Introduction

Plasmonic sensors employing metal–insulator-metal (MIM) waveguides (WGs) have developed as a promising technology for sensitive and label-free detection in diverse applications [1]. MIM WG-based sensors typically operate based on Surface Plasmon Polaritons (SPPs). SPPs are collective oscillations of electrons that occur at the interface between a metal and a dielectric material, such as air or an insulator. When light interacts with the metal–dielectric interface at a specific angle and wavelength, it can excite these SPPs, leading to enhanced electromagnetic fields near the interface. In a typical MIM WG configuration, two metal layers (gold or silver) sandwich a thin insulator (can be air, n = 1.0), forming a nanoscale WG structure. When illuminated with light, surface plasmons are excited at the metal-dielectric interfaces, leading to enhanced electromagnetic fields within the insulator layer [2]. The interaction between these surface plasmons and external stimuli, such as variations in refractive index or the presence of analytes, results in measurable alterations in the transmission characteristics of the guided light [3]. This phenomenon enables highly sensitive detection, making plasmonic sensors established on MIM WGs suitable for applications in biosensing, environmental monitoring, and chemical analysis [3,4,5,6]. Additionally, the compact nature of these devices and their compatibility with integrated circuits contribute to their potential for on-chip sensing platforms [7].

Plasmonic sensors exhibit a remarkable advancement in sensitivity, setting them apart as cutting-edge devices in the realm of sensing technologies. The interaction between incident light and plasmonic materials such as gold (Au) or silver (Ag) generates strong electromagnetic fields at the sensor surface, resulting in an enhanced sensitivity to variations in the surrounding environment [8]. This improved sensitivity proves invaluable in applications such as biological and chemical sensing, where minor deviations in molecular interactions or concentrations need to be accurately captured. The ability of plasmonic sensors to operate in real-time and provide precise, rapid responses positions them at the forefront of advancements in sensing technology, paving the way for innovations in fields ranging from medical diagnostics to environmental monitoring [9, 10].

The measurement of temperature in a photonic biosensor chip is of paramount importance due to its direct impact on the accuracy and reliability of bioanalytical results [11]. Photonic biosensor chips are widely utilized in fields such as medical diagnostics, environmental monitoring, and biotechnology for their ability to detect and analyze biological molecules with high sensitivity. Temperature influences the kinetics of biochemical reactions and the stability of biological samples, directly affecting the performance of biosensors [12]. Accurate temperature control is crucial in maintaining the optimal conditions for bioassays, ensuring reproducible and reliable results. In biological applications, the detection of minute shifts in temperature can be critical, as it directly affects the specificity and sensitivity of the biosensor. Furthermore, precise temperature monitoring is essential for preventing denaturation of biomolecules and maintaining the integrity of biological samples, which is especially vital in medical diagnostics and life sciences research [13].

In previous works related to plasmonic sensors, researchers proposed several interesting designs based on MIM WG for sensing applications [14,15,16,17,18]. However, those sensing devices can only be employed for one application at a time. Moreover, the light coupling mechanism to the MIM WG has not been widely explored [19]. In this work, a unique sensor design employed for the simultaneous detection of analytes and ambient temperature is proposed. Additionally, orthogonal mode coupler WGs are integrated to couple the light in and out of the MIM WG.

Device Design and Optimization

The sensor design intricately combines a MIM bus WG with circular cavities, seamlessly linked through a connecting WG. The key parameters defining this design include the width of the MIM bus WG and the connecting WG referred to as W, the cavity radius represented by R, and the length of the connecting WG indicated as L. To simplify the analysis, W is set at a fixed thickness of 50 nm, as it can effectively sustain a fundamental TM0 mode. Furthermore, the system incorporates silicon nitride (Si3N4) WG-based mode couplers to seamlessly convert the dielectric mode to the plasmonic mode. This integration facilitates efficient coupling of the plasmonic mode to the MIM WG and vice versa, enhancing the overall functionality of the system. The geometric parameters used in this study are listed in Table 1.

Table 1 Geometric parameters of the plasmonic sensor
Full size table

For MIM WG, Au is preferred as a metal that stands out as an optimal selection for plasmonic sensors, primarily attributed to its robust SPP in the visible and near-infrared regions of the electromagnetic spectrum [16]. This distinctive SPP response arises from the unique electronic structure of Au, enabling its electrons to resonate with light waves. Beyond its optical characteristics, Au demonstrates biocompatibility and chemical stability, rendering it a favorable choice for applications in biological sensing. The calculation of the permittivity of Au employs the Lorentz–Drude model, as denoted in Equation [20].

$$\varepsilon ={\varepsilon }_{\infty }-\frac{{\omega }_{p}^{2}}{{\omega }^{2}+j\omega \gamma };$$

where \(\varepsilon_\infty=9.0685\), \({\omega }_{p}=135.44 \times {10}^{14} rad/s\). The 3D model of the plasmonic sensing system is shown in Fig. 1a. The PDMS-filled cavity, designated as Ct, serves as a crucial component for temperature sensing. Simultaneously, another cavity, marked as Cb in Fig. 1b, is dedicated to biosensing applications. This dual-cavity system enhances the versatility of the device, allowing for both precise temperature monitoring and efficient biosensor functionality.

Fig. 1
figure 1

Graphical illustration of plasmonic sensor based on MIM WG. a Full 3D model. b 2D model of the sensing part

Full size image

The numerical investigation of the plasmonic sensor was methodically carried out using the finite element method (FEM) simulation tool, COMSOL Multiphysics. Leveraging the electromagnetic wave physics interface within the frequency domain (emw), this simulation approach allowed for a detailed exploration of the sensor’s behavior. In dissecting the WG cross-section, triangular mesh elements were employed with an “extremely fine” grid size, thoroughly applied throughout the entire sensor structure. The choice of meshing parameters was guided by a careful consideration of the dual factors: the imperative need for solution accuracy and the computational capabilities at hand. The intricacies of the problem were aptly addressed through this refined meshing strategy. To emulate an open geometry environment more realistically, scattering boundary conditions (SBC) were strategically implemented at the outer edges of the FEM simulation window. This thoughtful application of SBC enhances the simulation’s fidelity, approximating the sensor's response within an unconfined setting. By adopting these advanced simulation techniques, the study ensures a comprehensive exploration of the plasmonic sensor’s performance, laying the groundwork for insightful analyses and the potential optimization of its design.

PDMS, a mineral-organic polymer belonging to the siloxane family, is a widely used polymeric organosilicon chemical commonly known as silicone. This silicone-based organic polymer, PDMS, is recognized for its notable characteristics, including transparency across a broad wavelength range, a refractive index lower than fused silica, favorable elasto-optic and thermo-optic coefficients, biocompatibility, and minimal absorption loss. Moreover, PDMS displays advantageous mechanical characteristics attributed to its low Young’s modulus, resulting in a soft, deformable, and shrinkage-free material. An essential aspect of mass production is the cost-efficiency and simplicity of the manufacturing process. The high sensitivity of PDMS-based devices to temperature fluctuations is noteworthy, attributed to its elevated thermo-optic coefficient [12]. The correlation between the refractive index of the PDMS layer and temperature variation can be articulated as the refractive index of PDMS = 1.4176–4.5 × 10−4 T [21]; where T is the ambient temperature.

Light Coupling to MIM WG

Facilitating the connection of light to a MIM WG presents numerous challenges associated with designing and manufacturing these WGs [1]. One notable challenge involves mode matching, where aligning the incident light accurately with the WG's mode is essential for effective coupling [7]. This task becomes remarkably complex when dealing with diverse light sources and various WG structures. Another considerable concern is the considerable optical losses linked to MIM WGs, stemming from metal absorption and scattering, which can greatly affect light coupling efficiency, necessitating strategies to alleviate them. Moreover, the fabrication tolerances of MIM WGs are critical for their overall performance. Deviations in dimensions and geometries resulting from fabrication inaccuracies can hinder achieving consistent and efficient coupling. Additionally, MIM WGs often exhibit strong polarization dependence, meaning that light coupling efficiency may vary based on the incident light’s polarization orientation. This requires implementing polarization control and management techniques. Effectively addressing these challenges is essential to fully unleash the ability of MIM WGs in diverse photonic applications.

A proposed solution involves integrating an orthogonal mode coupler, which entails aligning the Si3N4 dielectric WG and the MIM plasmonic WG in a perpendicular positioning. This arrangement demonstrates remarkable efficiency in light coupling. The presented orthogonal coupling scheme provides a promising resolution to the momentum mismatch problem without the necessity for additional tapering. This advanced method aims to position the orthogonal momentum component of the Si3N4 WG (kx) with the momentum component of the MIM WG, contrasting with conventional methods like prism or Bragg grating, which target the longitudinal component. Importantly, the kx component exhibits a relatively small mismatch with the kSPP over a wide bandwidth. Spatially synchronizing the kx component with the kSPP component is crucial for efficiently guiding power into the MIM WG. This deliberate alignment facilitates the effective coupling of optical energy to the plasmonic slot WG, eliminating the necessity for additional tapering procedures.

The WG dimensions undergo optimization to accommodate the fundamental mode within the wavelength range of 1000 nm to 1500 nm. Consequently, the effective refractive index (neff) of the WG modes is computed for Si3N4 WG core width (Wcore) ranging from 600 to 800 nm, while maintaining a constant core height (Hcore) at 400 nm. As illustrated in Fig. 2a, it is evident that Wcore values of 700 nm or 750 nm are viable, enabling the support of the TE fundamental mode across the desired wavelength spectrum. Subsequently, attention is directed towards optimizing the interface between the orthogonal couplers and the MIM WG, as this section has the potential to introduce additional losses and light scattering. The separation between the dielectric WG and metal, denoted as ‘a,’ is maintained at a persistent value of 450 nm, while the length of the interface, labeled as ‘b,’ requires careful optimization. The transmission spectrum of the complete WG system, incorporating two couplers and a MIM WG with a length of 5000 nm, is graphed for ‘b’ within the range of 50 nm to 470 nm, as depicted in Fig. 2b. Notably, the transmission reaches its minimum value of − 2.94 dB when ‘b’ is set at 310 nm. Subsequently, ‘a’ undergoes further optimization to enhance transmission efficiency. However, Fig. 2c illustrates that ‘a’ does not exert a significant impact on transmission when ‘b’ is already optimized for peak performance. Furthermore, Fig. 2d illustrates the normalized H-field mapping at an operational wavelength of 1500 nm. The length and width of the Si3N4 WG are fixed at 3 µm and 0.7 µm, respectively. The length of the MIM WG segment is maintained at 4 µm. This conception showcases the transformation of a dielectric mode into a plasmonic mode and subsequently back to a dielectric mode at the output dielectric WG. Moreover, the transmission characteristics of the optimized design of an orthogonal mode coupler are investigated across the wavelength span from 1000 to 1500 nm. Remarkably, the transmission loss exhibits a nuanced range, fluctuating between − 2 dB and − 5.9 dB throughout the entire wavelength band, showcasing the device’s versatile performance characteristics.

Fig. 2
figure 2

a Effective refractive index of Si3N4 ridge WG. b Transmission spectrum of orthogonal couplers integrated with MIM WG versus ‘b’ at constant value of a = 450 nm. c Transmission spectrum of orthogonal couplers integrated with MIM WG versus ‘a’ at optimized value of b = 310 nm. d Normalized H-field mapping in the MIM WG connected to orthogonal couplers at an operational wavelength of 1500 nm. Note: a = 450 nm and b = 310 nm

Full size image

Optimization of the Sensor Geometry

In the initial phase of the device optimization, the length (L) of the connecting WG is held constant at 100 nm, establishing a stable baseline, while the radius (R) of the cavity undergoes a systematic exploration within the range of 200 nm to 250 nm, with an incremental step size of 10 nm. The transmission spectrum of the device is plotted across the wavelength range of 650 nm to 1000 nm where the refractive index of the ambient medium is fixed at 1.0, as vividly illustrated in Fig. 3. The nuanced relationship between R and the device’s resonance characteristics unfolds prominently in Fig. 3a. A discernible trend emerges as R experiences incremental augmentation from 200 to 250 nm—depicted in Fig. 3a—where the resonance dip noticeably undergoes a redshift. This transformation is not only visually evident but is also accompanied by discernible changes in the full width at half maximum and extinction ratio (ER) of the resonance dip. Notably, at a critical radius value of R = 230 nm, a resonance state characterized by a notably narrow and deep dip is achieved, featuring an ER of − 11.20 dB. This distinctive resonance state manifests at a specific wavelength, denoted as 882 nm, signifying a finely tuned optical response. The precision attained in this configuration highlights the intricate control over the device's resonance properties, paving the way for applications demanding specific optical characteristics.

Fig. 3
figure 3

Transmission spectrum of the device versus, a R at constant L = 100 nm, b L at constant R = 230 nm. Note: The refractive index of the ambient medium is 1.0. Upon introduction of bio-analytes into the cavity, the resonance will shift towards higher wavelengths within the spectrum

Full size image

In the second phase, the effect of L on the resonance state of the device is determined, therefore, R is steadfastly maintained at 230 nm, while L undergoes deliberate variation, ranging from 50 to 150 nm. This orchestrated manipulation of L unfolds a compelling narrative, clearly delineated in Fig. 3b. As the total size of the cavity (L+R) increases, the path length that light travels within the resonator also increases. According to wave theory, longer path lengths (L increases from 50 to 150 nm) result in longer wave periods. Consequently, the wavelength of the resonating light extends, while its frequency remains unchanged, leading to a shift towards longer wavelengths in the electromagnetic spectrum. This intriguing observation underscores a pivotal insight into the device's behavior—beyond the influence of R, the resonance state can be finely tuned by judiciously adjusting the L of the connecting WG.

Sensory Efficacy Evaluation of the Device

The refractive index sensor stands out as a powerful tool in the realm of label-free sensing, representing a change in basic assumptions in analytical techniques. Unlike traditional methods that rely on fluorescent or labeled markers to detect specific molecules, label-free sensing using refractive index measurements eliminates the need for additional chemical tags, thereby simplifying and accelerating the sensing process [22]. By exploiting the changes in refractive index induced by interactions between analytes and sensor surfaces, label-free refractive index sensors offer a direct and real-time approach to detecting molecular binding events [23, 24]. This not only streamlines experimental procedures but also minimizes the risk of perturbing the biological or chemical systems under investigation. Label-free refractive index sensing finds extensive applications in biochemistry, pharmaceuticals, and environmental monitoring, where it provides a non-invasive and highly sensitive means of studying molecular interactions, facilitating a deeper understanding of complex biological processes, and enabling the development of innovative diagnostic tools [4].

In the recent past, several interesting works on MIM WG-based plasmonic sensors have been proposed for refractive index sensing applications. For instance, the use of an elliptical resonator coupled to a straight MIM WG via a silicon layer is proposed for sensing applications [15]. Numerical investigation using the FDTD method is employed to study this structure. The results reveal multiple resonance modes in the transmission spectrum. The structure exhibits a sensitivity, reaching up to 550 nm/RIU, with a figure of merit (FOM) value of 282.5 RIU−1 at a resonance wavelength of 592 nm. Another interesting design comprised of a plasmonic refractive index sensor design employing a MIM WG coupled with concentric double rings resonator (CDRR), analyzed through numerical investigation is proposed [25]. By leveraging the unique supermodes of the CDRR, the FWHM of the resonant wavelength can be controlled, achieving a sensitivity of 1060 nm/RIU with a high figure of merit (FOM) of 203.8 in the near-infrared region. Further information on plasmonic sensors can be found in [17, 24, 26,27,28].

The plasmonic sensor based on MIM WG, integrated with a polydimethylsiloxane (PDMS) polymer, emerges as a groundbreaking solution for temperature sensing applications. In this advanced sensor design, one of the cavities is filled with PDMS polymer which contributes to the sensor’s thermal stability. The plasmonic resonance within the MIM WG experiences a shift corresponding to changes in temperature, offering a highly sensitive and responsive mechanism for temperature sensing. The incorporation of PDMS ensures the sensor's adaptability to various environments and surfaces. This innovative combination of plasmonic sensing and PDMS polymer not only enables precise temperature monitoring but also highlights the potential for developing robust and versatile temperature sensors for monitoring the thermal variations in the photonic chip for biosensing applications.

The significance of the high sensitivity of plasmonic sensors lies in their capacity to provide real-time and label-free detection with unparalleled precision [29]. This attribute is particularly advantageous in fields such as medical diagnostics, where the identification of biomolecules at low concentrations is critical for early disease diagnosis [15]. Additionally, in environmental monitoring, plasmonic sensors can detect pollutants or changes in environmental conditions at levels that may be otherwise challenging to measure [3, 29, 30]. The sensitivity of the biosensing module and temperature sensing module is calculated as.

$${S}_{bio}=\frac{\Delta \lambda }{\Delta n};\;\mathrm{and}\;{S}_{T}=\frac{\Delta \lambda }{\Delta T};$$

where ∆λ is the change in resonance wavelength, whereas ∆n and ∆T are the change in ambient refractive index and ambient temperature, respectively.

Discussion

In this section, three fundamental scenarios displaying the versatile applications of this device are studied, particularly in the realms of refractive index sensing and temperature sensing. Each scenario unfolds a unique narrative, demonstrating the device's adaptability and efficacy in addressing distinct challenges within these crucial domains. The ensuing discussions provide valuable insights into the potential impact of this device across a spectrum of practical applications, emphasizing its role as a versatile tool in the fields of refractive index and temperature sensing. The geometric parameters of the device that are used to characterize the device performance are as follows: R = R = 230 nm, W = 50 nm, L = 50 nm connected to Cb and L = 150 nm connected to Ct. This deliberate variation in length ensures distinct separation between resonance dips, mitigating the risk of undesirable overlap.

Case I: Plasmonic Sensor for Refractive Index Sensing Application

In the first scenario, the device functions as a highly sensitive refractive index sensor, enabling the label-free detection of analytes within a refractive index range spanning from 1.30 to 1.35. The ambient temperature is consistently maintained at 20 °C, ensuring a stable environment that produces a continual resonance dip. This dip is a result of the Ct being filled with PDMS polymer. As the refractive index of the cavity dedicated to biosensing increases, a noteworthy phenomenon unfolds. When the refractive index of the ambient medium surrounding a SPP wave is changed, it causes a shift in the SPP wave. This phenomenon occurs because the propagation of SPP waves is influenced by the refractive index of the surrounding medium. Specifically, the refractive index affects the phase velocity of the SPP wave, which is the speed at which the wavefronts propagate. When the refractive index increases in the Cb, the phase velocity of the SPP wave decreases, leading to a longer wavelength and hence a shift towards longer wavelengths in the spectrum. This shift in the SPP wave is a fundamental aspect of plasmonic sensing, where changes in the local refractive index, for example, due to the presence of analytes, can be detected by monitoring the wavelength shift of the SPP resonance.

The intricate interplay between the device and the varying refractive indices is visually depicted in Fig. 4a. The resonance wavelength versus the refractive index plot is shown in Fig. 4b which shows a linear relationship between the resonance wavelength and the ambient index. The line exhibits a slope of 737.71 nm/RIU, indicating the device's sensitivity.

Fig. 4
figure 4

a Transmission spectrum in the presence of varying medium refractive index, b resonance wavelength versus medium refractive index

Full size image

Case II: Plasmonic Sensor for Temperature Sensing Application

In the second scenario, the sensing device functions exclusively as a temperature sensor, with Cb possessing a refractive index of 1.0. This deliberate choice strategically displaces the resonance dip devised from Cb beyond the range of the intended transmission spectrum. Consequently, only a single dip is observed, originating from the resonant condition being met by Ct without biosensing, as depicted in Fig. 5a. With the rise in ambient temperature, the refractive index of PDMS decreases, inducing a noticeable blueshift in the resonance wavelength of the sensor. This correlation between resonance wavelength and temperature is graphically illustrated in Fig. 5b, highlighting a distinct slope of − 0.336 nm/°C. This slope serves as a quantitative indicator of the device's temperature sensing sensitivity, emphasizing its capability to precisely detect temperature variations.

Fig. 5
figure 5

a Transmission spectrum in the presence of varying ambient temperature, b resonance wavelength versus ambient temperature

Full size image

Case III: Plasmonic Sensor for Simultaneous Monitoring of Temperature and Refractive Index Sensing

In the presented scenario, the sensing device operates dually as a refractive index sensor and a temperature sensor. Investigating the impact of concurrent changes in temperature and refractive index on the transmission spectrum of the device, the refractive index is systematically varied within the biosensing-dedicated cavity from 1.30 to 1.36. Simultaneously, the temperature experienced a consistent shift of 10 °C, spanning from 20 to 80 °C. The Cb exhibited a discernible redshift in its resonance wavelength as the refractive index within the cavity increased from 1.30 to 1.36, as illustrated in Fig. 6a. Conversely, the ambient medium’s temperature underwent simultaneous variation from 20 to 80 °C. This led to a linear decrease in the refractive index of the PDMS polymer, inducing a blueshift in the resonance wavelength of Ct filled with PDMS, as depicted in Fig. 6a.

Fig. 6
figure 6

a Transmission spectrum of the device employed for simultaneous monitoring of analytes and temperature, b resonance state of Cb filled with refractive index = 1.30, c resonance state of the Ct filled with PDMS at an ambient temperature = 20 °C

Full size image

It is noteworthy that the resonance dips from both cavities (Cb and Ct) remained well-separated and did not interfere with each other. This unique characteristic enables the device to achieve simultaneous detection of analytes within the biosensing cavity and the ambient temperature. The distribution of the H-field within the device at the resonance wavelength of 1089 nm, corresponding to a refractive index of 1.30 within Cb is visually represented in Fig. 6b. Furthermore, Fig. 6c illustrates the resonance of Ct filled with PDMS at 1270.4 nm, specifically when the ambient medium temperature is 20 °C.

The Q-factor is a pivotal metric for evaluating the performance of plasmonic ring resonators across applications such as sensing and signal processing. A higher Q-factor is indicative of reduced losses and superior light confinement within the resonator, thereby amplifying sensitivity and resolution in sensing tasks. Achieving an elevated Q-factor necessitates careful design optimization aimed at minimizing dissipative losses while maximizing energy retention within the resonant structure. Essentially, the Q-factor denotes the degree of sharpness exhibited by the resonant peak within the device's transmission spectrum and can be calculated as

$$Q-factor=\frac{{\lambda }_{res}}{FWHM};$$

where λres is the resonance wavelength and FWHM is the full width at half maximum. The Q-factor of the biosensing module and temperature sensing module are 20.5 and 16.5, respectively.

Ultimately, the normalized H-field distribution of the plasmonic sensing system is revealed where the light is inserted through the dielectric mode couplers. This comprehensive depiction illustrates the field mapping corresponding to the resonance wavelengths of 1089 nm and 1270.4 nm. These wavelengths are specifically examined in two distinct scenarios: when Cb is filled with a refractive index of n = 1.30 and the ambient temperature is 20 °C. The nuanced variations in the H-field distribution are precisely displayed in Fig. 7a, b for the respective conditions, offering a comprehensive insight into the system’s performance under these specific parameters. Table 2 showcases a comparative analysis, delineating the performance parameters of the proposed sensor design against prior works.

Fig. 7
figure 7

Normalized H-field mapping in the sensor system for T = 20 °C and Cb is filled with n = 1.30, a wavelength = 1089 nm, b wavelength = 1270.4 nm

Full size image
Table 2 Sensitivity comparison of the proposed sensor design with previous works
Full size table

In the end, the illustrated optical characterization procedure is portrayed in Fig. 8. Initially, the tunable laser source is directed to couple with the input WG via a tapered optical fiber. To maintain the desired polarization, a 3-paddle polarization controller is strategically inserted between the laser source and the output end of the fiber (not depicted in the figure). Subsequently, the resulting output light is gathered from output WG and directed to the optical spectrum analyzer (OSA). Seamless data acquisition and analysis are facilitated by linking a computer to the OSA, allowing for the automation of measurement protocols and remote instrument control. This integration enables users to program specific measurement sequences, schedule tasks, and remotely manage the instrument, thus streamlining operations and enhancing overall efficiency. Following data acquisition, post-processing procedures are implemented to extract and analyze the spectral characteristics of the device, enabling comprehensive evaluation and providing further insights into its performance.

Fig. 8
figure 8

Anticipated optical characterization setup

Full size image

Concluding Remarks

Presented herein is the conceptualization of a plasmonic sensor, leveraging the MIM WG framework, specifically designed for unparalleled performance in refractive index and temperature sensing applications. The sensor’s intricate design incorporates a MIM bus WG as its core, seamlessly integrating two distinct cavities through connecting WGs. This architectural innovation eliminates the conventional necessity for a nanoscale gap between the cavity and the bus WG, enhancing the robustness and reliability of the sensor. Moreover, the existing literature fails to delve deeply into the intricacies of the light coupling mechanism, leaving a notable gap in understanding. This research endeavors to fill this void by elucidating a novel mechanism for launching light into the nanoscale MIM waveguide. Hence, the proposal of orthogonal mode couplers arises as an effective strategy for converting the dielectric mode to the plasmonic mode and vice versa with remarkable efficiency. Unlike conventional sensors, this advanced architecture allows for the concurrent operation of label-free biosensing and temperature measurement without mutual interference in the resonance states of either parameter. The device exhibits a refractive index sensitivity of 737.71 nm/RIU and a temperature sensitivity of − 0.336 nm/°C. Additionally, its Q-factor is determined at 20.5 for the refractive index sensing module and 16.5 for the temperature sensing module. In essence, the study conducted in this paper signifies a one step closer to the realization of plasmonic sensors for refractive index sensing application with accuracy.