Manuel Crespo-Ballesteros

Room: NW613 
Email: m.crespo@aston.ac.uk

In 2007, I received my degree in Electronics and Automation Engineering from the Technical University of Cartagena, Spain. In 2012, I completed my ‘Licenciatura’ (MSc) in Theoretical Physics at the University of Murcia and I obtained my PhD from the University of Birmingham in 2016. In that same year, I started working in the aerospace industry as an application engineer until 2018, when I joined the Aston Institute of Photonic Technologies (AiPT) as a research fellow. Since then, I have been working on Surface Nanoscale Axial Photonics (SNAP) technology. My research has been focused on microresonators, nonlinear optics and optical frequency combs.

When light is confined in tiny, microscopic structures, a myriad of new phenomena arise from  their small spatial dimensions. These structures are called microresonators and they are the main object of my research. In particular, I work with in our lab call SNAP microresonators. SNAP is the acronym for Surface Nanoscale Axial Photonics, a technology capable of fabricating microresonators with picometer precision. There are many techniques to create SNAP microresonators, but all of them consist in modifying the effective radius variation of a very smooth optical fibre made of extremely low loss silica. The maximum radius variation can be of just few nanometers! As tiny as it is, this variation confines light in the form of whispering gallery mode (WGM) traveling waves. In SNAP microresonators, we study the slow propagation of these WGMs along the fibre axis. The particular structure of the microresonators confines the light in the form of axial modes, in the same way described by quantum mechanical one-dimensional systems. In fact, the key equation that serves as the base of my research is the one-dimensional Schrödinger equation in all its variations.

These variations and the phenomena they represent constitute the lines of research I have followed in recent years at the Aston Institute of Photonic Technologies (AiPT). Here is a brief summary of them:

• When a quantum-mechanical system presents a structure of barriers and wells, the transmission coefficient through this structure will sharply peak at certain energies. The same occurs in SNAP systems where the microresonator and the tapers that couple light in and out of it act as a complex of barriers and potential wells. This phenomenon is called resonant tunneling and it has attracted the interest of both fundamental and applied science due to the fascinating properties of its underlying physics and its potential applications to high-speed electronics and communications. SNAP microresonators have well defined and spatially separated resonant modes which allows us to accurately select the resonant tunneling conditions.
• Another line of my research has been the study of nonlinear phenomena in SNAP microresonators. In particular, I have explored the idea of creating a moving SNAP microresonator to transport light by light. When a strong light pulse is coupled into an optical fibre, there exist solitonic solutions that travel along the optical fibre axis. According to the nonlinear Schrödinger equation, another light pulse of much weaker intensity will see the soliton as a moving potential well. This weak pulse can then be transported at designated points in the optical fibre. Furthermore, the great precision of our fabrication techniques allow us to modify the effective radius variation of the optical fibre to control the speed and direction of the moving soliton. This idea opens a myriad of possible applications such as optical computer elements as well as fundamental physics research such as the study of black holes analogues.
• What happens if, instead of a fix potential well in the Schrödinger equation, we have a time-dependent one? This means to temporarilly modify the parameters of our SNAP microresonators and, if we do it periodically, the SNAP resonator will parametrically oscillate. Any WGM in the resonator will then be modulated and will produce an optical frequency comb. So far, these kind of systems have been treated as lumped systems. In my research, I have consider the full Schrödinger equation with a time-dependent potential to study the formation of these optical frequency combs. What I have found out is that the role of the spatial distribution of these oscillations is critical. In fact, for a parabolic SNAP microresonator, there exist a spatial distribution that optimise the generation of combs in terms of number of lines and flatness of the comb spectrum. The advantage of using SNAP microresonators is that the spacing of the comb teeth can be very small (few tens of MHz), which applications in sensing and precision spectroscopy.

• BEng in Electronics and Automation Engineering, Technical University of Cartagena, Spain, 2007
• 'Licenciatura' (Integrated MSc) in Theoretical Physics, University of Murcia, Spain, 2012.
• PhD in Radar Systems, University of Birmingham, UK, 2016.