From the first femtoseconds to working devices

We are driven by curiosity about how energy moves through matter, and by what that understanding could make possible: solar cells that waste less sunlight, batteries that charge faster and last longer, and light sources that are purer and more efficient. Our approach is to build instruments that measure what could not previously be measured, to follow the physics wherever it leads, and to carry the best ideas from the laboratory into working devices and, through our spin-out companies, into industry.

Many of the processes that determine how well a solar cell or light-emitting material works are complete within a trillionth of a second and take place over a few nanometres. We build optical microscopes and spectrometers that follow these processes directly, with a time resolution below 10 femtoseconds and a spatial precision below 10 nanometres.

Our transient absorption microscopes track how excitons and charges move in space and time, and impulsive vibrational spectroscopy follows how molecular vibrations couple to electronic states. With these methods we identified transient delocalisation, a transport regime in which excitons in highly ordered polymer nanofibres travel about 300 nanometres, roughly one hundred times further than conventional hopping allows. We also showed that excitons in organic molecules can be decoupled from high-frequency vibrations, removing a loss pathway previously considered unavoidable.

We are extending these methods towards the single-photon limit: field-resolved detection of light pulses has reached yoctojoule (10⁻²⁴ J) sensitivity with attosecond precision. The same tools have been applied to biological light harvesting, where ultrafast spectroscopy of living cyanobacteria showed that electrons can be extracted from photosystems within picoseconds of excitation.

Batteries store energy by moving lithium ions into and out of microscopic particles in their electrodes. How quickly and how uniformly this happens determines how fast a battery can charge and how long it lasts, yet it has been difficult to observe directly. We developed charge photometry, an optical microscopy that follows individual particles inside a working battery as it charges and discharges.

Charge photometry is based on interference reflection microscopy. As lithium ions enter or leave an active particle, its refractive index changes, and the scattered light reports the local state of charge. Because this principle does not depend on the battery chemistry, the method applies to a wide range of electrode materials and can follow many single particles during realistic cycling.

With charge photometry we have followed phase transitions and lithium diffusion in individual particles of LiCoO₂, the most widely used commercial cathode material, and found different mechanisms on charge and discharge. We have visualised state-of-charge heterogeneities and cracking in high-rate anodes and nickel-rich cathodes, and shown that lithium enters graphite in avalanche-like steps linked to static disorder. The same approach has been applied to photo-redox phase engineering of transition-metal dichalcogenides and to electrochemical doping of conjugated polymers.

Charge photometry is being commercialised by illumion.

Our laboratories are in the state-of-the-art Ray Dolby Centre and Maxwell Centre at the Cavendish Laboratory. They house ultrafast laser systems and the optical microscopes and spectrometers that we design and build, charge-photometry instruments for operando battery studies, and facilities for making materials and devices.