Department of Physics John Anderson Research Colloquia

Prof K Long (Physics, ICL) and Prof A Giaccia (Oncology, Oxford) - 3pm, TL328

Cancer is the second most common cause of death globally. In 2018, 18.1 million new cancer cases were diagnosed, 9.6 million people died of cancer-related disease, and 43.8 million people were living with cancer.  Radiotherapy (RT) is used in 50% of cancer patients and is involved in 40% of cancer cures.  It is estimated that 26.9 million life-years could be saved in low- and middle-income countries if capacity could be scaled up. 

The beam characteristics that can be exploited in proton- and ion-beam therapy (IBT) facilities today are restricted to low dose rates, a small number of temporal schemes, and a small number of spatial distributions.  The use of novel beams with strikingly different characteristics has led to exciting evidence of enhanced therapeutic benefit.  This evidence, together with developments in our understanding of personalised medicine based on the biology of individual tumours, now provides the impetus for a radical transformation of IBT.

The ‘Laser-hybrid Accelerator for Radiobiological Applications’, LhARA, is conceived as a novel, uniquely flexible facility dedicated to the study of radiobiology.  The technologies that will be demonstrated in LhARA have the potential to allow particle-beam therapy to be delivered in a completely new regime, combining a variety of ion species in a single treatment fraction and exploiting ultra-high dose rates.  The laser-hybrid approach will allow the exploration of the vast “terra incognita” of the mechanisms by which the biological response is modulated by the physical characteristics of the beam.  We will describe the motivation for LhARA, present the status of its development and summarise the programme upon which the LhARA consortium has embarked to drive a step-change in clinical capability.

Dr Anton Idlerton (University of Edinburgh) 25^th October 2023, 3pm, TL328

Rachel Oliver (University of Cambridge) Date, location to be confirmed 

Porous semiconducting nitrides are effectively a new class of semiconducting material, with properties distinct from the monolithic nitride layers from which devices from light emitting diodes (LEDs) to high electron mobility transistors are increasingly made. The introduction of porosity provides new opportunities to engineer a range of properties including refractive index, thermal and electrical conductivity, stiffness and piezoelectricity. Quantum structures may be created within porous architectures and novel composites may be created via the infiltration of other materials into porous nitride frameworks.

Scanning transmission electron microscopy image of a single sub-surface porous layer, which the etchant has accessed via defect related channels.

A key example of the application of porous nitrides in photonics is the fabrication of high reflectivity distributed Bragg reflectors (DBRs) from alternating layers of porous and non-porous GaN.  These reflectors are fabricated from epitaxial structures consisting of alternating doped and undoped layers, in which only the conductive, doped layers are electrochemically etched. Conventionally, trenches are formed using a dry-etching process, penetrating through the multilayer, and the electrochemical etch then proceeds laterally from the trench sidewalls.  The need for these trenches then limits the device designs and manufacturing processes within which the resulting reflectors can be used. We have developed a novel alternative etching process, which removes the requirement for the dry-etched trenches, with etching proceeding vertically from the top surface through channels formed at naturally-occurring defects in the crystal structure of GaN (see Figure). This etch process leaves an undoped top surface layer almost unaltered and suitable for further epitaxy.

This new defect-based etching process provides great flexibility for the creation of a variety of sub-surface porous architectures on top of which a range of devices may be grown.  Whilst DBR structures enable improved light extraction from LEDs and the formation of resonant cavities for lasers and single photon sources, recent development also suggests that thick, sub-surface porous layers may enable strain relaxation to help improve the efficiency of red microLEDs for augmented reality displays.  Meanwhile, the option of filling pores in nitride layers with other materials provides new opportunities for the integration of nitrides with emerging photonic materials, such as the hybrid-perovskite semiconductors, with perovskites encapsulated in porous nitride layers demonstrating greatly improved robustness against environmental degradation.