Superlocalization of Excitons in Carbon Nanotubes at Cryogenic Temperature.

At cryogenic temperature and at the single emitter level, the optical properties of single-wall carbon nanotubes depart drastically from that of a one-dimensional (1D) object. In fact, the (usually unintentional) localization of excitons in local potential wells leads to nearly 0D behaviors such as photon antibunching, spectral diffusion, inhomogeneous broadening, etc. Here, we present a hyperspectral imaging of this spontaneous exciton localization effect at the single nanotube level using a super-resolved optical microscopy approach. We report on the statistical distribution of the trap localization, depth, and width. We use a quasi-resonant photoluminescence excitation approach to probe the confined quantum states. Numerical simulations of the quantum states and exciton diffusion show that the excitonic states are deeply modified by the interface disorder inducing a remarkable discretization of the excitonic absorption spectrum and a quenching of the free 1D exciton absorption.

Interplay of spectral diffusion and phonon-broadening in individual photo-emitters: the case of carbon nanotubes.

At cryogenic temperatures, the photoluminescence (PL) spectrum of nano-emitters may still be significantly broadened due to interactions with the environment. The interplay of spectral diffusion (SD) and phonon broadening in this context is still a debated issue. Singlewall carbon nanotubes (SWNTs) are a particularly relevant system to address this topic as they show intense spectral diffusion and undergo a high exciton-phonon coupling due to their one-dimensional geometry. Here, we investigate the correlations between the spectral diffusion of the main line and that of the wings in SWNTs quantitatively and demonstrate that the photoluminescence spectrum undergoes spectral jumps as a whole, without distortions. This behavior suggests that the spectral shape of SWNT PL is defined by exciton-phonon interactions and that spectral diffusion results in an additional flat broadening. The methodology developed here can be used to investigate a broad range of non-Lorentzian emitters undergoing spectral diffusion.

Exploiting One-Dimensional Exciton-Phonon Coupling for Tunable and Efficient Single-Photon Generation with a Carbon Nanotube.

Condensed-matter emitters offer enriched cavity quantum electrodynamical effects due to the coupling to external degrees of freedom. In the case of carbon nanotubes, a very peculiar coupling between localized excitons and the one-dimensional acoustic phonon modes can be achieved, which gives rise to pronounced phonon wings in the luminescence spectrum. By coupling an individual nanotube to a tunable optical microcavity, we show that this peculiar exciton-phonon coupling is a valuable resource to enlarge the tuning range of the single-photon source while keeping an excellent exciton-photon coupling efficiency and spectral purity. Using the unique flexibility of our scanning fiber cavity, we are able to measure the efficiency spectrum of the very same nanotube in the Purcell regime for several mode volumes. Whereas this efficiency spectrum looks very much like the free-space luminescence spectrum when the Purcell factor is small (large mode volume), we show that the deformation of this spectrum at lower mode volumes can be traced back to the strength of the exciton-photon coupling. It shows an enhanced efficiency on the red wing that arises from the asymmetry of the incoherent energy exchange processes between the exciton and the cavity. This allows us to obtain a tuning range up to several hundred times the spectral width of the source.

Widely Tunable Single-Photon Source from a Carbon Nanotube in the Purcell Regime.

The narrow emission of a single carbon nanotube at low temperature is coupled to the optical mode of a fiber microcavity using the built-in spatial and spectral matching brought by this flexible geometry. A thorough cw and time-resolved investigation of the very same emitter both in free space and in cavity shows an efficient funneling of the emission into the cavity mode together with a strong emission enhancement corresponding to a Purcell factor of up to 5. At the same time, the emitted photons retain a strong sub-Poissonian statistics. By exploiting the cavity feeding effect on the phonon wings, we locked the emission of the nanotube at the cavity resonance frequency, which allowed us to tune the frequency over a 4 THz band while keeping an almost perfect antibunching. By choosing the nanotube diameter appropriately, this study paves the way to the development of carbon-based tunable single-photon sources in the telecom bands.

Diameter-selective non-covalent functionalization of carbon nanotubes with porphyrin monomers.

We report on the spontaneous non-covalent functionalization of carbon nanotubes with hydrophobic porphyrin molecules in micellar aqueous solution. By monitoring the species concentrations with optical spectroscopies, we can follow the kinetics of the reaction and study its thermodynamical equilibrium as a function of the reagent concentrations. We show that the reaction is well accounted for by a cooperative Hill equation, reaching a molecular coverage close to a compact monolayer for a porphyrin concentration larger than a diameter-specific threshold concentration. The equilibrium constant is measured for 16 nanotube chiral species. The Gibbs energy of the reaction (of the order of -40 kJ mol(-1)) and its evolution with the nanotube diameter is consistent with theoretical calculations of the binding energy. This thermodynamical study shows a strong preferential binding of TPP molecules to larger diameter nanotubes. This original curvature selectivity can be used to induce diameter selective species enrichment.

Unifying the low-temperature photoluminescence spectra of carbon nanotubes: the role of acoustic phonon confinement.

At low temperature the photoluminescence of single-wall carbon nanotubes show a large variety of spectral profiles ranging from ultranarrow lines in suspended nanotubes to broad and asymmetrical line shapes that puzzle the current interpretation in terms of exciton-phonon coupling. Here, we present a complete set of photoluminescence profiles in matrix embedded nanotubes including unprecedented narrow emission lines. We demonstrate that the diversity of the low-temperature luminescence profiles in nanotubes originates in tiny modifications of their low-energy acoustic phonon modes. When low-energy modes are locally suppressed, a sharp photoluminescence line as narrow as 0.7 meV is restored. Furthermore, multipeak luminescence profiles with specific temperature dependence show the presence of confined phonon modes.

Detection of a biexciton in semiconducting carbon nanotubes using nonlinear optical spectroscopy.

We report the observation of the biexciton in semiconducting single-wall carbon nanotubes by means of nonlinear optical spectroscopy. Our measurements reveal the universal asymmetric line shape of the Fano resonance intrinsic to the biexciton transition. For nanotubes of the (9,7) chirality, we find a biexciton binding energy of 106 meV. From the calculation of the χ((3)) nonlinear response, we provide a quantitative interpretation of our measurements, leading to an estimation of the characteristic Fano factor q of 7 ± 3. This value allows us to extract the first experimental information on the biexciton stability and we obtain a biexciton annihilation rate comparable to the exciton-exciton annihilation one.

Strong exciton-photon coupling in microcavities containing new fluorophenethylamine based perovskite compounds.

We synthetize some new perovskite thin layers: p-fluorophenethylamine tetraiodoplumbate pFC(6)H(4)C(2)H(4)NH(3))(2)PbI(4) perovskite molecules, included in a PMMA matrix. We report on the optical properties of the perovskite doped PMMA thin layers and we show that these layers are much more stable under laser illumination and present a smaller roughness than the spin-coated (C(6)H(5)C(2)H(4)NH(3))(2)PbI(4) layers. These new layers are used as the active material in vertical microcavities and the strong-coupling regime is evidenced by a clear anti-crossing appearing in the angular-resolved reflectivity experiments at room temperature.

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.

Optical spectroscopy of two-dimensional layered (C(6)H(5)C(2)H(4)-NH(3))(2)-PbI(4) perovskite.

We report on optical spectroscopy (photoluminescence and photoluminescence excitation) on two-dimensional self-organized layers of (C(6)H(5)C(2)H(4)-NH(3))(2)-PbI(4) perovskite. Temperature and excitation power dependance of the optical spectra gives a new insight into the excitonic and the phononic properties of this hybrid organic/inorganic semiconductor. In particular, exciton-phonon interaction is found to be more than one order of magnitude higher than in GaAs QWs. As a result, photoluminescence emission lines have to be interpreted in the framework of a polaron model.

Many-body effects in electronic bandgaps of carbon nanotubes measured by scanning tunnelling spectroscopy.

Single-walled carbon nanotubes provide an ideal system for studying the properties of one-dimensional (1D) materials, where strong electron-electron interactions are expected. Optical measurements have recently reported the existence of excitons in semiconducting nanotubes, revealing the importance of many-body effects. Surprisingly, pioneering electronic structure calculations and scanning tunnelling spectroscopy (STS) experiments report the same gap values as optical experiments. Here, an experimental STS study of the bandgap of single-walled semiconducting nanotubes, demonstrates a continuous transition from the gap reduced by the screening resulting from the metal substrate to the intrinsic gap dominated by many-body interactions. These results provide a deeper knowledge of many-body interactions in these 1D systems and a better understanding of their electronic properties, which is a prerequisite for any application of nanotubes in the ultimate device miniaturization for molecular electronics, or spintronics.

Optical transitions in single-wall boron nitride nanotubes.

Optical transitions in single-wall boron nitride nanotubes are investigated by means of optical absorption spectroscopy. Three absorption lines are observed. Two of them (at 4.45 and 5.5 eV) result from the quantification involved by the rolling up of the hexagonal boron nitride (h-BN) sheet. The nature of these lines is discussed, and two interpretations are proposed. A comparison with single-wall carbon nanotubes leads one to interpret these lines as transitions between pairs of van Hove singularities in the one-dimensional density of states of boron nitride single-wall nanotubes. But the confinement energy due to the rolling up of the h-BN sheet cannot explain a gap width of the boron nitride nanotubes below the h-BN gap. The low energy line is then attributed to the existence of a Frenkel exciton with a binding energy in the 1 eV range.

Ultrafast carrier dynamics in single-wall carbon nanotubes.

Time-resolved carrier dynamics in single-wall carbon nanotubes is investigated by means of two-color pump-probe experiments. The recombination dynamics is monitored by probing the transient photobleaching observed on the interband transitions of the semiconducting tubes. This dynamics takes place on a 1 ps time scale which is 1 order of magnitude slower than in graphite. Transient photoinduced absorption is observed for nonresonant probing and is interpreted as a global redshift of the pi-plasmon resonance. We show that the opening of the band gap in semiconducting carbon nanotubes determines the nonlinear response dynamics over the whole visible and near-infrared spectrum.