Ionizing radiation induces femtosecond time scale modulations of a material’s optical properties

In this work, we continue the study of an optical based method for annihilation photon detection with the potential for a dramatic improvement in time resolution for time-of-flight positron emission tomography (ToF-PET). Previous work has shown that the refractive index of materials such as bismuth silicon oxide (BSO) and cadmium telluride (CdTe) can be modulated by the charge cloud created by annihilation photon interactions, though the ultrafast nature of the index modulation process remains untested. At the Linac Coherent Light Source (LCLS) at the SLAC National Accelerator Laboratory, the arrival time of X-ray pulses with photon energies between 0.5–10 keV is routinely detected with femtosecond scale time resolution. The ionizing interactions alter the local band structure in optically transparent insulators, changing the refractive index. Using a frequency chirped visible continuum probe pulse for a monotonic wavelength-to-time mapping, we measured the induced refractive index modulation, with interferometric sensitivity, and a sub-picosecond time resolution. In this work, we show that femtosecond scale resolution can be achieved for photon arrival time measurement using the refractive index modulation mechanism. This new detection concept has the potential to also achieve significantly improved timing capability for ToF-PET.