Photonic crystal slabs (PCSs), which generally consist of two-dimensional arrays of nanoholes in the top layer of a dual layer dielectric film, have been demonstrated as a promising platform for optical biosensing. Both the Fano resonance in a perfect PCS and the Lorentzian resonance in a micro-cavity resulted from an introduced defect in PCS have been studied. While, the use of resonance peak shift for detecting molecules owing to the change of the refractive index is a nonspecific biosensing technique. Biorecognition molecules, such as antibodies that can specific bond to interesting molecules, are conjugated on the PCS to improve the detection specificity. It is a widely adopted assumption that the conjugated molecules form into a uniform nanofilm in the PCS based biosensors, which covers either the entire surface of the dielectric layer or the entire sidewalls of nanoholes. However, the actual device performance is much lower than that obtained based on this assumption, which suggests the over-simplicity of the hypothesis above. It is of keen interest to reveal the actual arrangement and distribution of molecules on PCS for designing high-performance PCS biosensors. Here, we propose models and analysis of the distribution of nanofilms on PCS. We employed Raman scattering technique to experimentally reveal the actual various configurations of nanofilms, which support our theoretical modeling. The results obtained in this research can be essential for designing high-performance PCS based nanobiosensors.
Raman scattering spectroscopy is a unique tool to probe vibrational, rotational, and other low-frequency modes of a molecular system and therefore could be utilized to identify chemistry and quantity of molecules. However, the ultralow efficient Raman scattering, which is only 1/10<sup>9</sup> ~ 1/10<sup>14</sup> of the excitation light due to the small Raman scattering cross-sections of molecules, have significantly hindered its development in practical sensing applications. The discovery of surface-enhanced Raman scattering (SERS) in the 1970s and the significant progress in nanofabrication technique, provide a promising solution to overcome the inherent issues of Raman spectroscopy. It is found that In the vicinity of nanoparticles and their junctions, the Raman signals of molecules can be significantly improved by an enhancement factor as high as 10<sup>10</sup>, due to the ultrahigh electric field generated by the localized surface plasmons resonance (LSPR), where the intensity of Raman scattering is proportional to the |<strong>E</strong>|<sup>4</sup>. In this work, we propose and demonstrate a new approach combining LSPR from nanocapsules with densely assembled silver nanoparticles (NC-AgNPs) and guidemode- resonance (GMR) from dielectric photonic crystal slabs (PCSs) for SERS substrates with robustly high performance.
In recent decades, silicon photonics has attracted intensive research interest in optical communications due to its advantageous compact dimensions and high-volume manufacturability. Particularly, micro-ring resonators on silicon-oninsulator (SOI) platform have been widely exploited as a basic building block for a vast range of applications such as switches, modulators, and sensors. A majority of these applications involve light-matter interaction, which can be substantially enhanced by the high quality factor micro-ring resonators. However, conventional strip waveguide based micro-ring resonators suffer from the intrinsic dilemma in achieving high light confinement and strong light-matter interaction simultaneously. Subwavelength grating (SWG) waveguides, comprised of periodically interleaved high and low refractive index materials with a pitch less than one wavelength, have been demonstrated as a promising alternative. For SWG waveguides built on SOI wafers, the ratio of silicon and cladding materials can be engineered microscopically to achieve desired macroscopic properties. The control of these properties could potentially lead to significant performance improvements compared with conventional micro-ring resonators based photonic devices, such as filters and sensors. However, SWG waveguide based micro-ring resonators (SWGMRs) that have been demonstrated so far can only provide a moderate quality factor (~5600) with a large radius (e.g. 15 μm), which greatly jeopardize the wide spread research efforts in this area. In this paper, we propose to use trapezoidal silicon pillars to reduce the bend loss of SWGMRs to improve the quality factor. For the first time, we experimentally demonstrate the smallest SWGMR (the micro-ring radius equals to 5 μm) with an applicable quality factor as high as 11,500. This approach also can be applied to SWGMRs with larger radii for higher quality factors. We also experimentally demonstrated a 10 μm radius SWGMR that can provide a quality factor up to 45,000. Compared to SWGMRs built with conventional rectangular silicon pillars, the quality factors is increased by 4.6 times from a 5 μm radius SWGMR and 3 times from a 10 μm SWGMR radius, respectively.
Subwavelength grating (SWG) waveguide is an intriguing alternative to conventional optical waveguides due to its freedom to tune a few important waveguide properties such as dispersion and refractive index. Devices based on SWG waveguide have demonstrated impressive performances compared to those of conventional waveguides. However, the large loss of SWG waveguide bends jeopardizes their applications in integrated photonics circuits. In this work, we propose that a predistorted refractive index distribution in SWG waveguide bends can effectively decrease the mode mismatch noise and radiation loss simultaneously, and thus significantly reduce the bend loss. Here, we achieved the pre-distortion refractive index distribution by using trapezoidal silicon pillars. This geometry tuning approach is numerically optimized and experimentally demonstrated. The average insertion loss of a 5 μm SWG waveguide bend can be reduced drastically from 5.58 dB to 1.37 dB per 90° bend for quasi-TE polarization. In the future, the proposed approach can be readily adopted to enhance performance of an array of SWG waveguide-based photonics devices.
Integrating photonic waveguide sensors with microfluidics is promising in achieving high-sensitivity and
cost-effective biological and chemical sensing applications. One challenge in the integration is that an air gap would
exist between the microfluidic channel and the photonic waveguide when the micro-channel and the waveguide
intersect. The air gap creates a path for the fluid to leak out of the micro-channel. Potential solutions, such as oxide
deposition followed by surface planarization, would introduce additional fabrication steps and thus are ineffective in
cost. Here we propose a reliable and efficient approach for achieving closed microfluidic channels on a waveguide
sensing chip. The core of the employed technique is to add waveguide crossings, i.e., perpendicularly
intersecting waveguides, to block the etched trenches and prevent the fluid from leaking through the air gap. The
waveguide crossings offer a smooth interface for microfluidic channel bonding while bring negligible additional
propagation loss (0.024 dB/crossing based on simulation). They are also efficient in fabrication, which are patterned and
fabricated in the same step with waveguides. We experimentally integrated microfluidic channels with photonic crystal
(PC) microcavity sensor chips on silicon-on-insulator substrate and demonstrated leak-free sensing measurement with
waveguide crossings. The microfluidic channel was made from polydimethylsiloxane (PDMS) and pressure bonded to
the silicon chip. The tested flow rates can be varied from 0.2 μL/min to 200 μL/min. Strong resonances from the PC
cavity were observed from the transmission spectra. The spectra also show that the waveguide crossings did not induce
any significant additional loss or alter the resonances.