Introduction
Tamm-plasmon-polaritons (TPPs) are optical surface states that arise at the interface between a metallic film and a photonic crystal (PC). Unlike traditional surface plasmon polaritons (SPPs), which require specific conditions for excitation (such as total internal reflection in a prism-coupled system), TPPs can be directly excited in normal incidence configurations, making them ideal for biosensing applications.
The TPP biosensor operates on the principle that biomolecule adsorption on the sensor surface alters the refractive index, leading to a measurable shift in resonance conditions. This shift is detected through reflectance or transmission spectroscopy.
Theory of Tamm-Plasmon-Polariton Modes
The TPP modes are formed due to Bragg reflection from the photonic crystal and the surface plasmon resonance at the metal-dielectric interface. The dispersion relation of TPPs is given by:
$$
\cos(k_z d) = \frac{1}{2} \left( \frac{\eta + 1}{\eta} \right)
$$
where:
✔ $k_z$ is the propagation constant in the z-direction,
✔ $d$ is the thickness of the metal layer,
✔ $\eta$ is the impedance ratio between the metal and photonic crystal structure.
For an N-layer photonic crystal, the transfer matrix method (TMM) is used to derive the reflection coefficient $R(\lambda)$, given by:
$$
R(\lambda) = \left| \frac{M_{11} + M_{12}Z - M_{21}/Z - M_{22}}{M_{11} + M_{12}Z + M_{21}/Z + M_{22}} \right|^2
$$
where $M_{ij}$ are the elements of the transfer matrix, and $Z$ is the effective impedance of the system. The TPP resonance wavelength $\lambda_{\text{TPP}}$ is obtained from the minimum of $R(\lambda)$, which shifts due to refractive index changes induced by biomolecular binding.
Sensitivity and Detection Limit
The biosensor detects small changes in the refractive index ($\Delta n$) of the analyte layer by monitoring the shift in TPP resonance wavelength ($\Delta \lambda$). The sensitivity $S$ is defined as:
$$
S = \frac{\Delta \lambda}{\Delta n}
$$
For a high-performance biosensor, typical sensitivity values range from 500–1000 nm/RIU (refractive index unit). The limit of detection (LOD) is given by:
$$
\text{LOD} = \frac{\sigma}{S}
$$
where $\sigma$ is the spectral resolution of the detection system. A smaller LOD value indicates higher detection accuracy.
Fabrication and Design Considerations
✔ Metallic Layer: Typically gold (Au) or silver (Ag) is used due to their strong plasmonic response in the visible and near-infrared spectrum.
✔ Photonic Crystal: A dielectric multilayer stack (e.g., SiO₂/TiO₂) provides a high-quality Bragg reflector to support TPPs.
✔ Functionalization: The surface is coated with antibodies or aptamers to selectively capture biomolecules.
Experimental Observations
A recent study demonstrated a TPP-based biosensor for protein detection, achieving:
✔ Sensitivity of 750 nm/RIU, outperforming traditional SPR sensors.
✔ Selective detection of biomarkers at femtomolar concentrations.
✔ A real-time response with high reproducibility.
Advantages Over Traditional SPR Sensors
✔ Normal Incidence Operation: Unlike SPR, TPPs do not require Kretschmann or Otto configurations.
✔ Enhanced Sensitivity: The combined effects of plasmonic and photonic resonance enhance the response.
✔ Compact and Low-Cost: Suitable for lab-on-chip integration.
Applications of TPP Biosensors
✔ Medical Diagnostics: Detection of cancer biomarkers, viruses, and proteins.
✔ Environmental Monitoring: Sensing of toxins and pollutants in water and air.
✔ Pharmaceutical Industry: Real-time drug interaction studies.
Conclusion
Tamm-plasmon-polariton biosensors offer high sensitivity, ease of integration, and label-free detection, making them an excellent alternative to conventional plasmonic biosensors. Future research aims at miniaturization, multiplexing, and AI-driven spectral analysis for real-time diagnostics.
🔗 References for Further Reading:
Tamm-plasmon-polariton biosensor
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