Photoionization in KTN deflectors by light in the near-infrared imaging window

TL;DR

This study quantitatively characterizes near-infrared photoionization in KTN electro-optical deflectors, revealing wavelength-dependent charge dynamics crucial for optimizing deep tissue imaging applications.

Contribution

It provides the first detailed measurements of NIR photoionization rates in KTN deflectors, informing better operational strategies for biological imaging.

Findings

Photoionization rate decreases with increasing wavelength.

Charge decay exhibits multi-exponential behavior.

Data guides wavelength selection and duty cycle optimization.

Abstract

Electro-optical deflectors (EODs) offer unparalleled scanning speed for laser-scanning microscopy and other applications, but suffer from limited deflection range. EODs based on potassium tantalate niobate (KTN) crystals feature some of the highest number of resolvable spots. These deflectors rely on internal electric fields generated by trapped electrons to enable beam scanning. However, visible light induces rapid photoionization of trapped charges, thus KTN-based deflectors are typically continuously recharged with a bias voltage that effectively limits the range of the deflector. Recent work has proposed the use of KTN-based EODs for biological imaging with infrared excitation light, but quantitative data on near-infrared photoionization is lacking. Here, we present quantitative measurements of photoionization in KTN deflectors across the NIR-I and NIR-IIa biological imaging windows (700 - 1300 nm), a range that is particularly important for deep tissue imaging and nonlinear microscopy. Using a two-beam polarization interferometer, we measured trapped charge density as a function of photon fluence. We observed that the photoionization rate decreases dramatically with increasing wavelength. The charge density decay curves exhibit multi-exponential behavior that cannot be explained by a single-trap model without recapture, indicating the presence of multiple trap species or substantial recapture. These measurements provide critical quantitative guidance for selecting operating wavelengths and charge-scan duty cycles for KTN-based EODs in near-infrared imaging applications.