Operando Laser-PEEM Microscopy

• Operando laser-PEEM is a photoemission electron microscopy method that leverages laser excitation and in situ stimuli to spatially resolve electronic and structural properties.
• It integrates multiple imaging modes, including single-photon UV and multiphoton infrared regimes, to achieve high spatial, spectral, and momentum resolution.
• This technique enables direct correlation of electrical, optical, and environmental inputs with local electronic structure for advanced operando measurements.

Searching arXiv for papers on operando laser-PEEM, micro-ARPES-PEEM, and darkfield UV PEEM to ground the article in current literature. Operando laser-based photoemission electron microscopy (laser-PEEM) is a photoemission electron microscopy modality in which ultraviolet or infrared laser excitation is combined with PEEM imaging and spectromicroscopy while a sample or device is subjected to in situ stimuli such as electrical biasing, field cycling, heating, gas dosing, or optical pumping. In the cited literature, the term covers several experimentally distinct but conceptually related regimes: single-photon ultraviolet PEEM on buried ferroelectric capacitors with synchronized electrical characterization (Fujiwara et al., 11 Aug 2025), in-situ visualization of soft and hard dielectric breakdown in HfO$_2$-based devices (Fujiwara et al., 2023), femtosecond nonlinear PEEM at $\lambda = 1.55~\mu\text{m}$ for localized surface plasmon mapping (Mårsell et al., 2015), darkfield threshold UV-PEEM for magnetic dichroism imaging (Paleschke et al., 2024), and an angle- and energy-resolved micro-ARPES mode implemented in a LEEM/PEEM platform to improve the efficiency required for laser-based pump-probe photoemission microscopy with femtosecond time resolution (Neuhaus et al., 2023). Across these implementations, the unifying principle is the conversion of local electronic structure, internal fields, or near-field enhancement into spatially resolved photoelectron intensity, spectral, or momentum contrast under operating conditions.

1. Definition and operating principle

Laser-PEEM uses laser-driven photoemission as the signal-generation step and PEEM electron optics as the imaging system. In the ultraviolet single-photon regime, the relevant energy relation is

$E_{\mathrm{kin}} = h\nu - \Phi - E_{\mathrm{B}},$

where $E_{\mathrm{kin}}$ is the photoelectron kinetic energy at the analyzer entrance, $h\nu$ the photon energy, $\Phi$ the local work function, and $E_B$ the binding energy, with $E_B = 0$ at the Fermi level (Neuhaus et al., 2023). In operando ferroelectric-device work, continuous-wave lasers at 266 nm ($h\nu = 4.66$ eV) and 213 nm ($h\nu = 5.82$ eV) were used with photon energies comparable to typical electrode work functions, so imaging proceeded in a single-photon photoemission regime (Fujiwara et al., 11 Aug 2025). In threshold magnetic imaging, ultraviolet photons excited electrons from valence states to just above the work function, making the measurement highly sensitive to angularly resolved matrix-element effects and near-threshold asymmetries (Paleschke et al., 2024).

In the infrared strong-nonlinearity regime, the operative condition is instead the multiphoton threshold $\lambda = 1.55~\mu\text{m}$0. For Ag at 1.55 $\lambda = 1.55~\mu\text{m}$1m, with $\lambda = 1.55~\mu\text{m}$2 eV and $\lambda = 1.55~\mu\text{m}$3–4.7 eV, at least $\lambda = 1.55~\mu\text{m}$4 photons are required (Mårsell et al., 2015). In that limit, the yield scales as $\lambda = 1.55~\mu\text{m}$5, and with local field enhancement $\lambda = 1.55~\mu\text{m}$6, $\lambda = 1.55~\mu\text{m}$7, so local optical near-fields are converted into high-contrast electron-emission maps (Mårsell et al., 2015). This nonlinearity is central to plasmonic laser-PEEM, where only a subset of structures with sufficiently enhanced fields emit detectably.

The qualifier operando denotes simultaneous or tightly synchronized coupling of imaging to device-relevant stimuli. In the ferroelectric studies, PEEM was combined with AC/DC electrical characterization, including a Sawyer–Tower circuit for $\lambda = 1.55~\mu\text{m}$8–$\lambda = 1.55~\mu\text{m}$9 extraction, periodic endurance stressing, and local spectromicroscopy on intact capacitor stacks (Fujiwara et al., 11 Aug 2025). In the earlier breakdown study, electrical cycling and imaging were performed in the same instrument without sample transfer, so breakdown precursors and final filaments could be correlated directly with leakage-current evolution and $E_{\mathrm{kin}} = h\nu - \Phi - E_{\mathrm{B}},$0–$E_{\mathrm{kin}} = h\nu - \Phi - E_{\mathrm{B}},$1 behavior (Fujiwara et al., 2023). In ultrafast and magnetic contexts, operando relevance is tied to optical pumping, femtosecond probe compatibility, and momentum or dichroic selectivity under laboratory conditions (Neuhaus et al., 2023, Paleschke et al., 2024).

2. Electron optics, analyzers, and instrument configurations

The instrument architectures described in the literature are PEEM or SPELEEM systems with electrostatic imaging optics, optional energy filtering, and analyzer planes that can be reconfigured for real-space, momentum-space, or dispersive imaging. In a spectroscopic LEEM/PEEM, the electron-optical column can be tuned to image the real-space image plane, the momentum plane, or the hemispherical analyzer’s dispersive plane (Neuhaus et al., 2023). The objective lens accelerates and forms the primary image, the beam splitter passes emitted electrons into the spectroscopy path, a lens quadruplet positions the intermediate image plane and conjugate momentum plane, and the retarding lens plus inner lenses set the entrance conditions for the hemispherical imaging analyzer (Neuhaus et al., 2023). Projective lenses then image the analyzer exit onto a detector.

A standard laboratory PEEM can also be operated in threshold ultraviolet photoemission, with brightfield or darkfield imaging depending on contrast-aperture placement. In brightfield threshold PEEM, the image is formed by integrating over all emission angles within the objective’s acceptance. Darkfield PEEM translates a circular contrast aperture in the back focal plane off axis, selecting a specific in-plane momentum $E_{\mathrm{kin}} = h\nu - \Phi - E_{\mathrm{B}},$2 region before the real-space image is formed (Paleschke et al., 2024). This momentum selectivity is the basis for enhanced magnetic circular dichroism imaging in Fe(001), because it avoids cancellation between positive and negative contributions in $E_{\mathrm{kin}} = h\nu - \Phi - E_{\mathrm{B}},$3-space.

Commercial PEEM implementations in ultrafast plasmonics employed a Focus GmbH PEEM in ultra-high vacuum, with photoelectrons accelerated by 10–15 kV extraction voltage and imaged via electrostatic lenses. A high-pass imaging energy filter allowed energy-resolved PEEM, and detection used a double microchannel plate and CCD (Mårsell et al., 2015). In ferroelectric-device operando work, an aberration-corrected SPELEEM equipped with an energy analyzer was operated with a high voltage of $E_{\mathrm{kin}} = h\nu - \Phi - E_{\mathrm{B}},$4 kV between the sample and the objective lens during PEEM imaging (Fujiwara et al., 11 Aug 2025).

A particularly important extension for laser-based operando work is the angle-resolved micro-ARPES mode added to a LEEM/PEEM by installing a 5 $E_{\mathrm{kin}} = h\nu - \Phi - E_{\mathrm{B}},$5m-wide slit between the retarding lens and lens L1 near the analyzer entrance (Neuhaus et al., 2023). By decreasing the focal length of P1, moving the momentum plane into this slit, aligning with a deflector before the retarding lens, and relaying the slit into the analyzer with L1/L2, the system records a two-dimensional $E_{\mathrm{kin}} = h\nu - \Phi - E_{\mathrm{B}},$6 map in a single exposure (Neuhaus et al., 2023). This retains PEEM/LEEM spatial selectivity while making data acquisition in energy–momentum space much more efficient.

3. Imaging modes, contrast channels, and calibration

Laser-PEEM derives contrast from several physically distinct channels, and the cited work consistently emphasizes that interpretation requires explicit discrimination among them. In ferroelectric devices, contrast arises from variations in local work function $E_{\mathrm{kin}} = h\nu - \Phi - E_{\mathrm{B}},$7 and low-energy cutoff in the energy distribution curves, near-surface potential and band bending, internal electric fields due to ferroelectric polarization and defect distributions, and the local density of electronic states near $E_{\mathrm{kin}} = h\nu - \Phi - E_{\mathrm{B}},$8 (Fujiwara et al., 11 Aug 2025). In the breakdown-filament study, soft dielectric breakdown appeared as increased PEEM intensity without spectral-shape modulation, interpreted as an increased density of pre-existing defect states in Hf$E_{\mathrm{kin}} = h\nu - \Phi - E_{\mathrm{B}},$9Zr$E_{\mathrm{kin}}$0O$E_{\mathrm{kin}}$1, whereas hard dielectric breakdown produced an additional high-energy shoulder in the local EDC and decreased near-cutoff intensity, indicating new electronic states and local band-structure modification associated with a permanent conduction path (Fujiwara et al., 2023).

For polarization imaging through an oxide semiconductor top electrode, the dominant mechanism was not a work-function shift but modulation of the inelastic mean free path $E_{\mathrm{kin}}$2 by the polarization-induced internal electric field in the HZO layer (Fujiwara et al., 11 Aug 2025). The signal fraction from depths deeper than $E_{\mathrm{kin}}$3 follows

$E_{\mathrm{kin}}$4

and a small change in $E_{\mathrm{kin}}$5 was argued to account for the observed $E_{\mathrm{kin}}$6 differential contrast between $E_{\mathrm{kin}}$7 and $E_{\mathrm{kin}}$8 states through InZnO$E_{\mathrm{kin}}$9 (Fujiwara et al., 11 Aug 2025). This is significant because it shows that operando laser-PEEM can probe buried-state modulation through intact electrodes without invoking destructive sample preparation.

In threshold magnetic imaging, the key contrast quantity is the exchange-driven magnetic asymmetry $h\nu$0, defined from four intensities $h\nu$1 that depend on circular polarization and magnetization orientation. The darkfield method enhances real-space contrast by placing the aperture in $h\nu$2 regions where $h\nu$3 is large, rather than integrating over the full acceptance as in brightfield imaging (Paleschke et al., 2024). For Fe(001), in-plane magnetization produces MCD patterns with odd symmetry in $h\nu$4 and mirror symmetry in $h\nu$5, so brightfield cancellation suppresses net contrast, whereas darkfield momentum selection yields domain contrast of 3–4% and momentum-resolved $h\nu$6 up to 6% in experiment (Paleschke et al., 2024).

Momentum-resolved operando laser-PEEM requires additional calibration because projective distortions warp both energy and momentum axes. In the LEEM/PEEM micro-ARPES implementation, an image series was recorded while scanning the sample potential STV relative to the analyzer, and dense optical flow was used to extract displacement vectors $h\nu$7 between consecutive image pairs (Neuhaus et al., 2023). The distortion field was fitted to a radial/spiral parameterization,

$h\nu$8

with $h\nu$9 and $\Phi$0 (Neuhaus et al., 2023). After inverse distortion correction, detector coordinates were mapped to energy and momentum via empirical scale factors anchored by the Fermi edge and the free-electron cutoff parabola (Neuhaus et al., 2023). This calibration step is not ancillary; it determines whether the measured $\Phi$1 image can be interpreted quantitatively as band structure rather than as a distorted projection.

4. Operando integration with electrical, optical, and environmental stimuli

The most explicit operando integration is the coupling of laser-PEEM to electrical metrology in ferroelectric devices. In the ADECS implementation, the device under test was series-connected with a standard capacitor $\Phi$2, a function generator applied a drive voltage $\Phi$3, and an oscilloscope recorded the voltage across $\Phi$4, $\Phi$5. The ideal Sawyer–Tower relations are

$\Phi$6

where $\Phi$7 is the electrode area (Fujiwara et al., 11 Aug 2025). Real-system parasitic capacitances were explicitly quantified, with $\Phi$8 nF and $\Phi$9 nF, and the underestimation factor was given by $E_B$0 (Fujiwara et al., 11 Aug 2025). The system supported spectroscopic mode using STV, DC-based measurement via a source measure unit, and AC-based measurement using the Sawyer–Tower circuit, allowing direct correlation of image contrast, EDCs, hysteresis loops, endurance, leakage, and breakdown evolution in the same platform (Fujiwara et al., 11 Aug 2025).

Electrical operando measurements also impose timing and artifact constraints. In the ferroelectric study, 1 kHz was selected because 500 Hz and 1 kHz loops overlapped well, while 10 kHz induced phase shifts and loop distortions due to CR delay in $E_B$1 (Fujiwara et al., 11 Aug 2025). After hard breakdown, the DUT behaved like a resistor in series with $E_B$2, so RC charging produced a spurious overestimation of polarization in PUND loops; the relevant timescale was $E_B$3 (Fujiwara et al., 11 Aug 2025). In the earlier HZO breakdown study, cycling stress was applied in situ without laser irradiation, and PEEM images and EDCs were acquired before stress and after selected cycle counts, which permitted realistic electrical stressing without simultaneous optical perturbation (Fujiwara et al., 2023).

Optical operando integration is equally central in ultrafast and magnetic contexts. The angle-resolved LEEM/PE