Cavitation bubble wall pressure measurement by an electromagnetic surface wave enhanced pump-probe configuration

We report on the measurement of the pressure associated with a shock wave within a very thin layer (100 nm) in proximity of a boundarysurface. In the experiments, the shock wave was emitted by a cavitation bubble generated by a pulsed pump laser in water. We developed apump-probe setup based on the detection of the light scattered at the surface of a one-dimensional photonic crystal, which was purposelydesigned to sustain a surface electromagnetic wave in the visible range and to enhance the optical response. In order to better understand thephenomenon, we implemented numerical simulations to describe the light scattering intensity distributions through a modified Rayleigh’smethod. We report, with a LoD of about 0.1 MPa, the measurements of the pressure at a surface in the presence of a laser-induced cavitationbubble generated at different distances from the surface and for different pulse energies

Spectral analysis of organic LED emitters’ orientation in thin layers by resonant emission on dielectric stacks

Purposely tailored thin film stacks sustaining surface waves have been utilized to create a unique link between emission angle and wavelength of fluorescent dye molecules. The knowledge of the thin film stack’s properties allows us to derive the intrinsically emitted luminescence spectrum as well as to gain information about the orientation of fluorophores from angularly resolved experiments. This corresponds to replacing all the equipment necessary for polarized spectroscopy with a single smart thin film stack, potentially enabling single shot analyses in the future. The experimental results agree well with those from other established techniques, when analyzing the Rubrene derivative in a 2,4,6-tris(biphenyl-3-yl)-1,3,5-triazine (T2T) host used for the fabrication of optimized organic light-emitting diodes. The findings illustrate how resonant layered stacks can be applied to integrated spectroscopic analyses

Low-temperature stability and sensing performance of mid-infrared bloch surface waves on a one-dimensional photonic crystal

The growing need for new and reliable surface sensing methods is arousing interest in the electromagnetic excitations of ultrathin films, i.e., to generate electromagnetic field distributions that resonantly interact with the most significant quasi-particles of condensed matter. In such a context, Bloch surface waves turned out to be a valid alternative to surface plasmon polaritons to implement high-sensitivity sensors in the visible spectral range. Only in the last few years, however, has their use been extended to infrared wavelengths, which represent a powerful tool for detecting and recognizing molecular species and crystalline structures. In this work, we demonstrate, by means of high-resolution reflectivity measurements, that a one-dimensional photonic crystal can sustain Bloch surface waves in the infrared spectral range from room temperature down to 10 K. To the best of our knowledge, this is the first demonstration of infrared Bloch surface waves at cryogenic temperatures. Furthermore, by exploiting the enhancement of the surface state and the high brilliance of infrared synchrotron radiation, we demonstrate that the proposed BSW-based sensor has a sensitivity on the order of 2.9 cm–1 for each nanometer-thick ice layer grown on its surface below 150 K. In conclusion, we believe that Bloch surface wave-based sensors are a valid new class of surface mode-based sensors for applications in materials science

Advanced Optical Techniques for micro-Fluid Dynamics

The impact of microfluidic technologies in the field of life sciences has dramatically increased during the last years, due to the need to develop new products for real applications. The work reported in the present dissertation is focused on the development of optical systems for the study of fluid dynamic phenomena at the micro and nano-scale. The research activities were carried out in the frame of two main topics and, consequently, the dissertation is divided into two main parts. The first part of the dissertation is focused on the experimental study of cavitation in liquids, in which a bubble is generated by focusing a laser beam under extremely controlled conditions. The aim is addressing all different phases of the process. Cavitation is studied using a combined system of a fast camera, for the complete reconstruction of the plasma shape and bubbles dynamics, and a fiber optical hydrophone (FOHP), for the detection of the pressure shock waves in proximity of the bubble. The breakdown phenomenon and the bubble dynamics have been characterized when the optical arrangement of the focusing system is modified, e.g. using different expansion factors of the beam expander to change the focusing angle of the laser beam, or when the laser pulse energy is tuned. Data analysis shows a strong correlation between the number of plasma sites and the number of shock waves detected by the fiber hydrophone. The second part of the dissertation is focused on the study of a particular class of surface electromagnetic waves and on their use for sensing applications. Such waves are sustained at the surface of finite one-dimensional photonic crystals (1DPC) and are generally named Bloch surface waves (BSW). The robustness of optical sensors based on BSW has been investigated experimentally and numerically. The distributions of sensor characteristics caused by the fabrication uncertainties in dielectric layer thicknesses have been analysed. It is demonstrated that the performance of the surface wave sensors is sufficiently robust with respect to the changes of the photonic crystal layer thicknesses. Layer thickness optimization of the photonic crystal, carried out to achieve low limit of detection, leads to an improvement of the robustness of the surface wave sensors that is attributed to Bloch states lying deeper in the photonic band gap. The work reported in the dissertation demonstrates the use of an optical sensing platform based on BSW as a novel optical tool to probe in real time the fluid flow at a boundary wall of a microfluidic channel under dynamic conditions. Exploiting the properties of the BSW, we can put into evidence the temporal evolution of the interface during the injection of liquids with different refractive index (RI). We introduce ξ defined as the distance between the interface of the liquids of different RI and the 1DPC surface. Reconstructed experimental maps of ξ(z,t), as a function of time and of the position across the μ-fluidic channel, allow us to recovery the temporal evolution of the fluids interface in proximity of the wall. From the data analysis, the diffusion coefficient of a solute in water is measured and found in good agreement with the literature value. Moreover, biosensors based on BSWs have been studied and their practical application was demonstrated by detecting a specific glycoprotein, Angiopoietin 2, that is involved in angiogenesis and inflammation processes, and to detect clinically relevant concentrations of the breast cancer biomarker ERBB2 in cell lysates. The protocol used for the label-free detection of Angiopoietin 2 is described and the results of an exemplary assay are given, confirming that an efficient detection can be achieved. The limit of detection of the biochips for Angiopoietin 2, based on the protocol used, is 1.5 pg/mm^2 in buffer solution. To detect soluble ERBB2, we develoed an optical set-up which operates in both label-free and fluorescence modes. The resolution obtained in both modes meet international guidelines and recommendations (15 ng/mL ) for ERBB2 quantification assays, providing an alternative tool to phenotype and diagnose molecular cancer subtypes. During the last part of the doctorate period, the two research directions pursued during the first and the second part, devoted to bubble cavitation and to BSW based sensors, merged in a new innovative setup. Starting from the BSW concept, a new type of optical hydrophone based on BSW for the detection of pressure shock waves with a larger sensitivity than FOHP has been proposed and setup. Some preliminary and encouraging results are presented and discussed

A novel technique based on Bloch surface waves sustained by one-dimensional photonic crystals to probe mass transport in a microfluidic channel

We report on the use of an optical sensing platform based on Bloch surface waves sustained by one-dimensional photonic crystals as a novel optical tool to probe in real time the fluid flow at a boundarywall of a microfluidic channel under dynamic conditions. Understanding how fluid flow interacts withwall surfaces is crucial for a broad range of biological processes and engineering applications, such as sur-face wave biosensing. The proposed platform provides nanometric resolution with respect to the distancefrom the boundary wall sensor’s surface. Here, for the first time, we report on the experimental inves-tigation on the temporal evolution of the interface between two fluids with different refractive indicesunder convective and diffusive conditions. The temporal evolution of the fluids interface in proximity ofthe wall is recovered. From the data analysis, the diffusion coefficients of glucose and glycerol in waterare measured and found in good agreement with the literature. Tuning the one-dimensional photoniccrystals geometry and the Bloch surface wave’s dispersion has the potential to probe the fluid flow in anextremely wide range of distances from the microfluidic channel wall

Label-Free Monitoring of Human IgG/Anti-IgG Recognition Using Bloch Surface Waves on 1D Photonic Crystals

Optical biosensors based on one-dimensional photonic crystals sustaining Bloch surface waves are proposed to study antibody interactions and perform affinity studies. The presented approach utilizes two types of different antibodies anchored at the sensitive area of a photonic crystal-based biosensor. Such a strategy allows for creating two or more on-chip regions with different biochemical features as well as studying the binding kinetics of biomolecules in real time. In particular, the proposed detection system shows an estimated limit of detection for the target antibody (anti-human IgG) smaller than 0.19 nM (28 ng/mL), corresponding to a minimum surface mass coverage of 10.3 ng/cm2. Moreover, from the binding curves we successfully derived the equilibrium association and dissociation constants (KA = 7.5 × 107 M−1; KD = 13.26 nM) of the human IgG–anti-human IgG interaction

Enhanced Fluorescence Detection of Interleukin 10 by Means of 1D Photonic Crystals

In the present communication, we report on the exploitation of a Bloch surface wave-enhanced fluorescence scheme for the detection of Interleukin (IL)-10 in a protein-rich buffer mimicking a biological sample. IL-10 is a cytokine known for its potent anti-inflammatory and immunosuppressive effects. It is considered a valuable biomarker for prognostic prediction for both solid tumors and hematological malignancies, and recently, a distinguishing feature of hyperinflammation during severe viral infections. To demonstrate the validity of the technique, we transferred all the reagents and working concentrations used in a gold-standard technique, such as ELISA, to our assay, with a substantial reduction in the execution time and without using any enzymatic amplification during IL-10 recognition. We estimate a limit of detection (LoD) in terms of the concentration of IL-10 in solution of the order of 110 pg/mL (5.8 pM) with a 14% accuracy; in other terms, the presented technique is compatible with the assay range and resolution (1.6 pM) of commercial gold-standard ELISA kits. Moreover, such LoD successfully matches the concentrations reported in literature for IL-10 detection in COVID-19 patients, making the BSW-based sensors a viable solution for rapid and accurate screening of COVID-19-related molecules

Effects of reabsorption due to surface concentration in highly resonant photonic crystal fluorescence biosensors

Photonic crystal enhanced fluorescence biosensors have been proposed as a novel immunodiagnostic tool, due to the increased fluorescence excitation rates and angular redistribution of the emission. Among these, purely dielectric one-dimensional photonic crystals (1DPC) sustaining Bloch surface waves (BSW) at their truncation edge, have recently attracted much interest. We report for the first time on the time resolved experimental study of the effects of excess reabsorption of the BSW coupled fluorescence in the near infrared range around 800 nm. Temporally and angularly resolved measurements of the BSW coupled fluorescence emission permit to put into evidence a strong reabsorption of the fluorescence emission when using highly resonant 1DPC. The results suggest that, when designing 1DPC sustaining BSW for quantitative diagnostic assays, it is necessary to choose a compromise quality factor, to exploit the features arising from the electromagnetic field enhancement while avoiding reabsorption

Effect of thickness disorder on the performance of photonic crystal surface wave sensors

We investigated experimentally and numerically the robustness of optical sensors based on Bloch waves at the surface of periodic one- dimensional photonic crystals. The distributions of sensor characteristics caused by the fabrication uncertainties in dielectric layer thicknesses have been analyzed and robustness criteria have been set forth and discussed. We show that the performance of the surface wave sensors is sufficiently robust with respect to the changes of the photonic crystal layer thicknesses. Layer thickness optimization of the photonic crystal, carried out to achieve low limit of detection, leads to an improvement of the robustness of the surface wave sensors that is attributed to Bloch states lying deeper in the photonic band gap