Refractive index sensing using the metal layer in DVD-R discs

Surface topographic characterization of optical storage devices by Digital Holographic Microscopy

In this study, we have used Digital Holographic Microscopy (DHM) for surface topographic characterization of the optical storage devices. Optical storage devices like Compact Discs (CDs) and Digital Versatile discs (DVDs) are used worldwide for memory storage, grating, sensing and spectroscopy applications. These devices can store the information in binary form on a spiral track in the data recording layer. We demonstrate that these data tracks can be characterized through DHM and one can decode the binary data in transmission mode. The experimental results are shown for blank (without data) and burned (with data) areas of CDs and DVDs. The average width of the CD and DVD data track is experimentally found to be 600 ± 30 nm & 230 ± 20 nm, respectively and the thickness of the data recording track is obtained as 80 ± 10 nm. The obtained results are verified by Field Emission Scanning Electron Microscopy (FESEM) measurements and found a very close agreement between the two results. In addition, the proposed method can also be used for manufacturing defect identification and measurement.

Dual plasmonic modes in the visible light region in rectangular wave-shaped surface relief plasmonic gratings

Rectangular wave-shaped surface-relief plasmonic gratings (RSR-PGs) have been fabricated from a hybrid polymer by employing a simple nanoimprint photocuring lithography technique using a silicon template, followed by gold nanolayer metallization on top of the formed replica structure. By forming a one-dimensional (1D) plasmonic grating with a periodicity of approximately 700 nm, a reflectance spectral dip was experimentally observed in the visible light region, from 600 to 700 nm, with increasing incident angle from 45° to 60°. This dip can be associated with surface plasmon resonance (SPR) wave excitation, which is coupled with the diffraction order m =  - 2. The calculations of reflectance spectra simulation using the rigorous coupled wave analysis (RCWA) method have also been carried out, resulting in the appearance of an SPR dip in the range of 600-700 nm, for incident angles in the range of 45°-65°, which agrees with the experimental results. Interestingly, these RSR-PGs show richer plasmon characteristics than the sine-wave-shaped plasmonic gratings. The experimental and spectral simulation results revealed two different plasmonic excitation modes: long-range SPR and quasi-localized SPR (LSPR). While the long-range SPR was formed above the ridge sections along the grating structure surface, the quasi-localized SPR was locally formed inside the groove. In addition, for RSR-PGs with a narrow groove section, the long-range SPR seems to be coupled with the periodic structure of the grating, resulting in the appearance of plasmonic lattice surface resonance (LSR) that is indicated by a narrower plasmon resonance dip. These characteristics are quite different from those found in the sine wave-shaped plasmonic gratings. The present results may thus provide better insights for understanding the plasmon excitations in this type of rectangular plasmonic grating and might be useful for designing their structure for certain practical applications.

Large-Scale Functionalized Metasurface-Based SARS-CoV-2 Detection and Quantification

Plasmonic metasurfaces consist of metal-dielectric interfaces that are excitable at background and leakage resonant modes. The sharp and plasmonic excitation profile of metal-free electrons on metasurfaces at the nanoscale can be used for practical applications in diverse fields, including optoelectronics, energy harvesting, and biosensing. Currently, Fano resonant metasurface fabrication processes for biosensor applications are costly, need clean room access, and involve limited small-scale surface areas that are not easy for accurate sample placement. Here, we leverage the large-scale active area with uniform surface patterns present on optical disc-based metasurfaces as a cost-effective method to excite asymmetric plasmonic modes, enabling tunable optical Fano resonance interfacing with a microfluidic channel for multiple target detection in the visible wavelength range. We engineered plasmonic metasurfaces for biosensing through efficient layer-by-layer surface functionalization toward real-time measurement of target binding at the molecular scale. Further, we demonstrated the quantitative detection of antibodies, proteins, and the whole viral particles of SARS-CoV-2 with a high sensitivity and specificity, even distinguishing it from similar RNA viruses such as influenza and MERS. This cost-effective plasmonic metasurface platform offers a small-scale light-manipulation system, presenting considerable potential for fast, real-time detection of SARS-CoV-2 and pathogens in resource-limited settings.

Relative humidity sensor based on a silver nano-grating made from DVD discs

A relative humidity sensor based on a silver nano-grating was proposed. By stripping and cleaning commercially available CD and DVD discs, polycarbonate plates with different grating periods are obtained. These plates as templates are coated with a layer of sputtered silver film to form silver nano-gratings, which exhibit refractive index sensing sensitivities of 517 nm/RIU and 742.9 nm/RIU, respectively. The finite-difference time-domain simulation results conform the excited surface plasmon polariton modes and localized surface plasmon modes on the nano-grating. By spin coating a layer of humidity-sensitive porous silica with optimized thickness, the silver nano-grating shows a relative humidity detection sensitivity of 0.23 nm/%RH.

Resonant Subwavelength and Nano-Scale Grating Structures for Biosensing Application: A Comparative Study

Resonant-based sensors are attractive optical structures due to the easy detection of shifts in the resonance location in response to variations in the analyte refractive index (RI) in comparison to non-resonant-based sensors. In particular, due to the rapid progress of nanostructures fabrication methods, the manufacturing of subwavelength and nano-scale gratings in a large area and at a low cost has become possible. A comparative study is presented involving analysis and experimental work on several subwavelength and nanograting structures, highlighting their nano-scale features' high potential in biosensing applications, namely: (i) Thin dielectric grating on top of thin metal film (TDGTMF), which can support the excitation of extended surface plasmons (ESPs), guided mode resonance, or leaky mode; (ii) reflecting grating for conventional ESP resonance (ESPR) and cavity modes (CMs) excitation; (iii) thick dielectric resonant subwavelength grating exhibiting guided mode resonance (GMR) without a waveguide layer. Among the unique features, we highlight the following: (a) Self-referenced operation obtained using the TDGTMF geometry; (b) multimodal operation, including ESPR, CMs, and surface-enhanced spectroscopy using reflecting nanograting; (c) phase detection as a more sensitive approach in all cases, except the case of reflecting grating where phase detection is less sensitive than intensity or wavelength detection. Additionally, intensity and phase detection modes were experimentally demonstrated using off-the-shelf grating-based optical compact discs as a low-cost sensors available for use in a large area. Several flexible designs are proposed for sensing in the visible and infrared spectral ranges based on the mentioned geometries. In addition, enhanced penetration depth is also proposed for sensing large entities such as cells and bacteria using the TDGTMF geometry.

IP-Dip-Based SPR Structure for Refractive Index Sensing of Liquid Analytes

In this paper, we present a two-dimensional surface plasmon resonance structure for refractive index sensing of liquid analytes. The polymer structure was designed with a period of 500 nm and prepared in a novel IP-Dip polymer by direct laser writing lithography based on a mechanism of two-photon absorption. The sample with a set of prepared IP-Dip structures was coated by 40 nm thin gold layer. The sample was encapsulated into a prototyped chip with inlet and outlet. The sensing properties were investigated by angular measurement using the prepared solutions of isopropyl alcohol in deionized water of different concentrations. Sensitivity of 478-617 nm per refractive index unit was achieved in angular arrangement at external angle of incidence of 20°.

Tailoring optical discs for surface plasmon polaritons generation

The article reports on an optimization of gold submicron structures based on modified recordable blank digital versatile discs for surface plasmon polaritions excitation, mainly in near-infrared region. We have examined internal layers of commercially available DVD+R, DVD-R, DVD+RW and DVD-RW optical discs and we have elaborated a simple, inexpensive approach providing sharp resonances with efficiency reaching 95% for collimated excitation laser beams. We have experimentally and numerically confirmed the SPPs intensity being up to 220 times the intensity of the excitation laser beam. We have also directly measured thermal energy loss accompanying SPPs excitation.

The Sensitivity of Grating-Based SPR Sensors with Wavelength Interrogation

In this paper, we derive the analytical expression for the sensitivity of grating-based surface plasmon resonance (SPR) sensors working in wavelength interrogation. The theoretical analysis shows that the sensitivity increases with increasing wavelength and is saturated beyond a certain wavelength for Au and Ag gratings, while it is almost constant for Al gratings in the wavelength range of 500 to 1000 nm. More importantly, the grating period (P) and the diffraction order (m) dominate the value of sensitivity. Higher sensitivity is possible for SPR sensors with a larger grating period and lower diffraction order. At long wavelengths, a simple expression of P/|m| can be used to estimate the sensor sensitivity. Moreover, we perform experimental measurements of the sensitivity of an SPR sensor based on an Al grating to confirm the theoretical calculations.