Pieter Neutens

Electrical Excitation of Confined Surface Plasmon Polaritons in Metallic Slot Waveguides

We present the realization of an integrated electrical source of confined surface plasmon polaritons (SPPs) in metal-insulator-metal waveguides. Using an integrated light-emitting diode (LED) and subwavelength slits, we can couple light emitted by the LED directly into waveguided plasmon modes. Polarization-dependent measurements, decay length measurements, and spectral measurements are performed to confirm the electrical excitation of SPPs and characterize the properties of the plasmonic waveguide.

Bright and dark plasmon resonances of nanoplasmonic antennas evanescently coupled with a silicon nitride waveguide

In this work we investigate numerically and experimentally the resonance wavelength tuning of different nanoplasmonic antennas excited through the evanescent field of a single mode silicon nitride waveguide and study their interaction with this excitation field. Experimental interaction efficiencies up to 19% are reported and it is shown that the waveguide geometry can be tuned in order to optimize this interaction. Apart from the excitation of bright plasmon modes, an efficient coupling between the evanescent field and a dark plasmonic resonance is experimentally demonstrated and theoretically explained as a result of the propagation induced phase delay.

Harnessing plasmon-induced ionic noise in metallic nanopores.

The ionic properties of a metal-coated silicon nanopore were examined in a nanofluidic system. We observed a strong increase of the ionic noise upon laser light illumination. The effect appeared to be strongly mediated by the resonant excitation of surface plasmons in the nanopore as was demonstrated by means of ionic mapping of the plasmonic electromagnetic field. Evidence from both simulations and experiments ruled out plasmonic heating as the main source of the noise, and point toward photoinduced electrochemical catalysis at the semiconductor-electrolyte interface. This ionic mapping technique described is opening up new opportunities on noninvasive applications ranging from biosensing to energy conversion.

Angular dependence of bulk fluorescence noise in supercritical angle fluorescence (Conference Presentation)

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.

Supercritical Angle Fluorescence Characterization Using Spatially Resolved Fourier Plane Spectroscopy

Most fluorescent immunoassays require a wash step prior to read-out due to the otherwise overwhelming signal of the large number of unbound (bulk) fluorescent molecules that dominate over the signal from the molecules of interest, usually bound to a substrate. Supercritical angle fluorescence (SAF) sensing is one of the most promising alternatives to total internal reflection fluorescence for fluorescence imaging and sensing. However, detailed experimental investigation of the influence of collection angle on the SAF surface sensitivity, i.e., signal to background ratio (SBR), is still lacking. In this Letter, we present a novel technique that allows to discriminate the emission patterns of free and bound fluorophores simultaneously by collecting both angular and spectral information. The spectrum was probed at multiple positions in the back focal plane using a multimode fiber connected to a spectrometer and the difference in intensity between two fluorophores was used to calculate the SBR. Our study clearly reveals that increasing the angle of SAF collection enhances the surface sensitivity, albeit at the cost of decreased signal intensity. Furthermore, our findings are fully supported by full-field 3D simulations.

On-chip near field fluorescence excitation and detection with nanophotonic waveguides for enhanced surface sensitivity (Conference Presentation)

Fluorescence is a widely used transduction mechanism in bio-imaging, sensing or physical chemistry characterization applications. The ability to selectively excite desired molecules without generating considerable bulk background from nearby molecules is very important for all these applications. A near field excitation using an exponentially decaying evanescent field is often used to reduce the bulk background by selectively exciting molecules near to the surface. We propose an on-chip platform to improve the surface and bulk fluorescence separation by combining near-field excitation and near-field collection. We used the exponentially decaying evanescent tail of a Silicon Nitride rib waveguide to excite molecules and coupled the subsequent emission back via the same waveguide. We observe from the finite difference time domain simulation that both the excitation and coupling efficiency depend exponentially on the surface-molecule distance. Thus, combination of near field excitation and collection improves surface-bulk separation. A reduction by half in effective 1/e decay length was found experimentally for this combined near-field excitation and collection technique compare to the conventional only near-field excitation based technique . An analytical model is derived to find the optimum device efficiency for bio-sensing applications and established a general condition for sensor length to maximize the device efficiency and validated by experimental data. Finally, we used this platform for Fluorescence Correlation Spectroscopy and steady-state fluorescence anisotropy measurement. In this talk, I will present the fabrication, characterization and experimental results obtained using this proposed waveguide based platform.

All-dielectric nanoantennas for wavelength-controlled directional scattering of visible light(Conference Presentation)

Optical antennas have the prospect to redirect light rays into engineered directions at the subwavelength scale. They offer new options for photodetection, color routing, fluorescence emission and sensing applications. Previously, metallic nanoantennas based on localized surface plasmon resonance (LSPR) have commonly been exploited to fulfill this purpose, e.g. the gold Yagi-Uda array and split-ring resonator. However, the intrinsic ohmic losses of metals are large especially in the visible range, hindering further efficiency improvement for practical applications. In addition, the interaction of the metallic nanoantennas with the magnetic component of light is relatively weak, adding to their lack of highly tunable scattering directionality. In this presentation, we will demonstrate our recent experimental progress on all-dielectric nanoantennas made of amorphous silicon with tunable scattering directionality. Our nanoantennas are designed as V-shaped single element and fabricated using electron-beam lithography followed by dry reactive ion etching. It is illustrated that the scattering cross section of the silicon nanoantenna can be considerably higher than that of comparable Au antennas. In addition, the extinction coefficient of amorphous silicon is adequately low in the considered wavelength range, resulting in minimal absorption losses and an enhanced scattering efficiency. More interestingly, compared to Au nanoantennas that exhibit light scattering in a single particular direction, by carefully engineering the geometry of the silicon nano-antennas, their scattering can be effectively tuned into two opposite directions within the visible range (Supporting figure). Over a spectral range of less than 100 nm, the scattering directionality gradually shifts from the leftward to the rightward. More simulation results based on the finite difference time domain (FDTD) methods are available to perfectly match and corroborate our experimental measurement. Initial analysis of the underlying physics for the tunable scattering directionality will also be discussed. Such unique optical properties make the silicon nanoantenna promising candidates for novel low-loss optical devices that can enable unprecedented control over the scattering directionality.

Fluorescence excitation and detection on a chip using nanophotonic waveguides(Conference Presentation)

Often, in bio-sensing applications rely on fluorescence as the transduction mechanism. The emission from the fluorescently labeled molecules is detected and quantified to obtain the concentration. Most current techniques, such as ELISA, FISH, next generation DNA sequencing and others need one or several washing steps to remove the unreacted fluorescent molecules to reduce the background noise. To address that problem, we propose an integrated nano-photonic solution for on-chip fluorescence detection. The proposed platform is tailored for bio-sensing applications and has the potential to analyze bio-molecular interactions down to the single molecule level. The technology is based on near-field excitation and collection using PECVD Silicon Nitride (SiN) nano-photonic rib waveguides. SiN provides the combination of high-index-contrast and compatibility with CMOS processing technology, and unlike silicon, Silicon Nitride does not absorb in the visible wavelength window. The evanescent tail of the used SiN waveguide mode extends from 80 nm to 200 nm above the waveguide surface depending on the waveguide geometry, cladding material and excitation wavelength. Hereby, the evanescent field of the waveguide mode excites a very thin layer of molecules near to the surface. The subsequent emission from the excited fluorophore is collected in the near field by coupling to another waveguide. The coupling strength mainly depends on the distance between the waveguide and fluorophore. This way, both the excitation and collection efficiency have an exponential dependency on the distance between the molecule and waveguide surface. Therefore, exciting and collecting fluorescence using photonic waveguides improves the separation between the surface bound fluorescence signal from the bulk background noise, paving the way for wash free bio-sensing. Wash-free assays allow to examine the bio-molecular interactions in real time and simplify the sample/liquid handling system. Next to bulk fluorescence, autofluorescence generated in the SiN waveguide is another large contributor to noise. In the proposed design, the two separate single mode waveguides we use to excite fluorophores and collect the emission, are placed orthogonally in a cross configuration. The orthogonal placement of two Transverse Electric (TE) mode waveguides makes sure that the auto-fluorescence generated in the excitation waveguide is not coupled to the emission waveguide. As a result, we observe an improved signal-to-noise-ratio (SNR) which is a very critical parameter towards single molecule detection. In this talk, I will talk about the design, fabrication and characterization of the proposed cross-configured waveguide based fluorescence detection platform. Experimental results to quantify the excitation efficiency, collection efficiency and SNR will be discussed. A comparison will be shown between the Finite Difference Time Domain (FDTD) simulation and experimental results.

On-chip fluorescence excitation and collection by focusing grating couplers

Fluorescence detection is a commonly used technique to detect particles. Microscopes are used for the fluorescence detection of the micro-particles. However, the conventional microscopes are bulky. It is cumbersome to integrate all the equipment used for detection in one setup. They can be replaced by photonic chips for the detection of micro-particles such as cells. Most of the biological detection techniques require the utilization of the visible range of the spectrum. SiN as a waveguide material stands out for biological applications due to its transparency in the visible spectrum. Specifically designed grating couplers can be exploited to focus from inside SiN waveguides at a designated location above the chip. Those SiN focusing grating couplers can mimic microscope objectives for on-chip biological detection applications such as fluorescence and Raman spectroscopy. In this report, we present a 2D SiN focusing grating coupler. We study the effect of the grating design on the focus properties of visible light using finite-difference time-domain simulations.

Optical power distributions through fractal routing

Several applications in integrated optics require an equal distribution of power from a single input port among many photonic components, whether they be projection components or sensors. One method of achieving such a system is through using progressively more tightly coupled evanescent couplers to route power from a single feeding line [1]. While very compact, this approach requires careful design and characterization of evanescent couplers, and is vulnerable to process variations as the ratio of coupling has a non-linear relation to the couplers’ gap size. Fractals, widely present in nature, are recursive objects where each section is geometrically similar to its parent. They find applications in various fields [2], including RF antenna design and feeding [3]. In this paper we propose to use the fractal approach for spreading power evenly over an area using micro-machined photonic waveguides. In the fractal routing demonstrated in this work, an 1×2 multimode interference (MMI) coupler splits the power at each fractal stage. This provides several advantages. First, only one power splitter design is needed. Second, MMI couplers are well known, and more robust to process tolerances than evanescent couplers [3]. Third, they are symmetrical, and therefore provide a theoretically perfect power distribution independent of the fractal depth. We therefore demonstrate that a fractal routing provides a way to evenly and efficiently distribute power over a large area.