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

TL;DR

KTN electro-optical deflectors enable high-speed beam steering but suffer from trap photoionization under illumination. The authors employ a two-beam polarization interferometer to measure trapped-charge density across 700–1300 nm, revealing a strong wavelength dependence with decreasing photoionization rates at longer wavelengths. The observed decay is multi-exponential and not well described by a simple two-electron, no-recapture model, suggesting multiple trap species or recapture processes. These quantitative results provide actionable guidance for selecting NIR operating wavelengths and charge-scan duty cycles, supporting strategies such as discontinuous charging to extend deflector range in infrared imaging applications.

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.

Figures (5)

• Figure 1: KTN deflector operation. (a) Prior to illumination, injected electrons are trapped inside the KTN. Initially, after charging, light can be scanned across a large angle. Photoionization reduces trapped charge density and scan angle over time. Continuous charging (b) uses a biased sinusoid, which replenishes charge but adds a bias angle, limiting the maximum geometric scan angle. Discontinuous charging (c) alternates between a DC pulse and a zero-bias sinusoid. This provides no bias angle and increases the maximum geometric scan angle.
• Figure 2: Charge density measurement method. (a) Setup: a low-power probe beam (for phase-shifting interferometry) and a gated, tunable beam (for photoionization) are combined prior to the KTN. A phase-shifting Mach-Zehnder interferometer measures the KTN-induced differential phase. Example analysis: (b) Interferograms reflecting the phase retardation produced by the KTN are collected. The $y$-axis cross section location is indicated by a dashed blue line (c) A phase estimation algorithm converts interferograms into a wrapped phase profile along $y$. (d) A phase unwrapping algorithm is applied, and a parametric model is fit to the unwrapped phase to estimate the charge density.
• Figure 3: Remaining charge (percentage of initial charge) vs. time for a KTN crystal in the absence of a scanning laser. KTN charged to 450 V and operated at 28 ° C.
• Figure 4: Wavelength-dependent photoionization in KTN from 700 nm to 1300 nm in 50 nm increments. Remaining charge (percentage of initial charge) vs. photon fluence, shown on linear (top) and logarithmic (bottom) $x$-axes.
• Figure 5: Remaining charge (percentage of initial charge) vs. photon fluence at wavelengths from 700 nm to 1300 nm in 50 nm increments. Color indicates wavelength (see colorbar). Dashed lines show experimental data and solid lines show the best-fit (least-squares) model (Eqs. \ref{['eq:F1_sol']}--\ref{['eq:P_phi']}). $\sigma_1$ and $\sigma_2$ are fit independently at each wavelength.