Gordana N. Ostojic

Interband recombination dynamics in resonantly excited single-walled carbon nanotubes.

Wavelength-dependent pump-probe spectroscopy of micelle-suspended single-walled carbon nanotubes reveals two-component dynamics. The slow component (5-20 ps), which has not been observed previously, is resonantly enhanced whenever the pump photon energy coincides with an absorption peak and we attribute it to interband carrier recombination, whereas we interpret the always-present fast component (0.3-1.2 ps) as intraband carrier relaxation in nonresonantly excited nanotubes. The slow component decreases drastically with decreasing pH (or increasing H+ doping), especially in large-diameter tubes. This can be explained as a consequence of the disappearance of absorption peaks at high doping due to the entrance of the Fermi energy into the valence band, i.e., a 1D manifestation of the Burstein-Moss effect.

Excitons in carbon nanotubes with broken time-reversal symmetry.

Near-infrared magneto-optical spectroscopy of single-walled carbon nanotubes reveals two absorption peaks with an equal strength at high magnetic fields (>55 T). We show that the peak separation is determined by the Aharonov-Bohm phase due to the tube-threading magnetic flux, which breaks the time-reversal symmetry and lifts the valley degeneracy. This field-induced symmetry breaking thus overcomes the Coulomb-induced intervalley mixing which is predicted to make the lowest exciton state optically inactive (or dark).

Stability of High-Density One-Dimensional Excitons in Carbon Nanotubes under High Laser Excitation

Through ultrafast pump-probe spectroscopy with intense pump pulses and a wide continuum probe, we show that interband exciton peaks in single-walled carbon nanotubes (SWNTs) are extremely stable under high laser excitations. Estimates of the initial densities of excitons from the excitation conditions, combined with recent theoretical calculations of exciton Bohr radii for SWNTs, suggest that their positions do not change at all even near the Mott density. In addition, we found that the presence of lowest-subband excitons broadens all absorption peaks, including those in the second-subband range, which provides a consistent explanation for the complex spectral dependence of pump-probe signals reported for SWNTs.

Ultrafast dynamics of unbundeled single-walled carbon nanotubes

We have used ultrafast pump-probe spectroscopy to explore carrier dynamics in micelle-suspended single-walled carbon nanotubes. We observe two distinct relaxation regimes and attribute them to intraband carrier relaxation towards the band edge and radiative interband recombination

Long-lived dilute photocarriers in individually-suspended single-walled carbon nanotubes

We have observed slow (> 1 ns) carrier decays in single-walled carbon nanotubes. This previously-unreported signal appears in a dilute limit and is accompanied by polarization memory, which persists as long as the photocarriers exist

Mid-infrared ultrafast and nonlinear spectroscopy of semiconductors

Our recent experiments using 140 fs pulses of intense, coherent, and tunable mid-infrared (MIR) radiation produced by an optical parametric amplifier are described. These experiments explored MIR ultrafast and nonlinear optical phenomena in bulk semiconductors, InMnAs/GaSb ferromagnetic heterostructures, and single-wall carbon nanotubes.