The doctoral dissertation in the field of Photonics will be examined at the Faculty of Science, Forestry and Technology, Joensuu campus and online.
What is the topic of your doctoral research? Why is it important to study the topic?
My doctoral research, titled “Dielectric Electromagnetic Surface Waves in Space and Time: From Theory and Simulations to Experiments,” focuses on Bloch surface waves (BSWs). These waves are generated at the interface between a periodic multilayer dielectric material and a semi-infinite superstrate, demonstrating exceptional field confinement and enhancement at this interface. This property is important for applications such as nonlinear optics and sensing. Moreover, their nanometer-scale confinement makes them particularly well-suited for compact micro- and nanostructure platforms, enabling innovative solutions in environments where space and precision are critical.
What are the key findings or observations of your doctoral research?
Our research yielded three key findings. For pulsed excitation of Bloch surface waves (BSWs), we discovered that specific illumination configurations are required to achieve maximum coupling of the incident pulse to the BSWs. This condition is met when the spectral peak (or dip) of the BSW resonance coincides with the spectral peak of the illuminating Gaussian pulse. Under this condition, the resonant and non-reflected components exhibit distinct temporal separation, which can be advantageous for applications such as time-gated analysis or optical filtering. In addition, we developed closed-form expression for the temporal version of the Goos–Hänchen shift.
Using centrosymmetric structures, we identified a novel structural configuration that generates diffraction orders with transverse wave vectors matching the BSW wave vector. This resulted in the generation of a superposition of multiple BSWs, forming what we termed resonance-enhanced evanescent optical lattices. We further developed closed-form expressions to provide a comprehensive understanding of these lattices in the plane wave regime.
We investigated the generation of multiple BSWs using fields of finite extent, specifically Gaussian beams. Various beam configurations were explored, and the finite-field effects associated with BSW excitation were studied. This work provides valuable insights into practical implementations where finite beams are employed. These findings contribute to a deeper understanding of BSW behavior and open new avenues for their application in optics and photonics.
How can the results of your doctoral research be utilised in practice?
While a considerable body of literature exists on the experimental study of the ultrafast physics of Bloch surface waves (BSWs), the theoretical framework proposed in this work offers optimised experimental configurations for achieving superior results. For instance, we demonstrated that a clear temporal transition point between the resonant and non-resonant reflected intensities can be achieved when the peak profile of the incident field coincides with the spectral peak (or dip) of the BSWs. This finding has potential applications in temporal filtering and time-gated optical systems.
The resonance-enhanced evanescent optical lattices, formed through the superposition of multiple BSWs, could play a transformative role in optical trapping. These lattices provide a promising avenue for manipulating micro- and nanoparticles with high precision, offering new possibilities for advancements in optical manipulation technologies.
The generation of multiple BSWs using Gaussian beams enhances control over wave propagation and holds the potential to improve sensitivity in detecting nanoscale surface interactions in multiplexed configurations. Furthermore, this approach offers potential in applications such as biosensing and photonic circuits, where precise manipulation of surface waves is critical for performance and functionality.
What are the key research methods and materials used in your doctoral research?
The results of this dissertation stem from the combination of theoretical, numerical, and experimental tools, enabling a comprehensive study of Bloch surface waves (BSWs). Theoretical models were developed to understand BSW behavior, and these models informed the most appropriate numerical methods. The transfer matrix method (TMM), combined with the scattering matrix method, was employed to analyze homogeneous one-dimensional dielectric periodic layers supporting BSWs. Custom Kretschmann-Raether setups validated the ultrafast physics predictions of the theoretical and numerical results. Additional techniques were developed to address specific challenges, the angular spectrum decomposition was developed to model the finite fields (Gaussian beams). The Fourier modal method (FMM) method was developed to analyze grating-multilayer systems. All numerical methods were implemented in MATLAB.
The doctoral dissertation of Lewis Asilevi, MSc, entitled Dielectric electromagnetic surface waves in space and time: from theory and simulations to experiments will be examined at the Faculty of Science, Forestry and Technology, Joensuu Campus and online. The opponent will be Dr. Andriy Shevchenko, Aalto University, and the custos will be Professor Matthieu Roussey, University of Eastern Finland. Language of the public defence is English.