In this work we study various types of photonic waveguides for gas sensing in mid-infrared (MIR) region. MIR region which contains the absorption peaks of many gases is the most suitable region for gas detection. Operating near the absorption peak of the gas to be detected enhance the sensor performance significantly as both real and imaginary parts of the detected gas refractive index are maximized, which enhance the sensitivity and leads to lower detection limit. Here we focus on refractive index sensors that relies on the detection of the real part (n) of the refractive index. Refractive index sensors are strong candidates for integrated on chip sensors, where a small sample volume is needed. One of the main parameters in designing refractive index sensor is waveguide sensitivity which is defined as the ratio of the waveguide effective index change to the medium refractive index change, Swg=Δneff/Δnmed. Thus rigorous sensitivity analysis using full-vectorial finite difference mode solver have been carried out to determine the waveguide sensitivity of such waveguides to gaseous medium. We use silicon on sapphire (SOS) platform to operate in the MIR region from 2μm to 6μm. For each structure we make sensitivity analysis once with undoped silicon and once with doped silicon to show how converting the same structure from dielectric to plasmonic will affect its performance as sensor. The dependence of the effective index, sensitivity and mode loss of each waveguide on the different waveguide dimensions was studied. Finally, a comparison between the proposed waveguides is provided.
An integrated refractive index gas sensor working in the Mid infrared (MIR) region and utilizing suspended silicon waveguide is presented. Although many integrated refractive index gas sensors have been proposed in the literatures, their operating wavelength is limited to the near infrared range. Our proposed gas sensors can operate in the mid infrared up to 10μm, were many gases have their absorption fingerprints in order to enhance the sensing performance. A finite difference solver is used to perform the sensitivity analysis of the suspended silicon waveguide in the MIR range for gaseous medium. The analysis shows that a suspended silicon waveguide can achieve high waveguide sensitivity with a minimal mode loss. Thus, we designed a high performance Mach Zehnder Interferometer (MZI) gas sensor using a suspended silicon waveguide as the sensing arm. Three dimensional finite difference time domain (3D-FDTD) solver is used in the design and optimization of two designs. One for the wavelength interrogation scheme of detection and another one for the intensity interrogation scheme. The first design, exhibits high wavelength sensitivity S=7028 nm/RIU and can reach high figure of merit (FOM) of around 180 RIU-1 for both wavelength and intensity interrogation methods with only 250μm sensing arm length. The second design furtherly enhances the intensity interrogation FOM to reach 370RIU-1 at the same length. Intensity interrogation needs only a laser source and a detector. Hence, using our sensor in intensity interrogation based read-out offers compact, low cost and mass scale fabrication which makes our proposed sensor a good platform for lab on chip technology.
Interferometers are one of the basic devices in many photonics applications. Interferometers can be used in the design of optical filters, wavelength de-multiplexing (WDM), electro-optical modulators and optical sensors. They can also form the building block of optical digital signal processor (DSP). In this work, we propose novel integrated Michelson interferometer based on the Silicon on Insulator (SOI) technology with 220nm silicon device layer and working in the near infrared region. The Interferometer consists of input splitter directional coupler, two waveguide arms and directional coupler combiner with loop reflector. The interferometer transfer function and its parameters including the free spectral range (FSR), the full width half maximum (FWHM) and sensitivity were derived analytically. Using our proposed interferometer instead of the conventional Mach Zehnder Interferometer (MZI) as optical filter, electro-optical modulator or sensor will reduce the size of the device needed by a factor of two while achieving the same performance. Here, we use our Michelson Interferometer with four different path length differences resulting in FSR from 0.8nm to 6.4nm. A strip waveguide with 500nm width platform is used. These devices are suitable for optical filtering as well as wavelength de-multiplexing WDM applications. The simulation results of the proposed designs are extracted using Lumerical MODE and INTERCONNECT software tools that use scattering matrices of optical components to determine the transfer function of photonic integrated circuits (PICs). The designs were verified with three-dimensional finite-difference-time-domain (3D-FDTD) solver and show good agreement. Finally, the designs were fabricated using Electron Beam Lithography (EBL) and characterized showing also good matching with the numerical simulations results.
Gas sensors have been widely used for different applications including chemical detection, quality assurance, environmental monitoring and medical diagnostics. Optical gas sensors exhibit higher sensitivity and wider dynamic range than their electrical counterparts. This work demonstrates a novel design for a gas sensor based on conventional Silicon-on-insulator (SOI) platform. The sensor design is based on interferometer working in the near-infrared (NIR) region where directional couplers were used in splitting and combining the input power to and from the two arms of the interferometer with 50/50 splitting ratio at 1550nm. Slot-waveguide is used in the sensing arm of the interferometer and strip-to-slot and slot-to-stip converters with high coupling efficiency were used for transforming the optical mode. Finite difference eigenmode (FDE) solver was used to calculate its mode field profiles, effective index, and loss to optimize the waveguide dimensions and to achieve a waveguide sensitivity of 0.7 at 1550nm for 220nm silicon thickness. Three-dimensional finite-dif-ference-time-domain (3D-FDTD) method was used in the analysis and optimization of the proposed gas sensor. Results show significant improvement in the figure-of-merit (FOM) and reduction of device area. The sensor also exhibits low insertion loss (IL) leading to a low detection limit. The proposed sensor is easily fabricated using CMOS technology which is essential for mass-scale fabrication, and thus a low-cost sensor can be integrated with optical fiber communication systems and optoelectronic systems. Therefore, the proposed sensor has the potential to be a key component in lab-on-a-chip (LOC) systems.
Electro-optical modulator is a key component in data-communication, telecommunication and optical interconnects. In this paper we propose a novel electro-optical modulator design that utilizes Michelson Interferometer based on the widespread Silicon-on-insulator (SOI) technology with 220nm thickness of the silicon device layer. The proposed modulator is working at the telecommunication wavelength 1550nm. Due to its high Pockels coefficient and CMOS compatibility electro-optical polymer (EOP) is used as an active material where its refractive index changes with the applied electric field. The Michelson Interferometer consist of directional couplers which are used in splitting and combining the input power to and from the interferometer arms with 50/50 ratio at 1550nm. Slot waveguide with EOP clad is used in the interferometer arms to achieve high optical field confinement in the EOP which maximizes the mode effective index change of the interferometer arms when applying voltage. Finite Difference Eigen mode (FDE) solver was used to calculate the mode field profiles, effective index and loss of the slot waveguide. By optimizing the waveguide dimensions, we have achieved a waveguide sensitivity Swg=dneff/dnEOP of 0.9135 at 1550nm. Three-dimensional finite-difference-time-domain (3D-FDTD) method was used in the analysis and optimization of our Michelson Interferometer electro-optical modulator. Results show that our Michelson Interferometer modulator exhibit lower VπLπ product than previously published SOI based modulators. Moreover, the modulator exhibit low insertion loss (IL) leading to high extinction ratio (ER) in addition to its CMOS compatibility. Thus, our proposed modulator allows for compact, high performance and low cost modulators.
Silicon photonics have been approved as one of the best platforms for dense integration of photonic integrated circuits (PICs) due to the high refractive index contrast among its materials. Silicon on insulator (SOI) is a widespread photonics technology, which support a variety of devices for lots of applications. As the photonics market is growing, the number of components in the PICs increases which increase the need for an automated physical verification (PV) process. This PV process will assure reliable fabrication of the PICs as it will check both the manufacturability and the reliability of the circuit. However, PV process is challenging in the case of PICs as it requires running an exhaustive electromagnetic (EM) simulations. Our group have recently proposed an empirical closed form models for the directional coupler and the waveguide bends based on the SOI technology. The models have shown a very good agreement with both finite element method (FEM) and finite difference time domain (FDTD) solvers. These models save the huge time of the 3D EM simulations and can be easily included in any electronic design automation (EDA) flow as the equations parameters can be easily extracted from the layout. In this paper we present experimental verification for our previously proposed models. SOI directional couplers with different dimensions have been fabricated using electron beam lithography and measured. The results from the measurements of the fabricate devices have been compared to the derived models and show a very good agreement. Also the matching can reach 100% by calibrating certain parameter in the model.
The Mid Infrared MIR wavelength range offers many advantages in different applications. Chemical and biological detection are one of these applications, as it contains the absorption fingerprints of many gases and molecules. In addition integrated plasmonics are suitable platform for high sensitivity on chip sensors. In this paper we propose plasmonic Mach-Zehnder Interferometer (MZI) working as a gas sensor near the absorption fingerprints of many gases in the mid-infrared region. The proposed MZI contains a vertically stacked metal-insulator-metal (MIM) and metalinsulator (MI) waveguide. The sensitivity of MI waveguide is lower at higher wavelengths and also lower for gaseous medium than for liquid medium. In addition the losses of the MIM waveguide with oxide layer as insulator are much larger than the losses of the MI waveguide with gas as insulator which will result in poor visibility interferometers. Using a high index layer above the metal of the MI waveguide the sensitivity of the waveguide to gaseous in the mid infrared has been significantly enhanced. This layer also balances the intrinsic losses of both MI and MIM waveguides. The thickness and the refractive index of this layer have been optimized using finite difference modal analysis. Using this layer high sensitivity and high figure of merit (FOM) have been achieved for our MZI. This structure offers simple fabrication and low cost sensor that is suitable for rapid, portable and high throughput optical detection using multiplexed array sensing technique.
Localized Surface Plasmon Resonance (LSPR) that occur in plasmonic nanoparticles due to interaction with electromagnetic waves at wavelengths larger than the nanoparticles themselves has been exploited in many application like solar cells, cancer treatment and spectroscopy due to the enhanced scattering and absorption cross sections that LSPR provides. Being able to control the resonance peaks of scattering in real time using light can be a valuable tool for sensing-related applications as well especially if it happens in the near and Mid-IR spectrum where most of the biological molecules can be sensed as such spectrum contains strong characteristic vibrational transitions of many important molecules . In this work presented here, we used silicon nanoparticles and increased the concentration of free excess carriers in the nanoparticles by light generation until the free carrier concentration was large enough to cause LSPR similar to what we get with nanoparticles made of Noble metals. The LSPR generated by Si nanoparticles with high concentration of free carriers caused the resonance peak to happen in near and mid IR. Depending on the level of carrier concentration which can be changed dynamically in real time, we can control the scattering resonance peak characteristics and position as shown in our work. Successful fabrication of the Silicon nanosphere is demonstrated as well.
In this work we present novel and detailed dispersion the modal analysis of (SOS) strip waveguide in the mid-IR region. The effect of the various design parameters on each mode has been illustrated and carefully studied. The analysis has been extended to cover the fundamental and higher order TE and TM modes over the entire range of operation of this waveguides. The finite element method (FEM) and finite difference method have been both utilized to double verify the analysis. This dispersion analysis has been also utilized to propose novel functional devices in the MIR such as such as mode converter, switches, modulators and TE/TM-pass polarizer design based on the birefringence between the TE and TM mode.
A simple analytical model is developed to estimate the power loss and time delay in photonic integrated circuits fabricated
using SOI standard wafers. This model is simple and can be utilized in physical verification of the circuit layout to verify
its feasibility for fabrication using certain foundry specifications. This model allows for providing new design rules for the
layout physical verification process in any electronic design automation (EDA) tool. The model is accurate and compared
with finite element based full wave electromagnetic EM solver. The model is closed form and circumvents the need to
utilize any EM solver for verification process. As such it dramatically reduces the time of verification process and allows
fast design rule check.