Black gold: Enabling bright, high rep-rate electron beams

February 15, 2013 by Paul Preuss
In an electron gun, the photocathode emits electrons when illuminated by a laser pulse, which is immediately accelerated by a strong electric field. The same aperture provides the exit for the electrons and the entrance for the laser light. Credit: Alan Stonebreaker/APS

Free electron lasers (FELs) have proven their worth, but next-generation light sources will have to do better than produce ultrabright x-ray pulses 100 or so times a second. What's needed is megahertz rep rate, a million times a second. Since it's electrons that make the x-rays, the only way to achieve that kind of performance is with an electron gun that can deliver tight electron bunches with high charge, high energy, and a very high repetition rate – sources like Berkeley Lab's futuristic APEX, for which Howard Padmore of the Advanced Light Source (ALS) and his colleagues are designing the photocathodes.

Photocathodes produce electrons when hit by a laser beam, a process called photoemission; in accelerators the electron bunch is immediately accelerated inside a high-gradient cavity to minimize the effects of the like-charged particles blowing the bunch apart. With metal photocathodes, ultraviolet (UV) lasers are the usual choice to excite photoemission, with photon energy near the energy needed to from metals.

A set of groove-like, gold-floored nanocavities were carved in gold-plated aluminum from a template produced by electron beam lithography. The grooves cause an order of magnitude electric field enhancement at the mouth of the nanocavity and a like enhancement of the magnetic field at its base. Incidence of laser light is normal to the surface.

But surprisingly, at the necessary high laser intensities, infrared (IR) can be more efficient than even well-matched UV pulses. The secret: multiphoton processes using IR – those in which four photons, for example, arrive virtually simultaneously and excite emission of a single electron. With high-rep-rate IR lasers, electron bunches from the photocathode retain the laser's femtosecond (quadrillionth of a second) time structure.

Unfortunately metals reflect IR rather than absorb it, so under ordinary circumstances little of the incident light does work. To address this problem Alex Polyakov, Padmore's former graduate student now at the Molecular Foundy, worked with Jim Schuck and Stefano Cabrini to create a photocathode and electron amplifier, carving parallel grooves 15 nanometers wide, 60 nanometers deep, and just 100 nanometers apart (a nanometer is a billionth of a meter) in gold-plated aluminum – a pattern smaller than a laser's wavelengths.

The grooves served as resonators of plasmons – quantized vibrations in the oscillation of surface electrons – interacting with the incident laser light to amplify and control the photoemission of electrons. With colleagues at the Pacific Northwest National Laboratory the team showed that the device completely absorbed the laser light, eliminated reflection, and produced a million times more electrons than an IR laser on bare gold.

Colleagues of Padmore's at UCLA who had been developing IR-based photocathodes applied these plasmonic photocathode principles to the electron gun of a real accelerator. Instead of nanogrooves, Renkai Li of Pietro Musumeci's group drilled an array of nanoholes in copper. Theirs was the first demonstration of relativistic electron beams from a nanostructured in a high-gradient accelerating structure; with it they were able to perform the first measurements of beam quality.

Plasmonic photocathodes hold the promise of manipulating light fields on the nanoscale at will, in the same way that RF fields are used today, opening many new possibilities for the creation of ultrabright electron sources.

Explore further: Clocking Ultra-fast Electron Bunches

More information: Both reports appear in the same issue of Physical Review Letters at prl.aps.org/abstract/PRL/v110/i7/e076802 and  prl.aps.org/abstract/PRL/v110/i7/e074801 respectively.

Related Stories

Clocking Ultra-fast Electron Bunches

July 30, 2010

(PhysOrg.com) -- Brookhaven researchers have developed a device that acts like a high-tech stopwatch for speedy packs of electrons just trillionths of a second long. This new diagnostic tool could aid in the development of ...

New research improves quality of free electron laser

June 2, 2011

The free electron laser is the next step in the development of equipment to help us see the structure of materials. Nino Čutić at MAX-lab in Lund, Sweden, has done a PhD in further improving the test free electron ...

First X-ray lasing of SACLA

June 17, 2011

RIKEN and the Japan Synchrotron Radiation Research Institute (JASRI) have successfully produced a first beam of X-ray laser light with a wavelength of 1.2 Angstroms. This light was created using SACLA, a cutting-edge X-ray ...

Measuring individual atoms with compact X-ray lasers

November 30, 2012

To look at small objects typically requires big machines. For example, the study of single atoms with a laser requires x-ray radiation of such high energy that it is only produced by accelerating electrons in large facilities. ...

Recommended for you

Quantum matter stuck in unrest

July 31, 2015

Using ultracold atoms trapped in light crystals, scientists from the MPQ, LMU, and the Weizmann Institute observe a novel state of matter that never thermalizes.

New blow for 'supersymmetry' physics theory

July 27, 2015

In a new blow for the futuristic "supersymmetry" theory of the universe's basic anatomy, experts reported fresh evidence Monday of subatomic activity consistent with the mainstream Standard Model of particle physics.

Rogue wave theory to save ships

July 29, 2015

Physicists have found an explanation for rogue waves in the ocean and hope their theory will lead to devices to warn ships and save lives.

0 comments

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.