmetry analysis of
photo-induced non-thermal transient phases
Delayed formation of coherent LO phonon-plasmon coupled modes in n-type and p-type GaAs measured using a femtosecond coherent control technique
Coherent control experiments using a pair of collinear femtosecond laser
pulses have been carried out to manipulate longitudinal optical (LO)
phonon-plasmon coupled (LOPC) modes in both p- and n-type GaAs. By tuning the
interpulse separation, remarkably distinct responses have been observed in the
two samples. To understand the results obtained a phenomenological model taking
the delayed formation of coherent LOPC modes into account is proposed. The
model suggests that the lifetime of coherent LOPC modes plays a key role and
the interference of the coherent LO phonons excited successively by two pump
pulses strongly affects the manipulation of coherent LOPC modes.Comment: 19 pages, 6 figures, accepted for publication in Phys. Rev.
Ultrafast phonon dynamics of epitaxial atomic layers of Bi on Si(111)
Ultrathin bismuth (Bi) layers on Si(111)-7×7 undergo a structural phase transformation with reducing the number of atomic layers at 3 bilayers (BL). We investigate the phonon dynamics of the Bi films close to the phase transformation by pump-probe reflectivity measurements. Coherent A1g and Eg phonons at 3 and 2 THz are clearly observed for the Bi layers with thicknesses down to 3 BL, confirming their rhombohedral crystalline structure. The A1g frequency exhibits an abrupt redshift and splits into two components at 3 BL, which are attributed to the vertical motions of Bi atoms localized at the surface and subsurface bilayers. The Eg frequency, by contrast, shows a gradual blueshift with reducing the thickness, possibly due to the lateral compressive stress at the Bi/Si interface. Below 3 BL, no coherent phonon signal is detected, in agreement with the phase transformation to the black-phosphoruslike structure. Our observations indicate that the vertical vibrations are significantly softened at 3 BL, but become almost as hard as those in the bulk crystal by adding another bilayer
Manipulation of Squeezed Two-Phonon Bound States using Femtosecond Laser Pulses
Two-phonon bound states have been excited exclusively in ZnTe(110) via impulsive stimulated second-order Raman scattering, essentially being squeezed states due to phase coherent excitation of two identical components anticorrelated in the wave vector. By using coherent control technique with a pair of femtosecond laser pulses, the manipulation of squeezed states has been demonstrated in which both the amplitude and lifetime of coherent oscillations of squeezed states are modulated, indicating the feasibility to control the quantum noise and the quantum nature of phonon squeezed states, respectively
Characteristic features of the pseudogap and superconducting states of YBa2Cu3O7-x
The relaxation dynamics of the lattice and low-energy quasiparticles in a YBa2Cu3O7−x superconductor are studied by the light reflection technique with femtosecond temporal resolution in a wide temperature range. It is shown that, for T>Tc, there exist two temperature regions with qualitatively and quantitatively different excitation dynamics, and the transition between these regions is of a hysteretic nature. It is also found that the character of changes observed in the charge carrier relaxation dynamics in the superconducting state testifies to the presence of an anisotropic gap with nodes at the Fermi surface
On the Nature of Coherent Artifact
The coherent interaction of femtosecond laser pulses in the pump probe regime has been experimentally studied in the time domain by monitoring light reflection from a tellurium single crystal. The optical response of the probed medium exhibits periodic variations at a frequency equal to that of the exciting laser radiation. Experimental dependences of the observed coherent artifact on the pump/probe intensity ratio, the number of accumulated pulses, and the mutual orientation of the polarization vectors of electromagnetic fields and the crystallographic axes are well described by the proposed phenomenological model