Second harmonic microscopy of poled x-cut thin film lithium niobate: Understanding the contrast mechanism

Abstract

Thin film lithium niobate is of great recent interest and an understanding of periodically poled thin-films is crucial for both fundamental physics and device developments. Second-harmonic (SH) microscopy allows for the non-invasive visualization and analysis of ferroelectric domain structures and walls. While the technique is well understood in bulk lithium niobate, SH microscopy in thin films is largely influenced by interfacial reflections and resonant enhancements, which depend on film thicknesses and the substrate materials. We present a comprehensive analysis of SH microscopy in x-cut lithium niobate thin films, based on a full three dimensional focus calculations, and accounting for interface reflections. We show that the dominant signal in back-reflection originates from a co-propagating phase-matched process observed through reflections, rather than direct detection of the counter-propagating signal as in bulk samples. We can explain the observation of domain structures in the thin film geometry, and in particular, we show that the SH signal from thin poled films allows to unambiguously distinguish areas, which are completely or only partly inverted in depth.

Resource‐Efficient Low‐Temperature Synthesis of Microcrystalline Pb2B5O9X (X = Cl, Br) for Surfaces Studies by Optical Second Harmonic Generation

“…Finally, it should be noted that interfacial reflections and resonant enhancements in small structures might further influence signal strengths in SHG microscopy. [ 51 ]…”

High Quality Entangled Photon Pair Generation in Periodically Poled Thin-Film Lithium Niobate Waveguides

“…We used a diagnostic method, described in Ref. 16, to study and improve the poling process. After poling, the waveguide structure fabricated in collaboration with the University of Rochester, as reported in Ref.…”

“…We have recently developed a high-quality poling recipe for x-cut LNOI, and demon-strated two useful diagnostic methods using in-situ poling monitoring, and a non-destructive nonlinear microscopy technique. 15,16 This allows us to improve the poling process, and identify suitable waveguides without destroying them e.g., by cross-sectioning, and etching using dilute acids, as is typically performed in traditional poling diagnostics. In section III, we describe our experiments in performing SPDC, where we used a continuous-wave optical pump at 784.5 nm in order to generate frequencydegenerate photon pairs at 1569 nm.…”

Rejuvenating a Versatile Photonic Material: Thin‐Film Lithium Niobate

“…[121] Among these, quasiphase matching (QPM) may be considered as the most preferred approach. [144][145][146][147][148][149] The advantages include waveguide design and optimization freedom, as well as access to the largest nonlinear coefficient of LN (d 33 ). QPM is achieved in PPLN waveguides through inverting the crystalline domain polarity periodically to compensate for the wave-vector mismatch Δk between the pump and SHG optical modes.…”

“…More details on periodic poling of TFLN waveguides and various methods in order to study and improve domain inversion can be found in refs. [105,[144][145][146][147][148][149].…”