Optimally coupling light in an integrated Photonic crystal (PhC) cavity is challenging, but crucial for improving their sensing properties. Here we experimentally investigate the impact of side coupling and in-line coupling on the transmission properties of integrated silicon PhC based air-slot cavities by probing the near field of the cavity mode with a nano fiber tip. These cavities were fabricated with standard deep UV lithography. Positioning this nano-tip near and inside 130 nm wide PhC slot cavity modifies the dielectric map of the cavity which perturbs the intensity scattered from the cavity surface. We show that the mapping of the nano tip induced intensity variations provides some insight about the nature of the confinement of electric field of the various modes of slot cavities. Such intensity maps carry moreover information about the cavity light coupling, which is useful for maximizing the intensity of PhC slot cavity modes.
Silicon photonics is rapidly emerging as a mature technology platform for the fabrication of photonic integrated circuits. It builds on the technology base of the CMOS-world and allows to implement advanced photonic functions on a small footprint chip with high accuracy and yield. For operation at telecom wavelengths above 1 micrometer one typically uses silicon-on-insulator wafers with waveguides with a silicon core. For short-wavelength operation, below 1 micrometer, one can use a silicon nitride (SiN) core instead of a silicon core. This results in a platform for operation in the visible and near infrared, with moderately high refractive index contrast and low loss photonic components. Operation at short wavelengths can be beneficial for a variety of reasons, including the possibility to use low cost high performance sources and detectors and the compatibility with sensing in an aqeous environment.
The SiN CMOS-platform has been used to demonstrate a variety of spectroscopic sensing functions. In essence the SiN chips may contain sensing structures, whereby the evanescent tail of the guided light is interacting with the analyte, as well as spectrometric functions to read out the spectrum resulting from the interaction with the analyte. This approach has allowed to demonstrate refractive index biosensors, spontaneous Raman spectroscopy and surface-enhanced Raman spectroscopy. In the latter case the SiN waveguides are enriched with gold nano-antennas to enhance the local field strength seen by the analyte. The spectrometric functions can be based on arrayed waveguide gratings, echelle grating spectrometers or Fourier Transform spectrometers.
One of the most significant challenges facing physical and biological scientists is the accurate detection and identification of single molecules in free-solution environments. The ability to perform such sensitive and selective measurements opens new avenues for a large number of applications in biological, medical and chemical analysis, where small sample volumes and low analyte concentrations are the norm. Access to information at the single or few molecules scale is rendered possible by a fine combination of recent advances in technologies. We propose a novel detection method that combines highly sensitive label-free resonant sensing obtained with high-Q microcavities and position control in nanoscale pores (nanopores). In addition to be label-free and highly sensitive, our technique is immobilization free and does not rely on surface biochemistry to bind probes on a chip. This is a significant advantage, both in term of biology uncertainties and fewer biological preparation steps. Through combination of high-Q photonic structures with translocation through nanopore at the end of a pipette, or through a solid-state membrane, we believe significant advances can be achieved in the field of biosensing. Silicon microrings are highly advantageous in term of sensitivity, multiplexing, and microfabrication and are chosen for this study. In term of nanopores, we both consider nanopore at the end of a nanopipette, with the pore being approach from the pipette with nanoprecise mechanical control. Alternatively, solid state nanopores can be fabricated through a membrane, supporting the ring. Both configuration are discussed in this paper, in term of implementation and sensitivity.
We present a GaAs-based VCSEL structure, BCB bonded to a Si<sub>3</sub>N<sub>4</sub> waveguide circuit, where one DBR is substituted by
a free-standing Si<sub>3</sub>N<sub>4</sub> high-contrast-grating (HCG) reflector realized in the Si<sub>3</sub>N<sub>4</sub> waveguide layer. This design enables
solutions for on-chip spectroscopic sensing, and the dense integration of 850-nm WDM data communication transmitters
where individual channel wavelengths are set by varying the HCG parameters. RCWA shows that a 300nm-thick Si<sub>3</sub>N<sub>4</sub>
HCG with 800nm period and 40% duty cycle reflects strongly (<99%) over a 75nm wavelength range around 850nm. A
design with a standing-optical-field minimum at the III-V/airgap interface maximizes the HCG’s influence on the
VCSEL wavelength, allowing for a 15-nm-wide wavelength setting range with low threshold gain (<1000 cm<sup>-1</sup>).
The evanescent tail of the guided modes can efficiently excite Raman active molecules located in the cladding of a waveguide. Similarly, a significant fraction of the total emitted Stokes power is evanescently coupled to the same mode. Further, the enhancement effects inherent to the waveguide, alongside with the long interaction length, lead to an increased light-matter interaction, resulting in a higher sensitivity as required by spectroscopic applications, especially in the context of Raman spectroscopy. We calculate the spontaneous Raman scattering efficiency as a function of silicon-nitride strip waveguide dimensions and show that under typical conditions, the overall efficiency is approximately two orders of magnitude higher than in confocal configuration in the free space. We also report the experimental demonstration of the use of silicon-nitride based photonic waveguides in a lab-on-a-chip context for Raman spectroscopy. To the best of our knowledge, this is the first demonstration of Raman spectroscopy using photonic waveguides.
In recent years silicon photonics has become a mature technology enabling the integration of a variety of optical and optoelectronic functions by means of advanced CMOS technology. While most efforts in this field have gone to telecom and datacom/interconnect applications, there is a rapidly growing interest in using the same technology for sensing applications, ranging from refractive index sensing to spectroscopic sensing. In this paper the prospect of silicon photonics for absorption, fluorescence and Raman spectroscopy on-a-chip will be discussed. To allow spectroscopy in the visible and near infrared the silicon photonics platform is extended with silicon nitride waveguides.