Nonlinear anisotropic dielectric metasurfaces for ultrafast nanophotonics

We report on the broadband transient optical response from anisotropic nanobrick amorphous silicon particles, exhibiting Mie-type resonances. A quantitative model is developed to identify and disentangle the three physical processes that govern the ultrafast changes of the nanobrick optical properties, namely two-photon absorption, free-carrier relaxation, and lattice heating. We reveal a set of operating windows where ultrafast all-optical modulation of transmission is achieved with full return to zero in 20 ps. This is made possible due to the interplay between the competing nonlinear processes and despite the slow (nanosecond) internal lattice dynamics. The observed ultrafast switching behavior can be independently engineered for both or- thogonal polarizations using the large anisotropy of nanobricks thus allowing ultrafast anisotropy control. Our results categorically ascertain the potential of all-dielectric resonant nanophotonics as a platform for ultrafast optical devices, and reveal the pos- sibility for ultrafast polarization-multiplexed displays and polarization rotators

Two-dimensional electronic spectroscopy in the ultraviolet by a birefringent delay line

We introduce a 2D electronic spectroscopy setup in the UV spectral range in the partially collinear pump-probe geometry. The required interferometrically phase-locked few-optical-cycle UV pulse pair is generated by combining a passive birefringent interferometer in the visible and nonlinear phase transfer. This is achieved by sum-frequency generation between the phase-locked visible pulse pair and narrowband infrared pulses. We demonstrate a pair of 16-fs, 330-nm pulses whose delay is interferometrically stable with an accuracy better than λ/450. 2DUV maps of pyrene solution probed in the UV and visible spectral ranges are demonstrated

A comparative study of low-cost coating processes for green & sustainable organic solar cell active layer manufacturing

Owing to their facile integration into existing commercial products, high volume manufacturing of organic solar cells (OSCs) can be expected in the upcoming years. Therefore, it is important to evaluate the performance and sustainability of various active layer coating methods for OSCs. Herein, we compare four active layer coating processes: spin-coating, blade-coating, spray-coating and push-coating for poly(2,7-carbazole-alt-dithienylbenzothiadiazole):[6,6]-Phenyl-C71-butyric acid methyl ester (PCDTBT:PC71BM) active layers deposition. The optical, morphological and photovoltaic parameters of the active layers are studied. The suitability of each coating method for industrial manufacturing of PCDTBT:PC71BM OSCs is discussed in terms of environmental impact, necessary investments and running costs. Our results confirm that, despite producing high quality and high performance OSCs, spin-coating is unsuitable for high volume manufacturing due to the large amounts of materials and hazardous solvents wasted in the process. Blade-coating provides a good balance between low running costs, low environmental impact and decent performances but the process introduces lateral compositional gradients which could be detrimental for large area OSC processing. Spray-coating requires minimal initial investments but has relatively low performance and low manufacturing sustainability. Push-coating yields OSCs which perform as well as spin-coated ones, with a much lower environmental impact and cost. We should thus look forward to seeing whether this green and sustainable technology can develop into a large area coating process in the future

Ultrafast spectroscopy on water-processable PCBM: rod-coil block copolymer nanoparticles.

Using ultrafast spectroscopy, we investigate the photophysics of water-processable nanoparticles composed of a block copolymer electron donor and a fullerene derivative electron acceptor

Conformable nanowire-in-nanofiber hybrids for low-threshold optical gain in the ultraviolet

The miniaturization of diagnostic devices that exploit optical detection schemes requires the design of light-sources combining small size, high performance for effective excitation of chromophores, and mechanical flexibility for easy coupling to components with complex and non-planar shapes. Here, ZnO nanowire-in-fiber hybrids with internal architectural order are introduced, exhibiting a combination of polarized stimulated emission, low propagation losses of light modes, and structural flexibility. Ultrafast transient absorption experiments on the electrospun material show optical gain which gives rise to amplified spontaneous emission, with threshold lower than the value found in films. These systems are highly flexible and can conveniently conform to curved surfaces, which makes them appealing active elements for various device platforms, such as bendable lasers, optical networks and sensors, as well as for application in bioimaging, photo-crosslinking, and optogenetics.Comment: 50 pages, 17 figures, 1 table, ACS Nano, 202

A Unified Experimental/Theoretical Description of the Ultrafast Photophysics of Single and Double Thionated Uracils

Photoinduced processes in thiouracil derivatives have lately attracted considerable attention due to their suitability for innovative biological and pharmacological applications. Here, sub-20 fs broadband transient absorption spectroscopy in the near-UV are combined with CASPT2/MM decay path calculations to unravel the excited-state decay channels of water solvated 2-thio and 2,4-dithiouracil. These molecules feature linear absorption spectra with overlapping ππ* bands, leading to parallel decay routes which we systematically track for the first time. The results reveal that different processes lead to the triplet states population, both directly from the ππ* absorbing state and via the intermediate nπ* dark state. Moreover, the 2,4-dithiouracil decay pathways is shown to be strongly correlated either to those of 2- or 4-thiouracil, depending on the sulfur atom on which the electronic transition localizes

Charge Carrier Dynamics in Photocatalytic Hybrid Semiconductor-Metal Nanorods: Crossover from Auger Recombination to Charge Transfer

Hybrid semiconductor-metal nanoparticles (HNPs) manifest unique, synergistic electronic and optical properties as a result of combining semiconductor and metal physics via a controlled interface. These structures can exhibit spatial charge separation across the semiconductor-metal junction upon light absorption, enabling their use as photocatalysts. The combination of the photocatalytic activity of the metal domain with the ability to generate and accommodate multiple excitons in the semiconducting domain can lead to improved photocatalytic performance because injecting multiple charge carriers into the active catalytic sites can increase the quantum yield. Herein, we show a significant metal domain size dependence of the charge carrier dynamics as well as the photocatalytic hydrogen generation efficiencies under nonlinear excitation conditions. An understanding of this size dependence allows one to control the charge carrier dynamics following the absorption of light. Using a model hybrid semiconductor-metal CdS-Au nanorod system and combining transient absorption and hydrogen evolution kinetics, we reveal faster and more efficient charge separation and transfer under multiexciton excitation conditions for large metal domains compared to small ones. Theoretical modeling uncovers a competition between the kinetics of Auger recombination and charge separation. A crossover in the dominant process from Auger recombination to charge separation as the metal domain size increases allows for effective multiexciton dissociation and harvesting in large metal domain HNPs. This was also found to lead to relative improvement of their photocatalytic activity under nonlinear excitation conditions

Lévy defects in matrix-immobilized J aggregates : tracing intra-and intersegmental exciton relaxation

L.L. thanks the EC for financial support by the cofunded Amarout program and the Spanish Ministry for economy and competitiveness (plan nacional, Project MultiCrom (CTQ2014-58801)). G.C. acknowledges financial support by the European Research Council (ERC-2011-AdG No. 291198). D.G.L. and D.C. thank the UK EPSRC for funding via research grant EP/M025330/1 “Hybrid Polaritonics”One-dimensional J aggregates present narrow and intense absorption and emission spectra that are interesting for photonics applications. Matrix immobilization of the aggregates, as required for most device architectures, has recently been shown to induce a non-Gaussian (Lévy type) defect distribution with heavy tails, expected to influence exciton relaxation. Here we perform two-dimensional electronic spectroscopy (2DES) in one-dimensional J aggregates of the cyanine dye TDBC, immobilized in a gel matrix, and we quantitatively model 2DES maps by nonlinear optimization coupled to quantum mechanical calculations of the transient excitonic response. We find that immobilization causes strong non-Gaussian off-diagonal disorder, leading to a segmentation of the chains. Intersegmental exciton transfer is found to proceed on the picosecond time scale, causing a long-lasting excitation memory. These findings can be used to inform the design of optoelectronic devices based on J aggregates as they allow for control of exciton properties by disorder management.PostprintPostprintPeer reviewe

Ultraviolet Transient Absorption Spectrometer with Sub-20-fs Time Resolution

We describe a transient absorption (TA) spectroscopy system in the ultraviolet (UV) spectral range, for the study of the ultrafast optical response of biomolecules. After reviewing the techniques for the generation and characterization of ultrashort UV pulses, we describe the experimental setup of our ultrabroadband UV TA spectrometer. The setup combines sub-20-fs UV pump pulses tunable between 3.35 and 4.7 eV, with broadband white-light-continuum probe pulses in the 1.7–4.6 eV range. Thanks to the broad tunability of the pump pulses in the UV spectral range, the extremely high temporal resolution and the broad spectral coverage of the probe, this TA system is a powerful and versatile tool for the study of many biomolecules. As an example of its potential, we apply the TA spectrometer to track ultrafast internal conversion processes in pyrene after excitation in the UV, and to resolve an impulsively excited molecular vibration with 85-fs period

BODIPY-Based Molecules, a Platform for Photonic and Solar Cells

The 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY)-based molecules have emerged as interesting material for optoelectronic applications. The facile structural modification of BODIPY core provides an opportunity to fine-tune its photophysical and optoelectronic properties thanks to the presence of eight reactive sites which allows for the developing of a large number of functionalized derivatives for various applications. This review will focus on BODIPY application as solid-state active material in solar cells and in photonic devices. It has been divided into two sections dedicated to the two different applications. This review provides a concise and precise description of the experimental results, their interpretation as well as the conclusions that can be drawn. The main current research outcomes are summarized to guide the readers towards the full exploitation of the use of this material in optoelectronic applications