Excitation Polarization Research Articles

Resonance Fluorescence of an InGaAs Quantum Dot in a Planar Cavity Using Orthogonal Excitation and Detection.

The ability to perform simultaneous resonant excitation and fluorescence detection is important for quantum optical measurements of quantum dots (QDs). Resonant excitation without fluorescence detection - for example, a differential transmission measurement - can determine some properties of the emitting system, but does not allow applications or measurements based on the emitted photons. For example, the measurement of photon correlations, observation of the Mollow triplet, and realization of single photon sources all require collection of the fluorescence. Incoherent excitation with fluorescence detection - for example, above band-gap excitation - can be used to create single photon sources, but the disturbance of the environment due to the excitation reduces the indistinguishability of the photons. Single photon sources based on QDs will have to be resonantly excited to have high photon indistinguishability, and simultaneous collection of the photons will be necessary to make use of them. We demonstrate a method to resonantly excite a single QD embedded in a planar cavity by coupling the excitation beam into this cavity from the cleaved face of the sample while collecting the fluorescence along the sample's surface normal direction. By carefully matching the excitation beam to the waveguide mode of the cavity, the excitation light can couple into the cavity and interact with the QD. The scattered photons can couple to the Fabry-Perot mode of the cavity and escape in the surface normal direction. This method allows complete freedom in the detection polarization, but the excitation polarization is restricted by the propagation direction of the excitation beam. The fluorescence from the wetting layer provides a guide to align the collection path with respect to the excitation beam. The orthogonality of the excitation and detection modes enables resonant excitation of a single QD with negligible laser scattering background.

Polarization-dependent fluorescence of CdSe/ZnS quantum dots coupling to a single gold-silver alloy nanotube

The fluorescence intensities of CdSe/ZnS quantum dots on a silver nanowire and a gold-silver alloy nanotube with different molar ratios of gold and silver are investigated under excitation with different polarization angles. The ratio of the fluorescence intensity of perpendicular and parallel polarization excitations is measured to be 4.05 for a silver nanowire and decreases to 1.30, 1.10 and 1.07 for nanotube with Au:Ag molar ratios of 1:4.0, 1:2.5 and 1:1.5, respectively. The numerical simulation results show that the aforementioned experimental phenomena can be attributed to the variation in the distribution of the surface plasmon electromagnetic field under perpendicular polarized light excitation. The electromagnetic field is primarily distributed on the outer surface of the silver nanowire, while the field partially penetrates the interior of the silver nanotube and becomes completely concentrated at the interior of the gold nanotube. This research also found that the productivity and robustness of the chemical synthesis of the alloy nanotubes can be greatly improved by using a microwave reactor.

Excitation polarization dependence of optical coupled exciton states in a single coupled quantum dot molecule

The optically coupled exciton states have been investigated in terms of power dependent photoluminescence at the coupled GaAs quantum dots, which were laterally coupled along the [10] direction. The power dependent photoluminescence results are compared with two different excitation directions which are parallel to the direction of [10] and [110]. In coupled direction along the [10] direction, the power dependent redshift and coupled biexciton are observed, a fact that may be attributed to optical couplings via dipole-dipole interactions. Also, the redshift of excitons was manifested based on theoretical model.

High-Efficiency and Full-Space Manipulation of Electromagnetic Wave Fronts with Metasurfaces

Metasurfaces offered great opportunities to control electromagnetic (EM) waves, but currently available meta-devices typically work either in pure reflection or pure transmission mode, leaving half of EM space completely unexplored. Here, we propose a new type of metasurface, composed by specifically designed meta-atoms with polarization-dependent transmission and reflection properties, to efficiently manipulate EM waves in the full space. As a proof of concept, three microwave meta-devices are designed, fabricated and experimentally characterized. The first two can bend or focus EM waves at different sides (i.e., transmission/reflection sides) of the metasurfaces depending on the incident polarization, while the third one changes from a wave bender for reflected wave to a focusing lens for transmitted wave as the excitation polarization is rotated, with all these functionalities exhibiting very high efficiencies (in the range of 85%-91%). Our findings significantly expand the capabilities of metasurfaces in controlling EM waves, and can stimulate high-performance multi-functional meta-devices facing more challenging and diversified application demands.

Challenges in the Measurement of Passive Dissolution of Steel in Concrete

The corrosion rate of the steel reinforcement of concrete is measured and monitored in laboratory and civil structures by various techniques that include anodic or cathodic galvanostatic pulse, linear polarization resistance (using a given polarization rate), electrochemical impedance spectroscopy, etc. Considerable attention has been given to the suitability of those methods to measure the corrosion rate of steel in the active state, after passivity breakdown due to aggressive species contamination. However, fewer studies have evaluated the case of service in structures with required durability much in excess of one hundred years (as for example containment of radioactive waste) where the corrosion process is expected to remain in passive state, with anodic current densities typically lower than 10-8 A/cm2. The electrochemical measurement of such low corrosion rates is a challenging goal as it is difficult to separate polarization currents due to interfacial charge storage processes (which may be of actual or apparent non-Faradaic origin) to those related to the anodic, metal dissolution process itself. That separation often leads to the need for long duration of the measurements, which would make it impractical to monitor a large number of reinforcement locations in an actual structure. Understanding of the relevant measurement factors is needed to eventually develop more time efficient assessment methods for those systems.As part of developing that understanding, the objective of this work is the experimental and modeling analysis of steel in concrete in the passive state, in response to the application of cathodic cyclic polarization with polarization rates in the order of 0.001 mV/sec and polarization down to -300 mV with respect to the (passive) corrosion potential. This technique, implemented with higher polarization rates after appropriate adjustment as a refined form of Tafel extrapolation, is a possible candidate to monitor structures of interest. In order to interpret the passive corrosion process response to polarization excitation, a modified Randles interphase model analog was used. A novel feature of this study is the introduction of large amplitude time domain behavior of a constant phase angle element, which is then combined with the large amplitude response of the Faradaic current treated with a Butler-Volmer approach. The system steel/interface/concrete is described by a fractional order differential equation that is solved by the implementation of a finite differences aproach. Comparisons between the experimental results from multiple specimens at various scan rates and the response calculated by solution of the equations of the system were made, showing good agreement between both. Corrosion current densities obtained by numerically fitting the model to experimental results yielded values typically lower than 10-8 A/cm2 in agreement with other values reported in the literature for long term exposures of passive steel in concrete. Open issues on the applicability of these results for mechanistic interpretation of the processes at work on the rate of passive dissolution of steel in concrete are discussed.

Fluorescence Polarization Measurements to Probe Alignment of a Bithiophene Dye in One-Dimensional Channels of Self-Assembled Phenylethynylene Bis-Urea Macrocycle Crystals

This paper describes the use of polarized fluorescence microscopy to directly probe guest molecule orientation in bis-urea macrocycle crystals. These macrocycles assemble to afford one-dimensional (1D) microchannels ∼9 A in diameter that have previously been shown to exhibit normal Fickian diffusion and induce selective reactivity among the confined guest molecules. In the present work, we take advantage of the quasi-1D morphology of fiber-like microcrystals with the extended dimension corresponding to the channel axis to measure excitation and emission polarization values for a bithiophene guest. Guest fluorescence is shown to be polarized along the fiber axis with emission polarization values up to 0.729, indicating a high degree of orientational order within the 1D channels. The observed behavior is consistent with calculated results for the guest orientation and electronic transition dipole moment. The results indicate the value of functional fluorescent probes as a measure of guest confinement in low...

Tuning Plasmonic Enhancement of Single Nanocrystal Upconversion Luminescence by Varying Gold Nanorod Diameter.

Plasmonic enhancement induced by metallic nanostructures is an effective strategy to improve the upconversion efficiency of lanthanide-doped nanocrystals. It is demonstrated that plasmonic enhancement of the upconversion luminescence (UCL) of single NaYF4 :Yb3+ /Er3+ /Mn2+ nanocrystal can be tuned by tailoring scattering and absorption cross sections of gold nanorods, which is synthesized wet chemically. The assembly of the single gold nanorod and single upconversion nanocrystal is achieved by the atomic force microscope probe manipulation. By selecting two kinds of gold nanorods with similar longitudinal surface plasmon resonance wavelength but different diameters (27.3 and 46.7 nm), which extinction spectra are separately dominant by the absorption and scattering, the maximum UCL enhancement by a factor of 110 is achieved with the 46.7 nm-diameter gold nanorod, while it is 19 for the nanorod with the diameter of 27.3 nm. Such strong enhancement with the larger gold nanorod is due to stronger scattering ability and greater extent of the near-field enhancement. The enhanced UCL shows a strong dependence on the excitation polarization relative to the nanorod long axis. Time-resolved measurements and finite-difference time-domain simulations unveil that both excitation and emission processes of UCL are accelerated by the nanorod plasmonic effect.

Injection Locking and Polarization Switching Bistability in a 1550 nm VCSEL Subject to Parallel Optical Injection

We present experimental and theoretical results of bistable polarization switching in a single transverse mode 1550 nm vertical-cavity surface-emitting laser (VCSEL) subject to parallel optical injection. We focus our analysis on the bistability induced by power variation of the master laser found on the recently observed state of simultaneous injection locking of the parallel polarization mode and excitation of the orthogonal polarization mode, IL+PS. Experimental stability maps identifying, in the injected power-frequency detuning plane, where this state is observed, and corresponding bistable regions are reported for several bias currents. We find bistability between the IL+PS state and the single polarization mode injection locked solution. We also find bistability between the IL+PS and different periodic dynamics in the parallel polarization mode. The width of the hysteresis cycle increases when increasing the bias current or when increasing the wavelength of the optical injection beyond the solitary VCSEL wavelength. We theoretically confirm these types of bistability by numerically simulating the spin-flip model and by performing a linear stability analysis for the different stable states. Excellent agreement is found between experimental and theoretical results.

Self-orientation effect of liquid crystals on holographic polymer-dispersed liquid crystal and distributed feedback lasers

The average orientation of a liquid crystal (LC) director to the grating formation, morphology, and switching properties of a holographic polymer-dispersed liquid crystal (HPDLC) grating was systematically investigated in this study. The grating possessed high diffraction efficiency and low scattering with the LC director being parallel to the grating vector. The scanning electron microscope confirmed the well-defined morphology with the LC director being parallel to the grating vector. The grating was easily switched when the LC director was perpendicular to the grating vector. Moreover, polarization excitation was performed to investigate the polarization dependence behavior of the HPDLC-distributed feedback (DFB) laser. The results confirmed that the HPDLC grating is suitable as a laser oscillation when the LC director is parallel to the grating vector. Finally, the tuning range was obtained for the HPDLC DFB laser by applying an external electric field. The tunability, ease of fabrication, and mass production make the HPDLC DFB lasers suitable as smart laser sources for spectroscopy and communication.

Unidirectional transmission based on polarization conversion and excitation of magnetic or surface polaritons

We propose in this work combing a uniaxial crystal slab with a one-dimensional grating to realize unidirectional transmission (UDT). The physical mechanism for the UDT is attributed to polarization conversion with uniaxial crystal slab and excitation of magnetic polaritons (MPs) or surface plasmon polaritons (SPPs) in the grating region. Numerical simulations were performed by taking hexagonal boron nitride as the uniaxial crystal. The results reveal that UDT can be achieved for both TE and TM waves in the mid-infrared and the optical regions if the grating material is respectively selected as silicon carbide (SiC) and silver (Ag) with properly chosen values of the structure’s geometric parameters. This work may provide important guidelines for design of novel unidirectional transmission devices.

Pontamine fast scarlet 4B bifluorescence and measurements of cellulose microfibril angles.

Pontamine fast scarlet 4B is a red paper and textiles dye that has recently been introduced as a fluorescent probe for plant cell walls. Pontamine exhibits bifluorescence, or fluorescence dependent on the polarization of the excitation light: Because cellulose is aligned within the cell wall, pontamine-labelled cell walls exhibit variable fluorescence as the excitation polarization is modulated. Thus, bifluorescence measurements require polarized excitation that can be directly or indirectly modulated. In our confocal microscopy observations of various cellulose samples labelled with pontamine, we modulated excitation polarization either through sample rotation or by the confocal's scanfield rotation function. This variably rotated laser polarizations on Leica confocal microscopes, but not those from other makers. Beginning with samples with directly observable microfibril orientations, such as purified bacterial cellulose, the velamen of orchid roots and the inner S2 layer of radiata pine compression wood, we demonstrate that modelling the variations in pontamine fluorescence with a sine curve can be used to measure the known microfibril angles. We then measured average local microfibril angles in radiata pine samples, and showed similar microfibril angles in compression and normal (opposite) wood. Significantly, bifluorescence measurements might also be used to understand the degree of local cellulose alignment within the cell wall, as opposed to variations in the overall cellulose angle.

Polarization dependence of tip-enhanced Raman and plasmon-resonance Rayleigh scattering spectra

Tip-enhanced Raman scattering (TERS) spectroscopy has high sensitivity and high spatial resolution, although it shows low reproducibility due to the variable optical properties of the tips. In the present study, polarized scattering spectra of localized surface plasmon resonance (LSPR) at the apex of the tip induced by conventional dark field illumination were compared with the corresponding TERS spectra, generated by excitation using polarization not only parallel and perpendicular to the tip, but also vertical to the sample plane (z-polarization). The polarization-dependence of LSPR was consistent with that of the TERS. Thus, the optical properties of the tip can be easily optimized before TERS measurement by excitation polarization that induces the largest LSPR signal.

Polarization dependent femtosecond laser modification of MBE-grown III-V nanostructures on silicon

We report a novel, polarization dependent, femtosecond laser-induced modification of surface nanostructures of indium, gallium, and arsenic grown on silicon via molecular beam epitaxy, yielding shape control from linear and circular polarization of laser excitation. Linear polarization causes an elongation effect, beyond the dimensions of the unexposed nanostructures, ranging from 88 nm to over 1 um, and circular polarization causes the nanostructures to flatten out or form loops of material, to diameters of approximately 195 nm. During excitation, it is also observed that the generated second and third harmonic signals from the substrate and surface nanostructures increase with exposure time.

Ultrafast Zero-Bias Photocurrent in GeS Nanosheets: Promise for Photovoltaics

Ferroelectric semiconductors have been predicted to exhibit strong zero-bias shift current, spurring the search for ferroelectric semiconductors with band gaps in the visible range as candidates for so-called shift current photovoltaics with efficiencies not constrained by the Schockley–Queisser limit. Recent theoretical works have predicted that two-dimensional IV–VI monochalcogenides are multiferroic and capable of generating significant shift currents. Here we present experimental validation of this prediction, observing ultrafast shift currents by detecting terahertz electromagnetic pulses emitted by the photoexcited GeS nanosheets without external bias. We explore excitation fluence, orientation, and excitation polarization dependence of the terahertz emission to confirm that shift currents are indeed responsible for the observed emission. Experimental observation of zero-bias photocurrents puts GeS nanosheets forth as a promising candidate material for applications in third-generation photovoltaics ...

Ultrafast spectroscopy on DNA-cleavage by endonuclease in molecular crowding

The jam-packed intracellular environments differ the activity of a biological macromolecule from that in laboratory environments (in vitro) through a number of mechanisms called molecular crowding related to structure, function and dynamics of the macromolecule. Here, we have explored the structure, function and dynamics of a model enzyme protein DNase I in molecular crowing of polyethylene glycol (PEG; MW 3350). We have used steady state and picosecond resolved dynamics of a well-known intercalator ethidium bromide (EB) in a 20-mer double-stranded DNA (dsDNA) to monitor the DNA-cleavage by the enzyme in absence and presence PEG. We have also labelled the enzyme by a well-known fluorescent probe 8-anilino-1-naphthalenesulfonic acid ammonium salt (ANS) to study the molecular mechanism of the protein-DNA association through exited state relaxation of the probe in absence (dictated by polarity) and presence of EB in the DNA (dictated by Förster resonance energy transfer (FRET)). The overall and local structures of the protein in presence of PEG have been followed by circular dichroism and time resolved polarization gated spectroscopy respectively. The enhanced dynamical flexibility of protein in presence of PEG as revealed from excited state lifetime and polarization gated anisotropy of ANS has been correlated with the stronger DNA-binding for the higher nuclease activity. We have also used conventional experimental strategy of agarose gel electrophoresis to monitor DNA-cleavage and found consistent results of enhanced nuclease activities both on synthetic 20-mer oligonucleotide and long genomic DNA from calf thymus.

Polarization-dependent plasmon mode mapping of Ag nanowires based on two-photon excitation fluorescence of quantum dots

We show that the plasmon modes of Ag nanowires can be imaged by coating them with a layer of quantum dots (QDs), held off the nanowire surface by a nanoscale dielectric spacer layer. Parallel or perpendicular excitation polarization modulates the intensity maps of two-photon excitation fluorescence (TPEF), which exhibit Fabry–Pérot cavity modes at the excitation or fluorescence wavelength. We attribute this phenomenon to the QDs excited by propagating surface plasmon polaritons or localized surface plasmon modes. The results of the TPEF intensity maps are well explained by theoretical simulations, and the energy transfer process is also discussed.

Band edge photoluminescence of undoped and doped TlInS2 layered crystals

Intrinsic photoluminescence (PL) of undoped and of B-, Ag- or Er-doped TlInS2 layered single crystals was investigated by confocal spectroscopy. It is found that position and intensity of PL spectral peak at 2.4eV is vastly dependent on the excitation light incidence and polarization relative to crystallographic directions. For the normal incidence to the layer plane k||c, a significant Stokes shift between the PL peak in respect to exciton absorption energy is highlighted. The shift increases to 80meV around the crystal phase transformations region of 200K. We show that presence of B and Ag impurity do not diminish the intrinsic PL emission while Er atoms incorporation enhances it substantially and modifies a fine excitonic line structure in the ferroelectric phase at low T. The obtained results imply out-of-layer plain oriented 2D excitons in TlInS2 existing at various crystal phases which are influenced by related interlayer stresses.

Polarization-controlled optimal scatter suppression in transient absorption spectroscopy

Ultrafast transient absorption spectroscopy is a powerful technique to study fast photo-induced processes, such as electron, proton and energy transfer, isomerization and molecular dynamics, in a diverse range of samples, including solid state materials and proteins. Many such experiments suffer from signal distortion by scattered excitation light, in particular close to the excitation (pump) frequency. Scattered light can be effectively suppressed by a polarizer oriented perpendicular to the excitation polarization and positioned behind the sample in the optical path of the probe beam. However, this introduces anisotropic polarization contributions into the recorded signal. We present an approach based on setting specific polarizations of the pump and probe pulses, combined with a polarizer behind the sample. Together, this controls the signal-to-scatter ratio (SSR), while maintaining isotropic signal. We present SSR for the full range of polarizations and analytically derive the optimal configuration at angles of 40.5° between probe and pump and of 66.9° between polarizer and pump polarizations. This improves SSR by (or compared to polarizer parallel to probe). The calculations are validated by transient absorption experiments on the common fluorescent dye Rhodamine B. This approach provides a simple method to considerably improve the SSR in transient absorption spectroscopy.

Optimizing Excitation Polarization to Probe Fusion Pore Dynamics using TIRF Microscopy

During hormone or neurotransmitter release via exocytosis, fusion pores may open and close (flicker) repeatedly before resealing (“kiss & run” fusion) or dilating (full fusion) irreversibly. Pore dynamics regulate the amount and kinetics of cargo release, and determine the mode of recycling, but mechanisms that govern pore dynamics are not understood. This is in large part due to a lack of reconstituted assays with single-pore sensitivity and millisecond time resolution. We recently described a polarized total internal reflection (pTIRF) microscopy assay to monitor fusion of proteoliposomes to planar lipid bilayers supported on a soft polymer cushion with single molecule sensitivity and ∼15 ms temporal resolution. Fluorescently labeled small unilamellar vesicles, reconstituted with exocytotic/neuronal v-SNAREs (v-SUVs), fuse with a supported bilayer containing the cognate t-SNAREs (t-SBL). Each fusion event is accompanied by changes in the total fluorescence intensity surrounding the fusion site, as the lipid-linked labels diffuse from the liposome into the supported bilayer through the fusion pore. Analysis of the intensity changes, combined with a mathematical model, provides information on pore dynamics (Stratton et al. Biophys. J. 2016). In principle, three factors can contribute to intensity changes upon fusion: 1) dequenching of fluorophores, 2) evanescent field decay and 3) a change in the average orientation of the fluorophore dipole moments with respect to the excitation polarization field, as the fluorophores move from the round liposome into the flat bilayer. Here, we systematically varied the polarization of the excitation field and quantified its contribution to intensity changes for different lipid-linked fluorophores. Large increases facilitate detection of fusion events and quantification of lipid release kinetics.

Probing the Absorption and Emission Transition Dipole Moment of DNA Stabilized Silver Nanoclusters.

Using single molecule polarization measurements, we investigate the excitation and emission polarization characteristics of DNA stabilized silver nanoclusters (C24-AgNCs). Although small changes in the polarization generally accompany changes to the emission spectrum, the emission and excitation transition dipoles tend to be steady over time and aligned in a similar direction, when immobilized in PVA. The emission transition dipole patterns, observed for C24-AgNCs in defocused wide field imaging, match that of a single emitter. The small changes to the polarization and spectral shifting that were observed could be due to changes to the conformation of the AgNC or the DNA scaffold. Although less likely, an alternative explanation could be that several well aligned spectrally similar emitters are present within the DNA scaffold which, due to Förster resonance energy transfer (FRET) processes such as energy hopping, energy transfer, and singlet-singlet annihilation, behave as a single emitter. The reported results can provide more insight in the structural and photophysical properties of DNA-stabilized AgNCs.