Optical Transmission Through Sub-Wavelength Slits in Metals:from Theory to Applications

This thesis explores the properties of the transmission of light through subwavelength slits in metal films. Theoretical and experimental studies on the transmission through slits in metals are presented. In addition, the potential use of these slits for different applications is investigated. The theoretical aspect concentrates on the study of the transmission of light through slit arrays. It is observed that high transmission is due to the coupling of two modes: one whose properties depend on the slit geometry and the other one whose properties depend on the grating periodicity. To determine the exact nature of these hybrid modes an analytical model is derived. The simplified model gives accurate analytical expressions for the transmission and also for the dispersion relations of the hybrid modes responsible for high transmission. The dispersion relations give a clear picture of the role played by the different resonances in the transmission process. We find that the high transmission modes are hybrid in nature between that of a Fabry-Pérot mode and a surface plasmon polariton mode. These hybrid modes have either symmetric or antisymmetric profiles. These findings are important as they clarify the nature of the modes responsible for high transmission, and therefore can be useful as a design tool for metal gratings for various applications. The light transmission through single slits, slit and groove structures and slit arrays is optically characterized. Also, slit and groove structures covered by a thin dielectric layer are fabricated. With this type of structure, transverse electric (TE) polarized dielectric waveguide modes can be excited inside the dielectric film, which strongly increases the TE polarized transmission as compared to conventional slit and groove structures, which only gives high transmission for transverse magnetic (TM) polarization. These structures demonstrate an efficient way to enhance light transmission through subwavelength apertures as both TE and TM polarization can be efficiently transmitted. The transmission measurements for the different types of structures are compared with theory. This gives an idea of the validity of the theoretical tools considered, which is necessary to address the potential of subwavelength slits in metals for applications. Taking advantage of the theoretical and experimental work developed in this thesis, the use of slits in metals is considered for three different optical devices: biosensors, light emitting diodes and image sensors. In the case of biosensors, slit arrays whose optical transmission is highly sensitive to refractive index changes could be useful to design compact and portable sensors. Our work show that a high sensitivity to refractive index changes can be obtained in the case of suspended slit arrays. As an example, theoretically an adsorption of a 1.6 nm thick layer of bovine serum albumin on a slit array could provide a 3% intensity change. Slit arrays are also investigated for light emitting diodes in order to obtain transparent metal contacts. It is shown that light transmission through slit arrays on a high refractive index substrate, characteristic of light emitting diode, can be very efficient. Light can be extracted even for angles of incident larger than the critical angle due to grating coupling. Preliminary calculations show that a metal contact with arrays of slit can give twice the extraction efficiency of a planar GaP-air interface. Finally, slit arrays and slit and groove structures have been integrated as a post process onto pixels of image sensors. In the case of a pixel with a slit array, a polarization extinction ratio higher than 200 over a bandwidth of 200 nm is measured, which means that slit arrays can be used as very efficient micro-polarizers in view of polarization imaging. In the case of pixels with slit and groove structures, measurements show that the slit and groove structure could improve by a factor of eight the signal measured on the pixel. Such structure could be used to replace current optical systems or to improve device performances by increasing the signal to noise ratio and allowing polarization and colour selectivity

Observation of enhanced transmission for s-polarized light through a subwavelength slit

Enhanced optical transmission (EOT) through subwavelength apertures is usually obtained for p-polarized light. The present study experimentally investigates EOT for s-polarized light. A subwavelength slit surrounded on each side by periodic grooves has been fabricated in a gold film and covered by a thin dielectric layer. The excitation of s-polarized dielectric waveguide modes inside the dielectric film strongly increases the s-polarized transmission. Transmission measurements are compared with a coupled mode model and show good qualitative agreement. Adding a waveguide can improve light transmission through subwavelength apertures, as both s and p-polarization can be efficiently transmitted.Comment: 11 pages, 3 figures, submitted to Applied Physics Letter

Experimental evidence of cut-wire-induced enhanced transmission of transverse-electric fields through sub-wavelength slits in a thin metallic screen

Recent numerical studies have demonstrated the possibility of achieving substantial enhancements in the transmission of transverse-electric-polarized electromagnetic fields through subwavelength slits in a thin metallic screen by placing single or paired metallic cut-wire arrays at a close distance from the screen. In this Letter, we report on the first experimental evidence of such extraordinary transmission phenomena, via microwave (X/Ku-band) measurements on printed-circuit-board prototypes. Experimental results agree very well with full-wave numerical predictions, and indicate an intrinsic robustness of the enhanced transmission phenomena with respect to fabrication tolerances and experimental imperfections.Comment: 6 pages, 4 figures (slight revision

Enhancement of two photon processes in quantum dots embedded in subwavelength metallic gratings

We show a large enhancement of two-photon absorption processes in nanocrystal quantum dots and of light upconversion efficiency from the IR to the near-IR spectral regime, using a hybrid optical device in which near-IR emitting InAs quantum dots were embedded on top a metallic nanoslit array. The resonant enhancement of these nonlinear optical processes is due to the strong local electromagnetic field enhancements inside the nanoslit array structure at the extraordinary transmission resonances. A maximal two-photon absorption enhancement of more than 20 was inferred. Different high field regions were identified for different polarizations, which can be used for designing and optimizing efficient nonlinear processes in such hybrid structures. Combining nanocrystal quantum dots with subwavelength metallic nanostructures is therfore a promising way for a range of possible nonlinear optical devices.Comment: 14 pages, 7 figure

Light scattering by an array of electric and magnetic nanoparticles

Light scattering by an array of alternating electric and magnetic nanoparticles is analyzed in detailed. Specific geometrical conditions are derived, where such an array behaves like double-negative particles, leading to a suppression of the backscattered intensity. This effect is very robust and could be used to produce double-negative metamaterials using single-negative components

Transflective holographic film for head worn display

A display panel assembly comprises a transflective holographic screen, i.e., a transparent screen that reflects light from a projection system, comprising at least a volume hologram, a first protective element and a second protective element, each arranged in contact with the volume hologram such that the volume hologram is sandwiched between the first protective element and the second protective element. The display panel assembly further comprises a projection system focusing an image on the volume hologram comprising at least projection optics, mounting means arranged to fixedly mount the projection system relatively to the transflective holographic screen. The volume hologram comprises a plurality of diffractive patterns disposed in sequence across the volume hologram, each of the plurality of diffractive patterns being configured to diffuse the light rays from the projection system in a determined direction corresponding to the specific diffractive pattern and oriented towards a position of an intended eye of a user wearing the display panel assembly

Large area hole arrays fabricated by a templating method for refractive index sensing

Subwavelength hole arrays in thin gold films were fabricated in a low-cost and parallel fabrication process on 2" wafers. Polystyrene beads were used as an initial pattern template which is transferred into a thin gold film by sputter etching. Hole arrays with two different periodicities were characterized in optical transmission measurements in air and in liquids of varying refractive index. Despite the conical hole shape and long-range disorder of the arrays, the optical transmission spectra are similar to those obtained for precise hole arrays fabricated by serial writing methods, e.g. focused ion beam milling (FIB). We further show the use of the fabricated hole arrays for sensing of molecular binding events

Inexpensive and fast wafer-scale fabrication of nanohole arrays in thin gold films for plasmonics

In this paper, a fast and inexpensive wafer-scale process for the fabrication of arrays of nanoscale holes in thin gold films for plasmonics is shown. The process combines nanosphere lithography using spin-coated polystyrene beads with a sputter-etching process. This allows the batch fabrication of several 1000 μm2 large hole arrays in 200 nm thick gold films without the use of an adhesion layer for the gold film. The hole size and lattice period can be tuned independently with this method. This allows tuning of the optical properties of the hole arrays for the desired application. An example application, refractive index sensing, is demonstrated