Silicon Photonics Sensors for Biophotonic Applications - A Tutorial

Introduction

Over the past decades, the development of biosensor technology has led to the use of biosensors in a wide range of applications, such as food quality monitoring, drug discovery, environmental monitoring, medical diagnosis, and industrial process control. Biosensors offer the advantages of fast detection response, ease of use, high sensitivity and specificity, lower cost, reliability, and compactness.

The integration of silicon photonics technology with biosensors has significantly broadened the scope and applications of optical biosensors. Silicon photonics leverages the extremely high index contrast of silicon and the mature CMOS fabrication processes to develop compact and cost-effective photonic integrated circuits (PICs). In photonic technology, information is transmitted via light rather than electrons, enabling transmission in the THz spectrum.

Various silicon photonic-based biosensing devices have been developed, including interferometer-based (Mach-Zehnder interferometers), microring resonator-based, photonic crystal-based, and Bragg grating-based biosensors. These sensors rely on the evanescent wave (EW) field interacting with the analyte, causing changes in the effective refractive index (RI) that are detected through optical phase or wavelength shifts.

This tutorial will provide an overview of the operating principles, configurations, and performance enhancement strategies of silicon photonic-based biosensors. It will also discuss the integration of these sensors into lab-on-a-chip (LOC) platforms using techniques such as 3D printed molding, microfluidics, and optoelectronic integration.

Fig 1. Biorecognition elements

Operating Principles

Biosensors based on the evanescent field interaction work on the principle that the external RI region created by the interaction of the evanescent field with the surrounding environment is affected by the binding and accumulation of signaling molecules at receptors. This, in turn, disturbs the evanescent field and alters the characteristics of the guided light in the waveguide.

The sensitivity of these sensors can be distinguished into two categories: bulk sensitivity and surface sensitivity. Bulk sensitivity refers to the slope of the wavelength or phase shift with respect to changes in the bulk RI of the analyte solution. Surface sensitivity refers to the wavelength or phase shift with respect to changes in the thickness of the adlayer on the sensor surface.

Optical Biosensor Configurations

A. Interferometer-Based Biosensors

Interferometer-based biosensors, such as Mach-Zehnder interferometers (MZIs) and Young's interferometers (YIs), rely on highly sensitive waveguide interferometry techniques. In an MZI, the guided input beam is split into two symmetric arms and then rejoined, resulting in a phase difference that causes constructive or destructive interference. The evanescent field in the sensing arm interacts with the analyte, causing changes in the effective RI and optical phase difference at the output (Fig. 2a). In a YI, the split beams are not rejoined, and the output light intensity is detected in free space or on a CCD camera (Fig. 2b).

Fig. 2. (a) MZI biosensor. (b) YI biosensor

B. Resonant Microcavity-Based Biosensors

Microcavity resonator-based biosensors consist of a linear waveguide or tapered fiber in which input light couples to the cavity via an evanescent field. The coupled light travels in the form of whispering gallery modes (WGMs), and the wavelength of light in resonance condition is determined by the effective RI and the cavity radius (Fig. 3).

Fig. 3. Planner microcavity resonator-based sensors. (a) MRR-based sensor; light is coupled to MRR from the linear waveguide at resonant conditions. (b) Microdisk resonator-based sensor. (c) Microtoroid resonator-based sensor; the structure is coupled by a tapered fiber having a low-loss and ultrahigh-quality factor

C. Photonic Crystal-Based Biosensors

Photonic crystal (PhC) biosensors consist of a periodic structure with variations in RI that create defected modes. The strong light-matter interaction in the defected region results in sensitivity for biosensing applications. PhC sensors can be configured in 1D, 2D, and 3D structures (Fig. 4).

Fig. 4. PhC-based sensors. (a) 1-D configuration consists of different materials with alternative high and low RIs. (b) 2-D configuration; the periodicity of dielectric occurs in two directions. (c) 3-D configuration has permittivity modulation along all three directions

D. Bragg Grating-Based Biosensors

Bragg grating biosensors have a periodic structure with alternating high and low RI materials. The reflected guided light at the Bragg wavelength is affected by changes in the effective RI, which can be detected (Fig. 5).

Fig. 5. Bragg grating-based biosensors. (a) Bragg grating structure with side wall and top grating; R and T are the grating’s reflection and transmission with the numerous reflections through the structure. (b) Schematic of the phase-shifted Bragg grating structure, with the periodicity Λ, corrugated width of Δw, different RIs, and length a or b

Performance Enhancement Strategies

A. Fundamental Strategies

1. Strip Waveguides: The most common silicon waveguide is the strip waveguide. The quasi-TM mode exhibits better sensitivity than the quasi-TE mode due to the larger evanescent field component propagating around the waveguide (Fig. 6).

2. Slot Waveguides: Slot waveguides have two high-index silicon rails separated by a low-index slot region. This configuration strongly confines the light in the slot area, leading to enhanced light-analyte interaction and improved sensitivity (Fig. 7-8).

Fig. 6. Electric field intensity distributions of the strip waveguide. (a) TE-quasi-transverse electric mode. (b) TM-quasi-transverse magnetic mode. Most of the field intensity occurs above and below the waveguide core. The silicon waveguide has RIs, core ηsi = 3.48, cladding ηcladd = 1.7, and box η at wavelength λ = 1550 nm

Fig. 7. (a) Schematic of the MZI biosensor with slotted waveguide sensing arm and strip waveguide reference arm made up of silicon nitride. A grating coupler is efficient for coupling light between the optical fiber and the integrated waveguide. Light is converted using a mode converter between strip waveguide and slot waveguide. Electric field intensity distributions of the quasi-TE mode for (b) strip waveguide and (c) slot waveguide

Fig. 8 presents SOI waveguide types and an SOH-based modulator. (a) Describes three waveguide forms: the conventional silicon strip for tight light confinement, the slot waveguide with light partially confined by silicon slabs, and the strip-loaded slot waveguide connecting to metal electrodes for voltage application. (b) Showcases a 3-D view of the strip-loaded slot waveguide with EO organic cladding around the waveguide on a BOX layer. (c) Illustrates electric field distribution in the slot and an RC equivalent circuit for voltage application. (d) Details electron accumulation in a silicon strip load under a positive gate voltage, causing energy band bending in the strip loads, depicted for EF,C,V as Fermi, conduction, and valence bands, with a SiO2 substrate thickness of 2 μm and a strip load thickness of 60 nm

B. Advanced Approaches

1. Subwavelength Grating Waveguides: Subwavelength grating (SWG) waveguides have a periodic structure with a period much smaller than the wavelength. The SWG structure can be used to implement various wavelength-selective devices, such as Bragg gratings, ring resonators, and racetrack resonators, with improved performance (Figs. 9-12).

2. Vernier Effect-Based Systems: Vernier effect-based sensors utilize two cascaded microcavity ring resonators with slightly different free spectral ranges. The sensing ring interacts with the analyte, while the reference ring is protected, allowing for highly sensitive measurements (Fig. 13-14).

Fig. 9. SWG waveguide structure. (a) SWG schematic. (b) Distribution of electric field strength through SWG

Fig. 10. SWG waveguide structure. (a) SWG schematic. (b) Cross section view of SWG. (c) SWG taper top view

Fig. 11. SWG waveguide structure. (a) Schematic of BG implemented by interleaving two SWGs. (b) Layout of BG based on SWG. (c) Fabricated design. (d) SWG waveguide structure. (d) SWG ring resonator schematic

Fig. 12. SWG waveguide structure. (a) Layout SWG racetrack. (b) SWG racetrack schematic. (c) Schematic configuration of the designed 3-dB power splitter based on asymmetric directional coupler with sub-wavelength gratings

Fig. 13. Vernier Effect-based sensor configurations. (a) 3-D view of the Vernier effect system consisting of sensing and reference ring. (b) Microscopic view of two cascaded MRRs mounted on the SOI platform (sensing ring has an open window)

Fig. 14. Schematic of the combination of MZI and the ring resonator-based sensor having open at the sensing arm

Lab-on-a-Chip Implementation

A. Materials and Methods

1. 3D Printed Molding Fabrication: The silicon substrate can be removed using XeF2 etching to expose the photonic sensor to the microfluidic samples. The sensor chip can then be integrated with a 3D printed microfluidic package (Fig. 15).

2. Microfluidic Packaging Integration: The integration of microfluidics with biosensors offers benefits such as miniaturization, improved sensitivity, and fast response. Alignment and sealing techniques for the microfluidic packaging are crucial (Fig. 16-17).

3. On-Chip Optoelectronic Integration: Optoelectronic integrated circuits (OEICs) that combine photonic and electronic components on a single chip can enable high-speed optical interconnects and advanced sensing functionalities (Fig. 18, S4).

Fig. 15. (a) CMOS 45RFSOI process cross section; backside Si substrate etches exposes the sensing photonics to the fluidic samples. (b) Cross section of microfluidic package aligned to chip. (c) 3-D printed mold model

Fig. 16. Fabrication techniques. (a) PDMS/plastic molding process. (b) Monolithic devices fabrication using SL/3-D printing

Fig. 17 showcases microfluidic assembly and integration techniques: (a) A multilayer stack of a glass substrate and PDMS layers bonded via oxygen plasma etching, with alignment achieved through corner holes matched with a PCB plate. (b) Injection of colored dyes into the assembly through coupling tubes. (c) Advanced integration of microfluidic packaging with electronic circuits and sensors, featuring temporary attachment of sensing layers to electronics via flexible contacts. (d) A microfluidic chip employing lensless imaging techniques, designed for placement over a CCD image sensor for point-of-care testing.

Fig. 18. Integrated optical interconnect link with a silicon electrooptic modulator, silicon waveguide, and germanium-on-silicon PD. SEM images of (a) full optical link, (b) ring resonator modulator, and (c) PD

Light Coupling by Nanogratings

Efficient coupling of light into the integrated waveguide is essential for sensor performance. Diffraction grating structures etched directly on the waveguide can provide improved light in-coupling and better integration compared to traditional methods (Fig. 19-20).

Fig. 19. Experimental setup photograph including grating coupler, grating excitation on chip, and propagation of light

Fig. 20 depicts an ultracompact grating coupler efficiency measurement device, featuring a suspended waveguide with grating couplers on each end (a), an SEM image of the fabricated device (b), and an ion-exchanged glass where Ag+ ions and nanoparticles create a silver nanograting, coupling the laser beam into the waveguide (c).

Future Perspectives

The existing biosensor technology can be employed in various industries and fields, such as drug discovery, environmental monitoring, clinical diagnostics, and food safety. Future research should target customized sensor designs with improved sensitivity, robustness, and simplified readout schemes. Fully integrated, user-friendly systems capable of analyzing complex biospecimens are desired.

Conclusion

This tutorial has provided an overview of the operating principles, configurations, and performance enhancement strategies of silicon photonic-based biosensors. The integration of these sensors into lab-on-a-chip platforms using techniques like 3D printed molding, microfluidics, and optoelectronic integration was also discussed. The tutorial highlighted the advantages of advanced approaches, such as slot waveguides and subwavelength grating structures, for improving the sensitivity and performance of these biosensing devices. The future perspectives emphasize the need for customized, robust, and integrated biosensing systems that can enable widespread adoption in various applications.

Reference

[2] C. Dhote, A. Singh and S. Kumar, "Silicon Photonics Sensors for Biophotonic Applications—A Review," in IEEE Sensors Journal, vol. 22, no. 19, pp. 18228-18239, 1 Oct.1, 2022, doi: 10.1109/JSEN.2022.3199663.