Ultrafast Radiation Detection by Modulation of an Optical Probe Beam

Abstract

We describe a new class of radiation sensor that utilizes optical interferometry to measure radiation-induced changes in the optical refractive index of a semiconductor sensor medium. Radiation absorption in the sensor material produces a transient, non-equilibrium, electron-hole pair distribution that locally modifies the complex, optical refractive index of the sensor medium. Changes in the real (imaginary) part of the local refractive index produce a differential phase shift (absorption) of an optical probe used to interrogate the sensor material. In contrast to conventional radiation detectors where signal levels are proportional to the incident energy, signal levels in these optical sensors are proportional to the incident radiation energy flux. This allows for reduction of the sensor form factor with no degradation in detection sensitivity. Furthermore, since the radiation induced, non-equilibrium electron-hole pair distribution is effectively measured "in place" there is no requirement to spatially separate and collect the generated charges; consequently, the sensor risetime is of the order of the hot-electron thermalization time ≤ 10 fs and the duration of the index perturbation is determined by the carrier recombination time which is of order ~ 600 fs in, direct-bandgap semiconductors, with a high density of recombination defects; consequently, the optical sensors can be engineered with sub-ps temporal response. A series of detectors were designed, and incorporated into Mach Zehnder and Fabry-Perot interferometer-based detection systems: proof of concept, lower detection sensitivity, Mach-Zehnder detectors were characterized at beamline 6.3 at SSRL; three generations of high sensitivity single element and imaging Fabry-Perot detectors were measured at the LLNL Europa facility. Our results indicate that this technology can be used to provide xray detectors and xray imaging systems with single xray sensitivity and S/N ~ 30 at xray energies ~ 10 keV.

Ionizing photon interactions modulate the optical properties of crystals with femtosecond scale temporal resolution

“…nanocrystals) (Turtos et al 2016). On the other hand, ionization-induced ultrafast modulation phenomena such as x-ray and neutron induced electrooptic modulations are also studied for radiation detection (Vernon andLowry 2006, Nelson et al 2012), which is more closely aligned with our study of the direct ionization-induced modulation of a material's complex refractive index.…”

A promising new mechanism of ionizing radiation detection for positron emission tomography: modulation of optical properties

“…Although combining optics pump-probe measurement with radiation detection still remains a mostly unexplored research concept, rising efforts and attention have been drawn to this field as a promising way to dramatically improve the timing properties of radiation detection. Ultrafast low energy x-ray detection based on optical modulation has been achieved (Lowry et al 2004, Vernon andLowry 2006). Electro-optic neutron detectors have also been realized (Nelson et al 2010(Nelson et al , 2012.…”

“…Ultrafast low energy X-ray detection based on optical modulation has been achieved (Lowry et al 2004, Vernon and Lowry 2006). Electro-optic neutron detectors have also been realized (Nelson et al 2010, Nelson et al 2012).…”

Nevada Test Site-Directed Research and Development, FY 2007 Report

“…LLNL recently demonstrated a gallium arsenide (GaAs) material-based RadOptic sensor (Vernon, 2006;Lowry, 2005), using optical readout in the IR region. We saw an opportunity in LLNL's significant accomplishment to make many improvements in our project and utilized their experiment specifications as a stepping off place for our own work.…”