4D-STEM data for ptychography and tcBF#

STEM patterns are coherent for 4D-STEM geometries: the renderer propagates a probe wavefunction (condenser aperture and the column’s probe aberrations) through the specimen’s complex transmission function and records |FFT|² on the detector. The bright-field disk interferes, so a defocused probe gives a shadow image, and ptychography and tilt-corrected bright field (tcBF, “parallax”) reconstruct the twin’s data the way they do real DE data. de-twin produces the data and the ground truth; reconstruct with the tool of your choice.

from de_twin import DigitalTwin
from de_twin.state import Roi, ScanRequest, TemStem

twin = DigitalTwin("Dense Au on holey C", camera="Celeritas")
col = twin.column
col.set_tem_stem(TemStem.STEM)
col.set_convergence_mrad(15.0)
col.set_defocus_um(-0.5)  # tcBF: a few hundred nm to ~2 um of (under)focus
col.set_camera_length_mm(40.0)  # bright-field disk ~1/3 of the detector
col.set_spot_size(8)  # low current: no saturation, smaller source
req = twin.request(
    hw_roi=Roi(384, 384, 256, 256),
    hw_binning=(2, 2),
    frame_time_s=1e-4,
    total_frames=64 * 64,
    scan=ScanRequest(enabled=True, size=(64, 64), step_um=0.001, dwell_s=1e-4),
)

# raw frames exactly as the camera would deliver them (dark/gain-correct with twin.processor)
refs = twin.processor.references(req)
frames = [twin.processor.correct(raw, refs) for raw, meta in twin.frames(req)]
# ...or the noiseless data cube, (scan y, x, 128, 128) electrons / binned pixel / s
cube = twin.datacube(req)
gt = twin.ground_truth_ptychography(req)  # object t(r), probe (modes), positions, aberrations

For ptychography, use a focused (or slightly defocused) probe with a scan step well below the probe size so neighbouring probe positions overlap. The same data are served through the deapi and shared-memory faces.

Model#

  • Aberrations come from the column: C1 is the defocus (negative = underfocus), A1 the condenser stigmator, C3 1.2 mm uncorrected or whatever the probe corrector leaves; beam tilt adds coma.

  • Partial coherence: focal spread and the effective source size are treated as probe modes.

  • Sampling: detector pixels (after ROI and binning) integrate an oversampled k-grid, so a probe defocused by 1 µm fits the real-space window.

  • Model choice: RenderConfig.stem_model is "auto" by default: coherent when the binned pattern is at most 256² (or the scan oversamples the probe), else a fast kinematic disk model. Set "coherent" or "kinematic" to force one.

  • Speed: on a 24-core workstation a 64×64 scan with 128² patterns at 20 mrad takes about 4 s; a tcBF scan at 1 µm defocus about 1 min with 4 probe modes (coherent_max_modes=1 is faster).

The test suite reconstructs this data (tcBF defocus within 10 %, ptychographic phase correlation above 0.85 against the ground truth), which is how the model is checked.

On this page
Show Source