New computational method to estimate the effect of roughness on scattering loss and its implementation in a hybrid heterojunction optical modulator

In the past year, there has been an advancement in the development of optical active and passive Silicon-On-Insulator for Photonic Integrated Circuits (PICs) applications. Following the continuous miniaturisation trend in this technology the study of the loss performance of these devices also become an attractive subject. Among the many components silicon based optical modulators are specifically important for low loss and high bandwidth short reach interconnects. The general aim of this thesis is to design and computationally investigate the performance of a multilayer hybrid silicon modulator. The design involves a heterojunction structure that operates based on plasma dispersion effect taking the effect of surface roughness and scattering loss into account. For this purpose a novel numerical approach is developed to estimate scattering loss due to the roughness in general waveguide structures and also the proposed silicon based optical waveguide modulator in this work. The applicability can be however extended towards wider range of optical waveguide based devices including multilayer configurations. The method is based on 2D Fourier transform technique that is widely used in Magneto Resonance Imaging technique.Firstly, the effect of three forms of roughness is investigated in a general strip waveguide structure; isotropic, anisotropic and the mixture of isotropic and anisotropic. In each case the generated sidewall roughness is implemented in various SOI high contrast refractive index waveguides. The waveguide dimensions have been chosen to cover a large variety of waveguide sizes to evaluate the accuracy of the modelling technique. Two SOI waveguide samples have been fabricated. The first sample is 100nm, 1200nm and 1400nm in width and 500nm in height which are single mode in Mid Infra Red (MIR) region. The second sample contains waveguide with 220nm height and 330nm widths that is single mode in Near Infra Read (NIR) wavelength. Other dimensions are chosen from several published works. The calculated losses using FDTD show good agreement with all measured fabricated waveguide and the referred experimental works in the literature. The three dimensional model successfully explains the scattering loss dependence on the width of a high aspect ratio waveguide when the result is compared with a published work. While the measurement shows the loss reduces from 32dB/cm to 0.8dB/cm, the simulation results varies from 44dB/cm to 1dB/cm. The interdependence of scattering loss is also investigated against other theoretical approaches when the correlation length varies from 0 to 1000nm. The relatively low aspect ratio waveguide is chosen to have a fixed dimension of 220nm by 330nm and 5nm value.In order to study the scattering loss caused by roughness the roughness model is applied in the proposed modulator structure. The design involves a multimode strip like p doped silicon material wrapped by transparent ZnO as a naturally n doped active material. This forms a pn heterojunction that is implemented in one arm of a Mach Zehnder interferometer. The switch is designed to operate at 1.55µm and in depletion mode to avoid minority carrier life time effect in switching speed. The calculated capacitance switching speed of a pulse were less than 1 pf=cm and 90ps respectively. The resistivity v is higher compared to a general form of heterojunction due to the relativity larger ZnO/Si contact area of the device. The phase shifter is implemented in Mach Zehnder structure to change from phase modulation to amplitude modulation using a MMI structure. The calculated extinction ratio was as high as 23.7dB with the insertion loss of 2.5dB. Further simulation results shows that the 100nm change in the ZnO thickness can alter the effective index of refraction and loss performance of the devices. In an ideal situation, as the thickness increases from 50nm to 150nm the loss changes from 2 to 8dB/cm for TE mode. The involvement of sidewall roughness results higher insertion loss by at least by 0.2dB when the rms of the sidewall roughness increases by 7nm. As the ZnO coating thickness increases, the roughness effect is counterbalanced by almost 50%

Hot-wire polysilicon waveguides with low deposition temperature

We fabricated and measured the optical loss of polysilicon waveguides deposited using Hot-Wire Chemical Vapour Deposition (HWCVD) at a temperature of 240°C. A polysilicon film 220 nm thick was deposited on top of a 2000 nm thick PECVD silicon dioxide. The crystalline volume fraction of the polysilicon film was measured by Raman spectroscopy to be 91%. The optical propagation losses of 400, 500, and 600 nm waveguides were measured to be 16.9, 15.9, and 13.5 dB/cm, respectively, for transverse electric (TE) mode at the wavelength of 1550 nm. Scattering loss is expected to be the major contributor to the propagation loss

Scattering loss estimation using 2D Fourier analysis and modelling of sidewall roughness on optical waveguides

We report an accurate scattering loss 3D modeling technique of sidewall roughness of optical SOI waveguides based on Fourier and Finite Difference Time Domain (FDTD) analysis methods. The Fourier analysis method is based on the image recovery technique used in magnetic resonant imaging. Losses for waveguides with isotropic and anisotropic roughness are calculated for wavelengths ranging from 1550 nm to 3800 nm and compared with reported results in literature. Our simulations show excellent agreement with published experimental results and provide an accurate prediction of roughness-induced loss of 3D arbitrary shaped optical waveguides

Successful Treatment of Acute Invasive Rhino-Orbito-Cerebral Mucormycosis Associated with COVID -19: Case Report and Literature Review

Background and Aim: Mortality associated with post-coronavirus disease 2019 (COVID-19) rhino-orbital cerebral mucormycosis (ROCM) is an evolving concern. Association of COVID-19, corticosteroid therapy, and uncontrolled diabetes mellitus (DM) are considered predisposing factors for ROCM. We present two cases of successful treatment leading to patient survival for post-COVID-19, stage 4c-ROCM. Case Presentation: Two middle-aged men with poorly controlled DM were referred to our hospital for post-COVID-19 ROCM. They had received intravenous antivirals and dexamethasone as treatment in a primary center. Both patients had unilateral oculofacial pain and swelling followed by acute visual loss, unilateral proptosis, facial palsy, and trigeminal hypoesthesia. A computed tomography scan revealed opacity and bony erosion of paranasal sinuses (PNS). Obtained specimens confirmed mucormycosis agent on histopathological examination. After the failure of conservative treatments, retrobulbar injections of liposomal-amphotericin B, PNS debridement, and orbital exenteration, both patients developed blurred consciousness owing to the extension of the infection into the intracranial cavity. Brain magnetic resonance imaging revealed a right frontal lobe abscess in case 1 and a left frontal sinus abscess and involvement in case 2. Both underwent transcranial debridement and resection of necrotic tissue and drainage of the abscess. At follow-up, the patients were in good condition, and the fungal cultures were negative for mucormycosis agents. Conclusion: ROCM may be a complication of COVID-19 in high-risk patients. Control of the patient’s underlying systemic condition and prompt treatment with antifungal agents, along with timely aggressive resection of cerebral abscess and necrotic paranasal tissues, are the mainstays of management for ROCM

Silicon waveguides and devices for the mid-infrared

We report on the design, fabrication, and characterization of silicon-on-insulator rib and strip waveguides at wavelengths longer than 3.7µm. Propagation losses of 1.5±0.2 dB/cm at 3.73µm and 1.8±0.2 dB/cm at 3.8µm have been measured for rib waveguides, whilst submicron strip waveguides exhibited propagation losses of 4.6±1.1 dB/cm at the wavelength of 3.74µm. A 1×2 multimode interference (MMI) splitter and racetrack resonators based on submicron strip waveguides are also examined. Optical losses of 3.6±0.2 dB/MMI and a racetrack resonator Q-value of 8.2k are obtained at 3.74µm

Submicron silicon waveguides and optical splitters for mid-infrared applications

We report on the design, fabrication and characterisation of submicron silicon-on-insulator strip waveguides at a 3.74 μm wavelength. Experimental results for 1x2 multi-mode interference splitters are also given.</p

Compact Fabry-Perot electro-optic switch based on n-ZnO/p-Si heterojunction structure

We present a hybrid waveguide-based optical switch using n-ZnO/p-Si heterojunction diode structure. The n-ZnO/p-Si vertical diode is incorporated with a Fabry-Perot microcavity to realize optical switching through electrical biasing. The ZnO layer functions as n-type transparent oxide semiconductor and waveguide cladding. Under 4V reverse bias(depletion), simulation shows a modulation depth of 5.54dB and tunable red-shift of 4nm over a cavity length of 17µm. A Fabry-Perot microcavity of 20µm length has been fabricated and measurement shows a tunable blue-shift of 0.1nm under 10V forward bias condition (accumulation)