Supercritical angle fluorescence (SAF) is a surface sensitive detection mechanism that has been used to perform rapid, sensitive detection of biomolecules. However, the SAF biosensors that have been demonstrated so far have been either bulky and expensive or small and affordable while sacrificing on sensitivity . We present a novel interference based SAF biosensor technology that has the potential to be fast, small, inexpensive and mass manufacturable while being as sensitive as the current gold standard. This technology can therefore create biosensors in a wide range of form factors, from large, complex high multiplex devices to small handheld point of care solutions.
Supercritical angle fluorescence (SAF) is a near-field collection method that has surface sensitivities similar to or better than near-field excitation techniques like TIRF and waveguide based excitation. SAF is emitted by fluorophores that are a few hundred nanometers away from an interface, above the critical angle, into the higher index material. SAF decreases exponentially with increasing distance from the interface and is therefore more sensitive to molecules near the surface. Although a lot of research has used SAF for biosensing and microscopy, the angular dependence of SAF on both the surface and bulk fluorescence contributions hasn’t been experimentally studied. We present a method that measures the surface selectivity of SAF in the presence of bulk fluorophores. Two different fluorophores were used. One was bound to the surface and the other was suspended in the bulk. The spectrum was measured at discrete points in the back focal plane (BFP) and the contribution of the two fluorophores was extracted from it. The results of the experiment show the highest signal-to-noise ratio in the region just above the critical angle of 61.31º because of the higher signal intensity. However, for experiments where bulk exclusion is important, we observe the highest signal-to-bulk ratio at angles above 68˚ for a glass-water interface. Understanding the angular dependence on the sensitivity of a SAF biosensor enables tuning the collection angles towards specific applications and could lead to the creation of smaller, more sensitive devices.
Through the effect of Purcell enhancement, nanoantennas strongly modify the local density of optical states and control the emission of coupled emitters. These antennas determine, in addition to the emission spectrum and polarization, also the angular distribution of the emitted photons, i.e., the radiation pattern. Nearly all directional nanoantennas reported so far, rely on external excitation schemes such as a laser or scanning tunneling microscope (STM) tip, which severely hamper on-chip integration. Here, we experimentally demonstrate for the first time, unidirectional light emission from electrically driven in-plane two-wire nanoantennas in the shape of the letter V. The antenna wires are connected with narrow electrical leads which support electrical currents while preserving the resonant properties of the antenna [1]. A nanoscopic tunneling gap is formed at the feed point of the antenna through a controlled electromigration procedure. Strong far-field interference between the spontaneous dipolar light emission of the tunnel junction and the fundamental quadrupolar resonance of the antenna gives rise to a directional radiation pattern [2]. We show that this directivity can be actively tuned with the applied voltage, and passively tuned with the antenna geometry. The experimental findings are analyzed in detail through electro-optical characterization and extensive numerical simulations. These fully configurable ultra-fast tunneling nanoantennas seamlessly exploit light-matter interactions at the nanoscale and set a new paradigm for directing optical energy on chip using an extremely small footprint. [1] Kern et al. Nature Photonics (2015) 9, 582–586; [2] Vercruysse et al. ACS Nano (2014) 8(8), 8232−8241.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.