Sudents learning the electron microscopy facilities at HKU

Strengthening the coupling between photons and free electrons

23 June 2023

A new method can produce a hundredfold increase in light emissions from a type of free-electron—photon coupling, which could improve the efficiency of a range of technologies.

Creating different wavelengths of electromagnetic radiation – from those in the visible light spectrum to those in the terahertz, ultraviolet, and X-ray and regimes – is important for many technologies. These include solar panels, LEDs, lasers, and medical imagers.

Many methods for creating radiation use ‘free electrons’: beams of electrons that are accelerated until they produce photons. However, the travelling electrons are about 1,000 times smaller than the wavelengths of optical photons. This creates a mismatch that limits the coupling strength between the two, restricting the photon emission efficiency.

Now, a team of researchers at The University of Hong Kong (HKU), MIT and other universities say they have come up with an innovative way to make more efficient interactions between photons and electrons possible. The system is based on a concept called Smith-Purcell radiation, but uses new material structures to improve light emission at the target frequency by around a hundredfold, with the potential for much more.

Dr Yi Yang, Assistant Professor of the Department of Physics at HKU and a former postdoc at MIT, is a lead author of the research, published in the prestigious journal Nature. He says: “Our proof-of-concept experiment demonstrated an enhancement in photon emission, and had the potential to go further with more optimisation and refinement.”

Enhanced emission

Smith-Purcell radiation occurs when a beam of electrons is passed very close to parallel over a diffraction grating – a surface with a periodic pattern that creates phase matching and photon emission. The team refined this by using the concept of flatbands. These are structures that create ‘dispersionless’ areas, allowing more modes to mediate the interactions between the electrons and photons. Flatbands have been used in electronics and photonics research, but have not yet been used to manipulate free electrons and light together.

The team created such a system using a ‘photonic crystal’, which consists of a square lattice of shallow etched nanometre-scale holes in a silicon slab to replace the basic diffraction grating key for Smith-Purcell radiation. They then fired a beam of electrons from a repurposed scanning electron microscope (SEM) at a grazing angle over the surface of the crystal. This produced enhanced radiation, 100 times the emission of photons emitted outside the flatband from the same sample, and 30 times the emission of standard Smith–Purcell radiation generated with a control silicon grating of deeper etch depth but without flatbands.

Yang says: “Experimentally, it was difficult to achieve, as we needed to not only fire the electron beam exceptionally close to the photonic crystal and make sure the beam and crystal were well aligned, but we also needed to add a twisting angle between the beam and the crystal. But this innovation from my colleagues proved that flatbands do matter in increasing the interactions between the free electrons in the beam and the emitted photons.”

Not only does the system enhance emission, but it is also tuneable: desired wavelengths can be created by adjusting the speed of the electrons and the resonant structure. This is especially valuable for producing wavelengths that are difficult to produce efficiently, including terahertz waves, ultraviolet light, and X-rays.

The system can also be tuned in terms of the polarisation of the emitted light – a property that can be exploited for many technologies.

Vice versa

In this case, the team used electrons to enhance light. But in principle, it could be made to work the other way round: laser light could be used to accelerate electrons. The way these systems would increase electron velocity means they might even be harnessed to build miniature particle accelerators-on-a-chip, replicating accelerators that conventionally require huge facilities. Not only could this shrink fundamental physics facilities akin to the Large Hardon Collider, but it could also be used in medical applications like radiotherapy.

The system could also be tuned to use electrons to generate multiple entangled photons, which are sought-after phenomena for quantum technologies, including quantum computing and communications systems. Coupling many photons is a hard problem, but using free-electron beams to control them could be a way to make this simpler.

These potential applications are exciting, but it will take many more iterative steps to see any them come to fruition. The team first want to test the system in new regimes. In this paper they modified a scanning electron microscope, but next they want to try it with a transmission electron microscope, which has higher acceleration voltages.

Further steps towards useable technologies include optimising the photonic structures and creating interfaces between photonic and electrical components in order to make them work together in any chip-based technology. But the new work shows the effort is worthwhile: the gains are potentially huge.

Yi Yang began the work at MIT, but is now enjoying his new working environment at HKU: “I am very excited that HKU is actively expanding the capability and capacity of our electron microscopy facilities. My students are learning those advanced tools, and we hope that we can leverage these infrastructures to obtain exciting results in the future.”