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The ALS X-Ray Streak Camera: Bringing the Ultrafast and Ultrasmall into Focus Print
Wednesday, 26 March 2008 00:00

 

 

Studying the world of the ultrasmall and the ultrafast is at the frontier of scientific research. Two x-ray approaches can be used for ultrafast examinations. The first entails developing sources that have short x-ray pulses such as free-electron lasers and slicing sources, which will provide the ultrafast temporal information. The other approach is to develop a detector that is fast enough to resolve the ultrafast details of the dynamical processes. ALS researchers are taking the second path but adding a spatial resolution capability; that is, they are developing a high-speed x-ray streak camera with high spatial resolution to watch, in real time, the motion of the atoms in materials. So far, a temporal resolution of 233 fs and a spatial resolution of 10 mm have been demonstrated. This is the first time that such a high temporal resolution has been combined with high spatial resolution in a streak camera.

In an x-ray streak camera system, the photocathode converts the x-ray signal into a photoelectron analog signal. The electron lens focuses the electron to an image plane; while in transit, the electron beam is deflected to convert the temporal dimension to a spatial dimension using a fast deflection structure, which consists of meander strip-line sweep plates. The electron lens can be put before or after the sweep plates, depending on the space-charge effect. The sweep plates are driven by a fast ramp voltage from a photoconductive switch.

Capturing the Very Small
and the Very Fleet

People have been making cameras for over two thousand years. The first, and simplest, was the pinhole camera, discovered around 400 B.C. by the Chinese philosopher Mozi. This device had a very small hole in a thin material that focused light from an object through a point. Pinhole cameras are still around, and for a pinhole-to-film distance of 1 inch, a reasonable diameter for the pinhole itself is 0.22 millimeters, about the width of a human hair.

The ALS x-ray streak camera, the latest and most advanced descendent of the pinhole camera, still has a hole through which light flows, only its "pinhole," or slit, has a diameter of 10 micrometers (a micrometer, or micron, is one millionth of a meter and one thousandth the width of a human hair). Incoming photons pass through the slit and strike a photocathode, generating electrons. The electrons strike a recording device, and the location yields time, space, and intensity information. The ALS streak camera can look at objects with a time resolution of 233 quadrillionths of a second (233 femtoseconds) and a spatial resolution better than 10 microns. Using this camera to explore the origin of magnetic phenomena in thin films, multilayer constructs, and nanostructures could produce smaller magnetic bits and faster magnetic switching for data storage and other applications.

A streak camera is a detector that resolves the intensity of a photon signal as a function of time and space. Such detectors have very desirable characteristics for ultrafast x-ray dynamic studies due to the fact that the system records the full temporal response of a sample in a single shot. In addition, they can be integrated into complex end stations on any beamline. The ALS streak camera consists of a photocathode, a pair of deflection plates, a magnetic solenoid lens, and a CCD detector. What makes this streak camera unique is the use of a carefully designed extraction-mesh and sweep plate for acceleration and streaking, which reduces the different aberrations both in static mode and dynamic mode. The sweep plates are driven by voltage ramps with a fast rise time through a GaAs photoconductive switch triggered by an infrared (IR) fs laser pulse. Pulse-to-pulse jitter in this system was reduced to less than 60 fs. Another key improvement is the use of a large-aperture magnetic solenoid lens design that sits downstream of the sweep plates to reduce field aberrations of the electron optics. The magnetic lens is operated outside the vacuum chamber, which has significantly reduced vacuum and thermal issues.

The performance of the ALS streak camera has been measured using 266 nm UV light from a Ti:sapphire fs laser. The spatial resolution in static mode is measured by imaging the anode mesh without voltage ramps on the deflection plates by adjusting the current of the magnetic lens. A 1500 line/inch Cu mesh can be clearly imaged in the static mode of this streak camera. The period of mesh is 16.6 mm, and an effective opening size is 11 mm separated by a 5.6-mm-wide wire. The range with good focus of the magnetic lens is projected to be +/–3.6 mm on the photocathode plane. An electron lens with a large field of view is required to realize the desired resolution.

(a) Static image obtained with a Au-coated photocathode having a 25 mm opening in the time dispersion direction and a 1500/lines/inch anode assembly. (b) 1000-shots-averaged streaked image and projected beam profile. The dashed lines are Gaussian fits of the experimental results. This graph shows clearly that two pulses separated by 233 fs can be resolved. (c) The high temporal resolution and spatial resolution in the ALS streak camera can be maintained over the entire 53 ps time window.

For the dynamic mode, the ALS streak camera detector system can resolve two pulses separated by 233 fs with 10 mm spatial resolution with 1000 shots. The high resolutions in the temporal and spatial dimensions can be retained in a wide 53 ps temporal window. Furthermore, the focus current in this streak camera stayed unchanged for both the static and dynamic modes, revealing that the deflection dispersion is very small in the streak camera.

ALS X-Ray Streak Camera team. 1st row from left: J. Qiang, T. Young, H.J Shin, J. Feng, K. Opachich. 2nd row from left: G. Huang, M. Greave A. Comin, A. Bartelt. 3rd row from left: C. Coleman-Smith, W. Wan. Not pictured: J. Nasiatka, J. Byrd, H. Padmore, K. Engelhorn.

The latest improvement to the ALS streak camera is a built-in orthogonal sweep system. The updated system will be installed at an elliptically polarized undulator beamline at the ALS. This unique streak camera, with its bright synchrotron light source, is a powerful tool for the field of ultrafast x-ray spectroscopy of magnetic materials dynamics at subpicosecond temporal resolution. The next goal is to conduct x-ray probe experiments in reaction dynamics at the subfemtosecond resolution.

 


 

 

Research conducted by J. Feng, H.J. Shin, K. Engelhorn, J.R. Nasiatka, A.T. Young, A. Comin, H.A. Padmore (ALS); W. Wan, G. Huang, J. Byrd (Accelerator and Fusion Research Division, Berkely Lab).

Research Funding: U.S. Department of Energy, Offices of Science and Basic Energy Sciences (BES). Operation of the ALS is supported by BES.

Publication about the research: J. Feng, H.J. Shin, J.R. Nasiatka, W. Wan, A.T. Young, G. Huang, A. Comin, J. Byrd, and H.A. Padmore, "An x-ray streak camera with high spatio-temporal resolution," Appl. Phys. Lett. 91, 134102 (2007).