Designing Mid-Infrared Gold-Based Plasmonic Slot Waveguides for CO2-Sensing Applications

Abstract

Plasmonic slot waveguides have attracted much attention due to the possibility of high light confinement, although they suffer from relatively high propagation loss originating from the presence of a metal. Although the tightly confined light in a small gap leads to a high confinement factor, which is crucial for sensing applications, the use of plasmonic guiding at the same time results in a low propagation length. Therefore, the consideration of a trade-off between the confinement factor and the propagation length is essential to optimize the waveguide geometries. Using silicon nitride as a platform as one of the most common material systems, we have investigated free-standing and asymmetric gold-based plasmonic slot waveguides designed for sensing applications. A new figure of merit (FOM) is introduced to optimize the waveguide geometries for a wavelength of 4.26 µm corresponding to the absorption peak of CO2, aiming at the enhancement of the confinement factor and propagation length simultaneously. For the free-standing structure, the achieved FOM is 274.6 corresponding to approximately 42% and 868 µm for confinement factor and propagation length, respectively. The FOM for the asymmetric structure shows a value of 70.1 which corresponds to 36% and 264 µm for confinement factor and propagation length, respectively.

Design, Analysis, and Optimization of a Plasmonic Slot Waveguide for Mid-Infrared Gas Sensing

“…The obtained FOM for the proposed PSWG in this work is much higher than the one reported in [ 34 ], i.e., (3.2). At the same time, we note that the achieved FOM in this work is lower than the FOMs of (276.4 and 70.1) reported in [ 32 ], which is related to different sensor structures, though.…”

“…Therefore, the more field penetrates in the substrate, the less field is confined in the gap region resulting also in less electromagnetic energy in the gold. Interestingly, the L SPP even increases when the mode is leaky which is due to the fact that the dominant loss mechanism in the structure is the material loss in gold [ 32 ]. The comparison between the L SPP of the asymmetric PSWG (see Figure 6 b) and the symmetric one (see Figure 5 b) for the same w shows that the L SPP of the asymmetric one increases faster than that of the symmetric one.…”

“…The intrinsic attenuation can be described in terms of the so-called propagation length L SPP denoting the length, after which the intensity of the guided mode decreases by a factor of 1/ . The confinement factor ( ) and L SPP have been discussed in detail in our earlier paper [ 32 ].…”

Toward Complete Optical Coupling to Confined Surface Polaritons

“…The ability to confine and manipulate light at small length scales allows us to enhance the optical electromagnetic field by several orders of magnitude at specific spatial locations, and this, in turn, enables the design of improved optoelectronic devices, , chemical sensors, − and nonlinear photonic structures. − Surface polaritons play an important role in this context because they provide the means to realize a strong degree of spatial confinement, thanks to their small associated wavelengths compared with the light wavelength at the same frequency. In particular, visible and near-infrared plasmons can be trapped in regions spanning just a few nanometers in size, resulting in a large field enhancement that has been extensively leveraged for optical sensing and chemical identification , down to the single-molecule level − as well as for enhancing both the efficiency of light harvesters , and the nonlinear response of nanostructures. , As impressive as these applications might be, the full potential of light confinement is dramatically hindered by the mismatch between the light and polariton wavelengths, which results in an intrinsically weak in and out coupling of propagating light to polaritons and prevents a wider deployment of polariton-based devices.…”

Design of a High Q-Factor Label-Free Optical Biosensor Based on a Photonic Crystal Coupled Cavity Waveguide

“…Among the different fields of biosensing, optical sensors are powerful candidates to tackle most of the mentioned challenges. Optical sensors convert light into an electrical signal for a real-time analysis of a (preferably) small volume of analyte [ 6 , 7 , 8 ]. There is a wide variety of different optical sensor technologies, such as waveguiding devices, surface functionalization, and optofluidic integration.…”