Farshid Bahrami

An improved refractive index sensor based on genetic optimization of plasmon waveguide resonance.

Plasmon waveguide resonance (PWR) sensors are particularly useful for biosensing due to their unique ability to perform sensing with two different polarizations. In this paper we report a comprehensive performance comparison between the surface plasmon resonance (SPR) sensor and the PWR sensor in terms of the sensitivity and the refractive index resolution. Both sensors were optimized using a genetic algorithm to acquire their best performance for bulk sensing applications. The experimental results show that the PWR sensor has a refractive index resolution of 5 × 10(-7) RIU which is 6 times smaller than that of the optimized SPR sensor. The TE polarization in the PWR sensor has a resolution of 1.4 × 10(-6) RIU which is smaller than the SPR sensor. The polarization diversity in the PWR sensor is another advantage which can be used to improve the measurement reliability.

Dual Polarization Measurements in the Hybrid Plasmonic Biosensors

A novel affinity biosensor is proposed based on the hybrid plasmonic platform. The proposed biosensor benefits from the high sensitivity of the surface plasmon resonance (SPR), while at the same time, it is capable of performing measurements in both the TM and TE polarizations (p- and s-polarizations). Unlike the conventional SPR biosensors, the polarization diversity of the hybrid sensor allows for decoupling of the bulk index variations in the fluidic channels (due to variations in concentration, decomposition, temperature, and so on) from the surface properties of the attached molecules. Compatibility of the proposed hybrid plasmonic biosensor with standard Si-processing techniques and the simplicity of its design are other advantages of the sensor which makes its fabrication straightforward. The best figure of merit for the biosensor is defined based on the minimum detection limit and a genetic algorithm is used to optimize the device. A method of de-convolving the surface and bulk effects is also discussed.

Self-referenced spectroscopy using plasmon waveguide resonance biosensor

A plasmon waveguide resonance (PWR) sensor is designed, fabricated, and tested for self-referenced biosensing. The PWR sensor is able to support two different polarizations, TM and TE. The TM polarization has a large sensitivity to variations in the background refractive index while the TE polarization is more sensitive to the surface properties. The ability of the PWR sensor to simultaneously operate in both TM and TE modes is used to decouple the background index variations (bulk effects) from the changes in adlayer thickness (surface effects) via multimode spectroscopy. To benchmark the performance of the PWR, a conventional surface plasmon resonance (SPR) sensor is fabricated and tested under the same conditions.

Pd-based integrated optical hydrogen sensor on a silicon-on-insulator platform.

We have experimentally demonstrated a compact, integrated optical hydrogen sensor on a silicon-on-insulator platform. The sensor consists of silicon waveguide coated with a thin palladium film. The sensitivity and response time of the sensor was tested for volumetric hydrogen concentrations ranging from 0% to 4%. The proposed hydrogen sensor shows great potential as a building block for an optical nose capable of simultaneous detection of multiple gases as well as environmental effects such as temperature and humidity.

Analysis and Optimization of Hybrid Plasmonic Waveguide as a Platform for Biosensing

Hybrid plasmonic waveguides (HPWGs) have received attention worldwide for many different kinds of applications, including on-chip polarization control, enhanced nonlinear optical effects, and biosensing. The HPWG sensor can achieve detection limit lower than possible with purely plasmonic sensors. It can be also used for obtaining additional information about complex biological samples. We analyze the effects of various parameters on such a sensor, optimize the sensor design, and predict the optimum performance achievable for an HPWG sensor in the Mach-Zehnder configuration. We also compare the performance of the HPWG sensor to those of other plasmonic sensors.

Dual-Wavelength Spectroscopy of a Metallic-Grating-Coupled Surface Plasmon Resonance Biosensor

A novel approach is proposed to decouple the surface and bulk effects in surface plasmon resonance (SPR) biosensors. This method is based on a metallic-grating-based SPR in which a sensor is efficiently optimized to excite three surface plasmon waves at two different wavelengths. Decoupling surface and bulk effects can be realized using this technique by tracing the resonance angle variations corresponding to each excited mode. To optimize the sensor, a genetic algorithm is applied, and finally, the performance of the sensor is compared with the conventional single-interface SPR biosensor.

A Hybrid Waveguide Sensor for Highly Sensitive Biosensing

We propose a highly sensitive biosensor consisting of a metal-plane separated from a silicon layer by a nano-fludic channel. The sensor performance is estimated and a method to distinguish bulk and surface sensitivity is proposed. OCIS codes: (280.1415) Biological sensing and sensors; (310.6628) Subwavelength structures, nanostructures; (130.3120) Integrated optics. Surface plasmons (SP) are currently one of the most widely used techniques for affinity biosensing. Because of the high field confinement of SP, these sensors are extremely sensitive and the presence of gold as a part of the guiding structure makes them suitable for functionalization with a wide range of antigen and antibodies- a great advantage for biosensing. Commonly used SP based sensors depend on only one measurement (e.g. shift in reflection minimum in an attenuated total reflection configuration) and cannot differentiate between the change of adlayer thickness and change of bulk sample properties, for example variation of index of water with temperature. Though a reference channel can be used to alleviate this problem, even a small degree of mismatch between the reference and sensing arm may compromise the effectiveness of such a scheme (1). As a result very precise control of temperature is necessary for the proper operation of a SP sensor. Dielectric waveguide sensors capable of measuring the change of both TM and TE mode properties do not suffer from this limitation (2) but the difficulty of functionalization of the dielectric surface makes them less useful for biological applications (3). Here we propose a new biosensing scheme that uses the recently proposed hybrid guide (4-6) to combine the benefits of SP and dielectric waveguide sensors. 2. Analysis and Results Figure 1(a) shows the hybrid waveguide that acts as the guiding structure for the proposed biosensing scheme. It consists of a thin silicon film of thickness d separated from a gold surface by a nano-fluidic channel of thickness h. Figures 1(b) and (c) show the guided power density profiles calculated using finite element code Comsol Multiphysics for the TE and TM modes. The presence of both TE and TM modes offers the possibility of two independent measurements. The presence of the gold surface also makes the sensor very suitable for biological applications. Here we present only the sensitivity analysis for a one dimensional hybrid guide. Details of specific implementations (prism and grating coupling, Mach-Zehnder interferometer) will be provided in our presentation.

Multimode spectroscopy using dielectric grating coupled to a surface plasmon resonance sensor.

A new platform is proposed to solve one of the main shortcomings of surface plasmon resonance biosensors, namely, the cross sensitivity to surface and bulk effects. This approach is based on multimode spectroscopy in which three different modes are excited simultaneously. The proposed design consists of an SPR sensor loaded with a dielectric grating. The design parameters (dimensions and wavelength) are optimized with a genetic algorithm. The optimized design has two resonance modes excited with TM polarized light, each sensitive to surface effects, and one TE mode mostly sensitive to variations in the bulk fluid refractive index. Numerical and analytical methods are used to justify the simulation results, which are in good agreement. Finally, it is shown that, by applying three-mode spectroscopy, decoupling the properties of the attached biomaterial from the background index variations is possible with the proposed design.

Kinetic analysis of nanoparticle-protein interactions using a plasmon waveguide resonance

A plasmon waveguide resonance (PWR) sensor is proposed for studying the interaction between gold nanoparticles and proteins. The ability of the PWR sensor to operate in both TM and TE Polarizations, i.e. its polarization diversity, facilitates the simultaneous spectroscopy of the nanoparticles surface reactions using both polarizations. The response of each polarization to streptavidin-biotin binding at the surface of gold nanoparticles is investigated in real time. Finally, using the principles of multimode spectroscopy, the nanoparticles surface reactions are decoupled from the bulk solution refractive index variations. Schematic diagram of the NP-modified PWR sensor.

Design, fabrication, and testing of a Pd-based integrated optical hydrogen sensor

A palladium-based integrated optical hydrogen sensor on a silicon-on-insulator platform was designed, fabricated, and tested. The sensitivity and response time of the sensor was recorded for hydrogen concentrations varying from 0-4%.