Periodic Domain Inversion in Single Crystal Barium Titanate-on-Insulator Thin Film

Abstract

Experimentally achieving the first-ever electric field periodic poling of single crystal barium titanate oxide (BTO, or BaTiO₃) thin film on-insulator is reported. Owing to the outstanding optical nonlinearities of BTO, this result is a key step toward achieving quasi-phase-matching (QPM). First, the BTO thin film is grown on a dysprosium scandate substrate using pulsed laser deposition with a thin layer of strontium ruthenate later serving as the bottom electrode for poling. The characterization of the BTO thin film using x-ray diffraction (XRD) and piezo-response force microscopy to demonstrate single crystal, single domain growth of the film that enables the desired periodic poling, are presented. To investigate the poling quality, both non-destructive piezo force response microscopy and destructive etching-assisted scanning electron microscopy (SEM) are applied, and it is shown that high quality, uniform, and intransient poling with 50% duty cycle and periods ranging from 2 µm to 10 µm is achieved. The successful realization of periodic poling in BTO thin film unlocks the potential for highly efficient nonlinear processes under QPM that seemed far-fetched with prior polycrystalline BTO thin films which predominantly relied on efficiency-limited random or non-phase matching conditions and is a key step toward integration of BTO photonic devices.

Keywords: barium titanate‐on‐insulators; nonlinear frequency conversion; nonlinear optical materials; optoelectronics; periodic poling; quasi‐phase‐matching (QPM).

Figure 1

a) X‐ray diffraction (XRD) spectra for BTO/SRO/DSO, XRD 2θ‐ω coupled scan showing BTO is single crystalline with c‐axis oriented out‐of‐plane. b) XRD rocking curve at the BTO (002) peak with the full width at half maximum (FWHM) of 0.53° shows high crystal quality for BTO c) SEM image of the BTO/SRO/DSO cross‐section. The SRO is only 15 nm thick, so it is not visible d) shows the enlarged view of the cross ‐section to clearly demonstrate the three layers e) Energy dispersive X‐ray analysis (EDXA) for the chemical characterization of the sample f) showing the corresponding variation in elemental components along the cross‐section of the structure g) PFM phase image of the film surface showing well‐aligned domain orientations.

Figure 2

a) Variation of QPM period for SHG with the fundamental wavelength, insets show the modal profiles for the fundamental TE modes for frequencies at 600, 1500, and 3000 nm and their respective SH profiles. b) Poling set up: An arbitrary waveform generator (AWG) is used to generate a trapezoid‐like waveform that is amplified by a high voltage amplifier (HVA) and applied across the BTO thin film via a poling circuit. Thin film BTO on DSO with a finger electrode pattern on top (inset showing the AFM image of the electrodes patterned on our sample) and a ground SRO electrode beneath the BTO film are shown. The input waveform from the AWG and that observed at the oscilloscope (blue waveform corresponds to the applied field and red refers to the resulting poling current across a load) are also shown c) The optical microscope image of the patterned electrodes.

Figure 3

a–c) PFM phase images corresponding to under‐poling and d) over‐poling. (a,b) correspond to under‐poling resulting from applying a voltage less than the voltage required for complete domain inversion c) The domain inversion is complete as evident from the image contrast, but the duty cycle is only 45% d) Over‐poling which results from domain spreading beyond electrode fingers. e–i) PFM images for inverted domains with 50% duty cycle with different periods: e) 10 µm f) 7 µm g) 5 µm h) 2 µm i) 3 µm domain with complete electrode finger length imaged j) Aperiodic poling showing domain inversion corresponding to electrodes with varying widths and period.

Figure 4

a,b) SEM images and corresponding PFM images of the poling pattern revealed after selective HF etching for 150 s showing good uniformity in poling along the length and depth c) Tilted SEM image to show the smooth sidewalls of the poled region, which further endorses the high‐poling quality. d,e) SEM and corresponding PFM images of poling pattern revealed after selective HF etching for 300 s, showing good uniformity in poling along the length and depth f,g) Etching results into roughness which exacerbates the PFM image quality, and we conform the roughness through the AFM images, first one refers to before etching and second after etching h,i) SEM and PFM phase image of inverted domains after 3.5 months of poling. The image is achieved after selective etching of the sample in HF for 120 s.