The instrinsic photodielectric effect (PDE), which uses light to increase the permittivity of the dielectric used in capacitors, could aid in the development of photocapacitors that would enable the remote control of the dielectric response via photo-irradiation.

Researchers at the University of Nagoya have demonstrated the photo-induced enhancement of the dielectric permittivity in a ceramic with the composition LaAl_0.99Zn_0.01O_3-δ under visible-light irradiation.

Previous research has achieved extrinsic PDE using a variety of materials and photo-conductance, where light increased the material’s electrical conductivity. The rise in conductivity led to greater dielectric permittivity.

According to researcher Hiroki Taniguchi, this type of extrinsic PDE is not suitable for practical applications. A capacitor must be a good insulator; but under the extrinsic PDE, a capacitor’s insulating properties deteriorate. In addition, such a capacitor would only work with low-frequency alternating current.

Taniguchi and his colleagues have found an intrinsic PDE in LaAl_0.99Zn_0.01O_3-δ.

“We have demonstrated the existence of the photodielectric effect experimentally,” he said.

Researchers found that photo-irradiation with an incident energy of 3.4 electron volts (eV) enhanced the dielectric permittivity in LaAl_0.99Zn_0.01O_3-δ over a wide frequency range, from 100 hertz (Hz) to one megahertz (MHz). Dielectric permittivity at high frequency was enhanced by seven percent with the slight increment of tanδ in the high-frequency region.

Research showed that the change in dielectric permittivity in the high-frequency region only nominally depended on frequency and was not accompanied by an increase in dielectric loss, indicating an intrinsic PDE in LaAl_0.99Zn_0.01O_3-δ that was not due to photoconduction. Instead, researchers attributed the PDE to the dielectric response of photo-excited electrons trapped in deep in-gap states, working as effective polar displacements under an applied electric field.

The ceramic remained a good insulator, while its dielectric permittivity increased even at high frequencies.

The lack of a significant loss could mean that photo-irradiation directly altered the dielectric permittivity of the material, and did not increase conductance. According to Taniguchi, it is still unclear how the intrinsic PDE works, but it may have to do with defects in the material.

Light excites electrons into higher (quantized) energy states, but the quantum states of defects are confined to smaller regions. This could prevent photo-excited electrons from traveling far enough to generate an electric current. The researchers hypothesize that the electrons remain trapped, which leads to more electrical insulation of the dielectric material.

The findings of the present study could open the way to the development of innovative optical materials with dielectric functionalities. Further research will focus on enhancing the intrinsic PDE even more, minimizing any energy dissipation due to a drop of dielectric properties, and optimizing the material fabrication process, Taniguchi said. Additional research could also reveal novel materials better suited for other electronics applications.

The research was published in Applied Physics Letters (doi: 10.1063/1.4979644).