Tip-enhanced Raman spectroscopic imaging of localized defects in carbon nanotubes

We used tip-enhanced Raman spectroscopy to study defect induced D-band Raman scattering in metallic single-walled carbon nanotubes with a spatial resolution of 15 nm. The spatial extent of the D-band signal in the vicinity of localized defects is visualized and found to be about 2 nm only. Using the strong optical fields underneath the tip, we photogenerate localized defects and derive a relation between defect density and resulting D-band intensity

Chirality Changes in Carbon Nanotubes Studied with Near-Field Raman Spectroscopy

We report on the direct visualization of chirality changes in carbon nanotubes by mapping local changes in resonant RBM phonon frequencies with an optical resolution of 40 nm using near-field Raman spectroscopy. We observe the transition from semiconducting-to-metal and metal-to-metal chiralities at the single nanotube level. Our experimental findings, based on detecting changes in resonant RBM frequencies, are complemented by measuring changes in the G-band frequency and line shape. In addition, we observe increased Raman scattering due to local defects associated with the structural transition. From our results, we determine the spatial extent of the transition region to be Ltrans 40−100 nm

Subsurface Raman Imaging with Nanoscale Resolution

We report on chemically specific, subsurface imaging with high spatial resolution. Using tip-enhanced Raman spectroscopy, we probe carbon nanotubes buried beneath a host dielectric media. We demonstrate our ability to map and resolve specific vibrational modes with 30 nm spatial resolution for dielectric layers with different thicknesses

Nanoscale Vibrational Analysis of Single-Walled Carbon Nanotubes

We use near-field Raman imaging and spectroscopy to study localized vibrational modes along individual, single-walled carbon nanotubes (SWNTs) with a spatial resolution of 10−20 nm. Our approach relies on the enhanced field near a laser-irradiated gold tip which acts as the Raman excitation source. We find that for arc-discharge SWNTs, both the radial breathing mode (RBM) and intermediate frequency mode (IFM) are highly localized. We attribute such localization to local changes in the tube structure (n, m). In comparison, we observe no such localization of the Raman active modes in SWNTs grown by chemical vapor deposition (CVD). The direct comparison between arc-discharge and CVD-grown tubes allows us to rule out any artifacts induced by the supporting substrate

Near-Field Second-Harmonic Generation Induced by Local Field Enhancement

The field near a sharp metal tip can be strongly enhanced if irradiated with an optical field polarized along the tip axis. We demonstrate that the enhanced field gives rise to local second-harmonic (SH) generation at the tip surface thereby creating a highly confined photon source. A theoretical model for the excitation and emission of SH radiation at the tip is developed and it is found that this source can be represented by a single on-axis oscillating dipole. The model is experimentally verified by imaging the spatial field distribution of strongly focused laser modes

Controlling Nonequilibrium Phonon Populations in Single-Walled Carbon Nanotubes

We studied spatially isolated single-walled carbon nanotubes (SWNTs) immobilized in a quasi-planar optical λ/2-microresonator using confocal microscopy and spectroscopy. The modified photonic mode density within the resonator is used to selectively enhance or inhibit different Raman transitions of SWNTs. Experimental spectra are presented that exhibit single Raman bands only. Calculations of the relative change in the Raman scattering cross sections underline the potential of our microresonator for the optical control of nonequilibrium phonon populations in SWNT

Compression of Ultrashort Laser Pulses via Gated Multiphoton Intrapulse Interference Phase Scans

Delivering femtosecond laser light in the focal plane of a high numerical aperture microscope objective is still a challenge, despite significant developments in the generation of ultrashort pulses. One of the most popular techniques, used to correct phase distortions resulting from propagation through transparent media, is the multiphoton intrapulse interference phase scan (MIIPS). The accuracy of MIIPS however is limited when higher order phase distortions are present. Here we introduce an improvement, called Gated-MIIPS, which avoids shortcomings of MIIPS, reduces the influence of higher order phase terms, and can be used to more efficiently compress broad band laser pulses even with a simple 4f pulse shaper setup. In this work we present analytical formulas for MIIPS and Gated-MIIPS valid for chirped Gaussian pulses; we show an approximated analytic expression for Gated-MIIPS valid for arbitrary pulse shapes; finally we demonstrate the increased accuracy of Gated-MIIPS via experiment and numerical simulation.Comment: 11 pages, 7 figure

Probing Exciton Localization in Single-Walled Carbon Nanotubes Using High-Resolution Near-Field Microscopy

We observe localization of excitons in semiconducting single-walled carbon nanotubes at room temperature using high-resolution near-field photoluminescence (PL) microscopy. Localization is the result of spatially confined exciton energy minima with depths of more than 15 meV connected to lateral energy gradients exceeding 2 meV/nm as evidenced by energy-resolved PL imaging. Simulations of exciton diffusion in the presence of energy variations support this interpretation predicting strongly enhanced PL at local energy minima

Probing the ultrafast dynamics of excitons in single semiconducting carbon nanotubes

Excitonic states govern the optical spectra of low-dimensional semiconductor nanomaterials and their dynamics are key for a wide range of applications, such as in solar energy harvesting and lighting. Semiconducting single-walled carbon nanotubes emerged as particularly rich model systems for one-dimensional nanomaterials and as such have been investigated intensively in the past. The exciton decay dynamics in nanotubes has been studied mainly by transient absorption and time-resolved photoluminescence spectroscopy. Since different transitions are monitored with these two techniques, developing a comprehensive model to reconcile different data sets, however, turned out to be a challenge and remarkably, a uniform description seems to remain elusive. In this work, we investigate the exciton decay dynamics in single carbon nanotubes using transient interferometric scattering and time-resolved photoluminescence microscopy with few-exciton detection sensitivity and formulate a unified microscopic model by combining unimolecular exciton decay and ultrafast exciton-exciton annihilation on a time-scale down to 200 fs