Resonance surface plasmon spectroscopy by tunable enhanced light transmission through nanostructured gratings and thin films

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Yeh, Wei-Hsun
Major Professor
Andrew C. Hillier
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Chemical and Biological Engineering

urface plasmon resonance (SPR) is a powerful tool in probing interfacial events in that any changes of effective refractive index in the interface directly impact the behavior of surface plasmons, an electromagnetic wave, travelling along the interface. Surface plasmons (SPs) are generated only if the momemtum of incident light matches that of SPs in the interface. Observation of SPR can be achieved by either monitoring reflection via Kretchsmann configuration or enhanced transmission through nano-structured patterned substrates (for example, diffraction gratings). For Kretchsmann configuration, SPs resonate with particular frequencies of incident light and results in a decreased intensity in the reflection spectrum at corresponding wavelengths. For diffraction grating, enhanced transmission peaks at particular wavelengths is observed in that the SPs resonate with particular energies (eV) of incident light and tunnel through the nanostructure of gratings. This thesis focuses on tuning the behavior of SPs by changing the topology of diffraction gratings, monitoring the thickness of thin films by diffraction gratings, and use of dispersion images to analyze complex optical responses of SPs through diffraction gratings.

Chapter 1 covers the background/principle of SPR, comprehensive literature review, sensor applications, control of SPR spectral responses, and sensitivity of SPR. In Chapter 2, we illustrate a chirped grating with varying surface topology along its spatial position. We demonstrated that the features of nanostructure such as pitch and amplitude significantly impact the behavior of enhanced transmission. In addition, we also illustrate the sensing application of chirped grating and the results indicate that the chirped grating is a sensitive and information rich SPR platform. In chapter 3, we used a commercial DVD diffraction grating as a SPR coupler. A camera-mounted microscope with Bertrend lens attachment is used to oberserve the enhanced transmission. We demonstrate that this system can monitor the SPR responses and track the

thickness of a silicon monoxide film without using a spectrophotometer.

Surface plasmons are a result of collective oscillation of free electrons in the

metal/dielectric interface. Thus, the interaction of SPs with delocalized electrons from molecular resonance is complex. In chapter 4, we perform both experimental and simulation works to address this complex interaction. Detailed examination and analysis show nontypical SPR responses. For p-polarized light, a branch of dispersion curve and quenching of SPs in the Q band of zinc phthalocyanine are observed. For both p- and s-polarized light, additional waveguided modes are observed and the wavelength from different guided modes are dispersed.

Diffraction gratings can provide complicated optical information about SPs. Both front side (air/metal) and back side (metal/substrate) provide SPR signals simultaneously. In chapter 5, we use dispersion images to analyze the complicated optical responses of SPR from an asymmetrical diffraction grating consisting of three layers (air/gold/polycarbonate). We illustrate that clear identification of SPR responses from several diffraction orders at front side and back side can be achieved by the use of dispersion images. Theoretical prediction and experimental results show consistency. We also show that only the behavior of SPs from the front side is impacted by the deposition of Langmuir-Blodgett dielectric films.

In chapter 6, we construct a diffraction grating that has a fixed pitch and several amplitudes on its surface by using interference lithography. The purpose of this work is to examine how the amplitude impacts the behavior of transmission peaks. Different amplitudes are successfully fabricated by varying development time in the lithography process. We observed that largest (optimized) enhanced transmission peak shows as the amplitude approach a critical value. Transmission is not maximized below or beyond a critical amplitude. We also found that transmission enhancements are strongly affected by the diffraction efficiencies. A maximum

enhancement is observed as diffraction efficiency is largest where amplitude reaches the critical value. The experimental results are then compared to the simulation.

First, this work demonstrates that diffraction gratings have rich information of SPs. For example, rich optical responses of SPs can be acquired by the chirped grating. For another example, the information about the behavior of SPs can be acquired by tracking first order diffraction spots. All information can be utilized to monitor the thickness of ultra thin films formed on the gratings. Therefore, diffraction gratings represent a flexible and information-rich SPR platform. Second, the transmission peaks (or optical responses of SPs) can be tuned by the topology of the diffraction gratings. The resonant wavelengths of transmission peaks can be tuned by the pitches of gratings; the magnitude of peaks can be maximized by tuning the amplitudes of gratings. The control over transmission peaks allows ones to improve the performance of grating-based SPR sensors. Last, rich yet complex optical responses of SPs from diffraction gratings can be analyzed and indexed by the use of dispersion images. Complex optical responses originate from simultaneous excitations of SPs from metal/air and metal/polymer interfaces. By the use of dispersion images, enhanced transmission from the front side interface (metal/air) and reduced transmission from the back side interface (metal/polycarbonate) can be identified and different modes of SPs can be indexed.

Tue Jan 01 00:00:00 UTC 2013