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Research - Enhanced FluorescenceEnhanced Excitation and Extraction of fluorescenceEnhanced Excitation:Enhancing the output of fluorescent species is highly desirable for a wide range of biological applications including DNA sequencing, gene expression, single molecule detection and high resolution cell imaging. An extensive literature exists on the effects of metals on fluorophores, and on the use of a variety of metal surface configurations or metallic particle that involve plasmonic resonances to enhance fluorescence. A relatively smaller variety of dielectric structures, such as waveguides and optical resonators have been exploited for this purpose. The Guided-mode resonance (GMR) effect occurring in photonic crystal slabs leads to heightened energy density within the resonator under steady-state operation conditions (via continual reinforcement of the leaky modes by the externally incident illumination), the magnitude of which is directly related to the resonance quality factor. A high resonance Q-factor leads to high intensity near-fields with which fluorophores can interact and fluoresce with greater intensity, due to an enhanced absorption rate. The electric near-fields produced by a device in response to TM polarized incidence are shown in Fig. 1, which shows the electric field intensity (Ez2, Ex2). As is evident, the maximum near-field intensity occurs where the resonant mode is confined, with evanescent tails penetrating both the substrate and superstrate of the device. For the purpose of fluorescence enhancement, these enhanced fields represent greater incident energy density.
Fig 1: Calculated near-electric field intensity profiles for the resonant mode at λ=632.8 nm for TM polarized incidence showing enhancement of both Ez and Ex field components. The color scale associated with each figure represents the intensity of the electric field and is normalized to the unit intensity incident wave.
Example of enhancement in detection sensitivity: Detecting Cy-5 conjugated Streptavidin
Fig 2: Fluorescence scans off (left) and on (right) resonance measured for photonic crystal device, showing the enhanced fluorescence effect. The plot below the figure shows the intensity profile on the sensor surface as a function of position, as marked by the line on the fluorescence scans. Enhanced extraction: The existence of leaky modes that overlap the fluorescence emission spectrum opens up pathways for the emitted light to escape into free-space. Besides direct emission, the fluorescence can now couple to the overlapping leaky modes and Bragg scatter out of the structure, thereby greatly reducing the amount of light trapped as guided-modes, in comparison to an unpatterned substrate. If the dispersion of these overlapping leaky modes is close to the Г-point band-edge i.e. K|| (magnitude of in-plane wave vector) ~ 0, most of the emitted light will be extracted within small angles with the surfaced normal. More generally, appropriately engineering the leaky dispersion of the PC facilitates the funnelling of guided light into regions of space where it can be easily detected
In our group, we attempt to engineer the band structure of photonic crystals in order exploit both the enhanced excitation and enhanced extraction effects at once, which can result in greatly enhanced signal to noise ratio in the detection of molecular fluorescence:
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Research Areas