On the performance of label-free biosensors based on vertical one-dimensional photonic crystal resonant cavities

One-Dimensional Defect Layer Photonic Crystal Sensor for Purity Assessment of Organic Solvents

This paper presents the design and analysis of a one-dimensional defect layer photonic crystal (1D-DLPC) sensor for the assessment of the purity of chemical solvents with enhanced accuracy. Chemical solvents are frequently used in chemical processes as reaction mediums. It is essential to ascertain its purity since impurities can significantly affect the outcome of the reaction. The structure of the proposed one-dimensional defect layer photonic crystal sensor consists of a defect layer sandwiched between alternate layers of ZnO and SiO2 organized with a certain periodicity. It has been shown that the localized defect modes inside the structure can detect minute refractive index changes based on the degree of impurity of chemical solvents. Simulation studies have been performed through the transfer matrix method (TMM) and the performance of the design is evaluated using several metrics such as sensitivity, full width at half-maximum, figure of merit, quality factor, and dynamic range. Results indicate that the designed one-dimensional defect layer photonic crystal sensor has a significantly high efficiency and is suitable for detecting impure solvents.

Distributed Bragg Reflectors Employed in Sensors and Filters Based on Cavity-Mode Spectral-Domain Resonances

Spectral-domain resonances for cavities formed by two distributed Bragg reflectors (DBRs) were analyzed theoretically and experimentally. We model the reflectance and transmittance spectra of the cavity at the normal incidence of light when DBRs are represented by a one-dimensional photonic crystal (1DPhC) comprising six bilayers of TiO2/SiO2 with a termination layer of TiO2. Using a new approach based on the reference reflectance, we model the reflectance ratio as a function of both the cavity thickness and its refractive index (RI) and show that narrow dips within the 1DPhC band gap can easily be resolved. We revealed that the sensitivity and figure of merit (FOM) are as high as 610 nm/RIU and 938 RIU−1, respectively. The transmittance spectra include narrow peaks within the 1DPhC band gap and their amplitude and spacing depend on the cavity’s thickness. We experimentally demonstrated the sensitivity to variations of relative humidity (RH) of moist air and FOM as high as 0.156 nm/%RH and 0.047 %RH−1, respectively. In addition, we show that, due to the transmittance spectra, the DBRs with air cavity can be employed as spectral filters, and this is demonstrated for two LED sources for which their spectra are filtered at wavelengths 680 nm and 780 nm, respectively, to widths as narrow as 2.3 nm. The DBR-based resonators, thus, represent an effective alternative to both sensors and optical filters, with advantages including the normal incidence of light and narrow-spectral-width resonances.

Novel Photonic Bio-Chip Sensor Based on Strained Graphene Sheets for Blood Cell Sorting

A photonic biochip with a tunable response in the visible range is suggested for blood cell sorting applications. Multi-layers of ZnS and Ge slabs (as the main building blocks), hosting a cell in which bio-sample could be injected, are considered as the core of the sensor. In order to increase the sensitivity of the chip, the bio-cell is capsulated inside air slabs, and its walls are coated with graphene sheets. Paying special attention to white and red blood components, the optimum values for structural parameters are extracted first. Tunability of the sensor detectivity is then explored by finding the role of the probe light incident angle, as well as its polarization. The strain of the graphene layer and angle in which it is applied are also suggested to further improve the performance tunability. Results reflect that the biochip can effectively identify selected components through their induced different optical features, besides of the different figure of merit and sensitivity amounts that are recorded for them by the sensor.

Impact of Fabrication and Bioassay Surface Roughness on the Performance of Label-Free Resonant Biosensors Based On One-Dimensional Photonic Crystal Microcavities

Micro- and nanofabrication offer remarkable opportunities for the preparation of label-free biosensors exploiting optical resonances to improve sensitivity and reduce detection limit once specificity is imparted through surface biofunctionalization. Nonetheless, both surface roughness, peculiar of fabrication processes, and bioassay roughness, resulting from uneven molecular coverage of the sensing surfaces, produce light scattering and, in turn, deterioration of biosensing capabilities, especially in resonant cavities where light travels forth and back thousands to million times. Here, we present a quantitative theoretical analysis about the impact of fabrication and bioassay surface roughness on the performance of optical biosensors exploiting silicon-based, vertical one-dimensional (1D) photonic crystal resonant cavities, also taking noise sources into account. One-dimensional photonic crystal resonant cavities with different architectures and quality factors ranging from 10² to 10⁶ are considered. The analysis points out that whereas sensitivity and linearity of the biosensors are not affected by the roughness level, either due to fabrication or bioassay, the limit of detection can be significantly degraded by both of them, depending on the quality factor of the cavity and noise level of the measurement system. The paper provides important insights into performance versus design, fabrication, and readout of biosensors based on resonant 1D photonic crystal cavities for real-setting operation.

Engineering vertically interrogated interferometric sensors for optical label-free biosensing

In this work, we review the technology of vertically interrogated optical biosensors from the point of view of engineering. Vertical sensors present several advantages in the fabrication processes and in the light coupling systems, compared with other interferometric sensors. Four different interrelated aspects of the design are identified and described: sensing cell design, optical techniques used in the interrogation, fabrication processes, fluidics, and biofunctionalization of the sensing surface. The designer of a vertical sensor should decide carefully which solution to adopt on each aspect prior to finally integrating all the components in a single platform. Complexity, cost, and reliability of this platform will be determined by the decisions taken on each of the design process. We focus on the research and experience acquired by our group during last years in the field of optical biosensors.