Morphology-dependent Resonances Research Articles

Temperature compensation of dye doped polymeric microscale lasers

ABSTRACTIn this article, a new technique for building temperature compensated microscale lasers that are based on the morphology dependent resonances phenomenon is presented. The dome shaped resonator is made with a mixture of NBA 107 UV curable polymer and rhodamine 6 g dissolved in ethanol solution. Since polymers usually exhibit linear thermal expansion and thermo‐optic coefficients that are opposite in sign but similar in order of magnitude, it is possible to compensate for temperature over the dome shaped resonator by tuning the dye concentration and the ratio between the polymer and dye solution. Experiments with dye concentration between 10−2 to 10−3 M and polymer and dye solution ratios between 1:1 and 4:1 are conducted. The sensitivity for all cases is presented, and the quality factor Q is investigated. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017, 55, 789–792

Temperature measurements of micro-droplets using pulsed 2-color laser-induced fluorescence with MDR-enhanced energy transfer

In this work, a new measurement system is presented for studying temperature of micro-droplets by pulsed 2-color laser-induced fluorescence. Pulsed fluorescence excitation allows motion blur suppression and thus simultaneous measurements of droplet size, velocity and temperature. However, high excitation intensities of pulsed lasers lead to morphology-dependent resonances inside micro-droplets, which are accompanied by disruptive stimulated emission. Investigations showed that stimulated emission can be avoided by enhanced energy transfer via an additional dye. The suitability and accuracy of the new pulsed method are verified on the basis of a spectroscopic analysis and comparison to continuously excited 2-color laser-induced fluorescence.

Enhanced Raman Scattering with Dielectrics.

Dielectrics represent a new frontier for surface-enhanced Raman scattering. They can serve as either a complement or an alternative to conventional, metal-based SERS, offering key advantages in terms of low invasiveness, reproducibility, versatility, and recyclability. In comparison to metals, dielectric systems and, in particular, semiconductors are characterized by a much greater variety of parameters and properties that can be tailored to achieve enhanced Raman scattering or related effects. Light-trapping and subwavelength-focusing capabilities, morphology-dependent resonances, control of band gap and stoichiometry, size-dependent plasmons and excitons, and charge transfer from semiconductors to molecules and vice versa are a few examples of the manifold opportunities associated with the use of semiconductors as SERS-active materials. This review provides a broad analysis of SERS with dielectrics, encompassing different optical phenomena at the basis of the Raman scattering enhancement and introducing future challenges for light harvesting, vibrational spectroscopy, imaging, and sensing.

Pulsed Photothermal Heating of One-Dimensional Nanostructures

Pulsed lasers are used in tandem with one-dimensional nanostructures in a wide range of contemporary physical chemistry experiments, including four-dimensional transmission electron microscopy, scanning probe microscopy, and laser-assisted atom-probe tomography (APT). In this work, closed-form solutions for the pulsed photothermal heating of one-dimensional nanomaterials are compared with experimental time-of-flight APT ion spectra from both amorphous and crystalline silicon targets. Analytical results are given for targets with either a uniform cylindrical morphology or an arbitrary degree of conical tapering. Counterintuitively, increasing a conical specimen’s taper-angle is shown to lead to increases in the maximum temperature reached at the tip of the specimen. In particular, the heat source for tapered targets is affected by internal morphology-dependent cavity resonances that increase the maximum tip temperature relative to an untapered cylindrical structure. Experimental time-of-flight ion spectra ...

Optical forces exerted on a graphene-coated dielectric particle by a focused Gaussian beam

In this paper, we derive the analytical expression for the multipole expansion coefficients of scattering and interior fields of a graphene-coated dielectric particle under the illumination of an arbitrary optical beam. By using this arbitrary beam theory, we systematically investigate the optical forces exerted on the graphene-coated particle by a focused Gaussian beam. Via tuning the chemical potential of the graphene, the optical force spectra could be modulated accordingly at resonant excitation. The hybridized whispering gallery mode of the electromagnetic field inside the graphene-coated polystyrene particle is more intensively localized than the pure polystyrene particle, which leads to a weakened morphology-dependent resonance in the optical forces. These investigations could open new perspectives for dynamic engineering of optical manipulations in optical tweezers applications.

Fiber Optic Excitation of Silicon Microspheres in Amorphous and Crystalline Fluids

ABSTRACTThis study investigates the optical resonance spectra of free-standing monolithic single crystal silicon microspheres immersed in various amorphous fluids, such as air, water, ethylene glycol, and 4-Cyano-4’-pentylbiphenyl nematic liquid crystal. For the various amorphous fluids, morphology-dependent resonances with quality factors on the order of 105 are observed at 1428 nm. The mode spacing is always on the order of 0.23 nm. The immersion in various amorphous fluids affects the spectral response of the silicon microsphere and heralds this technique for use in novel optofluidics applications. Even though the nematic liquid crystal is a highly birefringent, scattering, and high-index optical medium, morphology-dependent resonances with quality factors on the order of 105 are observed at 1300 nm in the elastic scattering spectra of the silicon microsphere, realizing a liquid-crystal-on-silicon geometry. The relative refractive index and the size parameter of the silicon microsphere are the parameters that affect the resonance structure. The more 4-Cyano-4’-pentylbiphenyl interacting with the silicon microsphere, the lower the quality factor of the resonances is. The more 4-Cyano-4’-pentylbiphenyl is interacting with the silicon microsphere, the lower the mode spacing Δλ of the resonances is. The silicon microspheres wetted with nematic liquid crystal can be used for optically addressed liquid-crystal-on-silicon displays, light valve applications, or reconfigurable optical networks.

Plasmon-free SERS detection of environmental CO2 on TiO2 surfaces.

SiO2/TiO2 core/shell beads (T-rex) were designed, fabricated and tested for Raman detection of environmental CO2 under real-working conditions, as those encountered, for example, in solar-to-fuel conversion reactions. The exploitation of light trapping and morphology dependent resonances was crucial for extending the limit of detection of CO2 adsorbed on TiO2 surfaces. T-rex beads allowed for achieving surface enhanced Raman scattering (SERS) without using plasmonic metals showing high-efficiency, fast response and reproducibility in CO2 detection in both air and solvents. The dependence of SERS activity on Mie-type resonances was investigated through a systematic comparison of experimental data and numerical simulations, demonstrating that T-rex beads can be tailored for the detection of gaseous environmental pollutants on the basis of simple, Mie-scattering based calculations. Three-dimensional T-rex colloidal crystals were also successfully tested in precise, in situ, real time detection of CO2 as a function of different temperature-sweep cycles.

Colloidal synthesis of Cu-ZnO and Cu@CuNi-ZnO hybrid nanocrystals with controlled morphologies and multifunctional properties.

Metal-semiconductor hybrid nanocrystals have received extensive attention owing to their multiple functionalities which can find wide technological applications. The utilization of low-cost non-noble metals to construct novel metal-semiconductor hybrid nanocrystals is important and meaningful for their large-scale applications. In this study, a facile solution approach is developed for the synthesis of Cu-ZnO hybrid nanocrystals with well-controlled morphologies, including nanomultipods, core-shell nanoparticles, nanopyramids and core-shell nanowires. In the synthetic strategy, Cu nanocrystals formed in situ serve as seeds for the heterogeneous nucleation and growth of ZnO, and it eventually forms various Cu-ZnO hetero-nanostructures under different reaction conditions. These hybrid nanocrystals possess well-defined and stable heterostructure junctions. The ultraviolet-visible-near infrared spectra reveal morphology-dependent surface plasmon resonance absorption of Cu and the band gap absorption of ZnO. Furthermore, we construct a novel Cu@CuNi-ZnO ternary hetero-nanostructure by incorporating the magnetic metal Ni into the pre-synthesized colloidal Cu nanocrystals. Such hybrid nanocrystals possess a magnetic Cu-Ni intermediate layer between the ZnO shell and the Cu core, and exhibit ferromagnetic/superparamagnetic properties which expand their functionalities. Finally, enhanced photocatalytic activities are observed in the as-prepared non-noble metal-ZnO hybrid nanocrystals. This study not only provides an economical way to prepare high-quality morphology-controlled Cu-ZnO hybrid nanocrystals for potential applications in the fields of photocatalysis and photovoltaic devices, but also opens up new opportunities in designing ternary non-noble metal-semiconductor hybrid nanocrystals with multifunctionalities.

A Theoretical Study on Light Scattering from and the Morphology-dependent Resonances of Nanowires

A Theoretical Study on Light Scattering from and the Morphology-dependent Resonances of Nanowires

Tunable multiple Fano resonances in magnetic single-layered core-shell particles

We investigate multiple Fano, comblike scattering resonances in single-layered, concentric core-shell nanoparticles composed of magnetic materials. Using the Lorenz-Mie theory, we derive, in the long-wavelength limit, an analytical condition for the occurrence of comblike resonances in the single scattering by coated spheres. This condition establishes that comblike scattering response uniquely depends on material parameters and thickness of the shell, provided that it is magnetic and thin compared to the scatterer radius. We also demonstrate that comblike scattering response shows up beyond the long-wavelength limit and it is robust against absorption. Since multiple Fano resonances are shown to depend explicitly on the magnetic permeability of the shell, we argue that both the position and profile of the comblike, morphology-dependent resonances could be externally tuned by exploiting the properties of engineered magnetic materials.

Quantum spectra of Raman photon pairs from a mesoscopic particle

Quantum Langevin formalism with noise operators is used to provide quantum descriptions of photon pairs (the Stokes and anti-Stokes fields) emitted by a mesoscopic spherical particle composed of quantum particles in a double Raman configuration. The spectra of the fields obtained are sensitive to the dimension of the microsphere and can be controlled by pump and control laser fields. Spectral peaks due to quantum coherence are Stark shifted by the laser fields experiencing autofocusing inside the spherical particle, causing broadening of peaks as the size of the microsphere increases. The antinormal-order spectrum is found to be identical to the normal-order spectrum. The anti-Stokes spectrum is identical to the Stokes spectrum when the linear dispersion is neglected. Frequency-dependent dielectric functions of the Stokes and anti-Stokes spectra corresponding to the linear dispersions of the particle yield narrow morphology-dependent resonance gain peaks at certain frequencies of the Stokes and anti-Stokes spectra that depend not only on the particle size but also on the angle of observation.

Morphology dependent polymeric capillary optical resonator hydrostatic pressure sensor

A hydrostatic pressure sensor based on morphology dependent resonances in a polymeric tube is presented. By internal pressurization, normal tensions will increase the device's size and shrink its wall thickness, inducing a shift in the resonant wavelengths of the resonator. Numerical simulations indicate that there are two modal regimes of sensitivity and a maximum achievable sensitivity, related to the device's geometry, constitutive material and analysed mode order. A sensitivity as high as 0.36 ± 0.01 nm/bar has been experimentally found for a 1.8mm diameter PMMA tube with wall thickness of 80µm.

Scattering of a plane electromagnetic wave by a generalized Luneburg sphere–Part 2: Wave scattering and time-domain scattering

We calculated scattering of an electromagnetic plane wave by a radially inhomogeneous particle and a radially inhomogeneous bubble when the square of the refractive index profile is parabolic as a function of radius. Such a particle or bubble is called a generalized Luneburg lens. A wide variety of scattering phenomena can occur, depending on the value of the two adjustable parameters of the parabola. These phenomena, including transmission rainbows, the weak caustic for near-critical-angle scattering by a bubble, surface orbiting, the interior orbiting paths of morphology-dependent resonances, and the separation of diffraction are studied here using wave theory and time domain scattering. These phenomena are also compared with their appearance or absence for scattering by a homogeneous sphere.

Characterization of the evaporation of interacting droplets using combined optical techniques

A monodisperse droplet stream is injected into a high-temperature enclosure supplied with air heated up to 540 °C. The two-color laser-induced fluorescence (2cLIF) is used for measuring the droplet temperature. The liquid fuel is seeded by pyrromethene 597-C8, which is a temperature-sensitive fluorescent dye. Calibration tests are performed for different types of fuels including ethanol and several alkanes and some of their mixtures. Morphology-dependent resonances (MDRs) are identified as a possible adverse effect for temperature measurements. Due to MDRs, lasing of pyrromethene 597-C8 may occur within fluorescent droplets and affect drastically the fluorescence signal upon which temperature measurement relies. The determination of the droplet size and velocity is achieved by means of quantitative shadow imaging. A double cavity PIV laser is focused on a piece of PMMA doped with a fluorescent dye to produce the background illumination of the droplets. A PIV camera is used to capture the drop motion between the pulses of the laser cavities. A large range of initial distance parameters (the ratio between the inter-droplet distance and the droplet diameter) is explored for different liquid fuels (ethanol, isohexane, n-heptane, n-decane, n-dodecane) and their mixtures. To put forward the effects of the interactions between the droplets, size and temperature measurements are compared to the isolated droplet whose evolution can be predicted with the use of classical models. Comparisons reveal that the inter-droplet spacing and also the fuel volatility play an important role in the reduction of the heat and mass transfers for these interacting droplets. Finally, the ability of the 2cLIF techniques to address the case of multicomponent droplet is also demonstrated.

On-chip phase measurement for microparticles trapped on a waveguide.

Polystyrene microparticles are trapped on a waveguide Young interferometer and the phase change caused by the trapped particles is measured. This is a novel, on-chip method that can be used to count and characterize trapped particles. The trapping of single particles is clearly identified. Simulations show that the phase change increases with the diameter up to 7 μm, while for larger particles, morphology-dependent resonances appear. For 7 μm particles, a phase change of -0.13 rad is measured, while the simulated value is -0.28 rad. Extensive simulations are carried out regarding the phase change, waveguide transmission and the forces on the particles, and also regarding sources of the discrepancy between simulations and measurements.

Elastic scattering from a sapphire microsphere placed on a silica optical fiber coupler: Possible applications to biosensing

Elastic light scattering is performed in the original band of optical fiber communication at 1300 nm for a 500 μm sapphire microsphere placed on a silica optical fiber half coupler. The morphology dependent resonances (MDRs) are observed in the transverse magnetically (TM) polarized and transverse electrically (TE) polarized 0∘ transmission and 90∘ elastic scattering obtained from the sapphire microsphere. The TE and TM MDRs can be detected selectively with the use of a Glan polarizer. The TE and TM polarization selectivity provides the ability to select relative MDR to BG levels. The TM polarization provides higher MDR signal to background ratio (SBR) and is suitable for optical monitoring, biological sensing or any other optoelectronic application that requires a high resolution optical filter. The polar angular mode spacing of 0.36 nm of the resonances correlates well with the optical size of the sapphire microsphere. The autocorrelation of the 90∘ elastic scattering spectra also shows peaks at 0.36 nm. The spectral linewidths of the resonances are on the order of 0.1 nm, which corresponds to quality factors on the order of 104. A sapphire sphere with a radius of 500 μm and relative refractive index of 1.31, resonances will red-shift by 1.01 nm (0.077%). This shift is on the order of 10 linewidths, making sapphire biophotonic sensors an interesting alternative to silica biophotonic sensors.

Effect of Angular Velocity on Sensors Based on Morphology Dependent Resonances

We carried out an analysis to investigate the morphology dependent optical resonances shift (MDR) of a rotating spherical resonator. The spinning resonator experiences an elastic deformation due to the centrifugal force acting on it, leading to a shift in its MDR. Experiments are also carried out to demonstrate the MDR shifts of a spinning polydimethylsiloxane (PDMS) microsphere. The experimental results agree well with the analytical prediction. These studies demonstrated that spinning sensor based on MDR may experience sufficient shift in the optical resonances, therefore interfering with its desirable operational sensor design. Also the results show that angular velocity sensors could be designed using this principle.

(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.

Morphology-dependent resonances of spherical droplets with numerous microscopic inclusions

We use the recently extended superposition T-matrix method to study the behavior of a sharp Lorenz-Mie resonance upon filling a spherical micrometer-sized droplet with tens and hundreds of randomly positioned microscopic inclusions. We show that as the number of inclusions increases, the extinction cross-section peak and the sharp asymmetry-parameter minimum become suppressed, widen, and move toward smaller droplet size parameters, while ratios of diagonal elements of the scattering matrix exhibit sharp angular features indicative of a distinctly nonspherical particle. Our results highlight the limitedness of the concept of an effective refractive index of an inhomogeneous spherical particle.

Effects of nonsphericity on the behavior of Lorenz–Mie resonances in scattering characteristics of liquid-cloud droplets

By using the results of highly accurate T-matrix computations for randomly oriented oblate and prolate spheroids and Chebyshev particles with varying degrees of asphericity, we analyze the effects of a deviation of water-droplet shapes from that of a perfect sphere on the behavior of Lorenz–Mie morphology-dependent resonances of various widths. We demonstrate that the positions and profiles of the resonances can change significantly with increasing asphericity. The absolute degree of asphericity required to suppress a Lorenz–Mie resonance is approximately proportional to the resonance width. Our results imply that numerical averaging of scattering characteristics of real cloud droplets over sizes may rely on a significantly coarser size-parameter resolution than that required for ideal, perfectly spherical particles.