Optical Transmission Through Sub-Wavelength Slits in Metals

Abstract

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-Perot 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.