Excitation Polarization Research Articles

Compact Integrated Feeding Network for Excitation of Dual-Circular Polarization in Series-Fed Antenna Lattice

A novel approach for realization of a feeding network allowing for achieving dual-circular polarization in the slot-coupled microstrip antenna lattice has been proposed. The presented feeding network is an eight-port composed of four 3\mathchar"702DdB/90 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">°</sup> directional couplers, and when appropriately integrated with the four-port series-fed antenna lattice ensures a full symmetry of the antenna layout and produces simultaneously right-handed and left-handed circular polarization. The proposed low-profile, low-cost solution can be easily integrated with the antenna lattice, and it allows for reduction of the introduced dissipation losses comparing to the known solutions, i.e., external feeding networks such as Butler matrices or power combiner/divider networks connected via coaxial cables. The proposed concept has been examined by measurements of the manufactured feeding network integrated with the previously developed antenna lattice operating at 5.4 GHz.

Two-photon interband absorption coefficients in tungstate and molybdate crystals

Two-photon absorption (TPA) coefficients were measured in tungstate and molybdate crystals – BaWO4, KGW, CaMoO4, BaMoO4, CaWO4, PbWO4 and ZnWO4 upon different orientations of excitation polarization with respect to the crystallographic axes. Trains of 25ps pulses with variable radiation intensities of third (349nm) harmonics of passively mode-locked 1047nm Nd:YLF laser were used for interband two-photon excitation of the crystals.It was suggested that in the case, when 349nm radiation pumping energy exceeds the bandgap width (hν>Eg), the nonlinear excitation process can be considered as two-step absorption.The interband two-photon absorption in all the studied crystals induces the following one-photon absorption from the exited states, which affects the nonlinear process dynamics and leads to a hysteresis in the dependence of the transmission on the excitation intensity. This fact was taken into account under analysis of the experimental dependences of the reciprocal transmission on the excitation intensity. Laser excitation in the transparency region of the crystals caused stimulated Raman scattering (SRS) not for all the crystals studied. The measured nonlinear coefficients allowed us to explain the suppression of SRS in crystals as a result of competition between the SRS and TPA.

Angle-Resolved Plasmonic Properties of Single Gold Nanorod Dimers.

Through wet-chemical assembly methods, gold nanorods were placed close to each other and formed a dimer with a gap distance ~1 nm, and hence degenerated plasmonic dipole modes of individual nanorods coupled together to produce hybridized bonding and antibonding resonance modes. Previous studies using a condenser for illumination result in averaged signals over all excitation angles. By exciting an individual dimer obliquely at different angles, we demonstrate that these two new resonance modes are highly tunable and sensitive to the angle between the excitation polarization and the dimer orientation, which follows cos2φ dependence. Moreover, for dimer structures with various structure angles, the resonance wavelengths as well as the refractive index sensitivities were found independent of the structure angle. Calculated angle-resolved plasmonic properties are in good agreement with the measurements. The assembled nanostructures investigated here are important for fundamental researches as well as potential applications when they are used as building blocks in plasmon-based optical and optoelectronic devices.Electronic supplementary materialThe online version of this article (doi:10.1007/s40820-014-0011-7) contains supplementary material, which is available to authorized users.

Polarization features of optically pumped CdS nanowire lasers

High quality CdS nanowires suspended in air were optically pumped both below and above the lasing threshold. The polarization of the pump laser was varied while emission out of the end facet of the nanowire was monitored in a ‘head-on’ measurement geometry. Highest pump-efficiency and most efficient absorption of the pump radiation are demonstrated for an incident electric field being polarized parallel to the nanowire axis. This polarization dependence, which was observed both above the lasing threshold and in the regime of amplified spontaneous emission, is caused by an enhanced absorption for parallel polarized optical pumping. Measured Stokes parameters of the nanowire emission reveal that due to the onset of lasing the degree of polarization rapidly increases from approximately 15 to 85%. Both Stokes parameters and degree of polarization of the nanowire lasing emission are independent of the excitation polarization. The transversal lasing mode is therefore not notably affected by the polarization of the pumping beam, although the supply with optical gain is significantly enhanced for an excitation polarization parallel to the nanowire axis.

Nonlinear Optical Properties of Large-Sized Gold Nanorods

The nonlinear optical properties of single gold nanorods (GNRs) with a large diameter of ∼200 nm and a long length of ∼800 nm were investigated by using a focused femtosecond (fs) laser light with tunable wavelength. While the linear and nonlinear optical properties of small-sized GNRs have been extensively studied, the nonlinear optical properties of large-sized GNRs and the effects of high-order surface plasmon resonances remain unexplored. Second harmonic generation (SHG) or/and two-photon-induced luminescence (TPL) were observed in the nonlinear response spectra, and their dependences on excitation wavelength and polarization were examined. The scattering and absorption spectra of the small- and large-sized GNRs were compared by using the discrete dipole approximation method. It was found that the extinction of large-sized GNRs is dominated by scattering rather than absorption, which is dominant in small-sized GNRs. In addition, it was revealed that the excitation wavelength-dependent SHG of a GNR is governed by the linear scattering of the GNR and the maximum SHG is achieved at the valley of the scattering spectrum. In comparison, the excitation wavelength dependence of TPL is determined by the absorption spectrum of the GNR. The polarization-dependent SHG of a GNR exhibits a strong dependence on the dimension of the GNR, and it may appear as bipolar distributions parallel or perpendicular to the long axis of the GNR or multipole distributions.

Multiwavelength excitation Raman scattering of Cu2ZnSn(SxSe1−x)4 (0 ≤ x ≤ 1) polycrystalline thin films: Vibrational properties of sulfoselenide solid solutions

In this work, Raman spectroscopy and X-ray diffraction were applied together to evaluate the crystal structure and the phonon modes of photovoltaic grade Cu2ZnSn(SxSe1−x)4 thin films, leading to a complete characterization of their structural and vibrational properties. Vibrational characterization has been based on Raman scattering measurements performed with different excitation wavelengths and polarization configurations. Analysis of the experimental spectra has permitted identification of 19 peaks, which positions are in good accord with theoretical predictions. Besides, the observation of Cu2ZnSnS4-like A symmetry peaks related to S vibrations and Cu2ZnSnSe4-like A symmetry peaks related to Se vibrations, additional Raman peaks, characteristic of the solid solution and previously not reported, are observed, and are attributed to vibrations involving both S and Se anions.

Two-color two-photon excited fluorescence of 2-methyl-5-tert-butyl-p-quaterphenyl (DMQ): ab initio calculations and experimental determination of the molecular parameters.

The paper presents experimental and theoretical studies of two-photon excitation dynamics in 2-methyl-5-tert-butyl-p-quaterphenyl (DMQ) dissolved in cyclohexane/paraffin. Experimentally, a two-color two-photon (2C2P) excitation by two femtosecond laser pulses at 800 and 400 nm has been used in combination with the time-resolved detection of polarized molecular fluorescence. The fluorescence decay was found to be two-exponential, resulting in the molecular excited state lifetime of 753 ± 10 ps and the rotational correlation time of 724 ± 45 ps. Control over the excited and fluorescent photons polarization has been used for determination from experiment of seven independent molecular parameters. The experimental data were analyzed on the basis of the recent theoretical approach [Shternin, P. S., Gericke, K.-H., and Vasyutinskii, O. S. Mol. Phys. 2010, 108, 813-825] supported by ab initio computations of the DMQ electronic structure and transition dipole moments. The results obtained imply that the two-photon absorption tensor S is mostly diagonal and that the Szz tensor component onto the molecular long axis gives the major contribution of 93%. However, it was also found that a number of different symmetry two-photon transitions related to the dipole moment components dxdz and dydz are excited in the conditions of our measurements.

Measurement of Three-Dimensional Dipole Orientation of a Single Fluorescent Nanoemitter by Emission Polarization Analysis

We demonstrate theoretically and experimentally that the three-dimensional orientation of a single fluorescent nanoemitter can be determined by polarization analysis of the emitted light (while excitation polarization analysis provides only the in-plane orientation). The determination of the emitter orientation by polarimetry requires a theoretical description, including the objective numerical aperture, the 1D or 2D nature of the emitting dipole, and the environment close to the dipole. We develop a model covering most experimentally relevant microscopy configurations and provide analytical relations that are useful for orientation measurements. We perform polarimetric measurements on high-quality core-shell CdSe/CdS nanocrystals and demonstrate that they can be approximated by two orthogonal degenerated dipoles. Finally, we show that the orientation of a dipole can be inferred by polarimetric measurement, even for a dipole in the vicinity of a gold film, while in this case, the well-established defocused microscopy is not appropriate.

Plasmonic Enhancement of Luminescence of Fluorscein Isothiocyanate and Human Immunoglobulin Conjugates

Plasmonic enhancement of the luminescence of fl uorescein isothiocyanate and human immunoglobulin conjugates near silver nanoparticles was investigated as functions of the nanoparticle-conjugate distance and the excitation polarization. The maximum luminescence enhancement of 7.4 was achieved for p-polarized excitation and nanoparticle-conjugate distance 3.3 nm. The luminescence enhancement factor increased experimentally for p-polarized excitation and decreased for s-polarized excitation as compared with unpolarized excitation.

Polarization spectroscopy of a velocity-selected molecular sample.

This Letter demonstrates polarization spectroscopy of a velocity selected and vibrationally excited molecular sample. Specifically, the anisotropy induced by a circularly polarized IR pump beam tuned to the R(14.5)(1/2)v=1←v=0 transition of nitric oxide is observed using an IR probe resonant with the R(15.5)(1/2)v=2←v=1 hot band transition. Using two detectors in combination with the rapidly swept probe allows both the absorptive and dispersive components of the excited state polarization to be observed for the first time. The data are well described by simulations based upon a three-level density matrix model.

Surface Plasmon Polariton Propagation and Coupling in Gold Nanostructures

Surface plasmon polariton (SPP) propagation in chemically synthesized gold nanobars was investigated using scanning transient absorption microscopy. The SPP propagation lengths were correlated to the size and shape of the nanobars, which were determined by atomic force microscopy. The average propagation length was found to be 12 ± 4 μm, and the measured values were independent of the excitation polarization. Finite element calculations were performed for gold nanobars in the presence of a glass substrate to model the experimental results. Comparison between the experimental and theoretical results shows that both bound and leaky SPP modes can be excited. The leaky mode is detected in the larger nanobars (widths greater than 500 nm), and the bound mode is detected in smaller nanobars. Highly directional and efficient plasmon coupling between a gold nanobar and a nearby gold nanoplate was also observed in the experiments.

Fluorescence nanoscopy by polarization modulation and polarization angle narrowing

When excited with rotating linear polarized light, differently oriented fluorescent dyes emit periodic signals peaking at different times. We show that measurement of the average orientation of fluorescent dyes attached to rigid sample structures mapped to regularly defined (50 nm)(2) image nanoareas can provide subdiffraction resolution (super resolution by polarization demodulation, SPoD). Because the polarization angle range for effective excitation of an oriented molecule is rather broad and unspecific, we narrowed this range by simultaneous irradiation with a second, de-excitation, beam possessing a polarization perpendicular to the excitation beam (excitation polarization angle narrowing, ExPAN). This shortened the periodic emission flashes, allowing better discrimination between molecules or nanoareas. Our method requires neither the generation of nanometric interference structures nor the use of switchable or blinking fluorescent probes. We applied the method to standard wide-field microscopy with camera detection and to two-photon scanning microscopy, imaging the fine structural details of neuronal spines.

(Invited) Photoluminescence Enhancement of a Silicon Nanocrystal Plane Positioned in the Near-Field of a Silicon Nanowire

Semiconductor nanowires (NW) are good candidates for new optoelectronic or photovoltaic devices due to their exceptional ability to guide, scatter or absorb light, from near-ultraviolet to near-infrared. The occurrence of morphology-dependent optical resonances in nanowires opens a route to overcome the intrinsic limitations of some materials (for instance the limited absorption of silicon in the visible spectrum due its indirect band gap) and optimize their interaction with light.Most optical applications of nanowires were recently focused on photovoltaic devices, because of their enhanced light absorption efficiency, which is also tunable either by the NW diameter, structure or chemical composition. It could be interesting to use the near-field enhancement around the NW induced by the excited resonances, in order to increase the emission of quantum dots or molecules placed in the NW vicinity. Semiconductor NWs could thus be used as optical antennas instead of plasmonic nanostructures, despite a much weaker enhancement, but with the advantages of field enhancement in larger volumes and using a fully compatible CMOS technology.We investigate the influence of Si -NWs on the photoluminescence of a single plane of Si nanocrystals embedded in a silica matrix. The presence of an individual Si-NW on top of the silica layer, at about 3 nm above the Si-NCs plane, leads to a strong photoluminescence enhancement, by up to a factor of three, depending on the excitation light wavelength and polarization, and the nanowire diameter. This opens a route for improving light emitting devices using Si-based nanowires.

Solid-state 13C NMR study of the mobility of polysaccharides in the cell walls of two apple cultivars of different firmness

Solid-state 13C nuclear magnetic resonance (NMR) was used to compare differences in mobility of the cell wall polysaccharides of ‘Scifresh’ and ‘Royal Gala’ apples after 20weeks of storage. The texture of ‘Scifresh’ apples was markedly firmer than that of ‘Royal Gala’ at the end of storage. In a novel approach Two Pulse Phase Modulation (TPPM) decoupling was combined with cross polarisation (CP) and single pulse excitation (SPE) experiments. The resulting high resolution solid-state SPE spectra, unprecedented for apple cell walls, allowed a detailed insight into the physical and chemical properties of very mobile polysaccharides such as the arabinan and galactan side chains of the pectic polysaccharide rhamnogalacturonan I (RG-I). NMR showed that the cellulose rigidity was the same in the two cultivars, while arabinans were more mobile than galactans in both. Unexpectedly, arabinans in ‘Scifresh’ cell walls were more mobile than those in ‘Royal Gala’ which was unforeseen considering the greater firmness of the ‘Scifresh’ cultivar.

Quantitative Retardance Imaging using Quadri-Wave Lateral Shearing Interferometry (QWLSI)

We describe the use of quantitative phase imaging to enhance specifically the contrast of ordered components like fibers and membranes inside biological samples. Phase contrast imaging can be considered as a powerful method for the label-free and long-time duration imaging of semi-transparent biological samples. Recent techniques give access to a quantitative measurement of optical thickness. Changes in the refractive index inside the samples can be the main contrast sources in living cells. It is known that some biological structures inside cells are optically anisotropic and thus scatter light differently depending on the illumination light polarization. This property is widely used in polarized light microscopy to reveal ordered structures without staining or labeling but most techniques are purely qualitative or hard to implement when one wants to obtain quantitative measurements on living specimens. Measurement of phase shifts introduced with different incident polarization angles, would give access to quantitative values of both linear retardance and orientation of optical axes. We propose here to use QWLSI to measure a set of polarization-dependent phase shifts, in order to reveal collagen fibers local structure and some components like actin stress fibers in living cells samples. The high-resolution wave front sensor is mounted on a non-modified transmission microscope to measure characteristic optical path difference of the sample [1]. The very simple setup is composed by a single rotating polarizer placed in the illumination light path before the sample that leads to record one quantitative phase image for each excitation polarization angle. The set of those images, recorded in few seconds so as to deal with living samples, is numerically computed to obtain what we call Quantitative Retardance Images which represent the local retardance value of the sample and its optical axis local orientation.

New Methods for Molecular Motor and Cell Motility Research

Several novel methods for high-speed, high resolution microscopy have enabled new insights into function of molecular motors in vitro and in live cells. Several groups have developed methods to track the position and orientation of bi-functional organic fluorophores or quantum rods bound to specific motor subunits. We have now improved the time resolution by time-multiplexing excitation polarization every 100 μs and resolving structural changes by the consequent sudden changes in photon collection rates. In addition, we developed a multi-channel change-point method to detect structural states within the noisy signals and estimate rotations to within a few photons. These improvements enabled new insights into the walking mechanism of myosin V: we directly observe 10-20 ms periods of high mobility while myosin V detached heads search for actin. The gradual left-handed path of myosin V is produced by intermittent large side-ways motions among long stretches of straight walking where the lever arms mostly tilt in a plane very close to that containing the actin filament. For in vivo studies, detecting forces and motions of intracellular cargos have been compromised, until recently, by uncertainties about calibrating optical traps in cells. In the last year, four groups published novel methods for improving trap calibration in living cells. In our method, the thermal fluctuations of a phagocytosed bead are analyzed together with its motions upon sinusoidal excitation over a series of frequencies, giving trap sensitivity and local sub-diffusive viscoelastic parameters of the cytoplasm. This method is insensitive to active biological processes in the cell, as the forced response is used to resolve the elastic responses of the trap and the environment. Forces on these cargos indicate several kinesin and dynein motors operating near force balance. Supported by NIH grants P01GM087253 and R01GM086352.

Nanoscopy by Fluorescence Demodulation and Polarization Angle Narrowing

A new fluorescence super-resolution wide-field approach is presented. We first demonstrate that the angle range for exciting molecules of different orientations can be significantly narrowed by rotating a linear polarized excitation beam together with a wide-field de-excitationbeam of perpendicular polarization. This results in short periodical flashes from different standard markers in sub-diffraction limited areas after each other (excitation polarization angle narrowing, ExPAN). Many molecules per image pixel still cause periodic fluorescence modulation of different phases that are directly connected to the nanometric sample structure. Since fluorescence markers attached to more rigid sample structures often have preferred orientations this modulation and phase information can be used deduce high-resolution images resolving details such as gaps between nanospheres, closely spaced microtubules and detailed neuronal spine head structures (SPoD, super-resolution by polarization demodulation). The combination with two-photon excitation allows imaging tissue at deep depths (∼0.6 mm). Data acquisition times of only few hundreds of miliseconds are necessary. Wide-Field illumination is possible without the need of special fluorescence markers or the generation of nanometric interference structures. In our presentation we will explain the principles of this approach in detail and demonstrate its benefits in comparison to previous approaches.View Large Image | View Hi-Res Image | Download PowerPoint Slide

Effect of the Side‐Chain‐Distribution Density on the Single‐Conjugated‐Polymer‐Chain Conformation

The spatial arrangement of the side chains of conjugated polymer backbones has critical effects on the morphology and electronic and photophysical properties of the corresponding bulk films. The effect of the side-chain-distribution density on the conformation at the isolated single-polymer-chain level was investigated with regiorandom (rra-) poly(3-hexylthiophene) (P3HT) and poly(3-hexyl-2,5-thienylene vinylene) (P3HTV). Although pure P3HTV films are known to have low fluorescence quantum efficiencies, we observed a considerable increase in fluorescence intensity by dispersing P3HTV in poly(methyl methacrylate) (PMMA), which enabled a single-molecule spectroscopy investigation. With single-molecule fluorescence excitation polarization spectroscopy, we found that rra-P3HTV single molecules form highly ordered conformations. In contrast, rra-P3HT single molecules, display a wide variety of different conformations from isotropic to highly ordered, were observed. The experimental results are supported by extensive molecular dynamics simulations, which reveal that the reduced side-chain-distribution density, that is, the spaced-out side-chain substitution pattern, in rra-P3HTV favors more ordered conformations compared to rra-P3HT. Our results demonstrate that the distribution of side chains strongly affects the polymer-chain conformation, even at the single-molecule level, an aspect that has important implications when interpreting the macroscopic interchain packing structure exhibited by bulk polymer films.

Interpreting the Energy-Dependent Anisotropy of Colloidal Nanorods Using Ensemble and Single-Particle Spectroscopy

We report the use of polarized excitation spectroscopy in the study of the polarized optical properties of II–VI semiconducting nanorods. This technique provides a quantitative measure for analyzing the polarization-dependent electronic structure of ensembles of semiconducting nanoparticles in colloidal solutions. We develop a procedure to quantify the anisotropy of nanorod excitations and the influence of the dielectric environment which yields results in qualitative agreement with theoretical predictions of nanorod absorption properties. Excitation measurements of nanorod ensembles and single-particle excitation and emission polarization measurements are used to interpret ensemble measurements on the molecular frame. At the single-particle level, we find a large dispersion in the angles between excitation and emission polarization curves. This contrasts with the conventional picture of CdSe electronic transitions, in which the crystal axes dictate the symmetry of excitations. Such geometrically heteroge...

Two-photon excited fluorescence from a pseudoisocyanine-attached gold-coated tip via a thin tapered fiber under a weak continuous wave excitation

A simple tapered fiber based photonic-plasmonic hybrid nanostructure composed of a thin tapered fiber and a pseudoisocyanine (PIC)-attached Au-coated tip was demonstrated. Using this simple hybrid nanostructure, we succeeded in observing two-photon excited fluorescence from the PIC dye molecules under a weak continuous wave excitation condition. From the results of the tip-fiber distance dependence and excitation polarization dependence, we found that using a thin tapered fiber and an Au-coated tip realized efficient coupling of the incident light (~95%) and LSP excitation at the Au-coated tip, suggesting the possibility of efficiently inducing two-photon excited fluorescence from the PIC dye molecules attached on the Au-coated tip. This simple photonic-plasmonic hybrid system is one of the promising tools for single photon sources, highly efficient plasmonic sensors, and integrated nonlinear plasmonic devices.