Integration of photonic circuits on silicon offers a unique opportunity to address the scaling of inter- and intra-chip communications in an energy-efficient and cost-effective manner. Mode-division multiplexing (MDM) is deemed as one of the most promising technologies to increase aggregated data bandwidth and avoid a communication capacity crunch. In this invited talk, we review our latest advances on integrated silicon mode multiplexers, including new topologies based on subwavelength grating (SWG) metamaterials for extended broadband operation and higher-order mode support. Specifically, we report on an ultra-broadband multiplexer based on a phase shifter and a multimode interference (MMI) coupler both engineered with subwavelength metamaterials. Experimental measurements of a complete multiplexer-demultiplexer link show losses lower than 2 dB and crosstalk below -17 dB over a bandwidth of 245 nm (1427 – 1672 nm).
Silicon photonics is considered an enabling technology for the development of high-performance photon-pair sources for quantum information applications. However, the substantially higher pump intensity and narrow wavelength separation between the photon-pairs and the optical pump impose stringent requirements that lie beyond the capabilities of state-of-the-art silicon spectral filters.
In this invited talk we will present an overview of our recent advances in the field of waveguide Bragg gratings for photonic noise reduction in silicon photon-pair sources, including different strategies harnessing subwavelength and modal engineering to overcome the bandwidth-rejection trade-off in state-of-the-art implementations.
Bragg filters stand as a key building blocks of the silicon-on-insulator (SOI) photonics platform, allowing the implementation of advanced on-chip signal manipulation. However, achieving narrowband Bragg filters with large rejection levels is often hindered by fabrication constraints and imperfections. Here, we present a new generation of high-performance Bragg filters that exploit subwavelength and corrugation symmetry engineering to overcome bandwidth-rejection trade-off in state-of-the-art implementations. We experimentally show flexible control over the width and depth of the Bragg resonance, unlocking new tools for the implementation of notch filters with arbitrary bandwidth and rejection level. These results pave the way for the implementation of high-performance on-chip wavelength filters with a great potential for nonlinear-based applications, e.g. next generation Si-based photon-pair sources for quantum photonic circuits.
Silicon photonics is considered an enabling technology for next generation datacom applications, providing ultra-compact and high-bandwidth transceivers that are cost-effectively fabricated at the existing CMOS facilities. Among photonic devices developed in silicon, Bragg gratings are routinely used for the realization of key functionalities including wavelength filtering, dispersion engineering and sensing. However, the realization of Bragg filters that simultaneously provides narrowband operation and high rejection remains a challenge in the Si platform. Indeed, the small core size of Si wires, together with the high index contrast between the silicon and the oxide cladding results in a strong interaction of the optical mode with the Bragg structure. Several approaches have been proposed to implement narrowband Bragg filters in Si wires including ultra-small corrugations (a few nanometres), periodic claddings, sub-wavelength engineering or inter-mode coupling. Nevertheless, these filters typically have comparatively weak light rejection performance due to fabrication errors limiting the accurate control of the grating geometry over few millimeter-long waveguide structures.
In this work, we present a novel waveguide Bragg grating geometry that leverages the large index contrast between Si and air in membrane waveguides to overcome these limitations, yielding both narrow bandwidth and high rejection ratio. We use a novel waveguide corrugation geometry that radiates out the higher order modes, allowing effective single-mode operation for micrometric fully etched membrane waveguides. The high mode confinement of these waveguides results in weak interaction with the sidewall corrugation, thus narrowband operation is achieved. On the other hand, the high rejection ratio is achieved by combining reflection and radiation effects within the Bragg resonance. Based on this concept, we designed and experimentally demonstrated notch filters in single-etch suspended Si waveguides with cross-sections as large as 0.5 µm (height) by 1.1 µm (width). We show a narrow bandwidth of 4 nm for a 500 nm wide corrugation, with a high rejection ratio exceeding 50 dB for a filter length of only 700 µm
The silicon-on-insulator (SOI) platform allows for a miniaturization of optical elements at the micron size. It is now a mature technology, with high quality material and well-known fabrication processes. Another advantage stems in its compatibility with the CMOS facilities. The SOI platform is used already for numerus applications such as datacom, sensing or manipulation of quantum objects. Bragg filters are often used for on-chip the rejection of pump lasers. They can also be used for sensing purposes. By periodically modulating a standard waveguide width, it is possible to realize a 1-D photonic crystal with a forbidden wavelength band. In principle, the bandwidth and central wavelength of this bandgap can be tailored just by a proper design of the introduced corrugation. However, the very large index contrast of Si-wires makes the realization of narrowband rejection filters a technological challenge, requiring multiple etching steps or corrugation widths of a few tens of nanometers. Sub-wavelength nanostructuration of Si waveguides has shown to allow narrowband operation with a single-etch process, but reported rejection levels remained limited.
Here we present an innovative differential corrugation approach that allows the realization of narrowband rejection optical filters with relaxed fabrication constraints. By sub-wavelength engineering of the waveguide geometry we experimentally demonstrated simultaneous high rejection of 50dB and narrowband operations less than 3nm.
We propose new subwavelength designs based on subwavelength structures having two subperiods in each Bragg period. We also investigate the equivalent asymmetric structure to reduce the index contrast in the periods and further reduce the bandwidth without adding any new fabrication constraints. We have fabricated the sub-wavelength engineered filters in standard SOI wafer with a 220 nm thick Si guiding layer and bottom oxide layer of 2 µm. We have used electron beam lithography with 5 nm step-size and have patterned the structurse by dry etching with an inductively coupled plasma etcher. Finally, we have covered the devices with PMMA to provide symmetric cladding.
We report results showing that subwavelength Bragg filter geometries allow a drastic reduction of the operating optical bandwidth to the 0.6nm-2.5nm range if compared with regular Bragg filters (20 nm) while retaining still a strong rejection level of around 40dB. Similarly, each asymmetric version was observed to be bandwidth narrower.
To sum up, this paper is an investigation of advanced SOI waveguide Bragg mirrors. We report that the use of subwavelength corrugations and a judicious of waveguide Bragg asymmetry allow to push the extinction ratio/operating bandwidth beyond its traditional limit.
The large transparency window of silicon (1.1 - 8 µm wavelength range) makes it a promising material for the implementation of a wide range of applications, including datacom, nonlinear and quantum optics, or sensing in the near- and mid-infrared wavelength ranges. However, the implementation of the silicon-on-insulator (SOI) platform in the mid-infrared is restricted by the absorption of buried oxide layer for wavelengths above 4 µm. A promising solution is to combine silicon membranes and subwavelength nanostructuration to locally remove the buried oxide layer, thus allowing access to the full transparency window of silicon. Additionally, structuring silicon with features smaller than half of the wavelength releases new degrees of freedom to tailor material properties, allowing the realization of innovative high-performance Si devices.
Implementing Si membrane waveguides providing simultaneous single-mode operation at both near-infrared and mid-infrared wavelengths is cumbersome. Due to the high index contrast between Si and air cladding, conventional strip waveguides with cross-sections large enough to guide a mode in the mid-infrared are multi-mode in the near-infrared. Here, we exploit periodic corrugation to engineer light propagation properties of Si membrane waveguides allowing effective single-mode operation in near- and mid-IR. Single-mode propagation in the mid-IR is allowed by choosing a 500-nm-thick and 1100-nm-wide silicon waveguide. A novel waveguide corrugation approach radiates out the higher order modes in the near-IR, resulting in an effectively single-mode operation in near-IR. Based on this concept, we demonstrated Bragg filters with 4 nm bandwidth and 40 dB rejection.
This is a study of the central nervous system (CNS) cells, including brain micro vascular endothelial cells (BMV) that constitute the blood brain barrier, and C6 glial cells that are the predominant cell in the brain. The cells are exposed to various chemicals by non-invasive, label-free methods. Digital holographic microscopy (DHM) is a technique that records an interference pattern between an object and reference waves, so that the computationally reconstructed holographic image contains both amplitude and phase information, and 3D images are obtained. The measurement of cell cultures by digital holographic microscopy yields information about cell death mechanisms, since these processes are correlated with individual cell volume. Our in-house DHM combines a visible (red) laser source with a conventional microscope base, and LabVIEW-run data processing. Terahertz spectral signatures are associated with structural changes in molecules and provide complementary information about cells. Both CNS cells BMV and C6 cells are treated with the drug “Methamphetamine" (METH), which induces apoptosis in neuronal cells and exhibits decrease in cell volume, a characteristic of cells undergoing apoptosis (induced cell death). METH can cause CNS cell death by cross-talk between mitochondria-, endoplasmic reticulum-, and receptor-mediated apoptotic events, all of which results in drug induced changes in neuroplasticity and significant neuropathology. Doxorubicin (DOX), a popular anticancer drug, is used as a control. We observe that METH treatment resulted in more pronounced cell volume shrinkage in both the BMV and C6 cells, as compared to DOX-induced cell apoptosis.
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