532 nm wavelength laser diode

Light-induced deformation was quantified using a WayCon inductive
position transducer conditioned with a Solartron OD5 Module. The system
had a linear scale of 42 V/mm, enabling the measurement of tens of
nanometers with a high-resolution oscilloscope (Keysight DSOX2004A).
For measurements under illumination, a 532 nm wavelength laser diode
(Thorlabs, Inc.) was incorporated into the system. Light power was
controlled using a source-measured unit (Keithley 2400) and set to
50 mW. The light spot had a diameter of ∼2 mm, and the duration
of light irradiation was regulated using a switch. Raw data were processed
to eliminate slow thermal drift, and a Savitzky–Golay filter
was applied to smooth the data.

XRD measurements were conducted at the SpLine CRG BM25 beamline
at the ESRF The European Synchrotron (Grenoble, France) under the
proposal MA-5601. The X-ray beam had a wavelength of 0.8266 Å
(approximately 15 keV) with an energy resolution of ΔE/E ∼ 1 × 10^–4. A beam spot size of 0.5 mm × 0.5 mm was used to ensure representative
measurements of the crystalline domain structure. A lanthanum hexaboride
(LaB₆) standard was employed for instrument calibration
and wavelength refinement. Measurements were carried out in reflection
geometry using a SixC diffractometer in a vertical configuration.
XRD patterns were recorded over an angular range of 5–25°
(2Θ) with a step size of 0.0075° using a 2D photon-counting
X-ray 2D-MAXIPIX detector.^{38 (link)} Reciprocal
space maps (RSMs) were measured with the consideration of BaTiO₃ lattice parameter units. All XRD data were processed using
the BINoculars software.^{39 (link)} For measurements
under illumination, a 532 nm wavelength laser diode (Thorlabs, Inc.)
was coupled to the measurement stage. Light power was set to 30 mW,
and the light spot diameter was approximately 2 mm. Light irradiation
time was controlled using an electronic switch.

Domain mappings were conducted using a confocal Raman microscope
(Witec alpha-300R) with a 532 nm excitation laser (intrinsic Raman
light source) and a 100× objective lens. The confocal microscope
offered lateral and vertical resolutions of approximately 250 and
500 nm, respectively, with a Raman mode spectral resolution of 0.02
cm^–1. The domain walls of the samples were optically
aligned perpendicular to the x-axis and parallel
to the y-axis of the piezo-driven scan table. To
generate the movement of the ferroelectric domain walls it was necessary
to apply another laser diode light source coupled to the setup. This
additional laser light was focused on the sample surface, parallel
to the surface. Specifically, a 532 nm wavelength laser diode (Thorlabs,
Inc.) was utilized at a power of 50 mW and a light spot diameter of
∼2 mm (perpendicular to the y-axis). The reversibility
of light switching on ferroelectric domains during off–on–off
light cycles was investigated. To do that, the process started with
the confocal Raman microscopy (CRM) scanning of the sample surface
under dark conditions (pristine state). Upon completion of one-third
of the total scanning, the iris of the optical setup was opened, and
the next third of the scanning was collected. Subsequently, the iris
was closed, and the remaining third of the scanning was obtained.
Raman data were processed by the Witec Control Plus Software.