John Weiner

Quantitative determination of optical transmission through subwavelength slit arrays in Ag films: Role of surface wave interference and local coupling between adjacent slits

Measurement of the transmitted intensity from a coherent monomode light source through a series of subwavelength slit arrays in Ag films, with varying array pitch and number of slits, demonstrate enhancement (suppression) by as much as a factor of 6 (9) when normalized to that of an isolated slit. Pronounced minima in the transmitted intensity were observed at array pitches corresponding to λSPP, 2 λSPP and 3λSPP where λSPP is the wavelength of the surface plasmon polariton (SPP). Increasing the number of slits to more than four does not increase appreciably the per-slit transmission intensity. These results are consistent with a model for interference between SPPs and the incident wave that fits well the measured transmitted intensity profile.

Transient behavior of surface plasmon polaritons scattered at a subwavelength groove

We present a numerical study and analytical model of the optical near field diffracted in the vicinity of subwavelength grooves milled in silver surfaces. The Green’s tensor approach permits the computation of the phase and amplitude dependence of the diffracted wave as a function of the groove geometry. It is shown that the field diffracted along the interface by the groove is equivalent to replacing the groove by an oscillating dipolar line source. An analytic expression is derived from the Green’s function formalism, which reproduces well the asymptotic surface plasmon polariton SPP wave as well as the transient surface wave in the near zone close to the groove. The agreement between this model and the full simulation is very good, showing that the transient “near-zone” regime does not depend on the precise shape of the groove. Finally, it is shown that a composite diffractive evanescent wave model that includes the asymptotic SPP can describe the wavelength evolution in this transient near zone. Such a semianalytical model may be useful for the design and optimization of more elaborate photonic circuits, whose behavior in a large part will be controlled by surface waves.

Influence of film thickness on the optical transmission through subwavelength single slits in metallic thin films

Silver and gold films with thicknesses in the range of 120-450 nm were evaporated onto glass substrates. A sequence of slits with widths varying between 70 and 270 nm was milled in the films using a focused gallium ion beam. We have undertaken high-resolution measurements of the optical transmission through the single slits with 488.0 nm (for Ag) and 632.8 nm (for Au) laser sources aligned to the optical axis of a microscope. Based on the present experimental results, it was possible to observe that (1) the slit transmission is notably affected by the film thickness, which presents a damped oscillatory behavior as the thickness is augmented, and (2) the transmission increases linearly with increasing slit width for a fixed film thickness.

High-frequency response of subwavelength-structured metals in the petahertz domain

Electromagnetic plane waves, incident on and reflecting from a dielectric-conductor interface, set up a standing wave in the dielectric with the B-field adjacent to the conductor. It is shown here how the harmonic time variation of this B-field induces an E-field and a conduction current J (c) within the skin depth of a real metal; and that at frequencies in the visible and near-infrared range, the imaginary term sigmai of the complex conductivity sigma = sigma(r) + isigma(i) dominates the optical response. Continuity conditions of the E-field through the surface together with the in-quadrature response of the conductivity determine the phase relation between the incident E-M field and J(c). If slits or grooves are milled into the metal surface, a displacement current in the dielectric gap and oscillating charge dipoles at the structure edges are established in quadrature phase with incident field. These dipoles radiate into the aperture and launch surface waves from the edges. They are the principle source of light transmission through the apertures.

The physics of light transmission through subwavelength apertures and aperture arrays

The passage of light through apertures much smaller than the wavelength of the light has proved to be a surprisingly subtle phenomenon. This report describes how modern developments in nanofabrication, coherent light sources, and numerical vector field simulations have led to the upending of early predictions from scalar diffraction theory and classical electrodynamics. Optical response of real materials to incident coherent radiation at petahertz frequencies leads to unexpected consequences for transmission and extinction of light through subwavelength aperture arrays. This talk is a report on progress in our understanding of this phenomenon over the past decade.

Metal–Insulator–Metal Surface Plasmon Polariton Waveguide Filters With Cascaded Transverse Cavities

In this work we propose a new approach for the design of resonant structures aiming at wavelength filtering applications. The structure consists of a subwavelength metal-insulator-metal (MIM) waveguide presenting cascaded cavities transversely arranged in the midpoint between the input and output ports. An extra degree of freedom added to this design consists in tilting the cavities around their midpoints which, besides effectively increasing the quality factor of the cavity, helps extending its range of applications by tuning multiple wavelengths.

Surface plasmon propagation in novel multilayered metallic thin films

Multilayered Ag/Au/Ag/Au and Au/Ag/Au/Ag films with 200 nm of thickness (50 nm for each layer) were evaporated onto BK7 glass substrates. Sequences of slits (around 60-600 nm of width) were milled with a focused gallium ion beam in the films. We have undertaken a series of high-resolution measurements of the optical transmission through the slits. The transmission measurement setup consists of 488.0 nm (for the Ag/Au/Ag/Au film) and 632.8 nm (for the Au/Ag/Au/Ag sample) wavelength light beams from Ar ion and HeNe lasers, respectively, aligned to the optical axis of a microscope. The beam is focused onto the sample surface by a microscope objective in TM polarization (magnetic Hfield component parallel to the long axis of the slits). As well, theoretical estimates investigating the slits optical transmission were performed. The origin of the slits transmission is mainly attributed to plasmonic surface excitations. Based on the present results, it was possible to observe that (1) the transmission increases linearly with increasing slit width, and (2) the transmission of the multilayered structures is augmented in comparison with a single perforated metal film of equal thickness, for a fixed slit width. A very good correspondence between theory and experiment was observed.