Excitons In Carbon Nanotubes Research Articles

Bright, long-lived and coherent excitons in carbon nanotube quantum dots

Carbon nanotubes exhibit a wealth of unique physical properties. By virtue of their exceptionally low mass and extreme stiffness they provide ultrahigh-quality mechanical resonances, promise long electron spin coherence times in a nuclear-spin free lattice for quantum information processing and spintronics, and feature unprecedented tunability of optical transitions for optoelectronic applications. Excitons in semiconducting single-walled carbon nanotubes could facilitate the upconversion of spin, mechanical or hybrid spin-mechanical degrees of freedom to optical frequencies for efficient manipulation and detection. However, successful implementation of such schemes with carbon nanotubes has been impeded by rapid exciton decoherence at non-radiative quenching sites, environmental dephasing and emission intermittence. Here we demonstrate that these limitations may be overcome by exciton localization in suspended carbon nanotubes. For excitons localized in nanotube quantum dots we found narrow optical lines free of spectral wandering, radiative exciton lifetimes and effectively suppressed blinking. Our findings identify the great potential of localized excitons for efficient and spectrally precise interfacing of photons, phonons and spins in novel carbon nanotube-based quantum devices.

Model of Finite-Momentum Excitons Driven by Surface Plasmons in Photoexcited Carbon Nanotubes Covered by Gold Metal Films

Optical spectra of finite-momentum excitons in carbon nanotubes with gold nanostructures are theoretically studied. A Green function method is developed for self-consistently solving Maxwell equations including the quantum-mechanical nonlocal response of the nanotubes and the local response of the nanostructures. Excitons with finite momenta in the axis direction in the nanotubes are effectively excited by localized electric fields due to surface plasmons in the gold nanostructures and counteract the surface plasmons through depolarization fields, showing the crucial self-consistency of these effects.

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Excitonic homogeneous broadening in single-wall carbon nanotubes

We present a review on the excitonic homogeneous linewidth, with an emphasis on original experiments by means of nonlinear spectral hole-burning. We address three specific aspects of the excitonic homogeneous broadening: (i) the variation of the homogeneous linewidth with the sonication energy, revealing the influence of extrinsic processes such as exciton-defect interactions, (ii) the collision-induced broadening due to exciton–exciton interactions, investigated by power-dependent measurements, and (iii) the phonon-induced dephasing due to exciton–phonon interactions, deduced from temperature-dependent experiments. We discuss the specific properties of one-dimensional excitons in carbon nanotubes in the context of the homogeneous broadening.

Environment effect on cross-polarized excitons in carbon nanotubes

Effects of environmental dielectric materials on cross-polarized excitons of semiconducting carbon nanotubes are theoretically studied in an effective-mass scheme. When a nanotube is surrounded by a medium, the bright exciton, shifted to the high-energy side by depolarization effect, becomes weaker and disappears with the increase of the dielectric constant. When a nanotube contains a medium inside, the bright exciton becomes more eminent. The dark exciton, brightened due to electron-hole asymmetry, is enhanced with the increase of the dielectric constant both for outside and inside materials. Consequently, the relative intensity of the brightened dark exciton normalized by the bright exciton is strongly enhanced by environmental screening, in particular, for outside materials.

Brightening of the Lowest Exciton in Carbon Nanotubes via Chemical Functionalization

Using time-dependent density functional theory, we found that chemical functionalization at low concentrations of single-walled carbon nanotubes (SWNTs) locally alters the π-conjugated network of the nanotube surface and leads to a spatial confinement of the electronically excited wave functions. Depending on the adsorbant positions, the chemisorption significantly modifies the optical selection rules. Our modeling suggests that photoluminescent efficiency of semiconducting SWNT materials can be controlled by selective chemical functionalization.

Excitonic nonlinearities in single‐wall carbon nanotubes

AbstractExcitons are composite bosons that allow a fair description of the optical properties in solid state systems. The quantum confinement in nanostructures enhances the excitonic effects and impacts the exciton–exciton interactions, which tailor the performances of classical and quantum optoelectronic devices, such as lasers or single‐photon emitters. The excitonic nonlinearities exhibit significant differences between organic and inorganic compounds. Tightly bound Frenkel excitons in molecular crystals are for instance affected by an efficient exciton–exciton annihilation (EEA). This Auger process also governs the population relaxation dynamics in carbon nanotubes that share many physical properties with organic materials. Here, we show that this similarity breaks down for the excitonic decoherence in carbon nanotubes. Original nonlinear spectral‐hole burning experiments bring evidence of pure dephasing induced by exciton–exciton scattering (EES) in the k‐space. This mechanism controls the exciton collision‐induced broadening, as for Wannier excitons in inorganic semiconductors. We demonstrate that this singular behavior originates from the intrinsic one‐dimensionality of excitons in carbon nanotubes, which display unique hybrid features of organic and inorganic systems.

Redshift of Excitons in Carbon Nanotubes Caused by the Environment Polarizability

Optical excitations of molecular systems can be modified by their physical environment. We analyze the underlying mechanisms within many-body perturbation theory, which is particularly suited to study nonlocal polarizability effects on the electronic structure. Here we focus on the example of a semiconducting carbon nanotube, which observes redshifts of its excitons when the tube is touched by another nanotube or other physisorbates. We show that the redshifts mostly result from the polarizability of the attached ad system. Electronic coupling may enhance the redshifts, but depends very sensitively on the structural details of the contact.

Ultrafast energy transfer of one-dimensional excitons between carbon nanotubes: a femtosecond time-resolved luminescence study

Excitation energy transfer has long been an intriguing subject in the fields of photoscience and materials science. Along with the recent progress of photovoltaics, photocatalysis, and photosensors using nanoscale materials, excitation energy transfer between a donor and an acceptor at a short distance (≤1-10 nm) is of growing importance in both fundamental research and technological applications. This Perspective highlights our recent studies on exciton energy transfer between carbon nanotubes with interwall (surface-to-surface) distances of less than ∼1 nm, which are equivalent to or shorter than the size of one-dimensional excitons in carbon nanotubes. We show exciton energy transfer in bundles of single-walled carbon nanotubes with the interwall distances of ∼0.34 and 0.9 nm (center-to-center distances ∼1.3-1.4 and 1.9 nm). For the interwall distance of ∼0.34 nm (center-to-center distance ∼1.3-1.4 nm), the transfer rate per tube from a semiconducting tube to adjacent semiconducting tubes is (1.8-1.9) × 10(12) s(-1), and that to adjacent metallic tubes is 1.1 × 10(12) s(-1). For the interwall distance of ∼0.9 nm (center-to-center distance ∼1.9 nm), the transfer rate per tube from a semiconducting tube to adjacent semiconducting tubes is 2.7 × 10(11) s(-1). These transfer rates are much lower than those predicted by the Förster model calculation based on a point dipole approximation, indicating the failure of the conventional Förster model calculations. In double-walled carbon nanotubes, which are equivalent to ideal nanoscale coaxial cylinders, we show exciton energy transfer from the inner to the outer tubes. The transfer rate between the inner and the outer tubes with an interwall distance of ∼0.38 nm is 6.6 × 10(12) s(-1). Our findings provide an insight into the energy transfer mechanisms of one-dimensional excitons.

Elastic Exciton-Exciton Scattering in Photoexcited Carbon Nanotubes

We report on original nonlinear spectral hole-burning experiments in single wall carbon nanotubes that bring evidence of pure dephasing induced by exciton-exciton scattering. We show that the collision-induced broadening in carbon nanotubes is controlled by exciton-exciton scattering as for Wannier excitons in inorganic semiconductors, while the population relaxation is driven by exciton-exciton annihilation as for Frenkel excitons in organic materials. We demonstrate that this singular behavior originates from the intrinsic one-dimensionality of excitons in carbon nanotubes, which display unique hybrid features of organic and inorganic systems.

Fine structure of exciton levels in carbon nanotubes: A semianalytical approach

We propose a new approach toward excitons in carbon nanotubes whereby the matrix elements of the electron-hole Coulomb interaction are expanded into a series over the nanotube's one-dimensional reciprocal lattice vectors. We show that only a few terms of this expansion give a nonvanishing contribution to the Coulomb matrix elements. The proposed approach allows one to single out Fourier components of the Coulomb potential responsible for the intervalley coupling and formation of the exciton fine structure for each particular nanotube chirality.

Breakdown of exchange approximation for cross-polarized excitons in carbon nanotubes

Cross-polarized excitons in carbon nanotubes where polarization is perpendicular to the tube axis are calculated in a Tamm-Dancoff-type approximation and the results are compared with those obtained in the self-consistent-field method. For the lowest exciton, the oscillator strength for the optical absorption peak is considerably reduced and the energy is overestimated.

Asymptotic exchange coupling of quasi-one-dimensional excitons in carbon nanotubes

An analytical expression is obtained for the biexciton binding energy as a function of the interexciton distance and binding energy of constituent quasi-one-dimensional excitons in carbon nanotubes. This allows one to trace biexciton energy variation and relevant nonlinear absorption under external conditions whereby the exciton binding energy varies. The nonlinear absorption line shapes calculated exhibit characteristic asymmetric (Rabi) splitting as the exciton energy is tuned to the nearest interband plasmon resonance. These results are useful for tunable optoelectronic device applications of optically excited semiconducting carbon nanotubes, including the strong excitation regime with optical nonlinearities.

Optical absorption of charged excitons in semiconducting carbon nanotubes

In this article we examine the absorption coefficient of charged excitons in carbon nanotubes. We investigate the temperature and damping dependence of the absorption spectra. We show that the trion peak in the spectrum is asymmetric for temperatures greater than approximately 1K whereas the absorption peak arising from excitons is symmetric. We expect the positive and negative trion absorption line shapes to be identical, independently of the chiral index (n,m).

Effect of electron-hole asymmetry on cross-polarized excitons in carbon nanotubes

Excitonic absorption of semiconducting carbon nanotubes for polarization perpendicular to the tube axis is theoretically studied in an effective-mass approximation including higher-order terms describing trigonal warping, curvature, lattice distortion, and electron-hole asymmetry. Asymmetry between electrons and holes causes brightening of dark excitons leading to additional low-energy peaks in optical absorption spectra. In comparison with parallel polarization, chiral-angle dependence of excitation energy is weak because of the cancellation of higher-order corrections between relevant valence and conduction bands.

"Dark" and "Bright" Excitons in Carbon Nanotubes: New Media for Quantum Optics

It is well-known that strong Coulomb effects in carbon nanotubes lead to formation of the so-called bright and (forbidden one-photon optical transition) exciton states, and dramatically decrease the efficiency of one-photon light emission via trapping of the carriers by states. We suggest and demonstrate that the proper use of these bright and exciton states with distinctively different recombination times may turn the situation around: the use of quantum coherence and multiphoton schemes of excitation potentially not only allow one to efficiently manipulate the dark states, but can also create conditions for efficient light generation in different frequency regions, produce slow or fast light, implement quantum light storage, and media with a negative refractive index. We discuss possible applications of the quantum coherent effects to quantum-optical carbon nanotube devices that have a potential for suitable performance at elevated temperatures, because the binding energies of excitons in single-walled nanotubes are hundreds-of-meV high.

Room temperature terahertz electro-optic modulation by excitons in carbon nanotubes

We have investigated the electroabsorption of aligned micelle-wrapped single-walled carbon nanotubes induced by intense terahertz (THz) pulses by using THz-pump and optical-probe spectroscopy at room temperature. The electroabsorption signal is enhanced at exciton resonances near the optical communication wavelength. The signal showed a quadratic dependence on the THz electric field and instantaneously followed the THz oscillation (∼1 ps). The observed ultrafast electro-optic modulation with a THz bandwidth suggests the potential functionality of carbon nanotubes in high-speed electro-optic devices.

Environment Effects on Excitons in Semiconducting Carbon Nanotubes

Effects of environmental dielectric screening on excitons in carbon nanotubes are studied within a k · p scheme and a continuum model. They are shown to be sensitive to the effective distance between the nanotube and the dielectric medium relative to the diameter. For material surrounding the nanotube, the band gap decreases with the increase of the dielectric constant, but the energy of the ground exciton exhibits only a slight decrease for effective distance comparable to interlayer spacing of bulk graphite. The binding energy of excited exciton states disappears rapidly. For dielectric material inside the nanotube, effects are much weaker and excited exciton states remain as bound states even for very large dielectric constant.

Energy splitting between bright and dark excitons in carbon nanotubes

We have calculated the bright-dark energy splitting in semiconducting zigzag carbon nanotubes in a simple tight-binding model for π -electrons . The short-range cut-off of the Coulomb interaction is used as a fitting parameter. We have found that the splitting depends not only on the type of transition, E 11 or E 22 , but also on the structure, or the index, of carbon nanotube, and that family pattern appears in the cut-off parameter dependence of the splitting.

Existence of an upper limit on the density of excitons in carbon nanotubes by diffusion-limited exciton-exciton annihilation: Experiment and theory

Through an investigation of photoemission properties of highly photoexcited single-walled carbon nanotubes, we demonstrate that there is an upper limit on the achievable excitonic density. As the intensity of optical excitation increases, all photoluminescence emission peaks arising from different chirality single-walled carbon nanotubes showed clear saturation in intensity. Each peak exhibited a saturation value that was independent of the excitation wavelength, indicating that there is an upper limit on the excitonic density for each nanotube species. We propose that this saturation behavior is a result of efficient exciton-exciton annihilation through which excitons decay nonradiatively. In order to explain the experimental results and obtain excitonic densities in the saturation regime, we have developed a model, taking into account the generation, diffusion-limited exciton-exciton annihilation, and spontaneous decays of one-dimensional excitons. Using the model, we were able to reproduce the experimentally obtained saturation curves under certain approximations, from which the excitonic densities were estimated. The validity of the model was confirmed through comparison with Monte Carlo simulations. Finally, we show that the conventional rate equation for exciton-exciton annihilation fails to fit the experimentally observed saturation behavior, especially at high excitonic densities.