Yuriy A. Akimov

Hybrid dielectric-loaded plasmonic waveguide and wavelength selective components for efficiently controlling light at subwavelength scale

We analyze and design a hybrid dielectric-loaded plasmonic waveguide (HDLW) featuring a long propagation length and strong field confinement, for efficient control and confinement of light in the subwavelength area of λ2/160. The HDLW is then used to build compact wavelength selective components of high optical performance, including ring resonators (RR) and add-drop filters (ADF). In particular, we demonstrate RRs having a small ring radius of 2 μm, a low transmission loss of 0.8 dB, a high extinction ratio of 21 dB, and a free spectral range of 66 nm. Moreover, an ADF with a ring radius of 2 μm features a 12 dB extinction ratio, a transmission loss of 0.9 dB, and a channel isolation level of 10 dB at the resonant wavelength. The compact footprint and superior performance of these plasmonic components make them promising building blocks for future nanoscale electronic-photonic integrated circuits for data communication and sensing applications.

Submicrometer radius and highly confined plasmonic ring resonator filters based on hybrid metal-oxide-semiconductor waveguide

We numerically report the submicrometer radius (0.5 μm) and high confinement (mode area ~λ(2)/1200) plasmonic ring resonators for both all-pass and add-drop filters based on the hybrid metal-oxide-semiconductor (Ag-SiO(2)-Si) waveguide platform. The best tradeoff between the propagation length and the confinement of this hybrid plasmonic waveguide platform is also discussed and compared to the dielectric-loaded plasmonic waveguide counterpart. We show that the ring resonator all-pass filter features an extinction ratio as high as 23 dB with a transmission loss of 1.5 dB, and a wide free spectral range of 168 nm with a bandwidth of 14 nm. Moreover, the demonstrated add-drop filter achieves an extinction ratio larger than 12 dB with a channel isolation between the through and drop channels of 13.5 dB at the resonant wavelength. These demonstrated plasmonic devices reveal as potential building blocks for future nanoscale electronic-photonic integrated circuits.

The Potential of Graphene as an ITO Replacement in Organic Solar Cells: An Optical Perspective

Graphene possesses innate potential to replace indium tin oxide (ITO) as the transparent electrode in an organic solar cell device. From our transmittance study weighted with the air mass 1.5 global (AM1.5G) solar spectrum, it is found that a higher transparency may be obtained for up to four layers of graphene in comparison to ITO. Our findings suggest that replacing ITO with monolayer graphene in organic solar cells yields comparable performance. Due to the increased optical absorption, organic solar cells with four-layer graphene (with the same sheet resistance as ITO at 30 Ω/□) are capable of attaining at least 92% of the same organic photovoltaic device with an optimized ITO electrode for both normal and angular AM1.5G illumination.

Three-Dimensional Optoelectronic Model for Organic Bulk Heterojunction Solar Cells

This paper describes a 3-D optoelectronic device model for organic bulk heterojunction solar cells. Three-dimensional full-wave optical simulation enables us to incorporate different modern light trapping techniques, such as subwavelength nanostructures, in a typical organic bulk heterojunction solar cell, while 3-D electrical simulation allows us to handle localized enhancement or reduction of polaron/charge generation, recombination, and transport induced by modern light trapping techniques in the device. We calibrate our model with an experimental poly (3-hexylthiophene) (P3HT):phenyl-C60-butyric acid methyl ester (PCBM) organic bulk heterojunction solar cell by tuning only one free parameter as compared with other device models, which have multiple fitting parameters. A 3-D example of a silver nanoparticle array in a typical P3HT:PCBM organic bulk heterojunction cell is also demonstrated, and the current density-voltage relation is predicted with our model.

Photoluminescence via gap plasmons between single silver nanowires and a thin gold film

In this work, we investigate the one-photon photoluminescence from a system consisting of an Ag nanowire on an Au film with a ~6 nm dielectric spacer that supports a localized resonance known as the gap plasmon mode. Although the Ag nanowire and Au film exhibit weak photoluminescence on their own, the combined system produces an enhanced PL emission. Our analysis reveals that the strong PL emission in this system was due to the enhanced laser absorption in, and PL emission from Au, with the Ag nanowire acting as an efficient antenna. The PL due to the gap mode is sensitive to the polarization of the laser excitation with respect to the nanowire orientation, and shows a clear dipolar emission profile. PL emission wavelengths were found to depend only on the nanowire width, and independent of its length. These observations were supported by simulation results indicating that gap plasmons are excited by light polarized transverse to the nanowire length.

π–π interactions mediated self-assembly of gold nanoparticles into single crystalline superlattices in solution

The first attempt of employing π–π interactions for the self-assembly of colloidal gold nanoparticles into 3D single crystalline superlattices in solution is presented. It is demonstrated that simple capping ligand exchange with aromatic thiols leads to self-assembly of gold nanoparticles into fcc packed superlattices with well-defined facets and long-range ordering. Stimuli that can break the π–π interactions lead to disassembly of gold nanoparticles, allowing the design of reversible assembly and reconfiguration. The crystallization of gold nanoparticles is shown to be kinetically controlled by the concentration of aromatic thiols in solution, enabling efficient tuning of the long- and short-range ordering in nanoparticle lattices, accompanied with corresponding changes of the effective optical properties.

Impact of Nonuniform Electron Density on Plasmonic Properties of Metal Nanoparticles

The interfacial nonuniformity of the electron density that occurs in metals as a result of atomic imperfections can strongly affect the plasmonic properties of metallic nanostructures. Under certain conditions, it induces the bulk plasmon resonance in the transition area and can significantly change scattering and absorption of light by metallic nanostructures in a broad frequency range. This effect is numerically demonstrated for radially nonuniform spherical silver nanoparticles and analytically investigated with respect to the resonant coupling with the dipolar surface plasmons of the metal core.

Optical Enhancement with Plasmonic Nanoparticles in Organic Bulk-Heterojunction Solar Cells

This work discusses the enhancement of the optical absorption of the organic bulk-heterojunction solar cell with plasmonic silver nanoparticles.

Correlation Between Surface Charge Distribution and Circular Dichroism of Elementary Planar Nanoparticles

A correlation is observed between surface charge distributions and the circular dichroism (CD) signature of nanoparticles excited by circularly polarized waves. These surface charge distributions arise as a result of charge separation and depend on the polarization of the externally excited light. This correlation can be observed by deriving the surface charge distribution profile of excited localized surface plasmon polaritons (SPPs) in elementary metal nanoparticles under the influence of circularly polarized light. Nanoparticles with strong CD signatures are especially desired for sensing of chiral biomolecules as well as to aid in photochemical catalysis. We also found out that CD signatures can even be induced via angular rotation. This is true for elementary non-rotated nanoparticles which do not possess a CD signature. The use of elementary nanoparticles for sensing poses a huge advantage over complex nanostructures due to the ease of fabrication. The observed CD signature can also be validated in accordance with theory and simulation results.

Highly efficient tunable and localized on-chip electrical plasmon source using protruded metal-insulator-metal structure

A compact and highly efficient tunable and localized source of propagating surface plasmon-polaritons is proposed based on a protruded metal-insulator-metal (pMIM) structure. The protrusion along a segment of the pMIM forms a nanometer gap and allows a low voltage bias to generate a localized tunneling current. The tunneling current excited plasmons can be fully coupled to the metal-insulator-metal (MIM) waveguiding segment of the pMIM without leakage and propagate a long distance as the gap in the MIM waveguiding segment is much larger than the gap in the protruded segment of the pMIM. Eigenmode and numerical analyses show that by using MIM structures as a benchmark, the pMIM structure enhances the total amount of average power that is transferred from the tunneling current into the excitation of intrinsic eigenmodes of the MIM waveguiding segment. Depending on the magnitude of the applied voltage bias, the pMIM structure supports single, dual and multi modes for a typical Au-SiO2-Au design with a 500 nm-thick SiO2. Among all excited modes, the single mode operation allows highly efficient excitation of long travelling surface plasmon-polaritons (SPPs) of up to 30 µm. The electrical excitation of SPPs using pMIM structures opens up the possibility of integrating plasmon sources into nanoscale optoelectronic circuits to facilitate on-chip data communications.