Xudong Cui

Optical forces on metallic nanoparticles induced by a photonic nanojet

We investigate the optical forces acting on a metallic nanoparticle when the nanoparticle is introduced within a photonic nanojet (PNJ). Optical forces at resonance and off-resonance conditions of the microcylinder or nanoparticle are investigated. Under proper polarization conditions, the whispering gallery mode can be excited in the microcylinder, even at off resonance provided that scattering from the nanoparticle is strong enough. The optical forces are enhanced at resonance either of the single microcylinder or of the nanoparticle with respect to the forces under off-resonant illuminations. We found that the optical forces acting on the nanoparticle depend strongly on the dielectric permittivity of the nanoparticle, as well as on the intensity and the beam width of the PNJ. Hence, metallic sub-wavelength nanoparticle can be efficiently trapped by PNJs. Furthermore, the PNJs attractive force can be simply changed to a repulsive force by varying the polarization of the incident beam. The changed sign of the force is related to the particles polarizability and the excitation of localized surface plasmons in the nanoparticle.

Tuning the resonance frequency of Ag-coated dielectric tips

A finite element model was built to investigate how to optimize localized plasmon resonances of an Ag-coated dielectric tip for tip-enhanced Raman spectroscopy (TERS). The relation between the resonance frequency, the electric field enhancement and the optical constant of the dielectric tip was numerically investigated. The results show that increasing the refractive index of the dielectric tip can significantly red shift the localized plasmon modes excited on the Ag-coated dielectric tip, and consequently alter the field enhancement. Moreover, the influence of the width of the resonance on the Raman enhancement was also considered. When taking all the factors into account, we find that an Ag-coated low-refractive index dielectric tip provides the best Raman enhancement in the blue-green spectral range. This is consistent with our prior experimental results.

Metal-filled carbon nanotube based optical nanoantennas: bubbling, reshaping, and in situ characterization

Controlled fabrication of metal nanospheres on nanotube tips for optical antennas is investigated experimentally. Resembling soap bubble blowing using a straw, the fabrication process is based on nanofluidic mass delivery at the attogram scale using metal-filled carbon nanotubes (m@CNTs). Two methods have been investigated including electron-beam-induced bubbling (EBIB) and electromigration-based bubbling (EMBB). EBIB involves the bombardment of an m@CNT with a high energy electron beam of a transmission electron microscope (TEM), with which the encapsulated metal is melted and flowed out from the nanotube, generating a metallic particle on a nanotube tip. In the case where the encapsulated materials inside the CNT have a higher melting point than what the beam energy can reach, EMBB is an optional process to apply. Experiments show that, under a low bias (2.0-2.5 V), nanoparticles can be formed on the nanotube tips. The final shape and crystallinity of the nanoparticles are determined by the cooling rate. Instant cooling occurs with a relatively large heat sink and causes the instant shaping of the solid deposit, which is typically similar to the shape of the molten state. With a smaller heat sink as a probe, it is possible to keep the deposit in a molten state. Instant cooling by separating the deposit from the probe can result in a perfect sphere. Surface and volume plasmons characterized with electron energy loss spectroscopy (EELS) prove that resonance occurs between a pair of as-fabricated spheres on the tip structures. Such spheres on pillars can serve as nano-optical antennas and will enable devices such as scanning near-field optical microscope (SNOM) probes, scanning anodes for field emitters, and single molecule detectors, which can find applications in bio-sensing, molecular detection, and high-resolution optical microscopy.

Enhanced propagation in a plasmonic chain waveguide with nanoshell structures based on low- and high-order mode coupling

We studied the performance of a plasmonic chain waveguide by employing an array of nanoshell structures. The optical properties of the proposed structures are discussed in detail with respect to the mode coupling for both low-order resonances and high-order multipolar modes. We show (a) that the choice of nanoshell particles allows an easy tuning of the structures resonances according to given wavelength specifications and (b) that the resonances are insensitive to the chain length when high-order multipolar modes are involved. Moreover, chain waveguides that are operated on resonant multipolar modes provide propagation lengths up to 1.88 microm, which is beyond what is maximally achieved by conventional solid particle chains. This is attributed to the large field enhancement within metallic nanoshell structures, as well as to far-field effects, which play an important role in low-loss light guiding along nanoshell chains.

Shaping the nanostructures from electromigration-based deposition

Electromigration-based deposition (EMBD) is proposed for the fabrication of three-dimensional (3D) metallic nanostructures. The process is based on nanofluidic mass delivery at the attogram scale from metal-filled carbon nanotubes (m@CNTs) using nanorobotic manipulation inside a transmission electron microscope. By attaching a conductive probe to the sidewall of the CNT, it has been shown that mass flow can be achieved regardless the conductivity of the object surface. Experiments have also shown the influence of heat sinks on the geometries of the deposits from EMBD. By modulating the relative position between the deposit and the heat sinks, it becomes possible to reshape the deposits. As a general-purposed nanofabrication process, EMBD will enable a variety of applications such as nanorobotic arc welding and assembly, nanoelectrodes direct writing, and nanoscale metallurgy.

Spheres on pillars: Nanobubbling based on attogram mass delivery from metal-filled nanotubes

We report an experimental investigation into the controlled fabrication of metallic nanospheres on the tip of nanotubes. The fabrication process, nanobubbling, is based on nanofluidic mass delivery at the attogram scale using metal-filled carbon nanotubes (CNTs). Two methods have been investigated including electron-beam-induced bubbling (EBIB) and electromigration-based bubbling (EMBB). Under the irradiation of a high energy electron beam of a transmission electron microscope (TEM), the encapsulated metal is melted and extruded out from the tip of the nanotube; generating a metallic sphere. Our investigation showed that several factors including temperatures, nanotube tip breaking/opening, and electron-beam-induced reconstruction of the carbon shells are responsible to the sizes and shapes of the metallic spheres on the nanotubes. In the case that the encapsulated materials inside the CNT has a higher melting point than that of the beam energy can reach, electromigration-based mass delivery is an optional process to apply. Results show that under a low bias (2–2.5V), spherical nanoparticles can be formed on the tips of nanotubes. EMBB is a further development of the nanorobotic spot welding technique and a fundamental technology for thermal dip pen nanolithography. The sphere-on-pillar structures and the nanobubbling processes proposed will enable devices such as nanooptical antennas, scanning near-field optical microscope (SNOM) probes, scanning anodes for field emitters, and single molecule detectors, which can find applications in bio-sensing, molecular detection, and high-resolution optical microscopy.

Frequency-domain simulations of optical antenna structures

Optical antennas consisting of metallic parts are analyzed using the Multiple Multipole Program (MMP), a semi-analytic boundary discretization method. It is demonstrated that difficult numerical problems are caused because optical antennas exhibit strong material dispersion, loss, and plasmon-polariton effects that require a very fine discretization. In addition to standard dipole-type antennas, consisting of two pieces of metal, a new structure consisting of a single metal piece with a tiny groove in the center is analyzed. This structure takes advantage of the Channel Plasmon-Polariton (CPP) effect and exhibits a strong enhancement of the electric field in the groove. Furthermore, the groove type antenna exhibits two resonance peaks when its dimension is much smaller than the wavelength. It is demonstrated that the strengths and locations of the resonance peaks may be tuned within some range by tuning the length of the antenna.

Metallic and dielectric photonic crystal filter design using multiple multipole program and model-based parameter estimation methods

Ultra compact photonic crystal filters with and without metallic parts operating at telecommunication wavelengths are designed using the multiple multipole program combined with the model-based parameter estimation technique. Material loss is taken into account and measured material properties are employed for practical design considerations. Stochastic and deterministic optimization techniques are applied to obtain optimum filter characteristics.

Photonic integration for high density and multi-functionality in the InP material system

Monolithic photonic integration offers unsurpassed perspectives for higher functional density, new functions, high per-formance, and reduced cost for the telecommunication. Advanced local material growth techniques and the emerging photonic crystal (PhC) technology are enabling concepts towards high-density photonic integration, unprecedented per-formance, multi-functionality, and ultimately optical systems-on-a-chip. In this paper, we present our achievements in photonic integration applied to the fabrication of InP-based mode-locked laser diodes capable of generating optical pulses with sub-ps duration using the heterogeneous growth of a new uni-traveling carrier ultrafast absorber. The results are compared to simulations performed using a distributed model including intra-cavity reflections at the sections inter-faces and hybrid mode-locking. We also discuss our work on InP-based photonic crystals (PhCs) for dense photonic integration. A combination of two-dimensional modeling for functional optimization and three-dimensional simulation for real-world verification is used. The fabricated structures feature more than 3.5μm deep holes as well as excellent pattern-transfer accuracy using electron-beam lithography and advanced proximity-effects correction. Passive devices such as waveguides, 60° bends and power splitters are characterized by means of the end-fire technique. The devices are also investigated using scanning-near field optical microscopy. The PhC activity is extended to the investigation of TM bandgaps for all-optical switches relying on intersubband transitions at 1.55μm in AlAsSb/InGaAs quantum wells.

Multiple multipole program analysis of metallic optical waveguides

The focus of this paper is on the numerical analysis of ultra-small metallic waveguides at optical wavelengths that is very demanding 1) because the cross sections may be much smaller than the wavelength, 2) because strong plasmon-polariton effects must be accounted for, and 3) because of strong dispersion and material loss. After a short outline of available numerical methods with focus on eigenvalue solvers, the Multiple Multipole Program (MMP) - that is applied for obtaining the results shown in this paper - is outlined. Since the analysis of metallic waveguides leads to difficult complex eigenvalue problems, several techniques for solving such problems are introduced. Based on these procedures, simple plasmonic wires, metallic wires coupled with a dielectric fiber, partially coated optical fibers, and metallic waveguides with tiny V-grooves of only a few nanometers are analyzed. The impact of the material properties is demonstrated by comparing gold and silver wires with V-grooves. It is shown that such structures may exhibit Channel Plasmon-Polariton (CPP) modes with acceptable propagation lengths even when the grooves are only a few nm deep, but only within a narrow frequency range and only for metals with low loss in the desired frequency range. These modes show a strong field confinement within the groove that might be attractive for sensor applications. Furthermore, the partially coated optical fiber is attractive for optical nearfield microscopy and exhibits field enhancement due to wedge plasmon polariton and triple-point plasmon polariton effects.