Name: Dr Max Attwood
Position: UKRI Quantum Technology Career Development Fellow in the Department of Materials.
Research Focus: Developing organic materials for quantum technologies and a type of quantum sensor called the “maser”.
Can you tell us more about your research area?
My research fellowship centres around developing organic materials for quantum technologies, most pressingly, a type of quantum sensor called the “maser”. These masers are the microwave equivalent of a laser, and the word itself is an acronym for “microwave amplification by stimulated emission of radiation”. Such devices have the potential to amplify extremely weak radio/microwave signals within a narrow bandwidth with extremely high signal-to-noise ratios (SNR).
Masers, originally conceived in the 1950s, have historically been used as sensor components inside space-facing radio telescopes. However, due to their thirsty consumption of liquid helium, which was required to sustain their low-noise characteristics, these classical masers fell from favour after the 1970s, mainly being replaced by cheaper field-effect transistor devices.
Interest in these devices was reinvigorated in 2012 when a team of researchers led by Professor Mark Oxborrow and Professor Neil Alford of Imperial College London demonstrated the world’s first room-temperature solid-state maser.[1,2] Built using an organic crystal of para-terphenyl doped with the “workhorse” molecule known as pentacene, this work represented a significant shift in terms of the maser gain medium and underlying mechanism governing its operation.
When irradiated with yellow light, one electron in the outermost orbital of pentacene becomes spin-inverted, switching the “state” of the molecule from singlet to triplet. In this triplet state, electrons can exist in one of three distinct energy levels that emerge due to interactions between electrons of the same spin. For some molecules such as pentacene, electrons preferentially occupy the uppermost state due to a process of spin-inversion known as “spin-selective intersystem crossing”, thereby generating a strong population inversion. The difference in energy between the uppermost and lowest energy levels corresponds to a microwave frequency of 1.45 GHz. When a microwave photon of 1.45 GHz interacts with a crystal containing lots of triplet-state pentacene molecules, it “stimulates” an avalanche of electrons from the uppermost energy level to the lowest. This simultaneously produces an emission of photons at 1.45 GHz, effectively amplifying the stimulating photon by several orders of magnitude – enough to produce an electrical impulse in a coupled detector.
Thanks to these findings, masers can be used to help detect any phenomenon that might operate or occur at a resonant microwave frequency. This includes sensing extremely small deviations in magnetic field strength or perhaps a single electron spin-flip. I’m interested in the latter example because of its potential to work in tandem with quantum spin-based technologies, including memory devices and communication networks. Before masers can become competitive with contemporary devices, we must find materials that improve their energy efficiency and reduce the impact of non-signal (i.e., noise) photons naturally emitted by ambient objects. This means testing new workhorse molecules instead of pentacene and new host molecules instead of para-terphenyl.
What are the main aims of your current research?
My research aims to build a library of efficient organic maser systems capable of sensing at various frequencies and under significantly dulcified operating conditions compared to the current pentacene-based maser.
As a chemist, my job involves synthesising and analysing new materials with properties that are open and responsive to maser applications. For new host molecules, we look for systems that are chemically inert while also being able to encapsulate a wider range of “workhorse” molecules at a high concentration. In all cases, the more molecules, the better!
We manipulate these properties by changing the chemical structure of guest and host components and determining maser devices’ operating conditions, sensitivity, and output power. In a perfect world, every photon of absorbed light would produce a triplet state with a population inversion!
How could this research potentially benefit society?
A significant advantage of masers over contemporary high electron mobility transistor (HEMT) or superconducting interference device (SQUID) based microwave sensors is their remarkable signal-to-noise. To achieve low-noise figures, these devices require cryogenic refrigeration however, masers operate at room temperature.
High signal-to-noise means that we require fewer repeat measurements of signals to average out background signals. For example, in electron paramagnetic resonance (EPR) spectroscopy, a technique that uses microwaves to detect electron spins, the signal-to-noise ratios are proportional to where is the number of measurements. If we doubled the signal-to-noise ratios, this would result in 4x fewer scans.
This advantage could be used for equipment like nuclear magnetic resonance (NMR) spectroscopy or magnetic resonance imaging (MRI) scanners in hospitals. Currently, NHS patients can expect to wait for more than three months for an MRI scan, so enabling faster scan times could significantly enhance their availability.
What are the next steps in your research?
Finding materials that exhibit all the qualities required to build a compelling maser device is complex, and we usually must settle for a compromise.
Often, molecules that exhibit a high triplet yield following irradiation with light have short spin relaxation times. This is because the mechanism that enables the formation of triplets, known as “spin-orbit coupling”, also makes it easier for the spin of an electron to flip. Furthermore, we often find that our workhorse molecules are chemically incompatible with known host molecules. Such issues stem from the fact that polar molecules prefer to interact with other polar molecules (like each other).
Therefore, we often rely on glass-forming materials that provide excellent optically clear media form, but light excitation can cause triplet-state molecules to occupy a disordered and inhomogeneous molecular environment. The consequence is that the spin-spin relaxation time diminishes significantly, causing the spread of emission frequencies to expand, which reduces the output power for any frequency we might like to study. Known as “inhomogeneous broadening”, this effect is minimised in crystals which form highly ordered 3D structures at the molecular level.
One of my primary research goals is to synthesise a universal host that can encapsulate a wide range of candidate maser molecules and significantly expedite the discovery and screening process while also ensuring an ordered crystalline environment. I would also like to build on the available computational tools that can calculate and predict the properties of future maser candidates.
References:
[1] M. Oxborrow, J. D. Breeze, N. M. Alford, Nature 2012, 488, 353–356.
[2] J. Breeze, K.-J. Tan, B. Richards, J. Sathian, M. Oxborrow, N. M. Alford, Nat. Commun. 2015, 6, 6215.