AMOLF Institutional Repository
Not a member yet
1351 research outputs found
Sort by
Observation and Identifcation of a New OH Stretch Vibrational Band at the Surface of Ice
Get PDFWe study the signatures of the OH stretch vibrations at the basal surface of ice using heterodyne-detected sum-frequency generation and molecular dynamics simulations. At 150 K, we observe 7 distinct modes in the sum-frequency response, five of which have an analogue in the bulk, and two pure surface-specific modes can be identified at higher frequencies (~3530 and ~3700 cm-1). The band at ~3530 cm-1 has not been reported previously. Using molecular dynamics simulations we find that the ~3530 cm-1 band contains contributions from OH stretch vibrations of both fully coordinated interfacial water molecules and of water molecules with two donor and one acceptor hydrogen-bond
Realization of high-Q/V photonic crystal cavities defined by an effective Aubry- André-Harper bichromatic potential
Get PDFWe report on the realization of high-Q/V photonic crystal cavities in thin silicon membranes, with resonances around 1.55 mu m wavelength. The cavity designs are based on a recently proposed photonic crystal implementation of the Aubry-Andre-Harper bichromatic potential, defined from the superposition of two one-dimensional lattices with a non-integer ratio between their periodicity constants. In photonic crystal nanocavities, this confinement mechanism is such that optimized figures of merit can be straightforwardly achieved, in particular an ultra-high-Q factor and diffraction-limited mode volume. Several silicon membrane photonic crystal nanocavities have been realized with measured Q-factors in the 1 x 10(6) range, as evidenced by resonant scattering. The generality of the proposed designs and their easy implementation and scalability make these results particularly interesting for realizing highly performing photonic nanocavities on different material platforms and operational wavelengths
Nano-antenna enhanced two-focus fluorescence correlation spectroscopy
Get PDFWe propose two-focus fluorescence correlation spectroscopy (2fFCS) on basis of plasmonic
nanoantennas that provide distinct hot spots that are individually addressable through polarization, yet lie within a single diffraction limited microscope focus. The importance of two-focus FCS is that a calibrated distance between foci provides an intrinsic calibration to derive diffusion constants from measured correlation times. Through electromagnetic modelling we analyze a geometry of perpendicular nanorods, and their inverse, i.e., nanoslits. While we find that nanorods are not suited for nano-antenna enhanced 2fFCS due to substantial background signal, a nanoslit geometry is expected to provide a di tinct cross-correlation between orthogonally polarized detection channels. Furthermore, by utilizing a periodic array of nanoslits instead of a single pair, the amplitude of the cross-correlation can be enhanced. To demonstrate this technique, we present a proof of principle experiment on the basis of a periodic array of nanoslits, applied to lipid diffusion in a supported lipid bilayer
Thermodynamics of Computational Copying in Biochemical Systems
Get PDFLiving cells use readout molecules to record the state of receptor proteins, similar to measurements or copies in typical computational devices. But is this analogy rigorous? Can cells be optimally efficient, and if not, why? We show that, as in computation, a canonical biochemical readout network generates correlations; extracting no work from these correlations sets a lower bound on dissipation. For general input, the biochemical network cannot reach this bound, even with arbitrarily slow reactions or weak thermodynamic driving. It faces an accuracy-dissipation trade-off that is qualitatively distinct from and worse than implied by the bound, and more complex steady-state copy processes cannot perform better. Nonetheless, the cost remains close to the thermodynamic bound unless accuracy is extremely high. Additionally, we show that biomolecular reactions could be used in thermodynamically optimal devices under exogenous manipulation of chemical fuels, suggesting an experimental system for testing computational thermodynamics
Single-photon nanoantennas
Get PDFSingle-photon nanoantennas are broadband strongly scattering nanostructures placed in the near field of a single quantum emitter, with the goal to enhance the coupling between the emitter and far-field radiation channels. Recently, great strides have been made in the use of nanoantennas to realize fluorescence brightness enhancements, and Purcell enhancements, of several orders of magnitude. This perspective reviews the key figures of merit by which single-photon nanoantenna performance is quantified and the recent advances in measuring these metrics unambiguously. Next, this perspective discusses what the state of the art is in terms of fluoresent brightness enhancements, Purcell factors, and directivity control on the level of single photons. Finally, I discuss future challenges for single-photon nanoantennas
Indirect to direct bandgap transition in methylammonium lead halide perovskite
Get PDFMethylammonium lead iodide perovskites are considered direct bandgap semiconductors. Here we show that in fact they present a weakly indirect bandgap 60 meV below the direct bandgap transition. This is a consequence of spin–orbit coupling resulting in Rashba-splitting of the conduction band. The indirect nature of the bandgap explains the apparent contradiction of strong absorption and long charge carrier lifetime. Under hydrostatic pressure from ambient to 325 MPa, Rashba splitting is reduced due to a pressure induced reduction in local electric field around the Pb atom. The nature of the bandgap becomes increasingly more direct, resulting in five times faster charge carrier recombination, and a doubling of the radiative efficiency. At hydrostatic pressures above 325 MPa, MAPI undergoes a reversible phase transition resulting in a purely direct bandgap semiconductor. The pressure-induced changes suggest epitaxial and synthetic routes to higher efficiency optoelectronic devices
Plasmonic nanoantenna design and fabrication based on evolutionary optimization
Get PDFNanoantennas can tailor light-matter interaction for optical communication, sensing,
and spectroscopy. Their design is inspired by radio-frequency rules which partly break down
at optical frequencies. Here we find unexpected nanoantenna designs exhibiting strong light
localization and enhancement by using a general and scalable evolutionary algorithm based
on FDTD simulations that also accounts for geometrical fabrication constraints. The resulting
nanoantennas are "printed" directly by focused-ion beam milling and their fitness ranking is val-
idated experimentally by two-photon photoluminescence. We find the best antennas’ operation
principle deviating from that of classical radio wave-inspired designs. Our work sets the stage
for a widespread application of evolutionary optimization in nano photonics
Dendritic optical antennas: scattering properties and fluorescence enhancement
Get PDFWith the development of nanotechnologies, researchers have brought the concept of antenna to the
optical regime for manipulation of nano-scaled light matter interactions. Most optical nanoantennas
optimize optical function, but are not electrically connected. In order to realize functions that require
electrical addressing, optical nanoantennas that are electrically continuous are desirable. In this article,
we study the optical response of a type of electrically connected nanoantennas, which we propose to
call “dendritic” antennas. While they are connected, they follow similar antenna hybridization trends
to unconnected plasmon phased array antennas. The optical resonances supported by this type of
nanoantennas are mapped both experimentally and theoretically to unravel their optical response.
Photoluminescence measurements indicate a potential Purcell enhancement of more than a factor of 58
Organic Cation Rotation and Immobilization in Pure and Mixed Methylammonium Lead-Halide Perovskites
Get PDFThree-dimensional lead-halide perovskites have attracted a lot of attention due to their ability to combine solution processing with outstanding optoelectronic properties. Despite their soft ionic nature these materials demonstrate a surprisingly low level of electronic disorder resulting in sharp band edges and narrow distributions of the electronic energies. Understanding how structural and dynamic disorder impacts the optoelectronic properties of these perovskites is important for many applications. Here we combine ultrafast two-dimensional vibrational spectroscopy and molecular dynamics simulations to study the dynamics of the organic inethylammonium (MA) cation orientation in a range of pure and mixed trihalide perovskite materials. For pure MAPbX3 (X = I, Br, Cl) perovskite films, we observe that the cation dynamics accelerate with decreasing size of the halide atom. This acceleration is surprising given the expected strengthening of the hydrogen bonds between the MA and the smaller halide anions, hut can be explained by the increase in the polarizability with the size of halide. Much slower dynamics, up to partial immobilization of the organic cation, are observed in the mixed MAPb(ClxBr1-x)3 and MAPb(BrxI1-x)3 alloys; which we associate with symmetry breaking within the perovskite unit cell. The observed dynamics are essential for understanding the effects of structural and dynamical disorder in perovskite-based optoelectronic systems
