[Show abstract][Hide abstract] ABSTRACT: We report on the laser action in a microcavity where both the dielectric mirrors and the active material have a macromolecular nature, resulting in a full plastic laser device. Distributed Bragg reflectors (DBRs) are prepared by spin-coating of polyvinylcarbazole and cellulose acetate orthogonal solutions and the active layer consists of a highly fluorescent conjugated polymer poly(9,9-dioctylfluorenyl-2,7-diyl-co-1,4-benzo-(2,1'-3)-thiadiazole) (F8BT) spun film. A quality factor in the range 80–180 is achieved and the cavity mode is carefully tuned on the peak of the F8BT amplified spontaneous emission spectrum. Under pulsed optical pumping, laser emission is obtained with a surprisingly low lasing threshold (<20 μJ cm−2) for a full plastic DBR optical cavity. This result opens a simple and cheap way to obtain a new class of polymer lasers.
[Show abstract][Hide abstract] ABSTRACT: We report on the experimental investigation and theoretical modeling of the ultrafast nonlinear optical response exhibited by Cu2–xSe nanoparticles in a broad range of wavelengths, from the red to the near-infrared, under excitation with intense femtosecond-laser pulses. The ultrafast dynamics of the localized plasmon resonance of the system is mapped by pump–probe differential transmission measurements. Deviations from theoretical predictions based on standard two-temperature model (TTM) are revealed in the subpicosecond time-scale where the nonlinear phenomenon is more pronounced. The key-role played by nonthermalized carriers is investigated in detail on the basis of an extended TTM.
[Show abstract][Hide abstract] ABSTRACT: Heavily-doped semiconductor nanocrystals characterized by a tunable plasmonic
band have been gaining increasing attention recently. Herein, we introduce this
type of materials focusing on their structural and photo physical properties.
Beside their continuous-wave plasmonic response, depicted both theoretically
and experimentally, we also review recent results on their transient, ultrafast
response. This was successfully interpreted by adapting models of the ultrafast
response of gold nanoparticles.
Physics of Condensed Matter 06/2013; 86. · 1.28 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: CdSe/CdS dot/rods nanocrystals show interesting physical properties related to the band-alignment at the hetero-interface, which controls the band-edge electron delocalization over the rods. Here the differential transmission spectra of CdSe/CdS nanorod samples with different core sizes have been measured using excitation resonant to the core transition. The photo bleaching ratio between dot and rod transitions increases with the dot size, indicating a trend towards electron localization. This trend has been further quantified by performing effective mass calculations in which the conduction band misalignment was varied in order to reproduce the observed bleaching feature ratio. The best agreement was found for negligible conduction band misalignment for small dots of around 2.3 nm in diameter, and about -0.1 eV misalignment was estimated for the larger dots, above 3.5 nm in diameter. This shows that the band misalignment might be dependent on the geometry of the system, and we argue that this might be related to different strain developed at the hetero-interface.
Physical Chemistry Chemical Physics 04/2012; 14(20):7420-6. · 3.83 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Here we show an innovative, simple and reliable method to fabricate micro-lasers by self-assembly of rod-shaped nanocrystals. We use dot/rod core/shell CdSe/CdS nanorods to form optical micro-resonators by exploiting their self-organization into well-defined coffee stain rings. The fabrication process merely consists of capillary jet deposition of a nanorod solution onto a glass substrate, and is scalable, economic, and highly reproducible. Upon optical pumping of the micro-resonators we obtain laser emission in the red or in the blue-green spectral region, demonstrating lasing both from core and shell transitions, with low pumping thresholds. Modeling by full-wave numerical simulations according to generalized (i. e. scattering) formulation of laser theory demonstrates lasing from complex modes of the self-assembled cavity.
[Show abstract][Hide abstract] ABSTRACT: An innovative, simple and reliable method to fabricate micro-lasers by self-assembly of rod-shaped nanocrys-tals is demonstrated. Dot/rod core/shell CdSe/CdS nanorods are used to form optical micro-resonators by exploiting their self-organization into well-defined coffee stain rings. The fabrication process merely consists of capillary jet deposition of a nanorod solution onto a glass substrate, and is scalable, eco-nomic, and highly reproducible. Upon optical pumping of the micro-resonators, laser emission in the red or in the blue-green spectral region is obtained, demonstrating lasing both from core and shell transitions, with low pumping thresholds. Modeling by full-wave numerical simulations according to generalized (i. e. scattering) formulation of laser theory demonstrates lasing from complex modes of the self-assembled cavity.
[Show abstract][Hide abstract] ABSTRACT: The optical response of metallic nanostructures after intense excitation with femtosecond-laser pulses has recently attracted increasing attention: such response is dominated by ultrafast electron-phonon coupling and offers the possibility to achieve optical modulation with unprecedented terahertz bandwidth. In addition to noble metal nanoparticles, efforts have been made in recent years to synthesize heavily doped semiconductor nanocrystals so as to achieve a plasmonic behavior with spectrally tunable features. In this work, we studied the dynamics of the localized plasmon resonance exhibited by colloidal Cu(2-x)Se nanocrystals of 13 nm in diameter and with x around 0.15, upon excitation by ultrafast laser pulses via pump-probe experiments in the near-infrared, with ∼200 fs resolution time. The experimental results were interpreted according to the two-temperature model and revealed the existence of strong nonlinearities in the plasmonic absorption due to the much lower carrier density of Cu(2-x)Se compared to noble metals, which led to ultrafast control of the probe signal with modulation depth exceeding 40% in transmission.
[Show abstract][Hide abstract] ABSTRACT: Colloidal branched nanocrystals have been attracting increasing attention due to evidence of an interesting relationship between their complex shape and charge carrier dynamics. Herein, continuous wave photoinduced absorption (CW PIA) measurements of CdSe/CdS octapod-shaped nanocrystals are reported. CW PIA spectra show strong bleaching due to the one-dimensional (1D) CdS pod states (480 nm) and the zero-dimensional (0D) CdSe core states (690 nm). The agreement with previously reported ultrafast pump-probe experiments indicates that this strong bleaching signal may be assigned to state filling. Additional bleaching features at 520 and 560 nm are characterized by a longer lifetime and are thus ascribed to defect states, localized at the pod-core interface of the octapod, showing that some of the initially photogenerated carriers get quickly trapped into these long-lived defect states. However, we remark that a relevant part of electrons remain untrapped: this opens up the opportunity to exploit octapod shaped nanocrystals in photovoltaics applications, as electron acceptor materials, considering that several efficient hole extracting materials are already available for the realization of a composite bulk heterojunction.
Physical Chemistry Chemical Physics 09/2011; 13(33):15326-30. · 3.83 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We studied the optical properties of core-shell CdSe/CdS nanorods with various lengths and core diameters that were fabricated by wet chemical synthesis using the seeded growth method. We investigated the optical emission from thin films consisting of dense nanorod arrays, where we observed amplified spontaneous emission from states related either to the CdSe core or to the CdS shell depending on the nanorod's length. The optical gain of the nanorods was studied by transient absorption experiments and we found optical gain for the core and shell states of short rods, whereas for long rods, the optical gain of the core was quenched by defect states and we observed optical gain solely from the states of the shell material. (C) 2011 American Institute of Physics. [doi:10.1063/1.3549298]
[Show abstract][Hide abstract] ABSTRACT: We studied the carrier dynamics in colloidal octapod-shaped cadmium selenide/cadmium sulfide (CdSe/CdS) nanocrystals in the solution phase via pump-probe optical techniques with subpicosecond resolution. We could resolve bleaching from two different types of electronic states having distinct dynamics and assigned them to states delocalized in the pods and mildly localized in the core based on the good agreement of energies found with effective mass modeling. Contrary to other CdSe/CdS core/shell nanocrystals, such a mild localization has geometrical origins as the best agreement was found for negligible conduction band offset. Moreover, even though the large surface of the CdSe/CdS heterointerface results into a weak signature of electron trapping in the bleaching spectrum, we found that a relevant fraction of electrons do remain delocalized in pod states for long times and are thus available for diffusion in photovoltaic applications where the highly branched geometry is expected to advantageously yield to effective percolation in dense assemblies.
Journal of Physical Chemistry C. 01/2011; 115(18):9005-9011.
[Show abstract][Hide abstract] ABSTRACT: Two-photon pumped lasing of a poly(phenylenevinylene)-titania-silica (PPV-TiO2-SiO2) nanocomposite comprised of PPV synthesized within the void spaces of a multilayer nanoparticle TiO2 and SiO2 1D photonic crystal is reported for the first time. With this distributed feedback (DFB) device, we have discovered the surprising result that two photon lasing requires only a factor of 2 higher intensity than one photon lasing thereby allowing laser excitation densities far below those that cause material degradation. The lower excitation density makes possible laser operation in the high-repetition rate regime and the use of compact and cheap NIR pump sources. All this makes our device rather interesting for optical telecommunication applications.
[Show abstract][Hide abstract] ABSTRACT: We studied carrier dynamics in semiconductor nanocrystals consisting of a small CdSe dot embedded in an elongated, rod-shaped CdS shell, using the ultrafast pump-probe technique. We found clear evidence of a substantial suppression of the Auger nonradiative recombination in the biexciton regime. Moreover, a simple model of the dynamics in which biexcitons show no Auger recombination, and only holes are localized in the dot, fits well the differential transmission observed at all pump densities. The long biexciton lifetime results into an observed long-living gain having a peak that is red shifted with respect to the lowest energy absorption peak. We argue that the origin of the large relative gain observed at large fillings is related to the peculiar structure of the electronic levels, and in particular, to delocalization of electrons in the rod.
[Show abstract][Hide abstract] ABSTRACT: Colloidal semiconductor quantum rods have demonstrated many advantageous properties as light emitters such as high quantum yield, tunable emission wavelength, and polarized emission. This makes them an interesting optical gain material for laser applications. We report room-temperature gain lifetimes in core/shell CdSe/CdS quantum rods exceeding 300 ps, and show that the long gain lifetimes result from the significant reduction of Auger recombination in our quantum rods, even though the electrons are delocalized over the rod volume. We also fabricate devices by deposition of small droplets of quantum rod solution onto flat substrates. The evaporation dynamics of the droplets are governed by the coffee stain effect which leads to the formation of well defined micron-size stripes. These stripes consist of densely packed, laterally aligned quantum rods and provide optical feedback originating from the abrupt changes of refractive index at the stripe borders. We exploit the optical gain and the coffee stain mediated self-assembly and show that we can fabricate novel microlasers solely by deposition of droplets of quantum rod solutions on flat substrates.
[Show abstract][Hide abstract] ABSTRACT: We studied the delocalization of electron wave function in asymmetric CdSe/CdS nanocrystals, consisting of a spherical CdSe dot embedded in an elongated US shell, by means of a pump-probe technique. By comparing the transient spectra obtained upon pumping the band edge transition of the CdSe in CdSe/CdS heterostructure and in a bare CdSe dot, we observed the delocalization of electron wave function at the CdSe/CdS interface. (C) 2009 Elsevier Ltd. All rights reserved.
Superlattices and Microstructures 01/2010; 47(1):170-173. · 1.56 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The complex optical polarizability of a nano-oscillator determines the way it couples to light and to other nano-objects in its environment. Hence, its experimental evaluation at the single-particle level represents a crucial task in nano-optics. In this work we demonstrate that both phase and amplitude of a nanoresonator polarizability, which are embedded in its near-field response, can be decoupled by combining near-field and confocal far-field extinction imagings. The interpretation of our measurements based on a simple analytical model is further confirmed by finite-difference time-domain calculations. Introduction. Noble-metal nanoparticles have been the subject of an impressive amount of experimental studies over the last decade because of the broad variety of their applica-tions in nano-optics. Their appeal lies in the occurrence of localized surface plasmon LSP resonances at visible and near-infrared frequencies, which significantly influence the particle polarizability and produce large local-field enhance-ment up to several orders of magnitude and confinement on the nanometer scale. It has been shown that noble-metal nanoparticles can be employed as nanoresonators for visible-light enhancement 1–6 and that, in array structures, they can efficiently transport and guide light below the diffraction limit, 7,8 demonstrating the possibility of creating LSP-based networks for light manipulation at the nanoscale. 9 To achieve the best design for such complex systems both analytical modeling 10 and experimental far-field studies 11 have been carried out. Generally, the optical properties of an ellipsoidal nanopar-ticle can be described within the framework of the general-ized Mie theory or, for more complicated structures, by means of numerical simulations. In practice, plasmonic nan-oresonators are greatly affected by the particle morphology: small imperfections can induce shifts in the resonance fre-quency and affect the resonator quality factor. 12 For this rea-son, ensemble measurements on metal nanoparticles suffer from inhomogeneous broadening. The ideal solution for characterizing the optical performances of nano-oscillators would thus be addressing single nanoparticles. In this re-spect, the ability of retrieving the phase response of an indi-vidual nano-oscillator is of fundamental importance in novel applications based on coherent control of phase and polariza-tion to guide light via nanoparticle arrays. 13–15 In particular, light propagation in such plasmonic networks is effectively determined by the optical phase response of each single ele-ment, which influences the phase relation between different components of the propagating electromagnetic wave and rules the interference phenomena that lead to efficient propa-gation. A number of works in the literature have reported single-particle extinction or scattering measurements addressing the LSP spectral behavior by either far-field confocal optical microscopy 16–18 or scanning near-field optical microscopy SNOM. 19–23 Scattering and extinction cross sections in far-field experiments can be directly related to the absolute value and imaginary part of the particle polarizability, 24 respec-tively. Stoller et al. 18 developed a method to experimentally address the complex polarizability of single gold nanopar-ticles by combining coherent white-light illumination and differential interference contrast far-field microscopy. An-other far-field method to retrieve phase information from a nano-object has been theoretically proposed by Hwang et al. 25 and recently applied to nanocrystal quantum dots. 26 It is now established that near-field extinction spectra bear a strong signature from the phase of the particle polarizabil-ity as a result of interference between the radiation from the tip and that from the plasmon excited by the near-field com-ponent of the incoming light. 20 However, the particle near-field response is a function of both the phase and the ampli-tude of the LSP oscillation, hence an additional measurement would be required in order to experimentally separate the two contributions. Moreover, near-field experiments are in-fluenced by the complex spectral dependence of the propa-gating and evanescent light components in proximity of the aperture and can be affected by the probe-particle coupling, which might results in a shift of the particle resonance fre-quency. For these reasons, near-field techniques have not been applied so far to the task of experimentally reconstruct-ing the complex polarizability of single metal nanoparticles. In this work, we show that the combination of far-field and near-field imagings can be effectively exploited to ex-tract the phase response of an optical nano-oscillator and therefore to determine both the phase and the amplitude of its complex optical response. In particular, this method is demonstrated by applying extinction imaging to single gold nanorods. Experimental. In order to determine the particle complex polarizability, both the near-and far-field responses need to be evaluated in on-and off-resonant conditions. This can be achieved by i measuring the response of a particle at dif-ferent wavelengths or ii considering particles with different size at a fixed excitation wavelength. In the following we will apply the second solution on an ensemble of length-varying nanorods in order to disregard the frequency-dependent near-field response of the probe. This allows in-terpreting the results within a much simpler and reliable analytical model. The investigated sample consists of an array of Au nano-rods fabricated by electron-beam lithography on a fused
Physical Review B 10/2009; 80:153407. · 3.66 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Nanoparticle one-dimensional photonic crystals exhibit intense, broadband reflectivity coupled with a unique mesoporosity. The latter property allows for infiltration of the one-dimensional photonic crystal with functional materials, such as emitting polymers, which in turn can lead to the fabrication of composites whereby the emitter's emission can be modulated by the photon density of states of the photonic crystal. We exploit this interaction in order to produce efficient distributed feedback lasing from a composite poly(phenylene vinylene)-infiltrated nanoparticle one-dimensional photonic crystal.
[Show abstract][Hide abstract] ABSTRACT: We describe an aperture scanning near-field optical microscope (SNOM) using cantilevered hollow pyramid probes coupled to femtosecond laser pulses. Such probes, with respect to tapered optical fibers, present higher throughput and laser power damage threshold, as well as greater mechanical robustness. In addition, they preserve pulse duration and polarization in the near field. The instrument can operate in two configurations: illumination mode, in which the SNOM probe is used to excite the nonlinear response in the near field, and collection mode, where it collects the nonlinear emission following far-field excitation. We present application examples highlighting the capability of the system to observe the nonlinear optical response of nanostructured metal surfaces (gold projection patterns and gold nanorods) with sub-100-nm spatial resolution.
The Review of scientific instruments 04/2009; 80(3):033704. · 1.52 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Space resolved Stark spectroscopy is introduced as a non invasive optical technique for imaging electric field distribution in organic semiconductors. Stark spectroscopy relies on the electric field induced change in the absorption/reflection. It is shown that local monitoring of Stark shift with confocal spatial resolution provides quantitative information on the strength of the local field as well as charge distribution within the transport channel.