Electric fields inside guided-mode resonance filters (GMRFs) may be intensified by resonance effects. The electric field enhancement is investigated in two GMRFs: one is resonant at normal incidence, the other at oblique incidence. It is shown that the two GMRFs exhibit different behaviors in their electric enhancement. Differences between the electric field distributions of the two GMRFs arise because coupling between counterpropagating modes occurs in the first case. It is also shown that the order of the electric field of maximum amplitude can be controlled by modulation of the dielectric constant of the grating.
"Comparing Figure 1(a) and Figure 2(a), we find that with the TM mode electric field excited in the grating structure has less intensity than that of the TE mode, but has more sensitivity. When the resonant wavelength is incident into the grating, almost all of the energy is coupled into the waveguide layer [17,18]. The strongest part is 5 times stronger than the weakest part for the TE mode as shown in Figure 2(a), while for TM case is only 2.2. "
[Show abstract][Hide abstract] ABSTRACT: Sensitivity is a key factor in the performance of a sensor. To achieve maximum guided-mode resonant optical biosensor sensitivity, a comparison of biosensor sensitivity for Transverse Electric (TE) and Transverse Magnetic (TM) modes based on the distribution of electric fields is presented in this article. A label-free guided-mode resonant optical biosensor is designed using the quarter-wave anti-reflection method to reflect only a narrow band of wavelengths modulated by the adsorption of a biochemical material on the sensor surface at the reflected frequency. With the distribution of electric fields simulated according to the Rigorous Coupled Wave Analysis (RCWA) theory, it is found that the full width at half maximum of the TM mode is (∼4 nm) narrower than that of the TE mode (∼20 nm), and the surface sensitivity of the TE mode incident light is three times that of the TM mode. It is proposed in this article that the light mode plays an important role in the sensitivity of guided-mode resonant biosensors.
[Show abstract][Hide abstract] ABSTRACT: An outstanding challenge in biomedical sciences is to devise systems that can enable rapid, simultaneous and quantitative imaging of tens to hundreds of species at ultra-low concentrations. Central to addressing this challenge is the availability of a modality capable of sensitive, rapid, cost-effective and multiplexed sensing. Further, the microscopic heterogeneity of intact biological systems necessitates that the sensing be in an imaging format. Vibrational spectroscopic imaging (both Raman and infrared) is an attractive tool due to its potential to obtain rich chemical and structural information using relatively accessible instrumentation. Its applicability in devising such modality however is limited by current detection limits, throughput and speed of acquisition. This dissertation discusses design of novel nanostructured devices for enhancing the sensitivity, acquisition rate and multiplexing capabilities in vibrational spectroscopic imaging. First, Surface-enhanced Raman scattering (SERS)-based substrates will be discussed and nanostructured particle probes are proposed. Their optical tunability with structure is discussed in detail and preliminary fabrication and validation are presented. Next, design and fabrication of a new class of filters for narrow-band optical reflection in mid-infrared spectral regions using guided mode resonances is demonstrated. The design principles and methodology presented in this dissertation are expected to provide a rational approach in development of sets of probes and filters to enable rapid, ultrasensitive acquisition of unlimited number of molecular targets.
[Show abstract][Hide abstract] ABSTRACT: We fabricated and measured the far-field optical properties of a sub-wavelength Si3N4 (silicon nitride) two dimensional grating. Frequency-dependent transmission measurements from a white-light source revealed that both transverse magnetic (TM) and transverse electric (TE) modes were excited on the grating. We determined the dispersion relations of the modes by tilting the sample with respect to the incoming light beam and measuring the frequency shift of the absorption features. By comparing to a simple model, we determined the effective refractive index for the TM and TE modes and the geometrical constants for the grating. This information enables gratings with desired optical properties to be designed and fabricated. The application of the sub-wavelength grating for surface-enhanced Raman scattering (SERS) is demonstrated.
Applied Physics A 11/2011; 105(2). DOI:10.1007/s00339-011-6613-8 · 1.70 Impact Factor
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