Gated recording of holograms using rare-earth doped ferroelectric

Abstract

Rare earth doped ferroelectric materials are disclosed as reversible holographic recording medium (25) for use in two-photon recording systems. Such rare earth elements provide long-lived electronic states intermediate the ferroelectric material's valence and conduction bands. In some cases, these rare earth intermediate states have a sufficiently long life that low-power continuous wave ('cw') lasers (1) can be used to record interference patterns on them. Thus, two-photon holographic recording systems are also disclosed which do not require high-power, short pulse length, mode-locked or Q-switched lasers. Rather, the disclosed holographic recording systems employ cw lasers such as diode lasers. The rare earth dopants include praseodymium, neodymium, dysprosium, holmium, erbium, and thulium. These dopants provide ions having 4f excited states that give rise to absorptions in the near infra-red and visible spectral regions and typically have lifetimes on the order of 0.1 to 1 milliseconds. The disclosed two-photon holographic recording systems provide for absoption of a first photon which excites electrons of a holographic recording medium to a rare-earth intermediate state. Thereafter, upon absorption of a second photon, the electrons are promoted to the medium's conduction band where they are arranged according to the interference pattern provided by the recording system.