Metasurfaces for photonic sensors applications

Author(s)

Guilherme Simoneti de Arruda

• Document type: Doctoral Thesis
• Press: São Carlos.
• Institution: Universidade de São Paulo (USP). Escola de Engenharia de São Carlos (EESC/SBD)
• Defense date: 2025-07-04

Examining board members

Emiliano Rezende Martins; Luis Humberto da Cunha Andrade; Achiles Fontana da Mota; Jorge Ricardo Mejia Salazar; Gilliard Nardel Malheiros Silveira

• Advisor: Emiliano Rezende Martins

Abstract

Metasurfaces offer a novel approach for controlling and manipulating wave (e.g., light) beams and have found applications in various scientific areas, including sensing, imaging, augmented reality, and microscopy. They consist of an array of small resonators, known as meta-atoms, which control the amplitude and phase of the light beam. Photonic sensors are potential low-cost devices that allow for real time and in situ diagnosis (Point-of-Care devices), making them particularly appealing as diagnostics tools, especially in regions with inadequate healthcare and clinical laboratories. Metasurfaces have enormous potential to improve the performance of photonic sensors due to their ability to support electromagnetic resonances, which heavily depend on the surrounding media. Their promising sensing performance, compatibility with mass-production fabrication techniques and miniaturization capabilities make them ideal candidates for inexpensive Point-of-Care devices. However, the suitability of a sensing modality for streamline healthcare applications also depends on factors such as reproducibility, user-friendliness and cost. In particular, user-friendly, cost-effective sensors typically require using affordable materials and simple measuring techniques, which usually hinders the performance of the state-of-the art photonic sensors. This thesis proposes to quantify the impact of the most typical real-world application constraints associated with using low-cost compatible sensing devices, such as losses due to material absorption or surface scattering and misalignments between source and structure, on the sensing parameters of the metasurfaces, followed by design strategies to mitigate their impact. First, a model based on Temporal Coupled Mode Theory (TCMT) was derived to quantify the impact of the losses mechanisms on the Limit of Detection (LOD) (i.e., the minimum detectable measurand quantity) of photonic sensors. This model shows that the LOD is inversely proportional to the amplitude of the metasurfaces resonance, and that this amplitude is more affected by losses than the resonances Quality Factor (Q-factor), which is commonly used to estimate the losses impact on the sensors LOD. The model also highlights the conditions for optimising the LOD of the device, which happens by counterbalancing the Q-factor and amplitude. Next, TCMT was used to quantify the angular (misalignment) tolerance of distributed resonances in metasurfaces that support both Bound States in the Continuum (BICs), which are of great scientific interest due to their high Q-factor (high-Q) resonances, and Guided Mode Resonances (GMRs), which offers promising sensibility and are compatible with low-cost measurement systems. It was found that BICs are quite intolerant to misalignments between the metasurface and the source. Alternatively, a design strategy based on Fourier engineered metasurfaces supporting GMRs was proposed. The suggested structure features higher angular tolerance due to band planarisation, while still sharing the high-Q advantage of the BICs, thus offering a viable route towards high-Q resonances that are more suitable for applications. Afterwards, the concept of air-guided mode resonances (AGMRs) with Fourier control of the Q-factor was introduced. On the one hand, air modes can significantly improve the performance and functionality of metasurfaces, for example, by enhancing mode sensitivity and reducing material absorption. On the other hand, metasurfaces consisting of an array of double ridges can be Fourier engineered to support high-Q GMRs which are confined in air, thus supporting an AGMR. It was then experimentally demonstrated how to employ this design strategy to enhance the Q-factor of metasurfaces made of lossy materials. In particular, we demonstrated a Q-factor enhancement of 3.3x for resonances in the microwave regime, with potential for even better improvements depending on the application. A noteworthy acquired insight is that using the Fourier engineered metasurface is a mandatory requirement for exciting AGMRs, since regular gratings do not meet the criteria to support such modes. Subsequently, the idea of Fourier engineered metasurfaces supporting AGMRs was extended to structures consisting of a periodic array of dimer pillars, which supports resonances with electric field spatially localised outside the dielectric pillars (similar to the double ridge AGMR), and its suitability for improving the performance of photonics sensors was experimentally assessed. It was found that the resonances supported by the dimer structure have a sensing Figure of Merit (FOM) of one order of magnitude higher than that of resonances from structures supporting conventional GMRs, surface plasmons or even BICs. Also, an array of amorphous silicon (aSi) dimer pillars was fabricated and incorporated to a low-cost, user friendly photonic biosensing system that successfully measured the presence of tiny biomarkers of the Alzheimers disease (AD) at clinically relevant concentration levels (20 pg/ml) directly in human blood serum. Fourier engineering proved to be a valuable tool when designing metasurfaces, improving the performance of photonic sensors when appropriately used. The innovations presented in this theses make a major contribution in enhancing the capabilities of these devices. (AU)

• FAPESP's process: 20/15940-2 - Metasurfaces for photonic sensing applications
• Grantee: Guilherme Simoneti de Arruda
• Support Opportunities: Scholarships in Brazil - Doctorate