Nanoscale Waveguides Research Articles

Energy transfer effect of hybrid organic rubrene nanorod with CdSe/ZnS quantum dots: Application to optical waveguiding modulators

Organic rubrene (5,6,11,12-tetraphenyltetracene) nanorods (NRs) were fabricated through the physical vapor transport method for the study of active fluorescence optical waveguiding and its modulation. The functionalized CdSe/ZnS quantum dots (QDs) with blue and green emissions were partially attached to the surface of the rubrene NR. Using a high resolution laser confocal microscope (LCM), the nanoscale photoluminescence (PL) intensity of the pristine rubrene portion of the hybrid NR/QDs was observed to be considerably enhanced after it was attached with blue (or green) QDs. The nanoscale optical waveguiding characteristics of the hybrid NR/QDs were investigated in terms of the output LCM PL spectra as a function of propagation distance. We observed more efficient optical waveguiding characteristics from the hybrid rubrene NR/blue-QDs than from the pristine NR and the hybrid rubrene NR/green-QDs. These results can be analyzed in terms of the higher Förster resonance energy transfer efficiency for hybrid rubrene NR/blue-QDs system. The results and analysis were supported by a drastic decrease of exciton lifetime of QDs in the hybrid region as measured by time-resolved PL decay curves.

Plasmonic characteristics in nanoscale graphene resonator-coupled waveguides

In this paper, we numerically and theoretically investigate the propagation of surface plasmon polaritons in a graphene-based resonator-coupled waveguide system, consisting of a monolayer graphene ribbon coupling to two graphene sheets. The resonance wavelength of this system can be easily tuned by adjusting the chemical potential and the width of the graphene ribbon. Both resonance bandwidth and spectral transmission characteristics of the structure strongly depend on the coupling distance and overlap length between the graphene ribbon and graphene sheets. The structural symmetry is found to be another essential parameter. The presented results may pave the way toward the dynamic control of light propagation in graphene-based structures and the realization of tunable graphene-based optoelectronic devices.

Circuit optomechanics: concepts and materials

Nanophotonic integrated circuits offer unique advantages for studying the interaction of light fields with mechanical structures. Because nanoscale waveguides are closely size-matched to nanomechanical devices, strong optomechanical interactions arise which can be harnessed in optical systems. The additional mechanical degrees of freedom provided by optomechanical devices are of particular interest for material systems in which tunability of the optical properties is not readily available. Here, suitable materials for the realization of chip-based optomechanical circuits are discussed and analyzed in terms of performance and the achievable quality factors. In particular, materials that offer large electronic band gaps are of interest, because in this case broadband optical transparency is achieved, combined with reduced free carrier effects. Several device geometries that can be used for enhancing optical forces are presented which address both an increase in the field gradient and the net optical force through resonant enhancement. Combining a variety of optomechanical components into full circuits thus provides a new route toward functional nanophotonic circuits with applications in sensing and optical signal processing in a chip-scale framework.

Polarization selective phase-change nanomodulator.

Manipulating optical signals below the diffraction limit is crucial for next-generation data-storage and telecommunication technologies. Although controlling the flow of light around nanoscale waveguides was achieved over a decade ago, modulating optical signals at terahertz frequencies within nanoscale volumes remains a challenge. Since the physics underlying any modulator relies on changes in dielectric properties, the incorporation of strongly electron-correlated materials (SECMs) has been proposed because they can exhibit orders of magnitude changes in electrical and optical properties with modest thermal, electrical or optical trigger signals. Here we demonstrate a hybrid nanomodulator of deep sub-wavelength dimensions with an active volume of only 0.002 µm3 by spatially confining light on the nanometre length scale using a plasmonic nanostructure while simultaneously controlling the reactive near-field environment at its optical focus with a single, precisely positioned SECM nanostructure. Since the nanomodulator functionality hinges on this near-field electromagnetic interaction, the modulation is also selectively responsive to polarization. This architecture suggests one path for designing reconfigurable optoelectronic building blocks with responses that can be tailored with exquisite precision by varying size, geometry, and the intrinsic materials properties of the hybrid elements.

Multimodal plasmonics in fused colloidal networks.

Harnessing the optical properties of noble metals down to the nanometer-scale is a key step towards fast and low-dissipative information processing. At the 10-nm length scale, metal crystallinity and patterning as well as probing of surface plasmon (SP) properties must be controlled with a challenging high level of precision. Here, we demonstrate that ultimate lateral confinement and delocalization of SP modes are simultaneously achieved in extended self-assembled networks comprising linear chains of partially fused gold nanoparticles. The spectral and spatial distributions of the SP modes associated with the colloidal superstructures are evidenced by performing monochromated electron energy loss spectroscopy with a nanometer-sized electron probe. We prepare the metallic bead strings by electron beam-induced interparticle fusion of nanoparticle networks. The fused superstructures retain the native morphology and crystallinity but develop very low energy SP modes that are capable of supporting long range and spectrally tunable propagation in nanoscale waveguides.

Modeling light-sound interaction in nanoscale cavities and waveguides

AbstractThe interaction of light and sound waves at the micro and nanoscale has attracted considerable interest in recent years. The main reason is that this interaction is responsible for a wide variety of intriguing physical phenomena, ranging from the laser-induced cooling of a micromechanical resonator down to its ground state to the management of the speed of guided light pulses by exciting sound waves. A common feature of all these phenomena is the feasibility to tightly confine photons and phonons of similar wavelengths in a very small volume. Amongst the different structures that enable such confinement, optomechanical or phoxonic crystals, which are periodic structures displaying forbidden frequency band gaps for light and sound waves, have revealed themselves as the most appropriate candidates to host nanoscale structures where the light-sound interaction can be boosted. In this review, we describe the theoretical tools that allow the modeling of the interaction between photons and acoustic phonons in nanoscale structures, namely cavities and waveguides, with special emphasis in phoxonic crystal structures. First, we start by summarizing the different optomechanical or phoxonic crystal structures proposed so far and discuss their main advantages and limitations. Then, we describe the different mechanisms that make light interact with sound, and show how to treat them from a theoretical point of view. We then illustrate the different photon-phonon interaction processes with numerical simulations in realistic phoxonic cavities and waveguides. Finally, we introduce some possible applications which can take enormous benefit from the enhanced interaction between light and sound at the nanoscale.

Nanomagnonic devices based on the spin-transfer torque.

Magnonics is based on signal transmission and processing by spin waves (or their quanta, called magnons) propagating in a magnetic medium. In the same way as nanoplasmonics makes use of metallic nanostructures to confine and guide optical-frequency plasmon-polaritons, nanomagnonics uses nanoscale magnetic waveguides to control the propagation of spin waves. Recent advances in the physics of nanomagnetism, such as the discovery of spin-transfer torque, have created possibilities for nanomagnonics. In particular, it was recently demonstrated that nanocontact spin-torque devices can radiate spin waves, serving as local nanoscale sources of signals for magnonic applications. However, the integration of spin-torque sources with nanoscale magnetic waveguides, which is necessary for the implementation of integrated spin-torque magnonic circuits, has not been achieved to date. Here, we suggest and experimentally demonstrate a new approach to this integration, utilizing dipolar field-induced magnonic nanowaveguides. The waveguides exhibit good spectral matching with spin-torque nano-oscillators and enable efficient directional transmission of spin waves. Our results provide a practical route for the implementation of integrated magnonic circuits utilizing spin transfer.

Influence of van der Waals forces on the waveguide deformation and power limit of nanoscale waveguide devices

The ultrashort-range force, the van der Waals force (VWF), rises rapidly when one nanoscale waveguide (NW) is close to another one, and is stronger than the external transverse gradient force. We theoretically investigate the great influence of the VWF on the device performance in a typical NW optomechanical system consisting of a suspended silicon waveguide and a silica substrate including waveguide deformation stiction and failure mechanism. The device shows unique optically activated plastic and elastic behaviors and stiction due to the VWF. When the input optical power is above the critical power, the waveguide is sticking to the substrate and the deformation is plastic and unrecoverable, even though the total force is less than the yield strength of the waveguide material. This is important and helpful for the design and applications of optomechanical devices.

Low-loss ultra-subwavelength hybrid plasmonic waveguide based on metallic bump structures

Sharp metal edges (like wedge or triangular geometries) have been widely studied as low-loss surface plasmon polariton (SPP) waveguiding structures. However, fabrication difficulties lead to round-shaped tips which have larger mode size and shorter propagation length. As a solution, here we present an easy to realize structure for turning an edge-less bump metallic geometry (whose fabrication is less troublesome) into a hybrid one. Through investigating the geometrical dispersion, propagation lengths of up to half a millimetre and effective mode areas as small as λ2/330 are obtained for the 1.55 µm telecommunication wavelength. Gain-assisted SPP propagation is also considered by introducing gain in the InGaAsP material, for which critical gain as low as 11 cm−1 is calculated. Finally, obtained results suggest that the designed waveguide can find various applications in nanoscale waveguiding.

Influence of structural changes in a periodic antidot waveguide on the spin-wave spectra

We demonstrate that the magnonic band structure, including the band gap of a ferromagnetic antidot waveguide, can be significantly tuned by a relatively weak modulation of its structural parameters. We study the magnonic band structure in nanoscale spin-wave waveguides with periodically distributed small antidots along their central line by two independent computational methods, namely, a micromagnetic simulation and a plane-wave method. The calculations were performed with consideration of both the exchange and dipolar interactions. For the exchange dominated regime, we discuss, in details, the impact of the changes of the lattice constant, size, and shape of the antidots on the spin-wave spectra. We have shown that a precise choice of these parameters is crucial for achieving desired properties of antidot waveguides, i.e., a large group velocity and filtering properties due to existence of magnonic band gaps. We discuss different mechanisms of magnonic gap opening resulting from Bragg scattering or anticrossing of modes. We have shown that the dipolar interactions start to assert their role in the spin-wave spectrum when the waveguide is scaled up, but even for a period of few hundreds of nanometers, the magnonic band structure preserves qualitatively the properties found in the exchange dominating regime. The obtained results are important for future development of magnonic crystal based devices.

Quantum optical properties of a dipole emitter coupled to an ɛ-near-zero nanoscale waveguide

We study quantum optical properties of a dipole emitter coupled to a rectangular nanoscale waveguide with dielectric core and silver cladding. We investigate enhanced spontaneous emission and the photonic Lamb shift for emitters whose resonant frequencies are near the waveguide frequency cutoff where the waveguide behaves as an ɛ-near-zero metamaterial. Via a dyadic Green's function-based field quantization scheme, we calculate the photonic Lamb shift as well as the spontaneous emission enhancement and spectrum. Using realistic parameters for typical quantum emitters, we suggest experimentally realizable schemes to observe relatively large photonic Lamb shifts in waveguides.

Hybrid Nanowires: Vertically Aligned Hybrid Core/Shell Semiconductor Nanowires for Photonics Applications (Adv. Funct. Mater. 48/2013)

Single-crystal organic nanowires are fabricated and covered by an inorganic wide-bandgap semiconducting shell via chemical vapor deposition. The wires are vertically aligned by A. Borras and co-workers and supported on processable substrates. This method is widely applicable to a range of materials. The waveguide effect on the fluorescence emission of the organic cores and their application as nanoscale waveguides in the 550–750 nm range are explored on page 5981 using perylene and ZnO.

Graphene nano-ribbon waveguides of record-small mode area and ultra-high effective refractive indices for future VLSI

Electronics circuits keep shrinking in dimensions, as requested by Moore's law. In contrast, photonic waveguides and circuit elements still have lateral dimensions on the order of the wavelength. A key to make photonics have a microelectronics-like development is a drastic reduction of size. To achieve this, we need a low-loss nanoscale waveguide with a drastically reduced mode area and an ultra-high effective refractive index. For this purpose, we propose here several low-loss waveguide structures based on graphene nano-ribbons. An extremely small mode area (~10(-7)λ(0)(2), one order smaller than the smallest mode area of any waveguide that has ever been reported in the literature; here λ(0) is the operating wavelength in vacuum) and an extremely large effective refractive index (several hundreds) are achieved. As a device example, a nano-ring cavity of ultra-small size (with a diameter of ~10(-2)λ(0)) is designed. Our study paves the way for future VLSI (very-large-scale integration) optoelectronics.

Nanoscale All-Angle Waveguide Based on Plasmon Band Effect in Triangular Array of MWCNTs

In this paper, we present a nanoscale all-angle waveguide based on plasmon band effect in triangular array of multiwall carbon nanotubes (MWCNTs) which propagates the light beam in structure for any incident angle. We analyze photonic characteristics of the proposed MWCNT based nanoscale structure using finite-difference time-domain method. Multi pole Drude-Lorentz model is used for dispersive dielectric function of MWCNTs. Using this model, photonic properties of triangular MWCNT based photonic crystal like band structure, reflection and transmission coefficients are calculated and the effects of different structural parameters are studied for engineering the desired band structure. The MWCNT array with 20 nm lattice constant is designed in Silicon as a host. Considering coupled plasmonic-Bragg guided mode, we create a tunable optical waveguide by inserting engineered line defects of MWCNTs inside the structure.

Numerical Investigation of SPP Propagation at the Nano-scale MDM Waveguides with a Combiner

Numerical Investigation of SPP Propagation at the Nano-scale MDM Waveguides with a Combiner

Comparison of Atomistic and Continuum Methods for Calculating Ballistic Phonon Transmission in Nanoscale Waveguides

We compare two methods for the calculation of mode dependent ballistic phonon transmission in nanoscale waveguides. The first method is based on continuum acoustic waveguide theory and uses an eigenmode expansion to solve for phonon transmission coefficients. The second method uses lattice dynamics (LD)-computed mode shapes to excite guided phonon wavepackets in a nonequilibrium molecular dynamics (MD) simulation and calculates phonon transmission from the final distribution of system energy. The two methods are compared for the case of shear-horizontal (SH) phonons propagating in a planar waveguide with a T-stub irregularity, a geometry which has been proposed for the tuning of phonon transmission and nanostructure thermal conductance. Our comparison highlights advantages and disadvantages of the two methods and illustrates regimes when atomistic effects are prominent and continuum approaches are not appropriate.

Vertically Aligned Hybrid Core/Shell Semiconductor Nanowires for Photonics Applications

AbstractA family of 1D organic/inorganic core/shell materials formed by an inner organic nanowire (ONW) conformally covered with an inorganic wide band gap semiconductor (ZnO or TiO2) layer is presented. The developed procedure is a two‐steps vacuum methodology involving the formation of supported single crystal small‐molecule nanowires by physical vapor deposition and plasma enhance chemical vapor deposition (PECVD) of the inorganic shell. Critical characteristics of the last technique are the possibilities of low temperature and remote configuration deposition. Additionally, an initial step has to be included in order to create nucleation centers for the growth of the ONWs. The procedure and its general character in terms of the variability in organic core and inorganic shells composition and the applicability of the technique to different substrates are presented. The formation of the inorganic shell with no damage of the organic core single‐crystalline structure is demonstrated by high resolution transmission electron microscopy. The vertical alignment of the hybrid nanostructure is achieved thanks to the interaction of the 1D organic nanostructured surfaces and the glow discharge during the deposition of the inorganic shell by PECVD. The optical properties of these core/shell NWs are studied by fluorescence spectroscopy and microscopy, and their application as nanoscale waveguides in the 550–750 nm range addressed.

Semi-vector iterative method for modes of high-index-contrast nanoscale waveguides

An approximate semi-analytical iterative method is presented to find vector modes of high-index contrast single mode waveguides. Present method is developed to provide improvement over scalar analysis of Vopt method. To illustrate the accuracy and efficiency of this method, modal properties of silicon strip nanoscale waveguide are studied in detail and compared with other approximate and rigorous numerical analysis.

Metal-Coated Waveguide Stretches Wavelengths to Infinity

A zero refractive index at optical frequencies, as demonstrated in a new nanoscale waveguide, enables opportunities for better control and enhancement of light propagation in waveguides, as well as development of photonic nanocircuits.

207 Nanopore immobilization of DNA polymerase enhances single-molecule sequencing

A zero-mode waveguide (ZMW) is a nanoscale optical waveguide driven at a frequency below its cut-off. In this mode, the electric field, instead of traveling down the axis of the conducting cavity, decays exponentially. By fabricating waveguides with sub-wavelength diameters and illuminating them with laser light, the electric field in the waveguide is confined enough to enable single-molecule optical detection at micromolar concentration [1]. Immobilizing single DNA polymerases in ZMWs and using special phosphate-fluorescently labeled dNTPs form the basis for single-molecule real-time DNA sequencing, one of the most promising next-generation sequencing platforms [2]. In this method, the polymerase replicates the sample DNA, and as it incorporates new bases into the product strand, the labeled dNTPs emit a burst of light before the phosphate is cleaved off. The sequence of colors corresponds to the DNA sequence (see Figure 1 below from Eid et al., 2009). Because the ZMW aperture’s diameter is sub-diffraction-limit, it is impossible to optically distinguish one polymerase in a ZMW from two. Having only one polymerase in each waveguide is critical to sequencing accuracy. In its present state, experimenters use diffusion to fill ZMWs with polymerases, resulting in a Poisson distribution for filling ZMWs, and consequently a theoretical limit of 36.8% of ZMWs having only one polymerase [2]. We achieve full polymerase occupancy of ZMWs by fabricating the structures on an ultrathin silicon nitride membrane and drilling a nanopore at the base of each waveguide with an ion beam. A short DNA fragment with biotin on either end is conjugated to a streptavidin and then drawn into the nanopore with a voltage bias. There is then a free biotin at the base of the ZMW. A polymerase–streptavidin complex can diffuse into the ZMW and bind to the exposed biotin. Because the nanopore is too small to fit more than one molecule, only one ZMW will bind to a biotin in the nanopore. Upon flushing the ZMW chamber, the biotin-bound polymerase will remain trapped in the pore, and only a single polymerase will remain at the base of each waveguide.