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Optoelectronic detection of x-ray synchrotron pulses. Top: Layout of the coplanar stripline on a GaAs substrate surface. A DC voltage is applied across the gap. The x-ray pulse produces carriers that become a current pulse shorting the gap, creating a propagating voltage pulse. A time-delayed laser pulse creates photocarriers in the sampling gap, leading to a brief current proportional to the voltage at the gap. The current versus x-ray/laser pulse delay ( ) maps out the time profile of the original x-ray pulse, convoluted with carrier lifetime effects. Bottom: Layout of the synchrotron experiment. X-ray pulses are monochromated to 12 keV and focused by Kirkpatrick-Baez (KB) mirrors to a spot size and intensity of photons/s centered on the coplanar stripline gap, after passing through a 1 kHz chopper. The RF signal that controls the synchrotron electron bunches (351.933 MHz) is divided by four (87.9835 MHz) to provide the reference signal for the Ti:sapphire laser oscillator. The laser produces one laser pulse per x-ray pulse at a fixed time delay determined by a delay generator that controls a RF phase shifter. The laser pulse is focussed with a microscope objective onto a photoconducting gap between the striplines and a sampling electrode, with an average power of ∼80 mW. The collected current is converted to voltage by a preamplifier, and measured by a lock-in amplifier referenced to the chopper frequency.
Penetration depths into the GaAs substrate. Top: Cross-section view of GaAs substrate and the biased coplanar striplines, indicating the much greater penetration of 12 keV x-rays relative to the 800 nm laser light (not to scale). Middle: Exponential decay curves for the laser and x-ray radiation. Bottom: Vacancy density versus depth for 8 MeV protons, showing a nearly constant density for the top region exposed to x-rays.
Relative sampling current versus x-ray/laser pulse time delay. Top: Sampling current for semi-insulating GaAs, showing a pronounced signal (a drop from the baseline) caused by the x-ray generated voltage pulse passing the sampling gap. The width is consistent with 50 ps x-ray pulses convoluted with ∼200 ps carrier lifetime in GaAs. Bottom: Results for GaAs after bombardment with 8 MeV protons. The baseline current is much reduced after bombardment; the absolute deviation from baseline is smaller by 10−4. Three well-resolved structures are visible (a)-(c), corresponding to the x-ray induced voltage pulse and its reflections from impedance mismatches in the stripline. The bare line corresponds to the nominal 50 ps (FWHM) profile of the synchrotron pulses.
Generation and reflection of voltage pulses in the coplanar stripline. Top: Layout of the coplanar stripline circuit (not to scale). The central region hasa measured impedance of Z = 60 Ω, bracketed by regions of 40 Ω and 150 Ω. (a) X-ray pulse excitation near the center of the coplanar stripline circuit (above) creates voltage pulses that propagate in both directions along the wave guide. (b) The initial pulse is partially reflected at the first impedance mismatch, with positive amplitude. (c) The pulse is then reflected at the second impedance mismatch with negative amplitude.
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