Saturated Förster resonance energy transfer microscopy with a stimulated emission depletion beam: a pathway toward single-molecule resolution in far-field bioimaging

We theoretically demonstrate that the spatial resolution of stimulated emission depletion (STED) microscopy can be substantially enhanced without increasing the intensity of the STED beam. In our scheme, tiny nanobeads codoped with donor and acceptor molecules are used as fluorescent probes, in which F?rster resonance energy transfer (FRET) can occur with an ~100% efficiency between the donors and acceptors. Enhancement of the depletion of acceptors in the nanobeads with the doughnut-shaped depletion beam can lead to an increase of FRET efficiency in the outer area of the excitation spot, which itself is used for deexciting donor molecules and, consequently, enhancing the optical resolution.     

Near-field microscopy with a single-photon point-like emitter: Resolution versus the aperture tip?

We discuss theoretically the concept of spatial resolution in near-field scanning optical microscopy (NSOM) in light of a recent work [Opt. Express 17 (2009) 19969] which reported on the achievement of active tips made of a single ultrasmall fluorescent nanodiamond grafted onto the apex of a substrate tip and on their validation in NSOM imaging. Since fluorescent nanodiamonds tend to decrease steadily in size, we assimilate a nanodiamond-based tip to a point-like single photon source and compare its ultimate resolution with that offered by standard metal-coated aperture NSOM tips. We demonstrate both classically and quantum mechanically that NSOM based on a point-like tip has a resolving power that is only limited by the scan height over the imaged system whereas the aperture-tip resolution depends critically on both the scan height and aperture diameter. This is a consequence of the complex distribution of the electromagnetic field around the aperture that tends to artificially duplicate the imaged objects. We show that the point-like tip does not suffer from this “squint” and that it rapidly approaches its ultimate resolution in the near-field as soon as its scan height falls below the distance between the two nano-objects to be resolved.     

Can atomic force microscopy achieve atomic resolution in contact mode?

Atomic force microscopy operating in the contact mode is studied using total-energy pseudopotential calculations. It is shown that, in the case of a diamond tip and a diamond surface, it is possible for a tip terminated by a single atom to sustain forces in excess of 30 nN. It is also shown that imaging at atomic resolution may be limited by blunting of the tip during lateral scanning.     

Circularly polarized localized near-field radiation at the nanoscale

Diffraction-limited circularly polarized electromagnetic radiation has been widely used in the literature for various applications at both optical and microwave frequencies. With advances in nanotechnology, emerging plasmonic nano-optical applications, such as all-optical magnetic recording, require circularly polarized electromagnetic radiation beyond the diffraction limit. In this study, a plasmonic cross-dipole nano-antenna is investigated to obtain a circularly polarized near-field optical spot with a size smaller than the diffraction limit of light. A cross-dipole nano-antenna is composed of four metallic nano-rods placed at a perpendicular orientation with respect to each other. The performance of the nano-antenna is investigated through numerical simulations. In the first part of this study, the nano-antenna is illuminated with a diffraction-limited circularly polarized radiation. An optimal antenna geometry is specified to obtain an intense optical spot that satisfies two necessary conditions for circular polarization: a phase difference of 90° and a unit amplitude ratio between the electric field components in the vicinity of the antenna gap. In the second part of this study, the nano-antenna is illuminated with diffraction-limited linearly polarized radiation. It is shown that the phase difference between the electric field components can be adjusted by selecting either different antenna lengths or different gap distances in the vertical and horizontal directions. Due to the relatively short wavelength of surface plasma waves on the antenna, it is demonstrated that the phase difference can be sufficient to obtain circularly polarized light. An optimal physical configuration for the nano-antenna and the polarization angle of the incident light is identified to obtain a circularly polarized optical spot beyond the diffraction limit from diffraction-limited linearly polarized radiation.     

Kinetics of quasiballistic transport in nanoscale semiconductor structures: is the ballistic limit attainable at room temperature?

Electron transport in nanoscale semiconductor structures is theoretically investigated to answer the question of whether or not the ballistic limit is really attainable under room temperature operation. The semiclassical Boltzmann transport equation is solved analytically under the relaxation time approximation for n(+)-n-n(+) test structures. We demonstrate that the solution of the Boltzmann transport equation exhibits a boundary layer structure near the potential barrier and thus the scatterings in the active region cannot be neglected even in nanoscale structures, as far as they are operated at room temperature under high applied voltages.     

X‐ray microscopy at the advanced photon source

In modern third generation synchrotron sources, undulators have become the principal source of X-rays and today a brilliance close to 10²¹ photons/sec?mm²?mrad?0.1%BW is routinely attained for photon energies of 10 keV. However, generating brilliant beams of photons with energies of 50 keV and above leads to conflicting choices for the undulator parameters as the following analysis shows.     

Can the minimal supersymmetric standard model with light bottom squark and light gluino survive Z-peak constraints?

In the framework of the minimal supersymmetric model we examine the Z-peak constraints on the scenario of one light bottom squark (sbottom) ( approximately 2-5.5 GeV) and light gluino (approximately 12-16 GeV), which has been successfully used to explain the excess of bottom quark production in hadron collisions. Such a scenario is found to be severely constrained by the CERN LEP Z-peak observables, especially by R(b), due to the large effect of gluino-sbottom loops. To account for the R(b) data in this scenario, the other mass eigenstate of sbottom, i.e., the heavier one, must be lighter than 125 (195) GeV at 2sigma(3sigma) level, which, however, is disfavored by CERN LEP II experiments.     

Transmission electron microscopy investigation of the atomic structure of interfaces in nanoscale Cu–Nb multilayers

Multilayers of Cu–Nb have been grown on a Nb seed layer on a Si (100) substrate using a magnetron sputtering technique. The bilayer period (Λ) was varied from 10 to 2.4 nm. Cross-sectional transmission electron microscopy (XTEM) and high-resolution TEM (HRTEM) were used to study the detailed structure as a function of the bilayer period. Although the majority of the structures conformed to a Kurdjumov–Sachs (K–S) orientation relationship between the Cu and Nb layers, the structures exhibit considerable spatial variation. In some local regions, a Nishiyama–Wasserman (N–W) orientation relationship was found. In addition, considerable distortions were observed in both the Cu and Nb regions close to the interface. Using both HRTEM imaging and fast Fourier transform (FFT) of HRTEM images, early stage of the fcc to bcc transition in Cu was detected. The results suggest that, in multilayer structures, the detailed structure of the interface and large local distortions may play an important role in interface-controlled plasticity.     

A transmission electron microscopy study of the deformation behavior underneath nanoindents in nanoscale Al–TiN multilayered composites

Nanoscale multilayered Al–TiN composites were deposited using the dc magnetron sputtering technique in two different layer thickness ratios, Al : TiN = 1 : 1 and Al : TiN = 9 : 1. The Al layer thickness varied from 2 nm to 450 nm. The hardness of the samples was tested by nanoindentation using a Berkovich tip. Cross-sectional transmission electron microscopy (TEM) was carried out on samples extracted with focused ion beam from below the nanoindents. The results of the hardness tests on the Al–TiN multilayers with two different thickness ratios are presented, together with observations from the cross-sectional TEM studies of the regions underneath the indents. These studies revealed remarkable strength in the multilayers, as well as some very interesting deformation behavior in the TiN layers at extremely small length scales, where the hard TiN layers undergo co-deformation with the Al layers.     

Atomic resolution scanning tunneling microscopy imaging of Pt electrodes interfaced with β″-Al2O3

Solid electrolytes can be used as active catalyst supports to induce significant and reversible catalytic activity and selectivity enhancement via the effect of Non-Faradaic Electrochemical Modification of Catalytic Activity (NEMCA effect) or Electrochemical Promotion which has been recently reported for over fourty catalytic reactions. Atomically resolved Scanning Tunneling Microscopy was used to image the reversible electrochemically controlled dosing (backspillover) of sodium on Pt(111) interfaced to β″-Al₂O₃ at atmospheric pressure, which has been proposed as the cause of the NEMCA effect in the case of Na⁺ conductors. It was found that electrical current application between the Pt(111) monocrystal and a counter electrode also in contact with the β″-Al₂O₃ Na⁺-conducting solid electrolyte causes reversible migration (backspillover and spillover) of sodium which forms a (12×12) hexagonal structure on the Pt(111) surface. In addition to explaining the phenomenon of Electrochemical Promotion in Heterogeneous Catalysis when using Na-β″-Al₂O₃ solid electrolyte these observations provide the first STM confirmation that:

1. spillover-backspillover phenomena can take place over enormous (~mm) atomic distances, and
2. promoters can form ordered structures on catalyst surfaces under ambient conditions relevant to industrial practice.

     

Anisotropic light diffusion: an oxymoron?

Light propagation in anisotropic random media is studied in the steady-state and time domains. Solutions of the anisotropic diffusion equation are compared to results obtained by the Monte Carlo method. Contrary to what has been reported so far, we find that even in the "diffusive regime" the anisotropic diffusion equation does not describe correctly the light propagation in anisotropic random media.     

Capturing the non-equilibrium state in light–matter–free-electron interactions through ultrafast transmission electron microscopy

Ultrafast transmission electron microscope(UTEM) with the multimodality of time-resolved diffraction, imaging,and spectroscopy provides a unique platform to reveal the fundamental features associated with the interaction between free electrons and matter. In this review, we summarize the principles, instrumentation, and recent developments of the UTEM and its applications in capturing dynamic processes and non-equilibrium transient states. The combination of the transmission electron microscope ...     

Polarized light microscopy of the ferroelastic domains of YBa2Cu3O7−x

A short overview is given on recent polarized light microscopy studies of the orthorhombic ferroelastic domains and domain structures of YBa₂Cu₃O_7-x . The optical characteristics of large domains and lamellar domains with a high density of twin walls are pointed out, both for reflected and transmitted light. The polarizing microscope has allowed to observe the ferroelastic detwinning in situ and to determine the activation energy of the ferroelastic wall movements. The question of a potential ferroelectric phase transition is also discussed.     

Tuning the polarization states of optical spots at the nanoscale on the Poincaré sphere using a plasmonic nanoantenna

It is shown that the polarization states of optical spots at the nanoscale can be manipulated to various points on the Poincaré sphere using a plasmonic nanoantenna. Linearly, circularly, and elliptically polarized near-field optical spots at the nanoscale are achieved with various polarization states on the Poincaré sphere using a plasmonic nanoantenna. A novel plasmonic nanoantenna is illuminated with diffraction-limited linearly polarized light. It is demonstrated that the plasmonic resonances of perpendicular and longitudinal components of the nanoantenna and the angle of incident polarization can be tuned to obtain optical spots beyond the diffraction limit with a desired polarization and handedness.     

Search for one large extra dimension with the DELPHI detector at?LEP

Single photons detected by the DELPHI experiment at LEP2 in the years 1997–2000 are reanalysed to investigate the existence of a single extra dimension in a modified ADD scenario with slightly warped large extra dimensions. The data collected at centre-of-mass energies between 180 and 209 GeV for an integrated luminosity of ∼650 pb⁻¹ agree with the predictions of the Standard Model and allow a limit to be set on graviton emission in one large extra dimension. The limit obtained on the fundamental mass scale M _D is 1.69 TeV/c ² at 95% CL, with an expected limit of 1.71 TeV/c ². Deceased.     

Where is the energy of slow light stored?

We analyze the energy storage process of light propagating with slow group velocity in a sample where electromagnetically induced transparency (EIT) is created by a strong coupling field. We compare the formation of slow light in EIT and in self-induced transparency (SIT). For SIT, soliton-like propagation of light with essentially reduced group velocity takes place because of the temporary storage of an appreciable part of the pulse energy in the atoms. For EIT, no energy of the probe is stored in the atoms. This energy is transformed to the coupling field and leaves the sample with phase velocity c without absorption. Slow light is formed by a low frequency coherence induced at the input by the probe and coupling fields in a two-quantum excitation process. This coherence propagates as a “spin wave” with small group velocity, and at a large distance from the input, the coherence rules the process of the energy transformation from the coupling field to the probe, reproducing exactly the temporal profile of the probe at the input.     

Demonstration of fiber pulsed light source at 1.6μm with adjustable pulse duration

A novel practical 1.66-μm pulse light source with adjustable pulse duration is proposed. A 2.5-km Raman fiber is placed into a ring type Q-switched erbium-doped fiber laser (Q-EDFL), serving as both delay line fiber and Raman gain medium so that in addition to the wavelength shifted to 1.6μm, the pulse duration and the buildup time can be relatively extended. By properly controlling the fall edge of the acousto-optic switch (AOS), the pulse duration of 30-345 ns for～770-Hz repetition frequency with power of 1-1.6 W is achieved.