Midfield microscope: Exploring the extraordinary

Midfield microscope: Exploring the extraordinary

Docter, M.W.

Young, I.T. (promotor)
Garini, Y. (promotor)

Applied Sciences

2008-10-13

In this thesis the development of the midfield microscope is presented. This is a microscope in which the extraordinary transmission (EOT) through sub-wavelength hole-arrays is applied. Before trying to combine microscopy and EOT, we look at them separately. In chapter 1 an overview is given of the current microscope techniques. The main research questions posed are which qualities, like resolution, the midfield microscope will possess and more importantly whether it will be an addition to the current variety of microscopes. EOT has been subject of research ever since its discovery. A literature study is given in chapter 2, in which both experimental and theoretical results are given. Physical explanations are developed, like surface plasmons and coupled diffracted evanescent waves. For applications, like microscopy or lithography, these explanations are of less importance, as long as the device is working. Because of the lack of a unified theory to predict the transmitted spectra and intensity distribution, we analyze them ourselves. The influence of various parameters, like period or incidence angle, is measured on home-made arrays and is described in chapter 3. We confirmed experimentally that the simple surface plasmon model predicts the transmission spectrum only up to a certain level; experimental verification is an absolute must. Polarization measurements show that the polarization has no influence on the spectrum in 2D, which supports the idea of the array being an active device. The limited angular spread is supported by far-field measurements of the transmission for both Köhler and collimated illumination. Now that we measured the spectra, we should know whether the wavelengths corresponding to the peaks result in an interesting transmission pattern. A theoretical calculation, which is limited to the case of a (two-dimensional) slit-array, is done in chapter 4. The transmission pattern having the best contrast and highly localized spots is not the one corresponding to the peak intensity, but to an interference pattern because the wavelength smaller than the period. Fortunately the intensity is still enhanced (at that wavelength more light is transmitted through a periodic than through a random hole-array, chapter 3). Through an analytical approach was found that for a wavelength smaller than the period the contribution to the pattern is mostly due to the transmission (and not so much the surface waves). The theoretical predictions gave already enough information to formulate the working of the midfield microscope. In chapter 5 it is shown that the microscope is comparable with a confocal one, but instead of two pinholes the hole-array is used in the illumination pathway and a CCD in the imaging pathway. Based on the predicted size of lobes in the intensity pattern the midfield resolution is similar to the confocal one, because the point spread functions have a similar size. The possible use of increased resolution by means of structured illumination in possible combination with nonlinear techniques is discussed. In chapter 6 fluorescence measurements are described. Fluorescent molecules are used as probes for the local intensity. The measurement with continuous fluorescence results in an intensity distribution, similar to the one predicted earlier. This is the first time fluorescence is used to measure the transmitted intensity pattern through a hole-array. Moreover, it is a proof-of-principle that the midfield microscope is able to image a sample, in this case continuously labeled and placed in the transmitted "midfield" intensity distribution. Chapter 7 answers the research questions, conclude this thesis and gives recommendations for further research.

extraordinary transmission
sub-wavelength
microscope

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doctoral thesis

(c) 2008 M.W. Docter