A study published in Biosensors investigates surface plasmon resonance and lossy mode resonance in the bi-layer and tri-layer fiber optic sensing probes in the Kretschmann excitation configuration. This research will aid in understanding the underlying physics and development of biochemical sensors with a broader spectrum range.
Surface Plasmon Resonance (SPR)
In surface plasmon resonance (SPR), conduction band electrons collectively oscillate in resonance with the fluctuating electric field of incoming light, leading to the generation of intense plasmonic electrons through non-radiative excitation.
Surface plasmon resonance is produced by p-polarized light or transverse magnetic fields at the metal-dielectric interface. Since the momentum of the surface plasmon resonance mode is different from that of the incident light, direct optical excitation of the SPR mode is not possible.
However, various momentum matching approaches, such as passing light through a high refractive index prism and employing a grating, has been suggested to effectively excite surface plasmon resonance modes. In addition, the Otto and Kretschmann coupling arrangements are also viable options for SPR excitation. The Kretschmann configuration is often used because of its ease of implementation.
The emerging field of surface plasmon resonance has gained substantial scientific interest over the years due to its extensive spectrum of applications, including nano-antennas, imaging, and biosensing.
Lossy Mode Resonance (LMR)
Lossy mode resonance (LMR) is an optical phenomenon similar to surface plasmon resonance. It is sensitive to environmental fluctuations and can be utilized as a signal detector in biological sensors.
The LMR sensors can detect light with transverse electric and transverse magnetic polarization. In contrast to SPR, LMR excitation does not necessitate any particular polarization for the incoming light.
LMR is generated by coupling the lossy mode with the evanescent wave at a specific thickness of the thin film. Therefore, it is challenging to utilize LMR in sensing applications as selecting the appropriate material for the thin film is critical.
Indium tin oxide is one of the LMR-supporting conducting metal oxide (CMO) materials. It is a transparent material with a 3.6 eV optical band gap, which limits band-to-band transitions. Indium tin oxide (ITO) can have widely varying properties because its electrical and optical properties can be modified during manufacturing. This attribute may be used to change the LMR resonance wavelength.
Limited research has been done on lossy mode resonance sensors compared to surface plasmon resonance. It has been demonstrated that they can overcome the limits of conventional surface plasmon resonance sensors.
LMR-based sensors can achieve greater quality factors, and their sensitivity is steadily increasing as related research advances.
Investigation of Indium-Tin Oxide-Based (ITO) + Silver (Ag)-Based Bi-Layer and Tri-Layer Optical Fiber Sensors
This study investigates the characteristics of tri- and bi-layer indium-tin oxide-based optical fiber sensors simultaneously stimulating LMR and SPR. In tri- and bi-layer geometries, researchers investigated ITO + Ag + ITO and ITO + Ag configurations, respectively.
The detection accuracy and sensitivity of SPR and LMR were investigated as a function of analyte refractive index (RI) for various values of silver (Ag) layer thickness.
In the transmission spectrum, resonance dips were observed to determine the figure of merit (FOM), sensitivity and detection accuracy of SPR and LMR in the bi-layer and tri-layer optical fiber sensors.
Significant Findings of the Study
In the bilayer structure, the sensitivity and detection accuracy of the LMR dip was greater than that of the surface plasmon resonance. The SPR dip becomes insensitive as the silver layer thickness grows, leaving the LMR dip to be utilized for sensing.
The SPR dip can function as a reference at a very low thickness, converting the sensor to a self-referenced sensor. This arrangement can be employed in chemical and biological sensing by selecting the right thickness of Ag, while the same structure can also be used for wavelength filtering.
Two tri-layer geometry combinations were investigated, where two lossy mode resonance dips and one SPR were detected. The initial LMR dip in the first tri-layer configuration's visible area was less sensitive than the surface plasmon resonance dip due to variations in the thickness of the outermost indium-tin oxide layer.
The middle lossy mode resonance dip in the near-infrared region (NIR) was insensitive to changes in analyte RI. This insensitive LMR drop arises as the third indium-tin oxide layer's thickness increases.
The resonance wavelength of the SPR dip moves to the shorter wavelength side, whereas the resonance wavelength of the lossy mode resonance dip shifts to the longer wavelength side.
The primary contribution of this study is the observation and manipulation of the insensitive second LMR dip with varying indium-tin oxide layer thickness since this implies a change of resonance type between SPR and LMR.
This research offers the foundation of design standards for tri- and bi-layer ITO-based structures that enable LMR and SPR excitations.
Gaur, D.S., Purohit, A., Mishra, S.K., & Mishra, A.K. (2022). An Interplay between Lossy Mode Resonance and Surface Plasmon Resonance and Their Sensing Applications. Biosensors, 12, 721. https://www.mdpi.com/2079-6374/12/9/721