Silicon based nanophotonic device with electric control for optical switching

Abstract

Information processing at optical frequencies has its advantages,especially in terms of bandwidth. Photonic devices are rapidly emerging as saviors to meet bandwidth requirements in communication networks and high-speed computing. Compact photonics at real nanoscales can be the key to realize optical interconnects. The recent advances in the area of nanofabrication have given the ability to control the structure and properties of the devices to the desired levels. The miniaturization of the photonics devices is ideally limited by the diffraction limit of light which has been addressed with the proposal of novel guiding mechanisms. The coupling of light with collective oscillations of free electrons at a metal-dielectric interface is a potential candidate for nanoscale optical confinement beyond the diffraction limit. Such plasmonic waveguide with surface plasmon polariton (SPP) modes suffers from large metallic losses which limit its use in practical devices. Leaky mode confinement in a high refractive index layer underneath the HP confinement layer can further reduce the losses and can control propagation characteristics of the hybrid plasmonic waveguide for nanoscale devices. The large light-matter interaction can be harnessed for the application in electrical control of optical signals. A silicon-based nanophotonic hybrid plasmonic waveguide is designed, fabricated, and characterized. An 11-nm thick SiO2 on silicon with gold as top layer provides us an optical-confinement at the nanoscale with an effective mode area of λ2 /200 an acceptably low loss of 7 dB/cm. The role high index layer (silicon) is analyzed. The grating is introduced in the silicon layer which provides us an ultra-low dispersion of 10 pico-sec2 /m and it passes through zero-dispersion value many times over a broad range of wavelength. The proposed work can be useful in realizing photonic platform at nanoscales for a variety of on-chip functionalities including high data rate optical interconnects which require low dispersion at nanoscales. To electrically control the light in the proposed hybrid plasmonic waveguide, the field-effect transistor is proposed. The charge carrier dynamics along with the plasma dispersion effect in the silicon channel, through voltages applied on the gate and source-drain, results in the optical phase modulation in MOSFET and tunnel FET (TFET). A phase shift of π radian at a length of 1.2 mm and 0.21 mm is obtained in MOSFET and TFET, respectively. The proposed concept has the potential to enable the multifunctionality of the mature fieldeffect transistors. To further improve the electrical control at nanoscale we consider the combination of the strength of the resistive switch with that of the photonics which has resulted in an interesting optical switching element. A nanophotonic resistive switch based on silicon is designed, fabricated, and characterized. The proposed electrically writable resistive switch with optical readout capability is demonstrated. An optical hysteresis curve with 10 dB of extinction ratio is obtained for a 1550-nm wavelength of light. The top electrode (metal Au) is chosen such that the conduction bridge formation uses both electrochemical metallization and valency change mechanism. The device showed a good self-rectifying ratio which enables it for realization in fabrication with multi-layer stacking. The optical readout is less error-prone compare to electrical readout and large bandwidth operation. The proposed device shows an inherent stochastic property where the set (writing) voltage reduces in each set-reset cycle, which can be used for optical readout of synaptic weight for neuromorphic computations. On the analysis of the device, it is observed that the optical extinction ratio can be improved by increasing the metal diffusion in the active layer. An improved extinction ratio of 16 dB is demonstrated by using silver as the top electrode and highly doped p-Si in the bottom electrode. The titanium oxide is used as the controlling layer sandwiched between these two highly conducting electrodes. The top metal electrode material is chosen such that it has low ionization energy, low electronegativity, and support SPP propagation with low optical loss. The SET voltage of the device is improved from 10 V to 4 V by application of blue pump light. The pump light increases the photogenerated charges which in turn increases the leakage current. The scheme helps in avoiding undesirable overshoot of current hence improving device life span. The electroforming of the conduction bridge at lower voltages reduces the overall power consumption of the device.