Paraelectric Electrooptics

The electrooptic (EO) effect in centrosymmetric crystals is a quadratic (“Kerr”) effect. This effect is exhibited in particular in some oxygen perovskite crystals at the paraelectric (PE) phase in which the effect is exceptionally strong,  and when doped with impurity ions exhibit in addition the voltage controlled photorefractive effect. These characteristics make it a platform for constructing devices that operate throughout the entire VIS-NIR spectral range, implementing functions such as fast wavelength selective switching and wavelength tunability that cannot be implemented by the current photonic technologies. This merits presenting the EO effect at the PE in a class of its own for which we coined the term ‘Paraelectric Electrooptics

A quintessential example of these crystals is potassium tantalate niobate (KTN).  The composition of KTN is given by: KTa1-xNbxO3. Its ferroelectric phase transition (Tc) is determined by the relative Nb/Ta concentration and is given by: Tc [K] ≈682x+33.2 [REF1]. Upon approaching Tc the EO effect in KTN becomes exceptionally large  exhibiting electrically induced changes of the refractive index ≈10-2 [REF2]. However, already at elevated temperatures, the Nb ions emerge from the center of inversion of their respective unit cells forming small nano-dipoles. These, upon approaching Tc create large dipolar clusters that fluctuate in space and time exhibiting the behavior of a glass forming liquid [REF1]. Thus, KTN at the PE phase is a soft condensed matter in which dipolar clusters are embedded in a PE environment. Consequently, the EO effect in KTN at the PE phase is governed by the interplay between the electrically induced polarization in the PE lattice, and the dielectric response of the dipolar clusters. These clusters cause light that propagates in KTN to scatter. In addition, KTN is inherently photorefractive as it always contains traces of impurity ions that reside in the energy band gap and act as traps that are partially populated. Similar to other photorefractive crystals, this makes KTN prone to form optical damage when illuminated with light at visible wavelengths, as characteristic of photorefractive crystals. Thus, although the strong EO effect in KTN was discovered many years ago, the scattering and formation optical damage render it technologically useless.

Research that was done recently at the Hebrew University has shown that these showstoppers can be removed. It was shown that in KTN slightly doped with Li (KLTN) the exceptionally large EO effect was maintained, but the scattering was inhibited [REF15]. It was also shown that optical damage can be suppressed in KLTN by driving it with bipolar voltage. This was demonstrated by modulating a light beam at l=445 nm and intensity of 30 W/cm2, with bipolar driving voltage.  The intensity and contrast of the output beam was unchanged after six hours of continuous operation [REF5].

Oxygen perovskite that are doped with certain transition metal ions become photorefractive. When operated at the PE phase these crystals exhibit the “voltage controlled photorefractive effect”. A quintessential representative of these crystals is copper doped KLTN which exhibits an exceptionally strong voltage controlled photorefractive effect as it can be operated very close to the phase transition[REF4]. This makes KLTN an attractive candidate to be the medium for realizing devices that are based on electroholography [REF6]. Both fast wavelength selective switches and wavelength tunability are be implemented by electroholography. A laser with tunability range of 51 nm covering the entire C band is described in [REF10]. Fast wavelength selective switching based on electroholography are described in [REF11] and [REF12].

Finally, integrated photonic circuits in which PE EO devices, optical components, and nanophotonic circuits are interconnected by a mesh  of waveguides and operate in unison to perform complex functions can be constructed in KLTN substrates by employing a special fabrication technique – refractive index engineering described in  [REF8].