Ilya P. Radko

Efficient unidirectional nanoslit couplers for surface plasmons

The emerging field of plasmonics is based on exploiting the coupling between light and collective electronic excitations within conducting materials known as surface plasmons. Because the so-called surface plasmon polariton (SPP) modes that arise from this coupling are not constrained by the optical diffraction limit, it is hoped that they could enable the construction of ultracompact optical components1,2. But in order that such potential can be realized, it is vital that the relatively poor light–SPP coupling be improved. This is made worse by the fact that the incident light that is conventionally used to launch SPPs in a metal film 3,4,5,6 is a significant source of noise, unless directed away from a region of interest, which then decreases the signal and increases the system’s size. Back-side illumination of subwavelength apertures in optically thick metal films7,8,9,10,11,12,13 eliminates this problem but does not ensure a unique propagation direction for the SPP. We propose a novel back-side slit-illumination method that incorporates a periodic array of grooves carved into the front side of a thick metal film. Bragg reflection enhances the propagation of SPPs away from the array, enabling them to be unidirectionally launched from, and focused to, a localized point.

Laser-fabricated dielectric optical components for surface plasmon polaritons

Fabrication of dielectric optical components for surface plasmon polaritons (SPPs) by two-photon polymerization (2PP) is studied. This direct-write femtosecond laser technology provides a low-cost and flexible method for the fabrication and investigation of plasmonic structures and optical components. Using the 2PP technique, we fabricated narrow dielectric ridges with dimensions as small as 150 nm on metal surfaces. SPP excitation with the laser-fabricated structures and guiding along them are demonstrated.

Hybrid graphene plasmonic waveguide modulators.

The unique optical and electronic properties of graphene make possible the fabrication of novel optoelectronic devices. One of the most exciting graphene characteristics is the tunability by gating which allows one to realize active optical devices. While several types of graphene-based photonic modulators have already been demonstrated, the potential of combining the versatility of graphene with subwavelength field confinement of plasmonic waveguides remains largely unexplored. Here we report fabrication and study of hybrid graphene–plasmonic waveguide modulators. We consider several types of modulators and identify the most promising one for telecom applications. The modulator working at the telecom range is demonstrated, showing a modulation depth of >0.03u2009dBu2009μm−1 at low gating voltages for an active device area of just 10u2009μm2, characteristics which are already comparable to those of silicon-based waveguide modulators while retaining the benefit of further device miniaturization. Our proof-of-concept results pave the way towards on-chip realization of efficient graphene-based active plasmonic waveguide devices for optical communications.

Surface plasmon polariton beam focusing with parabolic nanoparticle chains

We report on the focusing of surface plasmon polariton (SPP) beams with parabolic chains of gold nanoparticles fabricated on thin gold films. SPP focusing with different parabolic chains is investigated in the wavelength range of 700-860 nm, both experimentally and theoretically. Mapping of SPP fields is accomplished by making use of leakage radiation microscopy, demonstrating robust and efficient SPP focusing into submicron spots. Numerical simulations based on the Greens tensor formalism show very good agreement with the experimental results, suggesting the usage of elliptical corrections for parabolic structures to improve their focusing of slightly divergent SPP beams.

Efficiency of local surface plasmon polariton excitation on ridges

We investigate experimentally and numerically the efficiency of surface plasmon polariton excitation by a focused laser beam using gold ridges. The dependence of the efficiency on geometrical parameters of ridges and wavelength dependence are examined. The experimental measurements accomplished using leakage radiation microscopy. The numerical simulations are based on Greens tensor approach.

Refracting Surface Plasmons with Nanoparticle Arrays

Variously shaped structures formed with a 100-nm-period square lattice of gold nanoparticles placed on a gold film are shown to possess an effective refractive index of about 1.08 for SPPs propagating through them.

Modulation of surface plasmon coupling-in by one-dimensional surface corrugation

Surface plasmon-polaritons have recently attracted renewed interest in the scientific community for their potential in sub-wavelength optics, light generation and non-destructive sensing. Given that they cannot be directly excited by freely propagating light due to their intrinsic binding to the metal surface, the light - plasmon coupling efficiency becomes of crucial importance for the success of any plasmonic device. Here, we present a comprehensive study on the modulation ( enhancement or suppression) of such a coupling efficiency by means of one-dimensional surface corrugation. Our approach is based on simple wave interference and enables us to make quantitative predictions which have been experimentally confirmed at both the near-infrared and telecom ranges.

Localized field enhancements in fractal shaped periodic metal nanostructures

Fractal shaped structures formed with a 100-nm-period square lattice of gold nanoparticles placed on a gold film are characterized by using far-field nonlinear scanning optical microscopy, in which two-photon photoluminescence (TPL) excited with a strongly focused laser beam (in the wavelength range of 730 - 790 nm) is detected. The TPL images recorded for all wavelengths exhibit diffraction-limited (~ 0.6 mum) bright spots corresponding to the field intensity enhancement of up to 150, whose positions are dictated by the incident light wavelength and polarization. We relate these field enhancements to the occurrence of constructive interference of surface plasmons (SPs), which are excited by the incident radiation (due to scattering by nanoparticles) and partially reflected by fractal shaped boundaries due to a difference in the SP effective index at a flat and periodically corrugated gold surface. The conjecture on SP index difference is verified with observations (using leakage radiation microscopy) of SP focusing by circular and waveguiding by rectangular areas filled with periodically arranged nanoparticles.

Transfer function and near-field detection of evanescent waves

We consider characterization of a near-field optical probe in terms of detection efficiency of different spatial frequencies associated with propagating and evanescent field components. The former are both detected with and radiated from an etched single-mode fiber tip, showing reciprocity of collection and illumination modes. Making use of a collection near-field microscope with a similar fiber tip illuminated by an evanescent field, we measure the collected power as a function of the field spatial frequency in different polarization configurations. Considering a two-dimensional probe configuration, numerical simulations of detection efficiency based on the eigenmode expansion technique are carried out for different tip apex angles. The detection roll-off for high spatial frequencies observed in the experiment and obtained during the simulations is fitted using a simple expression for the transfer function, which is derived by introducing an effective point of (dipolelike) detection inside the probe tip. It is found to be possible to fit reasonably well both the experimental and the simulation data for evanescent field components, implying that the developed approximation of the near-field transfer function can serve as a simple, rational, and sufficiently reliable means of fiber probe characterization.

Adiabatic bends in surface plasmon polariton band gap structures

Propagation and interaction of surface plasmon polaritons (SPPs) excited in the wavelength range 700-860 nm with periodic triangular arrays of gold bumps placed on gold film surfaces are investigated using a collection near-field microscope. We observe the inhibition of SPP propagation into the arrays within a certain wavelength range, i.e., the band gap (BG) effect. We demonstrate also the SPP propagation along a 30 degrees bent channel obtained by an adiabatic rotation of the periodic array of scatterers. Numerical simulations using the Lippmann-Schwinger integral equation method are presented and found in reasonable agreement with the experimental results.