Achieving Picosecond Control of Hard X-rays with MEMS Optics

$Fig. 1. MEMS oscillator tuned to the APS storage ring frequency for manipulating hard x-ray pulses at 300 picoseconds. Left: Scanning electron microscopy images of the MEMS device after multiple rounds of micromachining using focused ion beam (FIB). Tuning of the MEMS device resonance frequency was critical and was carried out using the CNM FIB. A systematic milling program was used to etch a set of symmetric rectangles away from the edges of the silicon crystal farthest from the oscillation axis of the torsional flexures, so that the device is tuned to the storage ring frequency while avoiding introducing an undesirable modal response in the device. Right: Demonstration of 300-ps diffractive time window in a delay scan covering two consecutive pulses in the APS 324-bunch mode where the two x-ray pulses are 11.37 ns apart. This scan was done with a MEMS device oscillating at 271.555 kHz, the exact APS storage-ring frequency. For detail, see P. Chen. et al., Nat. Commun. 10, 1158 (2019).$ Things happen fast at the nanoscale. The spatiotemporal study of important materials-related processes such as structural change, phase transition, diffusion, ionic transport, fluid flow, strain propagation, and many others is crucial for understanding structure and function at the nanoscale and mesoscale, but requires experiments using the pulsed nature of synchrotron-based x-ray sources. The ultrafast, time-resolved, hard x-ray imaging, scattering, and spectroscopy techniques often employed in such investigations can be limited by the temporal resolution and timing patterns of the sources. Achieving temporal manipulation at the pulse length or 100-picosecond time scales at the synchrotron facilities is possible only with difficult and expensive modifications to the facility technologies that can compromise other capabilities of the x-ray source. A team of researchers at the U.S. Department of Energy’s (DOE’s) Argonne National Laboratory has developed another way of accessing ultrafast time scales by using microelectromechanical system (MEMS)-based photonic devices to achieve dynamic control of the hard x-ray pulses.

Most previous work on MEMS photonic devices has concentrated on communication and imaging applications in visible and infrared wavelengths, with only limited exploration of the potential of x-ray devices, despite some demonstration of their possibilities by this same group of researchers. Here, leveraging on the experience of Argonne Center for Nanoscale Materials (CNM) researchers on MEMS and nanofabrication, the Advanced Photon Source (APS) and CNM team created MEMS resonators of different resonant frequencies consisting of thin silicon single-crystal chips (Fig. 1). The single-crystal chips diffract or transmit x-rays by changing the orientation relative to the x-ray beam; the team carried out the time-resolved x-ray measurements at the X-ray Science Division 7-ID-C beamline at the APS. The team, consisting of scientists from the APS and CNM, both Office of Science user facilities at Argonne, published the work in Nature Communications

As the device oscillates around the Bragg angle 𝜃_B, a diffractive time window (DTW) opens, and the timing between the DTW and the x-ray pulse can be manipulated. When the DTW is wider than the x-ray pulse, the MEMS device can act as a pulse-picking chopper; when narrower, it will create x-ray pulses shorter than the incident pulses, enabling higher temporal resolution. If the DTW is close to the x-ray pulse in width, x-ray streaking can be obtained, making sub-pulse-resolution possible in experiments of single-pulse duration.

To function as a practical dynamic x-ray optical device, the MEMS device must be tuned to the x-ray storage-ring frequency, which is accomplished by using a focused ion beam to trim mass from the crystal until its resonance frequency matches the frequency of the synchrotron source or one of the source’s harmonics. One set of devices (designated as P0/2) was tuned to a resonance frequency of 135.777 kHz, which is half the APS operating frequency of P0=271.555 kHz; the second set was tuned to the full APS frequency and designated as P0 devices.

Adjusting the DTW to time scales in the sub-nanosecond range was done by applying higher excitation voltages to the MEMS oscillator to increase its oscillation amplitude. With the P0/2 devices, varying the excitation voltage from 50 to 100 V caused a steady decrease in the DTW, from several nanoseconds to just below 0.5 nanoseconds. Shorter time scales are impractical because voltages above ~100 V would make the oscillation approaching the mechanical limit of the MEMS devices. With the higher-frequency P0 MEMS devices, the researchers found that higher excitation voltages were necessary to obtain the same oscillation amplitudes when the devices were operated in air at a normal pressure of about 100 kPa because of fluid dynamic damping effects. Operating the devices in a vacuum environment, however, significantly reduced the necessary excitation voltage to achieve reduced DTW values.

With a P0 device at a pressure of 1.32 kPa and an excitation voltage of 50 V, a DTW of approximately 300 picoseconds was achieved. Currently, this is limited somewhat by noise and physical factors such as the achievable amplitude of the device, but a higher frequency may be able to overcome this to demonstrate dispersing/streaking behavior, providing a means of performing time-domain research beyond the pulse-length limitation without the need for storage ring modifications.

The ultrafast MEMS-based x-ray optics demonstrated in this work opens a variety of exciting new possibilities to manipulate and control single hard x-ray pulses. The MEMS devices can operate in pulse-picking modes with extremely fine spatiotemporal resolution at typical high-energy synchrotron light sources. The APS and CNM research team is now exploring a variety of MEMS-based optics that will provide new opportunities to manipulate x-rays at current synchrotrons and also offers advantages for upgraded low-emittance sources such as the APS Upgrade Project. ― Mark Wolverton

See: Pice Chen, Il Woong Jung, Donald A. Walko, Zhilong Li, Ya Gao, Gopal K. Shenoy, Daniel López, and Jin Wang, “Ultrafast photonic micro-systems to manipulate hard x-rays at 300 picoseconds,” Nat. Commun. 10, 1158 (2019). DOI: 10.1038/s41467-019-09077-1

Author affiliation: Argonne National Laboratory

Correspondence: *wangj@aps.anl.gov

This research is supported by the Accelerator and Detector Research (ADR) Program of the U.S. Department of Energy (DOE) Office of Science-Basic Energy Sciences. The use of the Center for Nanoscale Materials and Advanced Photon Source was supported by the U.S. Department of Energy Office of Science-Basic Energy Science under Contract No. DE-AC02-06CH11357. Critical technical support from Tim Mooney and Michael Hu of the APS are gratefully acknowledged.

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Published Date

03.20.2019