34 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.
Study of ultrafast processes in matter by means of time-resolved electron diffraction and microscopy
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
New Evidence for a Nonclassical Behavior of Laser Multimode Light
In this work, we present new experimental evidence of a nonclassical behavior of a multimode Fabry–Perot (FP) semiconductor laser by the measurements of intensity correlation functions. Due to the multimode quantum state occurrence, instead of expected correlations between the intensities of the laser modes (a semiclassical theory), their anticorrelations were revealed
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