Asymmetric Dielectric Environment Research Articles

Tuning Multipolar Mie Scattering of Particles on a Dielectric-Covered Mirror.

Optically resonant particles are key building blocks of many nanophotonic devices such as optical antennas and metasurfaces. Because the functionalities of such devices are largely determined by the optical properties of individual resonators, extending the attainable responses from a given particle is highly desirable. Practically, this is usually achieved by introducing an asymmetric dielectric environment. However, commonly used simple substrates have limited influences on the optical properties of the particles atop. Here, we show that the multipolar scattering of silicon microspheres can be effectively modified by placing the particles on a dielectric-covered mirror, which tunes the coupling between the Mie resonances of microspheres and the standing waves and waveguide modes in the dielectric spacer. This tunability allows selective excitation, enhancement, suppression, and even elimination of the multipolar resonances and enables scattering at extended wavelengths, providing transformative opportunities in controlling light-matter interactions for various applications. We further demonstrate with experiments the detection of molecular fingerprints by single-particle mid-infrared spectroscopy and with simulations strong optical repulsive forces that could elevate the particles from a substrate.

Laser-Induced Chirality of Plasmonic Nanoparticles Embedded in Porous Matrix.

Chiral plasmonic nanostructures have emerged as promising objects for numerous applications in nanophotonics, optoelectronics, biosensing, chemistry, and pharmacy. Here, we propose a novel method to induce strong chirality in achiral ensembles of gold nanoparticles via irradiation with circularly-polarized light of a picosecond Nd:YAG laser. Embedding of gold nanoparticles into a nanoporous silicate matrix leads to the formation of a racemic mixture of metal nanoparticles of different chirality that is enhanced by highly asymmetric dielectric environment of the nanoporous matrix. Then, illumination with intense circularly-polarized light selectively modifies the particles with the chirality defined by the handedness of the laser light, while their "enantiomers" survive the laser action almost unaffected. This novel modification of the spectral hole burning technique leads to the formation of an ensemble of plasmonic metal nanoparticles that demonstrates circular dichroism up to 100 mdeg. An unforeseen peculiarity of the chiral nanostructures obtained in this way is that 2D and 3D nanostructures contribute almost equally to the observed circular dichroism signals. Thus, the circular dichroism is neither even nor odd under reversal of direction of light propagation. These findings will help guide the development of a passive optical modulator and nanoplatform for enhanced chiral sensing and catalysis.

Flexible engineering of circular dichroism enabled by chiral surface lattice resonances

Engineering the chiroptical responses of artificial nanostructures is vital for realizing applications in the fields of optical devices, enantioselective separation, and bio-sensing. Here, by utilizing the nano-kirigami based meta-molecule arrays, the flexible engineering of circular dichroism (CD) is achieved in the near-infrared wavelength region by the excitation of chiral surface lattice resonances (SLRs). It is found that the chiral SLRs can be flexibly tailored by a tiny structural perturbation. As a result, the wavelength, intensity, and sign of CD peak/dip can be abruptly engineered. Specifically, a CD peak with the value of +0.44 is evolved into a CD dip with an intensity of −0.66 when the etching silt length of the meta-molecules is simply decreased by 190 nm. Importantly, such CD reversal is experimentally demonstrated with the nano-kirigami method without requiring the inversion of geometric chirality. Moreover, it is found that the asymmetric dielectric environment around the meta-molecules can significantly suppress the chiral SLRs, providing an insightful understanding of the chiral SLR. Such flexible tailoring of the CD with chiral SLRs paves a versatile way toward the manipulation of chiral light–matter interactions and chiroptical functional devices.

Theoretical study of extremely narrow plasmonic surface lattice resonances observed by MIM nanogratings under normal incidence in asymmetric environments

Nanoarray structures can support plasmonic surface lattice resonances (SLRs) with extremely narrow linewidths and huge electric field enhancement features, which are attractive applications in nanolasers, biochemical sensors, and nonlinear optics. However, current nanoarray structures located in an asymmetric dielectric environment with a refractive index contrast of 1.00/1.52 of the superstrate/substrate excite much poorer SLRs under normal incidence, which largely limits their application range. In this work, we report extremely narrow SLRs supported by one-dimensional metal–insulator–metal nanograting in asymmetric dielectric environments. The simulation results show that an SLRs with linewidth of 3.26 nm and quality factor of 233.2 can be excited under normal incidence. This high-quality SLRs is attributed to the interference formation between the out-of-plane dipole resonance mode and the out-of-plane quadrupole resonance mode. We also show that the resonance wavelength and quality factor can be tuned by changing the structure geometry and period, and we calculate the normal incidence SLRs quality factor to be up to 248 in 1.33/1.52 and 250 in 1.45/1.52. We expect the SLRs of this work to find potential applications in asymmetric dielectric environments.

Quality factor enhancement of plasmonic surface lattice resonance by using asymmetric periods

We report that using asymmetric lattice periods can enhance the quality factor of plasmonic surface lattice resonances (SLRs) in two-dimensional array of metal-insulator-metalnanopillars in asymmetric dielectric environment. Simulation results show that by adopting appropriate asymmetric lattice periods, the SLR quality factor can be enhanced by 24\\% compared with the scenario of symmetric periods. We find that the SLR quality factor is optimized when the resonance wavelength is closest to the Rayleigh cutoff wavelength. We also find that the SLRs effect is polarization sensitive in the proposed structure. We expect this work will advance the engineering of SLRs especially in asymmetric dielectric environments, and will promote their applications in sensing.

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.

Fabrication and ultraviolet-visible-near infrared absorption properties of silver nano arrays based on aluminum

In this paper, the highly ordered periodic silver nanosphere arrays are fabricated by vacuum evaporation based on anodic aluminum oxide (AAO) template. The diameter and spacing of silver nanosphere in the arrays are adjusted just by controlling the thickness of evaporation. Furthermore, this can effectively modulate the absorption peaks and bandwidths in ultraviolet-visible-near-infrared regions. The measurement results of absorption spectra show that the nano-arrays have obvious electromagnetic wave absorption characteristics in the ultraviolet, visible and near-infrared bands. The finite-difference time-domain theoretical simulation combined with experiments is used to analyze the physical mechanism of light absorption characteristics in different wavebands. The ultraviolet strong absorption is due to the Fano resonance induced by asymmetric dielectric environment of silver and aluminum; the visible absorption originates from local surface plasmon resonance of silver nanoparticles; the near-infrared strong absorption is attributed to the surface lattice resonance of silver nanosphere arrays.

High-performance asymmetric optical transmission based on coupled complementary subwavelength gratings

Asymmetric transmission (AT) devices are fundamental elements for optical computing and information processing. We here propose an AT device consisting of a pair of coupled complementary subwavelength gratings. Different from previous works, asymmetric dielectric environment is employed for unidirectional excitation of surface plasmon polaritons (SPPs) and thus asymmetric optical transmission, and near-field coupling effect inherent in the coupled complementary structure is exploited to enhance forward transmission and AT behavior, and determine operation bandwidth as well. The influence of asymmetric dielectric environment, effect of vertical and lateral couplings, interactions of electric- and magnetic-dipole moments and the realization of Kerker conditions, are investigated in depth to unearth the AT mechanism and performance. High-performance AT with large forward transmittance of 0.96 and broad bandwidth of 174 nm is achieved at wavelength 1250 nm. Our work helps people to gain a better understanding of near-filed coupling effect in coupled complementary structures, expand their application fields, and it also offers an alternate way to high-performance AT devices.

Measurement of lateral and axial resolution of confocal Raman microscope using dispersed carbon nanotubes and suspended graphene

A confocal Raman microscope (CRM) facilitates visualization of the spatial distribution of molecular bonds or phonon modes at the submicron level and has been extensively used in the characterization of nanomaterials and devices. The lateral and axial resolution is a key specification that defines the performance of CRM, however, the interpretation of spatial resolution in the literature is often ambiguous, making it often difficult to directly compare Raman images obtained under different conditions. In this report, a convenient and reliable measurement protocol using dispersed carbon nanotubes and suspended graphene as test specimens is proposed to facilitate the determination of the lateral and axial resolutions of a CRM. Spatial resolution values comparable to the results based on Rayleigh criterion calculations were obtained using Raman mapping images of test specimens. This was achieved without the need for complex deconvolution processes or the consideration of an asymmetric dielectric environment.

Narrow plasmonic surface lattice resonances with preference to asymmetric dielectric environment.

Plasmonic surface lattice resonances (SLRs) supported by metal nanoparticle arrays exhibit narrow linewidths and enhanced localized fields and thus are attractive in diverse applications including nanolasers, biochemical sensors and nonlinear optics. However, it has been shown that these SLRs have much worse performance in a less symmetric environment, hindering their practical applications. Here, we propose a novel type of narrow SLRs that is supported by metal-insulator-metal nanopillar arrays and that has better performance in a less symmetric dielectric environment. When the dielectric environment is as asymmetric as the air/polymer environment with a refractive index contrast of 1.0/1.52, the proposed SLRs have high quality factors of 62 under normalincidence and of 147 under oblique incidence in the visible regime. We attribute these new SLRs to Fano resonance between an in-plane dipole and an out-of-plane quadrupole (or dipole) that are respectively supported by the two metal ridges under normal (or oblique) incidence. We also show that the resonance wavelength can be tuned by varying the geometric sizes or by changing the angle of incidence. By doing this, we clarify the conditions for the formation of the proposed SLRs and illustrate their attractive merits in sensing applications. We expect that this new SLR can open up applications in asymmetric dielectric environments.

Periodic Arrays of Dewetted Silver Nanostructures on Sapphire and Quartz: Effect of Substrate Truncation on the Localized Surface Plasmon Resonance and Near-Field Enhancement

Substrate-supported plasmonic nanostructures are distinct from their colloidal counterparts, in that they are surrounded by an asymmetric dielectric environment composed of the substrate material a...

High Q Plasmonic Lasing of Band Edge Modes in an Asymmetry Environment

We theoretically propose a novel plasmonic laser based on the band edge modes of one-dimensional plasmonic crystal in an asymmetric dielectric environment. We find that the quality factors of the band edge modes are high and comparable to that of the collective resonances of surface plasmon in symmetric background. Moreover, surface plasmon lasing of single mode is numerically demonstrated with a full-wave Maxwell-Bloch approach. And it is also shown simultaneously lasing of both radiative and non-radiative modes due to spatial hole burning, enabling the realization of coherent source of both light and surface plasmon in a single platform. Finally, we discuss the dependence of the sensitivity of the plasmonic laser mode on the geometrical and material parameters. Our model provides an easy-to-use approach for future silicon-based on-chip applications such as low-threshold plasmonic laser and solid-state lighting emission.

Asymmetric plasmonic grating for optical sensing of thin layers of organic materials

In this paper the performance of an optical organic sensor based on a plasmonic grating immersed in an asymmetric dielectric environment is investigated. Metal gratings offer the possibility to tailor the spectral response, in dependence of the geometrical parameter settings and variation of the refractive index of the surrounding media. It is then possible to obtain a reflection spectrum displaying a sharp dip that can be efficiently exploited to improve the detection of small quantities of organic materials. In particular, using a finite difference time domain (FDTD) based method, we demonstrate that the designed plasmonic grating, made of gold strips placed on a silicon substrate and covered by a generic organic material (analyte), exhibits an improved sensitivity for the detection of the overlayer thickness (analyte quantity). Nevertheless, this plasmonic sensor can also be used to determine the typology of the generic organic layer in dependence of the refractive index change.

Channel and wedge plasmon modes of metallic V-grooves with finite metal thickness

We investigate numerically the effect of a finite metal film thickness on the propagation characteristics of the channel Plasmon polariton (CPP) and wedge plasmon polariton (WPP) modes, both in a symmetric and asymmetric environment. We observe that decreasing the metal thickness results in an improvement of the field localization near the groove tip and an increase of the losses for both types of mode. This behavior stems from the typical symmetric charge distribution of both modes across the metal film. When considering an asymmetric dielectric environment, the CPP mode is found to evolve into short range Plasmon modes propagating along the groove walls, in contrast to the WPP mode which remains essentially confined at the tip apex. These results can be useful to tailor the properties of such plasmon modes, using the metal thickness as the variable parameter.

Light absorption by polaritons in an interface layer with strong excitonic response: effects of retardation and total reflection

Light absorption spectra by interface layer with strong excitonic response in asymmetric dielectric environment are calculated for various angles of incidence of the external light. Appreciable frequency shift of the absorption resonances in the total reflection regime is found. It is shown that higher polarition modes may reveal themselves in absorption spectra of high-ordered excitonic layers with strong exciton-photon interaction. In this case, doublet and triplet structures of absorption spectra emerge for s- and p-polarized light, respectively. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The two substrate-water molecules are already bound to the oxygen-evolving complex in the S2 state of photosystem II.

The first direct evidence which shows that both substrate-water molecules are bound to the O(2)-evolving catalytic site in the S(2) state of photosystem II (PSII) is presented. Rapid (18)O isotope exchange measurements between H(2)(18)O incubated in the S(2) state of PSII-enriched membrane samples and the photogenerated O(2) reveal a fast and a slow phase of exchange at m/e 34 (which measures the level of the (16)O(18)O product). The rate constant for the slow phase of exchange ((34)k(1)) equals 1.9 +/- 0.3 s(-1) at 10 degrees C, while the fast phase of exchange is unresolved by our current experimental setup ((34)k(2) >or= 175 s(-1)). The unresolvable fast phase has left open the possibility that the second substrate-water molecule binds to the catalytic site only after the formation of the S(3) state [Hillier, W., and Wydrzynski, T. (2000) Biochemistry 39, 4399-4405]. However, for PSII samples depleted of the 17 and 23 kDa extrinsic proteins (Ex-depleted PSII), two completely resolvable phases of (18)O exchange are observed in the S(2) state of the residual activity, with the following rate constants: (34)k(1) = 2.6 +/- 0.3 s(-1) and (34)k(2) = 120 +/- 14 s(-1) at 10 degrees C. Upon addition of 15 mM CaCl(2) to Ex-depleted PSII, the O(2) evolution activity increases to approximately 80% of the control level, while the two resolvable phases of exchange remain the same. In measurements of Ex-depleted PSII at m/e 36 (which measures the level of the (18)O(18)O product), only a single phase of exchange is observed in the S(2) state, with a rate constant ((36)k(1) = 2.5 +/- 0.2 s(-1)) that is identical to the slow rate of exchange in the m/e 34 data. Taken together, these results show that the fast phase of (18)O exchange is specifically slowed by the removal of the 17 and 23 kDa extrinsic proteins and that the two substrate-water molecules must be bound to independent sites already in the S(2) state. In contrast, the (18)O exchange behavior in the S(1) state of Ex-depleted PSII is no different from what is observed for the control, with or without the addition of CaCl(2). Since the fast phase of exchange in the S(1) state is unresolved (i.e., (34)k(2) > 100 s(-1)), the possibility remains that the second substrate-water molecule binds to the catalytic site only after the formation of the S(2) state. The role of the 17 and 23 kDa extrinsic proteins in establishing an asymmetric dielectric environment around the substrate binding sites is discussed.