Phase Retardation Effects Research Articles

Tailoring femtosecond LSP resonance and lifetime in a nanoresonator via phase retardation.

The manipulation of femtosecond plasmon resonance and lifetime in a nanoantenna is crucial for the realization of integrated and miniaturized plasmonic circuits. Here, we have used FDTD simulations to study the plasmonic resonance and lifetime variation of the far-end (PA) and near-end (PB) hotspots of size-gradient nanoresonators. We found that the near-field spectrum of PA is red-shifted compared to PB due to the phase retardation effect. By capturing the ultrafast dynamics of both hotspots, we confirm that these phenomena are governed by the transient evolution of the plasmonic field in the forced oscillation regime. Furthermore, the lifetimes of plasmonic hotspots scale directly with their near-field intensities. Meanwhile, the lifetime τA is always larger than that of τB in the same nanoresonator because of the smaller non-radiative damping of hotspot PA. Our results provide a basis for the miniaturization of plasmonic nanoresonators.

A confined self-assembly approach towards chiral photonic materials with circularly polarized structural colors for information storage and encryption

Chiroptical materials with tunable circular polarization performances have gained wide interest because of promising applications in optical devices and information communication. However, limited by their intrinsic chirality, such materials suffer from serious deficiencies in dynamical manipulation on circularly polarized light (CPL). Here, we demonstrate a kinetic trapping approach to capture multi-domain cholesteric mesophases for polarization-dependent structural coloration. The resultant photonic structures depicted by limited solvent evaporation display light control from selective left-handed circularly polarized light reflection to dual reflection, as a result of forming highly tilted and distorted domains with a phase-retardation effect. Notably, a positive linear correlation is established between the circular polarization behavior and structural orientation regulated by the solvent evaporation. The tunable circular polarization structural colors enable the application of the photonic materials for complex encryption in terms of polychromatic coding arrays. The confined-anisotropic-fabrication method of photonic structures paves a foundation toward advanced functional materials with promising applications in light science and technology.

The anapole state excited by an oblique incidence

Anapole states supported by high-refractive-index dielectric nanoparticles have mostly been studied under normal incidence, but this work explores the oblique incidence excitation. For a single silicon nanodisk, as the incident angle (θ) increases, the anapole wavelength undergoes a gradual blueshift, while the wavelength of maximum near-field enhancement remains almost unchanged with increasing E-field enhancement factor (|E/E 0 |) due to phase retardation effect caused by oblique incidence, and some unique features in field distributions differed from normal excitation are exhibited. In the case of a silicon nanodisk array, the anapole state and near-field enhancement are affected by near-field coupling and the phase retardation effect is weakened. With increasing θ, the coupling between the units is enhanced, and the anapole wavelength and maximum field enhancement wavelength both blue shift. The field distributions in anapole wavelength and maximum enhancement wavelength have obvious near-field coupling characteristics. Oblique incident excitation gives us a deeper understanding of anapole state and may have potential applications in nanophotonics.

Evidence of the retardation effect on the plasmonic resonances of aluminum nanodisks in the symmetric/asymmetric environment.

A single metallic nanodisk is the simplest plasmonic nanostructure, but it is robust enough to generate a Fano resonance in the forward and backward scattering spectra by the increment of nanodisk height in the symmetric and asymmetric dielectric environment. Thanks to the phase retardation effect, the non-uniform distribution of electric field along the height of aluminum (Al) nanodisk generates the out-of-plane higher-order modes, which interfere with the dipolar mode and subsequently result in the Fano-lineshape scattering spectra. Meanwhile, the symmetry-breaking effect by the dielectric substrate and the increment of refractive index of the symmetric dielectric environment further accelerate the phase retardation effect and contribute to the appearance of out-of-plane modes. The experimental results on the periodic Al nanodisk arrays with different heights confirm the retardation-induced higher modes in the asymmetric and symmetric environment. The appearance of higher modes and blueshifted main dips in the transmission spectra prove the dominant role of out-of-plane higher modes on the plasmonic resonances of the taller Al nanodisk.

Observation and Manipulation of Visible Edge Plasmons in Bi2Te3 Nanoplates.

Noble metals, like Ag and Au, are the most intensively studied plasmonic materials in the visible range. Plasmons in semiconductors, however, are usually believed to be in the infrared wavelength region due to the intrinsic low carrier concentrations. Herein, we observe the edge plasmon modes of Bi2Te3, a narrow-band gap semiconductor, in the visible spectral range using photoemission electron microscopy (PEEM). The Bi2Te3 nanoplates excited by 400 nm femtosecond laser pulses exhibit strong photoemission intensities along the edges, which follow a cos4 dependence on the polarization state of incident beam. Because of the phase retardation effect, plasmonic response along different edges can be selectively exited. The thickness-dependent photoemission intensities exclude the spin-orbit induced surface states as the origin of these plasmonic modes. Instead, we propose that the interband transition-induced nonequilibrium carriers might play a key role. Our results not only experimentally demonstrate the possibility of visible plasmons in semiconducting materials but also open up a new avenue for exploring the optical properties of topological insulator materials using PEEM.

Theoretical Study of Local Surface Plasmon Resonances on a Dielectric-Ag Core-Shell Nanosphere Using the Discrete-Dipole Approximation Method**Supported by the Anhui Provincial Natural Science Foundation under Grant Nos 1308085QA19 and 1408085QA15, the Key Scientific Research Foundation of Anhui Provincial Education Department under Grant Nos KJ2013A180, KJ2015A223, KJ2015ZD28 and AQKJ2015B017, and the Young Foundation of Anqing Normal University under Grant Nos KJ201313

The local surface plasmon resonances (LSPRs) of dielectric-Ag core-shell nanospheres are studied by the discretedipole approximation method. The result shows that LSPRs are sensitive to the surrounding medium refractive index, which shows a clear red-shift with the increasing surrounding medium refractive index. A dielectric-Ag core-shell nanosphere exhibits a strong coupling between the core and shell plasmon resonance modes. LSPRs depend on the shell thickness and the composition of dielectric-core and metal-shell. LSPRs can be tuned over a longer wavelength range by changing the ratio of core to shell value. The lower energy mode ω− shows a red-shift with the increasing dielectric-core value and the inner core radius, while blue-shifted with the increasing outer shell thickness. The underlying mechanisms are analyzed with the plasmon hybridization theory and the phase retardation effect.

The Study of Tunable Local Surface Plasmon Resonances on Au-Ag and Ag-Au Core-Shell Alloy Nanostructure Particles With DDA Method

The local surface plasmon resonances (LSPR) of bimetallic Au-Ag core-shell nanostructure particles are studied using discrete-dipole approximation (DDA) method and plasmon hybridization theory. It is found that LSPR is sensitive to the surrounding medium refractive index, showing a distinct redshift with increasing the surrounding medium refractive index. Au-Ag core-shell nanostructure exhibits a strong coupling between the core and shell plasmon resonance modes. The coupled resonance mode wavelengths show dependence on the layer thickness and the composition of core and shell metal. LSPR can be tuned over an extended wavelength range by adjusting the ratio of core to shell. The lower energy mode ω − of Au-Ag core-shell nanoparticle shows a redshift with increasing Ag shell thickness or Au core radius, while the higher energy mode ω + shows the opposite behaviors. In addition, Ag-Au core-shell nanostructure compound particles are also studied with the same method, whose properties are different from that of Au-Ag core-shells. For the sake of clarity, the wavelength shifts of LSPR are plotted as functions of surrounding media refractive index, core radius, shell thickness, and core-shell ratio with figures of merits (FOM). The underlying mechanisms are analyzed with the plasmon hybridization theory and phase retardation effect.

Macroscopic Layers of Chiral Plasmonic Nanoparticle Oligomers from Colloidal Lithography

Optical near-field coupling between closely spaced plasmonic metal nanoparticles is important to a range of nanophotonic applications of high contemporary interest, including surface-enhanced molecular spectroscopy, nanooptical sensing, and various novel light-harvesting concepts. Here we report on monolayers of chiral heterotrimers and heterotetramers composed of closely spaced silver and/or gold nanodisks of different heights fabricated through facile hole-mask colloidal lithography. These quasi-three-dimensional oligomers are interesting for applications because they exhibit hot gaps and crevices of nanometric dimensions, a pronounced circular dichroism, and optical chirality in the visible to near-infrared wavelength range, and they can be produced in large ensembles (>109) of identical orientation. We analyze the optical properties of the samples based on simulation results and find that the circular dichroism is due to strong near-field coupling and intricate phase retardation effects originating in the three-dimensional character of the individual oligomers.

A full-color reflective display using polymer-stabilized blue phase liquid crystal

We demonstrate a full-color-capability reflective display using red, green, and blue sub-pixels of polymer-stabilized blue phase liquid crystal (BPLC) with different pitch lengths. Vivid colors originate from three-dimensional BPLC photonic crystalline structure. Surface alignment plays a key role to generate uniform and saturated colors in the voltage-off state. Analogous grayscale is achieved by the electric-field-induced unwinding of double-twist structure. This working principle is drastically different from the phase retardation effect of conventional liquid crystal displays. Moreover, the submillisecond response time enables crisp video displays without image blurring. Potential applications for reflective 3D display are also analyzed.

Tunable properties of localized surface plasmon resonance wavelength of gold nanoshell

The effects of shell thickness, inner core size, and dielectric constants of core and embedding medium on the localized surface plasmon resonance wavelength of gold nanoshell are investigated by Mie theory. The results show that the extinction peak of gold nanoshell is first blue-shifted and then red-shifted with the increase of shell thickness, that the shift shell thickness corresponding to quadrupolar wavelength is greater than that corresponding to dipolar wavelength, and that the ratio of the shift shell thickness to the inner core size decreases with the increase of inner core size, and increases with the increase of dielectric constant of inner core or embedding medium. The resonance peaks in the scattering spectra have similar phenomena to those in extinction spectra as the shell thickness increases. The shift of the spectral peak is ascribed to the plasmon hybridization and phase retardation effect.

Near- and Far-Field Effects on the Plasmon Coupling in Gold Nanoparticle Arrays

Understanding the plasmon coupling between metal nanoparticles under light irradiation remains a challenging issue for optimizing plasmonic devices for chemical and biological sensing. Here, the optical properties of dense spatially ordered two-dimensional arrays of 50 nm gold nanoparticles are investigated in this aim. Microspectrophotometry experiments are carried out on square arrays and parallel chains, elaborated by electron beam lithography, having different periodicities ranging from 80 to 170 nm. The wavelength, width, and amplitude of the localized surface plasmon resonance (SPR) are quantitatively monitored as a function of both the incident light polarization and the interparticle distance. The experimental findings are then compared with calculations based on the discrete dipole approximation. They match remarkably well these numerical results, not only for the spectral location and the width of the SPR but also for the absolute value of the extinction cross section of the nanoparticles that is linked with the local field enhancement. The variation of the SPR band characteristics as a function of the periodicity of the arrays is investigated in terms of near-field and far-field coupling effects by discussing the topography of the field amplitude and phase in the arrays. Moreover, in order to highlight the influence of phase retardation and radiative effects on the plasmon coupling, the optical properties of equivalent arrays scaled by 1/10, which can be qualitatively described by electrostatic dipolar interactions, are also studied by numerical calculations. The discrepancies then observed between the two scales are interpreted. The contributions of the radiative and nonradiative dampings to the width and magnitude of the plasmon resonance are also investigated.

Two-photon absorption spectrum of silver nanoparticles

We have investigated the spectral dispersion of real and imaginary parts of third-order nonlinear optical susceptibility of silver nanospherical particles in the aqueous solution using Z-scan and antiresonant ring interferometric nonlinear spectroscopic (ARINS) techniques by femtosecond pulses from the modelocked Ti:Sapphire laser over the spectral range of 720–820nm. The observed characteristic of imaginary part shows a peak at ∼766nm which coincides with the peak of spectral dispersion of real part induced thermally. The observed peak in the dispersion of nonlinear absorption coefficient at ∼766nm is attributed to the two-photon absorption. This two-photon resonance peak is attributed to l=2 plasmon (quadrupolar mode) arising from the phase retardation effects in the spherical nanoparticles present in our polydispersed sample with their diameters (80±20nm) beyond the quasi-static limit.

Systematic study on visible light collimation by nanostructured slits in the metal surface**Project supported by the National Natural Science Foundation of China (Grant Nos. 60736041 and 10874238), and the NationalKey Basic Research Special Foundation of China (Grant No. 2007CB613205).

We present a systematic experimental investigation on visible light collimation by a nanostructured slit flanked with a pair of periodic array of grooves in gold thin film. A wide variety of aspects are considered, such as the polarization state, the transport path of incident light, the groove—groove spacing, the groove width and depth. Our results clearly show that the relationship between the collimation wavelength and the periodicity of the slit-groove structure accords well with the surface plasmon dispersion model proposed by previous researchers. Furthermore, the surface plasmon wave phase retardation effect induced by the surface structure is also verified via the measurement for samples with different groove widths and depths. These results indicate that the detailed geometry of the groove structure has obvious impacts on the collimation effect and the angular distribution of the diffraction light in the subwavelength plasmonic system.

Enhanced second-harmonic generation from individual metallic nanoapertures

We demonstrate the ability of single-subwavelength-size nanoapertures fabricated in a gold metal thin film to enhance second-harmonic generation (SHG) as compared to a bare metal film. Nonlinear microscopy imaging with polarization resolution is used to quantify the SHG enhancement in circular and triangular nanoaperture shapes. The dependence of the measured SHG enhancement on circular aperture diameters is seen to originate from both phase retardation effects and field enhancements at the nanoaperture edge. Triangular nanoapertures exhibit superior SHG enhancement compared with circular ones, as expected from their noncentrosymmetric shape.

Dispersion of metal-insulator-metal plasmon polaritons probed by cathodoluminescence imaging spectroscopy

Cathodoluminescence imaging spectroscopy is used to excite and characterize the resonant modes of Fabry-Perot resonators for surface plasmon polaritons confined in a metal-insulator-metal (MIM) geometry. The smallest MIM plasmon wavelength derived from the observed mode pattern is found to be 160 nm in cavities with a 10 nm ${\text{SiO}}_{2}$ layer for a free-space wavelength of 645 nm. The measured wavelength agrees well with values from analytical dispersion relation calculations. Calculations of the excitation probability show that the resonant excitation of MIM plasmons depends strongly on the electron energy due to phase retardation effects resulting from the finite electron velocity.

Effect of Phase Structure on Enzymatic Degradation in Poly(l-lactide)/Atactic Poly(3-hydroxybutyrate) Blends with Different Miscibility

Thin films of poly(L-lactide) (PLLA)/atactic poly(3-hydroxybutyrate) (ataPHB) blends with different miscibility were prepared and characterized by using differential scanning calorimetry (DSC) and atomic force microscopy (AFM). The DSC analysis suggested that the blend thin films exhibited different phase structures, such as miscible, partially miscible, and immiscible depending on the blending ratio as well as molecular weight of ataPHB component. The different miscibility was further confirmed by the surface morphological observation by AFM. Both the immiscible and partially miscible blends of PLLA/ataPHB revealed the formation of phase-separated morphology of PLLA and ataPHB components, whereas the homogeneous surface morphology was observed for the miscible blend. On the basis of the changes in the depth profile from the surface level of the thin films, the enzymatic degradation rates of the PLLA and ataPHB domains were determined in the presence of either PHB depolymerase or proteinase K, respectively. The erosion rate of PLLA/ataPHB blends was strongly dependent on the blend composition and the degree of dispersion of the two components. The enzymatic degradation behaviors were discussed in terms of phase structure, molecular mobility, and retardation effect of the components in the blends.

Analysis of Light Modulation in the Fringe-Field Switching Liquid Crystal Mode

The light modulation mechanism in conventional liquid crystal displays (LCDs) presents either phase retardation or polarization rotation effects only. However, in the fringe-field switching (FFS) mode, both effects exist depending on the electrode positions when the electrode width and distance is larger than 2 μm. Interestingly, when the electrode width and distance are reduced to less than 2 μm, only the polarization rotation effect exists. Further, reduction of the electrode width and distance gives rise to improved response time and light efficiency in the FFS mode.

Observation of red-shifted strong surface plasmon scattering in single Cu nanowires

Surface plasmon scattering spectra of chemically produced single Cu nanowires were obtained using a total internal reflection microscope. In particular, we have observed a strong surface plasmon peak in the far red and a red-shift of the surface plasmon resonance with increasing nanowire diameter. We believe that the most reasonable origin for the red-shift of comparably large diameter nanowires is the phase retardation effect.

Polariscope for simultaneous measurement of the principal axis and the phase retardation by use of two phase-locked extractions

A novel polariscope with electro-optic modulation that is capable of simultaneous measurement of the principal axis and the phase retardation of an optical linear birefringent medium by means of two phase-locked extractions is described. A phase compensator is used to suppress the transmission phase-retardation effect of the beam splitter, thereby enhancing the precision of the measuring performance. The validity of the proposed design is demonstrated by measurement of the principal axis and phase retardation of a quarter-wave plate sample. There are absolute errors of 0.25 degrees on average and 0.58 degrees at maximum in the principal-axis measurement and of 0.75 degrees (0.83%) on average and 3.11 degrees at maximum in the phase-retardation measurement. Meanwhile, the retardation error lies within a 5% uncertainty range of a commercial wave plate. The root-mean-square resolutions for the principal-axis angle and phase-retardation measurements are 0.042 degrees and 0.081 degrees, respectively. Finally, the dynamic ranges of the principal-axis angle measurement and the phase-retardation measurement extend as far as 180 degrees.

Plasmonic Properties of Concentric Nanoshells

The plasmonic properties of a concentric nanoshell, or “nanomatryushka”, are investigated. The plasmon resonant modes are analyzed in terms of the plasmon hybridization model, where new resonances occur due to a hybridization of the plasmon modes of the inner metallic shell with those of the outer metallic shell. The plasmon resonant spectra of experimentally realized concentric nanoshells are analyzed in terms of dipolar and higher order multipolar hybridized plasmon modes. A size-dependent asymmetry in the splitting of the hybridized plasmon states is observed, which is related to phase retardation effects due to the nanostructure's finite size.