36 research outputs found
Multiphoton-pumped UV-Vis transient absorption spectroscopy of 2D materials: basic concepts and recent applications
2D materials are considered a key element in the development of
next-generation electronics (nanoelectronics) due to their extreme thickness in
the nanometer range and unique physical properties. The ultrafast dynamics of
photoexcited carriers in such materials is strongly influenced by their
interfaces, since the thickness of 2D materials is much smaller than the
typical depth of light penetration into them and the mean free path of
photoexcited carriers. The resulting collisions of photoexcited carriers with
interfacial potential barriers of 2D materials in the presence of a strong
laser field significantly alter the overall dynamics of photoexcitation,
allowing laser light to be directly absorbed by carriers in the
conduction/valence band through the inverse bremsstrahlung mechanism. The
corresponding ultrafast carrier dynamics can be monitored using
multiphoton-pumped UV-Vis transient absorption spectroscopy. In this review, we
discuss the basic concepts and recent applications of this spectroscopy for a
variety of 2D materials, including transition-metal dichalcogenide monolayers,
topological insulators, and other 2D semiconductor structures
Plasmon-enhanced electron-phonon coupling in Dirac surface states of the thin-film topological insulator Bi2Se3
Raman measurements of a Fano-type surface phonon mode associated with Dirac
surface states (SS) in Bi2Se3 topological insulator thin films allowed an
unambiguous determination of the electron-phonon coupling strength in Dirac SS
as a function of film thickness ranging from 2 to 40 nm. A non-monotonic
enhancement of the electron-phonon coupling strength with maximum for the 8 -
10 nm thick films was observed. The non-monotonicity is suggested to originate
from plasmon-phonon coupling which enhances electron-phonon coupling when free
carrier density in Dirac SS increases with decreasing film thickness and
becomes suppressed for thinnest films when anharmonic coupling between in-plane
and out-of-plane phonon modes occurs. The observed about four-fold enhancement
of electron-phonon coupling in Dirac SS of the 8 - 10 nm thick Bi2Se3 films
with respect to the bulk samples may provide new insights into the origin of
superconductivity in this-type materials and their applications
Effect of Mn doping on ultrafast carrier dynamics in thin films of the topological insulator Bi2Se3
Transient reflectivity (TR) measured at laser photon energy 1.51 eV from the
indirectly intersurface coupled topological insulator Bi2-xMnxSe3 films (12 nm
thick) revealed a strong dependence of the rise-time and initial decay-time
constants on photoexcited carrier density and Mn content. In undoped samples (x
= 0), these time constants are exclusively governed by electron-electron and
electron-phonon scattering, respectively, whereas in films with x = 0.013 -
0.27 ultrafast carrier dynamics are completely controlled by photoexcited
electron trapping by ionized Mn2+ acceptors and their dimers. The shortest
decay-time (~0.75 ps) measured for the film with x = 0.27 suggests a great
potential of Mn-doped Bi2Se3 films for applications in high-speed
optoelectronic devices. Using Raman spectroscopy exploiting similar laser
photon energy (1.58 eV), we demonstrate that due to indirect intersurface
coupling in the films, the photoexcited electron trapping in the bulk enhances
the electron-phonon interaction strength in Dirac surface states
Plasmon-enhanced electron-phonon coupling in Dirac surface states of the thin-film topological insulator Bi 2 Se 3
Acoustic phonon dynamics in thin-films of the topological insulator Bi2Se3
Transient reflectivity traces measured for nanometer-sized films of the
topological insulator Bi2Se3 revealed GHz-range oscillations driven within the
relaxation of hot carriers photoexcited with ultrashort laser pulses of 1.51 eV
photon energy. These oscillations have been suggested to result from acoustic
phonon dynamics, including coherent longitudinal acoustic phonons in the form
of standing acoustic waves. An increase of oscillation frequency from ~35 to
~70 GHz with decreasing film thickness from 40 to 15 nm was attributed to the
interplay between two different regimes employing traveling-acoustic-waves for
films thicker than 40 nm and the film bulk acoustic wave resonator (FBAWR)
modes for films thinner than 40 nm. The amplitude of oscillations decays
rapidly for films below 15 nm thick when the indirect intersurface coupling in
Bi2Se3 films switches the FBAWR regime to that of the Lamb wave excitation. The
frequency range of coherent longitudinal acoustic phonons is in good agreement
with elastic properties of Bi2Se3
Ultrafast carrier dynamics in thin-films of the topological insulator Bi2Se3
Transient reflectivity measurements of thin films, ranging from 6 to 40 nm in
thickness, of the topological insulator Bi2Se3 revealed a strong dependence of
the carrier relaxation time on the film thickness. For thicker films the
relaxation dynamics are similar to those of bulk Bi2Se3, where the contribution
of the bulk insulating phase dominates over that of the surface metallic phase.
The carrier relaxation time shortens with decreasing film thickness, reaching
values comparable to those of noble metals. This effect may result from the
hybridization of Dirac cone states at the opposite surfaces for the thinnest
films
Effect of carrier recombination on ultrafast carrier dynamics in thin films of the topological insulator Bi2Se3
Transient reflectivity (TR) from thin films (6 - 40 nm thick) of the
topological insulator Bi2Se3 reveal ultrafast carrier dynamics, which suggest
the existence of both radiative and non-radiative recombination between
electrons residing in the upper cone of initially unoccupied high energy Dirac
surface states (SS) and holes residing in the lower cone of occupied low energy
Dirac SS. The modeling of measured TR traces allowed us to conclude that
recombination is induced by the depletion of bulk electrons in films below ~20
nm thick due to the charge captured on the surface defects. We predict that
such recombination processes can be observed using time-resolved
photoluminescence techniques