Generation Of Ultrafast Optical Fiducials For Shock Wave Experiments

Generation of ultrafast optical fiducials for shock wave experimentsBrian W. DodsonSandia National Laboratories, Shock Wave and Explosive PhysicsDivision 1131, Albuquerque, New Mexico 87185AbstractRecent advances in high time -resolution optical diagnostic instrumentation for shock waveexperiments in condensed media (especially time -resolved spectroscopy) have resulted in newchallenges concerning the timing of such experiments. We present a technique for detectingthe passage of a shock wave by the generation of an optical fiducial signal, which can bedetected and recorded directly by the optical recording device (typically a streak camera).This technique, which is based on Stress -Induced Defeat of Total Internal Reflection(SIDTIR), requires only simple apparatus and set up, and offers fiducial transition times asshort as 50 psec in a reasonable experimental configuration.IntroductionThe recent recognition1,2,3 by Western shock wave researchers that significant and un-usual chemistry can take place during shock loading of condensed media has stimulated a sub-stantial interest in performing time -resolved experiments which do not simply measure stressor particle histories, as do quartz gauge4 or VISAR5 techniques, but rather measure materialproperties such as electrical or physicochemical properties in the shock environment. One ofthe more difficult of such desirable experiments involves subnanosecond resolution spectros-copy, with the intent of investigating the molecular environment in and immediately followingthe shock front. The aim here would be to measure a spectrum (emission, absorption, orRaman) from the time the shock wave is incident on the measurement plane and for severalnanoseconds thereafter, and infer physical and chemical conditions from time -dependentspectral changes.We expect that induction times will be associated with the spectral changes, because oflifetimes of excited states, overcoming potential barriers, etc. Therefore, to unambiguouslyinterpret time- resolved spectral measurements, it is necessary to provide an independent in-dication of when the shock wave reaches the measurement plane. As subnanosecond measurementsare of interest only on samples which exhibit extremely fast shock rise times, a simple yes -no fiducial will suffice for timing calibration. The most useful fiducial signal would bepurely optical, so that it can be directly recorded on the spectrographic streak camera,avoiding the need to compare two separate time bases to subnanosecond accuracy.Such a subnanosecond fiducial is provided by the SIDTIR (Stress -Induced Defeat of TotalInternal Reflection) technique, which is described in the remainder of this paper. Followinga brief review of total internal reflection, we discuss the stress- related material proper- -ties which form the basis for the SIDTIR technique, and establish operating parameters fora practical system. We then discuss the limitations and potential of realistic experimentalconfigurations.Theory of SIDTIRTotal internal reflection6 can occur when light is obliquely incident on an interfacebetween an optical medium with high index of refraction n2 and a medium with a smaller indexof refraction n1. In the geometry shown in Fig.