Novel Non-linear Optical-Fibre Sources for Time-resolved Molecular Dynamics: Towards the Next Generation of Ultrafast Spectroscopy

Lead Research Organisation: Heriot-Watt University
Department Name: Sch of Engineering and Physical Science

Abstract

Developing detailed understanding of molecular interactions with light is of great importance. This is highly relevant, for example, to the fundamental biological processes of vision and photosynthesis, and also in photoresistive pathways (as seen in systems such as DNA and the melanin pigments) that protect living organisms from damage by ultraviolet (UV) light. Understanding light-molecule interactions is also of critical relevance for many other species, including photostabilizers, photochromic polymers, light harvesting complexes, sunscreens, photodynamic therapy drugs and molecules relevant to atmospheric/interstellar photochemistry. Advancing experimental techniques to improve the study of such systems is therefore imperative. In particular, learning more about the fundamental mechanisms that redistribute excess absorbed energy in molecules - and ultimately how to better utilize them - is of profound interest.

The use of "ultrafast" femtosecond laser pulses with temporal durations comparable to the timescales of molecular motion is a powerful method for studying light-matter interactions. Excess energy redistribution is followed in real time using "pump-probe" techniques: pump absorption effectively starts a dynamical "clock" on the overall process and the system is then interrogated at a series of precisely controlled delay times by the probe, mapping out the relaxation pathways. Time-resolved photoelectron imaging (TRPEI) is an extremely powerful variant of this general approach, yielding highly differential energy- and angle-resolved information offering deep insight into the underlying photophysics. A key requirement for TRPEI is the use of tuneable UV femtosecond pulses for both pump (excitation) and probe (ionization). Operating in this spectral region is, however, extremely inefficient and this places restrictions on the feasibility and scope of many studies. A rapidly emerging new technology for providing greatly improved (100-1000x) gains in UV generation efficiency makes use of hollow-core photonic crystal fibres (HC-PCFs). These also offer access to short-wavelength spectral regions (<200 nm) that are not easily realized via more conventional means. The key aim of this project is to harness the advantages afforded by HC-PCFs and undertake detailed, systematic studies of excess energy redistribution in model chromophore motifs (the light-absorbing centres in larger biomolecules). The selected motifs have all been implicated in providing UV photo-protective function and the highly-differential nature of TRPEI, supported by state-of-the-art quantum chemistry calculations, will yield much new insight into the fundamental mechanisms mediating such processes. Our study will also reveal principles relating more generally to the interplay between molecular structure, dynamics, and photochemical function that are broadly applicable to a far wider range of species - including those that may be exploited commercially.

The project brings together four researchers with complementary skills in ultrafast lasers, non-linear optics, molecular dynamics and cutting-edge computation. HC-PCF sources will be integrated into a TRPEI set-up, creating a unique state-of-the-art instrument. Detailed evaluation the device will include development of a novel single-wavelength pump-probe (SWPP) scheme that provides an expanded "view" along relaxation pathways and yields enhanced dynamical information. This opens up exciting new avenues of investigation and we will take advantage of this in using SWPP-TRPEI to perform studies of excess energy redistribution in three distinct molecular motifs providing starting models for chromophores found in nature (as detailed in the Objectives section). Our work represents a major step forward in realizing a next generation of low-cost table-top light sources for ultrafast spectroscopy and we anticipate that the dissemination of our findings will have lasting impact on this major research field.

Planned Impact

The proposed work is fundamental in nature and it is therefore expected that the economic and societal benefits stemming from the research output will primarily be realized in the longer term. This is in addition to the academic benefits, many of which will be much more immediate. Our research will significantly advance existing methodologies for the study of energy redistribution within the excited states of molecular systems - including those with biological, environmental and/or medical relevance, yielding important new mechanistic insight. This has potential implications for a wide range of practical applications where the interplay between structure and dynamics influences light-matter interactions and associated photochemical or biological function. In addition, the tuneable HC-PCF-based spectrometer we will develop will also be of possible interest to the applied photonics community. In order to facilitate timely uptake of our findings we will exploit a wide variety of routes to maximize exposure to relevant parties. These include presenting our work at large, interdisciplinary conferences that attract several thousand delegates as well as making use of well-established industry-academia networking events at the local, national and international levels. These networking events span a number of different themes: from exploratory collaboration between universities and companies, to forums discussing policy roadmaps for major funding initiatives. It is critical for the continuity and timeliness of any follow-up research that such opportunities are fully exploited throughout the duration of the project, rather than simply at its conclusion.

There will, in addition, be very immediate impact stemming from our work in the form of highly trained personnel with technical skills in the use of cutting edge techniques in the areas of laser physics, non-linear optics, ultra-high vacuum science, molecular spectroscopic, rigorous data analysis and theoretical computational techniques. They will also have well-developed generic and widely transferrable communication, presentation and problem solving skills. They will, therefore, be ideally positioned to contribute to the growth or creation of new research projects (both pure and applied) as well as high-tech companies, enhancing innovative capacity and possible revenue generation for the UK economy. Enhanced public awareness and engagement with our research efforts will be achieved through consciously non-specialist postings on university web pages, university open days and via "case study" reports produced at regular intervals throughout the project and made available in the public domain. It is anticipated that these studies will also be beneficial for attracting future research students to the key areas of ultrafast lasers and optics, molecular spectroscopy and dynamics, and theoretical photochemistry. It will also have the potential to advertise the existence of the project to any interested parties in the commercial sector.

Publications

10 25 50
 
Description We successfully achieved a key first milestone - namely the first practical demonstration of hollow-core photonic crystal fiber technology for time-resolved spectroscopy applications. This was published in J. Phys. Chem. Lett. (vol. 18, p 715 (2019)). This stimulated much interest from others working in the molecular spectroscopy and dynamics community and the work is clearly of high impact, as illustrated in an invited review we were asked to submit on this topic - see Phys. Chem. Chem. Phys. (vol. 23, p 10736 (2021)). A second key milestone was the demonstration of pump-probe photoelectron imaging measurements with extremely short time resolution (10 fs) - an advance on the previous state of the art in this field. This was published in Chem. Sci. (vol. 13 p 9586 (2022)), which also included the first proof-of-capability study of molecular excess energy redistribution dynamics in this short optical pulse regime. This is an important result and we anticipate its impact will become highly significant over the next few years. Further upgrades that expand the wavelength tunebility and output power have also been made subsequently to the setup and further research outputs will follow in due course.
Exploitation Route The successful demonstration of non-linear optical fibre sources for time-resolved spectroscopic applications has potentially wide-reaching implications for researchers working in a number of scientific fields and we anticipate wider uptake of the approach over the next few years. In particular, the improved temporal resolution our approach offers will open up many new and interesting possibilities for investigating excess energy redistribution dynamics. such fundamental light-molecule interactions are hugely important for many processes of biological, environmental, and technological relevance.
Sectors Chemicals

Electronics

Pharmaceuticals and Medical Biotechnology

 
Description Some concepts from this project have served as an instructive tool in undergraduate/postgraduate and lectures courses/material, providing an educational impact. Some of the research findings have lead directly to invitations to submit review articles (e.g. Phys. Chem. Chem. Phys., 23, 10736, (2021) and J. Mol. Spec., 395, 111807, (2023)). The methods we have developed and pioneered during this project are now starting to be adopted by other leading research groups around the world. As such, the work is developing significant academic impact.
First Year Of Impact 2023
Sector Education
Impact Types Societal

 
Description DTP Scholarship for Seb Jackson
Amount £65,000 (GBP)
Organisation Heriot-Watt University 
Sector Academic/University
Country United Kingdom
Start 09/2020 
End 03/2024
 
Title AIR Network 
Description Many charged particle imaging measurements rely on the inverse Abel transform (or related methods) to reconstruct three-dimensional (3D) photoproduct distributions from a single two-dimensional (2D) projection image. This technique allows for both energy- and angle-resolved information to be recorded in a relatively inexpensive experimental setup, and its use is now widespread within the field of photochemical dynamics. There are restrictions, however, as cylindrical symmetry constraints on the overall form of the distribution mean that it can only be used with a limited range of laser polarization geometries. The more general problem of reconstructing arbitrary 3D distributions from a single 2D projection remains open. Here, we demonstrate how artificial neural networks can be used as a replacement for the inverse Abel transform and-more importantly-how they can be used to directly "reinflate" 2D projections into their original 3D distributions, even in cases where no cylindrical symmetry is present. This is subject to the simulation of appropriate training data based on known analytical expressions describing the general functional form of the overall anisotropy. Our arbitrary image reinflation (AIR) neural network can be utilized for a range of different examples, potentially offering a simple and flexible alternative to more expensive and complicated 3D imaging techniques 
Type Of Material Data analysis technique 
Year Produced 2022 
Provided To Others? Yes  
Impact Only just published - still to be determined 
URL https://github.com/HWQuantum/AIR
 
Title Image de-noising using machine learning 
Description Artificial neural networks transform noisy charged particle images unsuitable for quantitative analysis into statistically reliable data in excellent agreement with benchmark references. The approach has significant potential use within the field of chemical dynamics, particularly for the extraction of subtle features originating from photofragment vector correlations or photoelectron circular dichroism. 
Type Of Material Data analysis technique 
Year Produced 2021 
Provided To Others? Yes  
Impact Still to be determined 
URL https://github.com/HWQuantum/Charged-Particle-Denoising
 
Description Contributed Talk at MOLEC Meeting 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Professional Practitioners
Results and Impact Talk by Seb Jackson (PhD student attached to project) at MOLEC meeting in Hamburg, Germany, August 2022
Year(s) Of Engagement Activity 2022
 
Description Contributed Talk at SDG Meeting 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Postgraduate students
Results and Impact Talk by Seb Jackson (PhD student attached to project) at SDG meeting in Durham, UK, Jan 2023
Year(s) Of Engagement Activity 2023
 
Description Invited Talk at APS Meeting 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Professional Practitioners
Results and Impact Invited talk by Dave Townsend at APS meeting, held Chicago, IL, USA, March 2022 (Special Session: 25 years of Velocity Map Imaging)
Year(s) Of Engagement Activity 2021,2022
 
Description Invited Talk at ISMS Meeting 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Postgraduate students
Results and Impact Invited talk by Dave Townsend at International Symposium on Molecular Spectroscopy, Urbana-Champaign, IL, USA, June 2022 (designated Journal of Molecular Spectroscopy Special Review Lecture).
Year(s) Of Engagement Activity 2022
 
Description Leicester talk 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach Local
Primary Audience Postgraduate students
Results and Impact Invited departmental seminar at University of Leicester, November 2019
Year(s) Of Engagement Activity 2019
 
Description New Horizons in Chemical Physics talk 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Professional Practitioners
Results and Impact Invited talk by Dave Townsend at New Horizons in Chemical Physics meeting, held Oxford, April 2019
Year(s) Of Engagement Activity 2019
 
Description Talk - UCL departmental seminar 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach Local
Primary Audience Professional Practitioners
Results and Impact Seminar at University College London Chemistry Department
Year(s) Of Engagement Activity 2023
 
Description York Talk 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach Local
Primary Audience Postgraduate students
Results and Impact Invited talk at Department of Chemistry, University of York (21st November 2018)
Year(s) Of Engagement Activity 2018