Optical interconnects have been proposed to be the next generation interconnect solution to overcome the impending interconnect bottleneck. Large optical devices have hindered integration of electrical and optical components. Plasmonics have enabled nanophotonic components with sub-micron scale optical devices with similar size range as electronics and they promise to bridge the size gap between optical and electrical components. Surmounting research is suggesting that the electronics industry is starting to accept more variety materials in the fabrication process, the most important of which is graphene. The modulator is composed of a thin layer of silicon nitride – a few nm thick – sandwiched between two graphene sheets that are both electrically connected to the signal. Thin Al2O3 layers separate the graphene sheets from the ground electrodes on top and bottom. The electric field generated by applying a maximum of 5V on the graphene sheets changes the fermi level of graphene to switch between a highly lossy metal-like material and a dielectric material. Operating in the mid infrared regime, around 5 μm wavelength, when the Fermi level is located in the band gap, optical absorption is high. When the Fermi level is located away from the bandgap, absorption is minimized. Simulations show that the modulator exhibits over 7 dB / μm extinction ratio and less than 0.1 dB / μm propagation loss. By designing for 3 dB extinction ratio and less than 0.1 dB propagation loss, the footprint of the modulator is only 80 nm x 400 nm for feasible integration in future electronic chips without competing for space.
To overcome the classic sensitivity vs selectivity trade-off often associated with sensors used in diagnostic applications, signature spectroscopic information that is characteristic of the molecules to be sensed can be exploited. Raman spectroscopy offers such information and is suitable for biological fluids. It is considered a label-free sensing method that inherently has excellent specificity. Sensitivity on the other hand is generally low unless amplification of the generally weak Raman signal is achieved. Surface enhanced Raman Scattering (SERS) employs localized surface plasmons on metallic nanoparticles to amplify this signal by several order of magnitude. In this work, SERS substrates were prepared by growing silver nanoparticles using electrodeposition on silicon nanowires that were prepared using metal assisted chemical etching. Experimental results agree with finite difference time domain (FDTD) simulation results. Using pyridine as a probe molecule, Raman signal intensity was found to correlate well with the pyridine concentration in the range 10-6 M to 10-9 M, indicating its applicability as a quantitative sensor. Very low concentration of pyridine, 10-11 M, was detected although at this low concentration the detection is only qualitative. The enhancement factor was calculated to reach 1011. Spot-to-spot, sample-to-sample, and batch-to-batch variation was studied to ensure repeatability, which had been a long-standing issue of low-cost SERS substrates. In addition, experiments over several days highlight the robustness of these SERS substrates. This work bolsters the use of SERS as a low cost sensing method with good sensitivity and specificity for a plethora of applications without compromising on repeatability or robustness.
High reflection losses combined with low absorption capabilities and high velocity surface recombination are the main problems that deteriorate the efficiency of thin silicon solar cells. Therefore, Low cost and easy scalable fabrication of wide band, angle and self-cleaning antireflection coatings are of great importance for different optical applications especially solar cells. Random textured silicon nanocones are fabricated through electroless metal assisted chemical etching (EMACE) combined with ambient oxidation. Theoretical studies using Finite difference Time Domain (FDTD) simulation guided the experimental procedures in terms of dimensions and tolerance to reach the optimum dimensions and superior optical properties. The Optical numerical and experimental studies are revealed wide antireflection properties and strong trapping effects up to 60° through the entire visible wavelength. The textured structure modified the hydrophobicity of the solar cell into hydrophobic surface with self-cleaning properties.
Raman scattering is an excellent analysis tool because a wealth of information can be obtained using a single measurement. It can also be configured as a diagnostic tool as a label free sensing method. In that case, enhancing the Raman signal is important to improve the sensitivity and detect low concentrations of analytes. A nanoparticle showing a particular Raman enhancement shows a much higher enhancement when it is on a nanowire. This was also confirmed experimentally. We report on a simple fabrication method of silver nanoparticles and silicon nanowires decorated with these nanoparticles. The nanowires were fabricated using metal assisted chemical etching. The nanoparticles were formed using electrodeposition. Samples were then immersed in Pyridine. An enhancement factor of around 6 to 8×105 was observed for silver nanoparticles alone. By depositing the same nanoparticles on silicon nanowires, the enhancement factor jumped 10-fold to 7×106. Finite Difference Time Domain simulations showed that a range of enhancement factors is possible up to 109.
The objective of this work was to develop an integrated general purpose label-free optical sensor using standard photolithography on silicon-on-insulator platform for lab on chip applications. Shallow silicon waveguides have weak confinement in the silicon with lots of field in the cladding. This is advantageous in sensor applications due to the high light matter interaction. Here, we use our shallow strip waveguide platform to design a sensor employing a multimode interference (MMI) section. Utilizing a multi-mode section as short as 4 mm, the sensor exhibits sensitivity ranging from 417 nm / RIU to 427 nm / RIU with a figure of merit from 32 to 133.