Kristjan Leosson

Surface plasmon polariton based modulators and switches operating at telecom wavelengths

We report design, fabrication, and characterization of thermo-optic Mach–Zender interferometric modulators and directional-coupler switches whose operation utilizes the long-range surface-plasmon-polariton waveguiding along 15-nm-thin and 8-μm-wide gold stripes embedded in polymer and heated by electrical signal currents. The devices are characterized at the light wavelength of 1.55 μm, featuring low driving powers ( 30dB), moderate response times (∼1ms), and the total (fiber-to-fiber) insertion loss of ∼13dB (for modulators) and ∼11dB (for switches) when using single-mode fibers.

Integrated optical components utilizing long-range surface plasmon polaritons

New optical waveguide technology for integrated optics, based on propagation of long-range surface plasmon polaritons (LR-SPPs) along metal stripes embedded in dielectric, is presented. Guiding and routing of electromagnetic radiation along nanometer-thin and micrometer-wide gold stripes embedded in polymer via excitation of LR-SPPs is investigated in the wavelength range of 1250-1650 nm. LR-SPP guiding properties, such as the propagation loss and mode-field diameter, are investigated for different stripe widths and thicknesses. A propagation loss of /spl sim/6 dB/cm, a coupling loss of /spl sim/0.5 dB (per facet), and a bend loss of /spl sim/5 dB for a bend radius of 15 mm are evaluated for 15-nm-thick and 8-/spl mu/m-wide stripes at the wavelength of 1550 nm. LR-SPP-based 3-dB power Y-splitters, multimode interference waveguides, and directional couplers are demonstrated and investigated. At 1570 nm, coupling lengths of 1.9 and 0.8 mm are found for directional couplers with, respectively, 4- and 0-/spl mu/m-separated waveguides formed by 15-nm-thick and 8-/spl mu/m-wide gold stripes. LR-SPP-based waveguides and waveguide components are modeled using the effective-refractive-index method, and good agreement with experimental results is obtained.

Polymer-based surface-plasmon-polariton stripe waveguides at telecommunication wavelengths

Long-range surface-plasmon-polariton (LR–SPP) waveguiding along thin gold stripes embedded in polymer is investigated in the wavelength range of 1510–1620 nm. LR–SPP intensity distributions at the output are measured for different stripe widths and thicknesses. Coupling loss of ∼0.5 dB is achieved when exciting the fundamental LR–SPP mode along 10-nm-thick stripes of 6–10 μm width with a polarization maintaining fiber. LR–SPP propagation loss of 6–8 dB/cm is estimated (at 1550 nm) and attributed to scattering from inhomogeneities of the metal stripe and polymer cladding.

Compact Bragg gratings for long-range surface plasmon polaritons

By introducing periodic thickness modulation of thin metal stripes embedded in a dielectric, compact and efficient Bragg gratings for long-range surface plasmon polaritons (LR-SPPs) operating around 1550 nm are realized. Reflection and transmission spectra of the gratings having different lengths (from 20 to 160 /spl mu/m), heights (tens of nanometers), and widths of the metal ridges forming the grating were measured, and the reflectivity of up to 60% and bandwidths ranging from 5 to 40 nm are demonstrated. By using a simple lossless-uniform-grating description, the effective refractive-index modulation in LR-SPP gratings is estimated to be of the order of 10/sup -2/, and two different approaches for these calculations are compared. The LR-SPP loss incurred in the investigated gratings is also discussed.

Long-range surface plasmon polariton nanowire waveguides for device applications.

We report an experimental study of long-range surface plasmon polaritons propagating along metallic wires of sub-micrometer rectangular cross-sections (nanowires) embedded in a dielectric. At telecom wavelengths, optical signals are shown to propagate up to several millimeters along such nanowires. As the wires approach a square cross-section, the guided mode becomes more symmetric and can, for example, be tuned to match closely the mode of a standard single-mode optical fiber. Furthermore, symmetric nanowires are shown to guide both TM and TE polarizations. In order to illustrate the applicability of plasmonic nanowire waveguides to optical circuits, we demonstrate a compact variable optical attenuator consisting of a single nanowire that simultaneously carries light and electrical current.

Bend loss in surface plasmon polariton band-gap structures

Using near-field optical microscopy, we investigate propagation of surface plasmon polaritons (SPPs) excited in the wavelength range of 720–830 nm at a corrugated gold-film surface with areas of 200-nm-wide and 45-nm-high scatterers arranged in a 410-nm-period triangular lattice containing line defects with double bends. We find that, for ∼2-μm-wide line defects and the wavelength of ∼740 nm, the double bend losses for bend angles of 15° and 30° are below 2 and 10 dB, respectively. Our data indicate that the bend loss increases approximately quadratically with the bend angle. We also demonstrate splitting and combining of two SPP line-defect modes in a 20-μm-long Y junction.

Long-range dielectric-loaded surface plasmon polariton waveguides operating at telecommunication wavelengths

We report on the realization of long-range dielectric-loaded surface plasmon polariton waveguides (LR-DLSPPWs) consisting of straight and bent subwavelength dielectric ridges deposited on thin and narrow metal stripes supported by a dielectric buffer layer covering a low-index substrate. Using imaging with a near-field optical microscope and end-fire coupling with a tapered fiber connected to a tunable laser at telecommunication wavelengths (1425-1545 nm), we demonstrate low-loss (propagation length ∼500 μm) and well-confined (mode width ∼1 μm) LR-DLSPPW mode guiding and determine the propagation and bend loss.

Evanescent-wave fluorescence microscopy using symmetric planar waveguides.

We describe a new evanescent-wave fluorescence excitation method, ideally suited for imaging of biological samples. The excitation light propagates in a planar optical waveguide, consisting of a thin waveguide core sandwiched between a sample in an aqueous solution and a polymer with a matching refractive index, forming a symmetric cladding environment. This configuration offers clear advantages over other waveguide-excitation methods, such as superior image quality, wide tunability of the evanescent field penetration depth and compatibility with optical fibers. The method is well suited for cell membrane imaging on cells in culture, including cell-cell and cell-matrix interaction, monitoring of surface binding events and similar applications involving aqueous solutions.

Compact Z-add-drop wavelength filters for long-range surface plasmon polaritons

We design, fabricate and investigate compact Z-add-drop (ZAD) filters for long-range surface plasmon polaritons (LR-SPPs) at telecom wavelengths. The ZAD filter for LR-SPPs consists of two ridge gratings formed by periodic gold thickness modulation at the intersections of three zigzag-crossed gold stripes embedded in polymer. We investigate influence of the grating length and crossing angle on the filter characteristics and demonstrate a 10 masculine- ZAD filter based on 80-microm-long gratings that exhibit a 15-dB dip (centered at ~ 1.55 microm) in transmission of the direct arm along with the corresponding ~ 13-nm-wide transmission peak in the drop arm.

Ultracompact metasurface in-line polarimeter

In-line polarimeters perform nondestructive polarization measurements of optical signals, and play a critical role in monitoring and controlling the polarization environment in, for example, optical networks. While current in-line polarimeters are constructed with multiple optical components, either fabricated into an optical fiber or using free-space optics, we present here a novel architecture conducive to monolithic on-chip integration. This enables the scalable fabrication of high-performance polarization sensors with exceptional stability, compactness, and speed. The method relies on the detection of the highly polarization-dependent scattered field of a subwavelength antenna array known as a metasurface, and is shown here to provide polarization state measurements matching those of a state-of-the-art commercial polarimeter.