Stanford researchers create particle accelerator on a chip much smaller scale, these Bremsstrahlung X-rays are the heart of medical X-ray equipment being used today, but the equipment required to generate them is big and expensive.



Appropriately enough, Neil Sapra, under Jelena Vuckovic of the Ginzton Laboratory at Stanford, whose namesake



How it works The chip operates using Dielectric Laser Acceleration (DLA), which takes advantage of the incredibly high electric fields (> 100,000,000 V/m — stronger than lightning) and oscillating nature of pulsed lasers. At a high level, the laser creates a "surface wave" that oscillates up and down near a nanostructured electrical insulator such as silicon dioxide (SiO 2 ). The surface wave is actually made of alternating sections of opposing electric fields so that you get regions of positive and negative electron acceleration. The surface wave also periodically flips which regions accelerate and which decelerate, so if the electrons have the right speed, they can hit only accelerating areas. DLA principle of operation. Inspired by Figure 1(b) from [1]. The pulsed laser is incident from the left, and the light blue is an electrical insulator. The steps below better explain what is actually happening to electrons, with numbers corresponding to the figure above from left to right: The laser is off. Electron 1 (yellow) is incident from the bottom of the figure The laser is on with a positive peak electric field. Electron 1 (yellow) enters the first DLA section and experiences an accelerating (green) force. Electron 2 (purple), initially traveling at the same speed as Electron 1, is incident from the bottom of the figure The laser is on with a negative peak electric field, causing the locations of the accelerating (green) and decelerating (red) forces to switch places. Electron 1 enters the second DLA section and experiences an accelerating force again. Electron 2 enters the first DLA section and experiences a decelerating force The laser is on with a positive peak electric field, causing the locations of the accelerating (green) and decelerating (red) forces to switch back to their locations in step (2). Eelectron 1 enters the third DLA section and experiences an accelerating force again. Electron 2 enters the second DLA section and experiences a decelerating force again The laser is off. Electron 1 and Electron 2 do not experience any force but are now traveling at different speeds The design and results Up until now, the laser has been part of a large experimental apparatus with focusing optics. What the authors has done is bring the laser light on-chip and used a special technique called Inverse Design where, instead of playing around with a chip layout themselves to get the best performance, they specify what level of performance they want ahead of time, and then use machine learning to try and find a layout that meets that performance, subject to basic physical constraints like Maxwell's Equations. In this case, their desired performance criterion is to maximize the accelerating force in the upward direction while minimizing the left-right forces that would steer the electron either into or away from the nanostructured surface.



After enforcing some additional constraints, like a minimum feature size of 80 nanometers, they created an actual silicon chip and tested its performance. What they found is that the average energy increase of 330 electron volts (eV), or about 53 attojoules. This seems like a small number, but for a single electron is huge; using this Beginning and ending electron energy distributions. Summary of Figure 4(a) of [2] A sketch of their results is shown above, with the gray curve corresponding to the distribution of electron energies out of the chip with the laser off, and the blue curve corresponding to after the laser is turned on. It's hard to interpret, because an equal number of electrons see energy increases (accelerated) as decreases (decelerated), but this is only because there was no attempt to control for when electrons entered the chip (an equal mix of yellow and purple electrons from the first figure). The peak energy for both curves is about 83.4 keV. The mean energy increase on top of this, which the authors call the "shoulder" is 330 eV, while the maximum increase, shown by when the distributions of "on" and "off" curves again cross, is 1.21 keV.



Since the accelerator stage was only 30 μm long, the theoretical gain over one meter is 40.3 MeV, which is over 99.99% the speed of light starting from rest. That is huge and way more than needed for medical purposes.



Other thoughts In an The team plans to add a thousand stages of acceleration by the end of 2020 to reach target velocity. This is really challenging and I wonder if it's not more suited for a startup. As you scale the number of stages, you start fighting the following effects: Your sensitivity to incoming electron energy increases You need independent pulsed lasers to more easily tune specific acceleration sections The stage's physical design needs to change (get longer) to accommodate the accelerating electrons, introducing design and fabrication complexity. Also, as a consequence of (3), this design doesn't work well if the electron is at low power, say ≤ 10 keV, because you would need either an impossibly small surface feature size, or a very long wavelength pulsed laser, to do the acceleration. As a result, this chip still needs to be paired with a bulkier linear "pre-accelerator". Still, this is a very thought provoking paper that I could see laying the groundwork for ultra-compact X-ray sources in a decade or so.



The final work appeared in the journal Science as References J. McNeur, et al., "Elements of a dielectric laser accelerator". arXiv:1604.07684 N. Sapra, et al., "On-chip integrated laser-driven particle accelerator". arXiv:1905.12822 You may have heard about "particle accelerators" as these giant, underground loops, like the Large Hadron Collider on the French/Swiss border, that are used to probe the deepest questions of physics. These gigantic constructs accelerate individual electrically-charged particles, like protons and electrons, to a large fraction of the speed of light. Sometimes, these particles are smashed into each other to create other particles of interest, or they're wiggled in a specific way to create X-rays called Bremsstrahlung . On asmaller scale, these Bremsstrahlung X-rays are the heart of medical X-ray equipment being used today, but the equipment required to generate them is big and expensive.Appropriately enough, Neil Sapra, under Jelena Vuckovic of the Ginzton Laboratory at Stanford, whose namesake Edward Ginzton developed Stanford's own particle accelerator in the 1940s, reported in early January on the creation of an electron particle accelerator contained on a single silicon chip.The chip operates using, which takes advantage of the incredibly high electric fields (> 100,000,000 V/m — stronger than lightning) and oscillating nature of pulsed lasers. At a high level, the laser creates a "surface wave" that oscillates up and down near a nanostructured electrical insulator such as silicon dioxide (SiO). The surface wave is actually made of alternating sections of opposing electric fields so that you get regions of positive and negative electron acceleration. The surface wave also periodically flips which regions accelerate and which decelerate, so if the electrons have the right speed, they can hit only accelerating areas.The steps below better explain what is actually happening to electrons, with numbers corresponding to the figure above from left to right:Up until now, the laser has been part of a large experimental apparatus with focusing optics. What the authors has done is bring the laser light on-chip and used a special technique calledwhere, instead of playing around with a chip layout themselves to get the best performance, they specify what level of performance they want ahead of time, and then use machine learning to try and find a layout that meets that performance, subject to basic physical constraints like Maxwell's Equations. In this case, their desired performance criterion is to maximize the accelerating force in the upward direction while minimizing the left-right forces that would steer the electron either into or away from the nanostructured surface.After enforcing some additional constraints, like a minimum feature size of 80 nanometers, they created an actual silicon chip and tested its performance. What they found is that the average energy increase of 330 electron volts (eV), or about 53 attojoules. This seems like a small number, but for a single electron is huge; using this relativistic electron energy calculator , an increase in energy from 83.44 keV to 83.77 keV corresponds to a velocity increase of over 200,000 m/s!A sketch of their results is shown above, with the gray curve corresponding to the distribution of electron energies out of the chip with the laser off, and the blue curve corresponding to after the laser is turned on. It's hard to interpret, because an equal number of electrons see energy increases (accelerated) as decreases (decelerated), but this is only because there was no attempt to control for when electrons entered the chip (an equal mix of yellow and purple electrons from the first figure). The peak energy for both curves is about 83.4 keV. The mean energy increase on top of this, which the authors call the "shoulder" is 330 eV, while the maximum increase, shown by when the distributions of "on" and "off" curves again cross, is 1.21 keV.Since the accelerator stage was only 30 μm long, the theoretical gain over one meter is 40.3 MeV, which is overstarting from rest. That is huge and way more than needed for medical purposes.In an interview about this paper for Photonics.com, it was reported thatThis is really challenging and I wonder if it's not more suited for a startup. As you scale the number of stages, you start fighting the following effects:Also, as a consequence of (3), this design doesn't work well if the electron is at low power, say ≤ 10 keV, because you would need either an impossibly small surface feature size, or a very long wavelength pulsed laser, to do the acceleration. As a result, this chip still needs to be paired with a bulkier linear "pre-accelerator". Still, this is a very thought provoking paper that I could see laying the groundwork for ultra-compact X-ray sources in a decade or so.The final work appeared in the journalas N. Sapra, et al., On-chip integrated laser-driven particle accelerator