The coherent dynamics of photoexcited green fluorescent proteins

The coherent dynamics of vibronic wave packets in the green fluorescent protein is reported. At room temperature the non-stationary dynamics following impulsive photoexcitation displays an oscillating optical transmissivity pattern with components at 67 fs (497 cm-1) and 59 fs (593 cm-1). Our results are complemented by ab initio calculations of the vibrational spectrum of the chromophore. This analysis shows the interplay between the dynamics of the aminoacidic structure and the electronic excitation in the primary optical events of green fluorescent proteins.Comment: accepted for publication in Physical Review Letter

Evidence for the Band-Edge Exciton of CuInS2 Nanocrystals Enables Record Efficient Large-Area Luminescent Solar Concentrators

AbstractTernary I‐III‐VI2 nanocrystals (NCs), such as CuInS2, are receiving attention as heavy‐metals‐free materials for solar cells, luminescent solar concentrators (LSCs), LEDs, and bio‐imaging. The origin of the optical properties of CuInS2 NCs are however not fully understood. A recent theoretical model suggests that their characteristic Stokes‐shifted and long‐lived luminescence arises from the structure of the valence band (VB) and predicts distinctive optical behaviours in defect‐free NCs: the quadratic dependence of the radiative decay rate and the Stokes shift on the NC radius. If confirmed, this would have crucial implications for LSCs as the solar spectral coverage ensured by low‐bandgap NCs would be accompanied by increased re‐absorption losses. Here, by studying stoichiometric CuInS2 NCs, it is revealed for the first time the spectroscopic signatures predicted for the free band‐edge exciton, thus supporting the VB‐structure model. At very low temperatures, the NCs also show dark‐state emission likely originating from enhanced electron‐hole spin interaction. The impact of the observed optical behaviours on LSCs is evaluated by Monte Carlo ray‐tracing simulations. Based on the emerging device design guidelines, optical‐grade large‐area (30×30 cm2) LSCs with optical power efficiency (OPE) as high as 6.8% are fabricated, corresponding to the highest value reported to date for large‐area devices

The key role of interband transitions in hot-electron-modulated TiN films

Titanium nitride (TiN) is an emerging new material in the field of plasmonics, both for its linear and nonlinear optical properties. Similarly to noble metals, like, e.g., gold (Au), the giant third-order optical nonlinearity of TiN following excitation with fs-laser pulses has been attributed to the generation of hot electrons. Here we provide a numerical study of the Fermi smearing mechanism associated with photogenerated hot carriers and subsequent interband transitions modulation in TiN films. A detailed comparison with Au films is also provided, and saturation effects of the permittivity modulation for increasing pump fluence are discussed

Unfolding the Origin of the Ultrafast Optical Response of Titanium Nitride

Ultrafast plasmonics is driving growing interest for the search of novel plasmonic materials, overcoming the main limitations of noble metals. In this framework, titanium nitride (TiN) is brought in the spotlight for its refractory properties combined with an extremely fast electron-lattice cooling time (<100 fs) compared to gold (approximate to 1 ps). Despite the results reported in literature, a clear-cut explanation of the origin of the ultrafast and giant optical response of TiN-based materials upon excitation with femtosecond laser pulses is still missing. To address this issue, an original model is introduced, capable of unfolding the modulation of TiN optical properties on a broad bandwidth, starting from the variations of electronic and lattice temperatures following ultrafast photoexcitation. The numerical analysis is validated on ultrafast pump-probe spectroscopy experiments on a simple structure, a TiN film on glass. This approach enables a complete disentanglement of the interband and intraband contributions to the permittivity modulation. Moreover, it is also shown that, varying the synthesis conditions of the TiN film, not only the static, but also the dynamical optical response can be efficiently tuned. These findings pave the way for a breakthrough in the field: the design of TiN-based ultrafast nanodevices for all-optical modulation of light

Disclosing Early Excited State Relaxation Events in Prototypical Linear Carbon Chains

One-dimensional (1D) linear nanostructures comprising sp-hybridized carbon atoms, as derivatives of the prototypicalallotropeknown as carbyne, are predicted to possess outstanding mechanical,thermal, and electronic properties. Despite recent advances in theirsynthesis, their chemical and physical properties are still poorlyunderstood. Here, we investigate the photophysics of a prototypicalpolyyne (i.e., 1D chain with alternating single and triple carbonbonds) as the simplest model of finite carbon wire and as a prototypeof sp-carbon-based chains. We perform transient absorptionexperiments with high temporal resolution (<30 fs) on monodispersedhydrogen-capped hexayne H (C C)(6)Hsynthesized by laser ablation in liquid. With the support of computationalstudies based on ground state density functional theory (DFT) andexcited state time-dependent (TD)-DFT calculations, we provide a comprehensivedescription of the excited state relaxation processes at early timesfollowing photoexcitation. We show that the internal conversion froma bright high-energy singlet excited state to a low-lying singletdark state is ultrafast and takes place with a 200 fs time constant,followed by thermalization on the picosecond time scale and decayof the low-energy singlet state with hundreds of picoseconds timeconstant. We also show that the time scale of these processes doesnot depend on the end groups capping the sp-carbonchain. The understanding of the primary photoinduced events in polyynesis of key importance both for fundamental knowledge and for potentialoptoelectronic and light-harvesting applications of low-dimensionalnanostructured carbon-based materials

Mode-matching in multiresonant plasmonic nanoantennas for enhanced second harmonic generation

Boosting nonlinear frequency conversion in extremely confined volumes remains a key challenge in nano-optics, nanomedicine, photocatalysis, and background-free biosensing. To this aim, field enhancements in plasmonic nanostructures are often exploited to effectively compensate for the lack of phase-matching at the nanoscale. Second harmonic generation (SHG) is, however, strongly quenched by the high degree of symmetry in plasmonic materials at the atomic scale and in nanoantenna designs. Here, we devise a plasmonic nanoantenna lacking axial symmetry, which exhibits spatial and frequency mode overlap at both the excitation and the SHG wavelengths. The effective combination of these features in a single device allows obtaining unprecedented SHG conversion efficiency. Our results shed new light on the optimization of SHG at the nanoscale, paving the way to new classes of nanoscale coherent light sources and molecular sensing devices based on nonlinear plasmonic platforms.Comment: 14 pages, 4 figure

Nano-engineered electron–hole exchange interaction controls exciton dynamics in core–shell semiconductor nanocrystals

A strong electron–hole exchange interaction (EI) in semiconductor nanocrystals (NCs) gives rise to a large (up to tens of meV) splitting between optically active ('bright') and optically passive ('dark') excitons. This dark–bright splitting has a significant effect on the optical properties of band-edge excitons and leads to a pronounced temperature and magnetic field dependence of radiative decay. Here we demonstrate a nanoengineering-based approach that provides control over EI while maintaining nearly constant emission energy. We show that the dark–bright splitting can be widely tuned by controlling the electron–hole spatial overlap in core–shell CdSe/CdS NCs with a variable shell width. In thick-shell samples, the EI energy reduces to <250 μeV, which yields a material that emits with a nearly constant rate over temperatures from 1.5 to 300 K and magnetic fields up to 7 T. The EI-manipulation strategies demonstrated here are general and can be applied to other nanostructures with variable electron–hole overlap

Novel hybrid organic/inorganic 2D quasiperiodic PC: from diffraction pattern to vertical light extraction

Recently, important efforts have been dedicated to the realization of a fascinating class of new photonic materials or metamaterials, known as photonic quasicrystals (PQCs), in which the lack of the translational symmetry is compensated by rotational symmetries not achievable by the conventional periodic crystals. As ever, more advanced functionality is demanded and one strategy is the introduction of non-linear and/or active functionality in photonic materials. In this view, core/shell nanorods (NRs) are a promising active material for light-emitting applications. In this article a two-dimensional (2D) hybrid a 2D octagonal PQC which consists of air rods in an organic/inorganic nanocomposite is proposed and experimentally demonstrated. The nanocomposite was prepared by incorporating CdSe/CdS core/shell NRs into a polymer matrix. The PQC was realized by electron beam lithography (EBL) technique. Scanning electron microscopy, far field diffraction and spectra measurements are used to characterize the experimental structure. The vertical extraction of the light, by the coupling of the modes guided by the PQC slab to the free radiation via Bragg scattering, consists of a narrow red emissions band at 690 nm with a full width at half-maximum (FWHM) of 21.5 nm. The original characteristics of hybrid materials based on polymers and colloidal NRs, able to combine the unique optical properties of the inorganic moiety with the processability of the host matrix, are extremely appealing in view of their technological impact on the development of new high performing optical devices such as organic light-emitting diodes, ultra-low threshold lasers, and non-linear devices