Quantum critical electro-optic and piezo-electric nonlinearities.

Although electro-optic (EO) nonlinearities are essential for many quantum and classical photonics applications, a major challenge is inefficient modulation in cryogenic environments. Guided by the connection between phase transitions and nonlinearity, we identify the quantum paraelectric perovskite SrTiO3 as a strong cryogenic EO [>500 picometers per volt (pm/V)] and piezo-electric material (>90 picocoulombs per newton) at T = 5 K, at frequencies to at least 1 megahertz. Furthermore, by tuning SrTiO3 toward quantum criticality, we more than double the EO and piezo-electric effects, demonstrating a linear Pockels coefficient above 1000 pm/V. Our results probe the link between quantum phase transitions, dielectric susceptibility, and nonlinearity, unlocking opportunities in cryogenic optical and mechanical systems and providing a framework for discovering new nonlinear materials. Editor's summary: Optical materials that exhibit a large electro-optic response, or Pockels effect, find industrial applications in areas of optical communication, sensing, and computing. There is also a need for such materials in developing quantum technologies, but the electro-optic response at cryogenic temperatures tends to be low. Ulrich et al. and Anderson et al. identified strontium titanate as a promising material to meet the challenge. The ability to engineer a foundry-friendly material with such a strong electro-optic response operating at cryogenic temperature should be useful for developing advanced functional quantum devices for low-temperature applications. —Ian S. Osborne [ABSTRACT FROM AUTHOR]