160 research outputs found
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 micro-cavity, 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
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 ultra narrow 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, multi-peak luminescence profiles with
specific temperature dependence show the presence of confined phonon modes
Chirality dependence of the absorption cross-section of carbon nanotubes
The variation of the optical absorption of carbon nanotubes with their
geometry has been a long standing question at the heart of both metrological
and applicative issues, in particular because optical spectroscopy is one of
the primary tools for the assessment of the chiral species abundance of
samples. Here, we tackle the chirality dependence of the optical absorption
with an original method involving ultra-efficient energy transfer in
porphyrin/nanotube compounds that allows uniform photo-excitation of all chiral
species. We measure the absolute absorption cross-section of a wide range of
semiconducting nanotubes at their S22 transition and show that it varies by up
to a factor of 2.2 with the chiral angle, with type I nanotubes showing a
larger absorption. In contrast, the luminescence quantum yield remains almost
constant
Environmental effect on the carrier dynamics in carbon nanotubes
Carrier dynamics is investigated in both luminescent and non luminescent samples of single wall carbon nanotubes -obtained by laser ablation- by means of two-color pump-probe experiments. The recombination dynamics is monitored by probing the transient photobleaching observed on the interband transitions of semi-conducting nanotubes. Interband and inter-subband relaxation times are about one order of magnitude slower in isolated nanotubes than in ropes of nanotubes bringing evidence of the environment influence on the carrier dynamics. The relaxation dynamics is non-exponential and is interpreted as a consequence of the inhomogeneity of the sample. Slow components up to 250 ps are measured which is significantly greater than values previoulsy reported in HiPCo nanotubes. These observations show the great dependence of the electronic properties of carbon nanotubes on the synthesis method and on their environment
Elastic exciton-exciton scattering in photoexcited carbon nanotubes
International audienceWe 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
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 photo-bleaching observed on the first interband transition of the semi-conducting tubes. The carrier dynamics takes place on a one picosecond time scale which is one order of magnitude slower than in graphite. Transient photo-induced absorption is observed with exactly the same dynamics for non-resonant probe conditions and is interpreted as a global red shift of the plasmon resonance. We show that the opening of the band gap in semi-conducting carbon nanotubes determines the non-linear response dynamics over the whole visible and near-infrared spectrum
Raman probing of uniaxial strain in individual single-wall carbon nanotubes in a composite material
International audienceThe temperature dependence of the Raman spectrum of a gelatine-based composite material doped with single-walled carbon nanotubes (SWNT@gelatin) is reported. A significant up-shift of the G-mode frequency is observed when the temperature is decreased from room temperature to 20~K. This frequency shift is significantly stronger than the one found for pure thermal effects. In contrast, the features of the radial breathing modes (frequencies and width) display no significant change in the same temperature range. These results are well understood by considering a uniaxial strain on the nanotube induced by the thermal expansitivity mismatch between the nanotube and the surrounding matrix
Excitation transfer and luminescence in porphyrin-carbon nanotube complexes
Functionalization of carbon nanotubes with hydrosoluble porphyrins (TPPS) is
achieved by "-stacking". The porphyrin/nanotube interaction is studied by
means of optical absorption, photoluminescence and photoluminescence excitation
spectroscopies. The main absorption line of the porphyrins adsorbed on
nanotubes exhibits a 120 meV red shift, which we ascribe to a flattening of the
molecule in order to optimize interactions. The porphyrin-nanotube
complex shows a strong quenching of the TPPS emission while the
photoluminescence intensity of the nanotubes is enhanced when the excitation
laser is in resonance with the porphyrin absorption band. This reveals an
efficient excitation transfer from the TPPS to the carbon nanotube
Quantum efficiency of energy transfer in noncovalent carbon nanotube/porphyrin compounds
International audienceWe report on the quantum yield of excitation energy transfer in non-covalently bound nan- otube/porphyrin compounds. Evidence for energy transfer is gained from photoluminescence exci- tation experiments. We perform a quantitative evaluation of the transfer quantum yield in the case of (6,5) nanotubes through three independent methods : quantitative PLE measurements, evalu- ation of the luminescence quenching of the donor (porphyrin) and ultrafast transient absorption measurements. The latter shows a tremendous increase of the porphyrin recovery rate upon incor- poration in the compound. All these measurements consistently lead to an exceptional quantum yield efficiency
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