Operando Optical Microscopy

• Operando optical microscopy is a technique that captures spatially resolved images of materials and interfaces under external stimuli, directly correlating structural, chemical, and functional changes.
• It integrates advanced modalities—including linear, nonlinear, and multimodal approaches—to achieve sub-diffraction imaging and nanoscale localization in dynamic environments.
• The method provides actionable insights into phase transitions, redox kinetics, and defect formation in systems such as batteries, catalysis, and soft matter for device optimization.

Operando optical microscopy is the real-time application of advanced optical imaging modalities to quantitatively interrogate materials, interfaces, and devices under working (operational) conditions. Unlike ex situ or in situ methods, operando approaches acquire spatially resolved data while the system is being driven by external stimuli (electric field, temperature, chemical gradients, etc.), thus permitting direct correlation of structural, chemical, and functional evolution with device performance. The technique has matured substantially, encompassing linear, nonlinear, and multimodal optical strategies that span length scales from single molecules to macroscopic ensembles. It is pivotal in soft matter physics, electrochemical energy storage, catalysis, electronic devices, and biological systems.

1. Fundamental Principles and Resolution Limits

Operando optical microscopy exploits the interplay between the sample's intrinsic optical properties and imaging system performance, governed by diffraction and lens parameters. The numerical aperture (NA) defines the resolving power:

$\mathrm{NA} = n \sin \theta$

where $n$ is the medium’s refractive index, and $\theta$ is the half-angle of illumination. The system’s point spread function (PSF), uniquely shapes image resolution and is expressed as:

$$\mathrm{PSF}(r) = \left[ \frac{2 J_1(ra)}{ra} \right]^2$$

with $J_1$ a Bessel function and $a = \frac{2\pi \mathrm{NA}}{\lambda}$, $\lambda$ being the wavelength. Lateral resolution follows:

$$r_\mathrm{lateral} \approx 0.442\,\frac{\lambda}{\mathrm{NA}}$$

These optical constraints, together with advances in objective lens design and engineered PSFs (via spatial light modulators), enable sub-diffraction imaging and precise nanoscale localization, underpinning all operando modalities (Lee et al., 2011).

2. Advanced Modalities: Linear, Nonlinear, and Polarization Techniques

Linear fluorescence confocal microscopy (FCM) utilizes focused laser excitation and pinhole filtering for three-dimensional sectioning. Its axial resolution:

$A_z \approx 1.55\,\frac{n^2}{\mathrm{NA}^2}$

permits millisecond-scale volumetric imaging of dynamic phenomena, including colloidal aggregation and liquid crystal director reconfigurations in real time. Fluorescence confocal polarizing microscopy (FCPM) modulates excitation polarization to probe molecular orientation anisotropy:

$I \propto \cos^4 \beta$

where $\beta$ is the angle between excitation polarization and chromophore dipole. These methods reconstruct complex director fields (e.g., torons in cholesteric LCs) during manipulations such as laser trapping.

Nonlinear modalities—multi-photon excitation, multi-harmonic generation (SHG, THG), and coherent anti-Stokes Raman scattering (CARS)—enable intrinsic 3D sectioning, minimal photodamage, and label-free imaging. CARS intensity scales by third-order susceptibility and beam intensities:

$I_\mathrm{CARS} \propto |\chi^{(3)}|^2 I_p^2 I_s$

where $\chi^{(3)}$ is the sample’s third-order nonlinear response, $I_p$ and $I_s$ the pump and Stokes intensities. Polarized CARS further resolves orientational order in liquid crystal and soft matter phases. Stimulated Raman scattering (SRS) selectively amplifies chemical signals and achieves high specificity for functional imaging (e.g., solvent penetration in lipid matrices under active cycling) (Lee et al., 2011).

3. Operando Electrochemical Imaging

Operando optical microscopy for electrochemical systems harnesses wide-field, plasmonic, scattering, and luminescence modalities to visualize local redox and ionic transport (Lemineur et al., 2021, Kanoufi, 2021). Plasmonic-based imaging (SPR, LSPR) detects changes in electron density and local refractive index during charge transfer; impedance maps are extracted from plasmonic intensity modulations:

$$Z^{-1}(x, y, \omega) = j\omega a \,\frac{A_0(x, y, \omega)}{A_V}$$

Interferometric scattering (iSCAT) records changes in dielectric function and lattice structure during ion insertion/extraction, offering sub-5 nm resolution of phase boundaries and order–disorder transitions in battery electrodes (Merryweather et al., 2020). Optical fiber sensors and photonic crystal structures extend operando capabilities, correlating spectral shifts with mechanical and chemical changes. The Bragg-Snell relation quantifies structural evolution using periodicity ($d_{hkl}$) and the effective refractive index ($n_\mathrm{eff}$), which is sensitive to cycling-induced changes:

$$m\lambda = 2d_{hkl}\sqrt{n_\mathrm{eff}^2 - n_\mathrm{sol}^2\sin^2\theta}$$

Fiber Bragg grating sensors track strain and stress via:

$\lambda_B = 2n_\mathrm{eff}A$

(Grant et al., 2022, Lonergan et al., 2023).

4. Real-Time Tracking of Phase Transitions, Defects, and Heterogeneities

Operando techniques resolve phase front propagation, ordering transitions, kinetic phase separation, and cracking at single-particle level, directly linking electrochemical cycling to mechanical and compositional evolution (Merryweather et al., 2021, 2207.13073). For fast lithium-ion anodes, interferometric imaging charts front velocities ($600-760$ nm/s) and crack propagation ($\sim187$ nm/s), elucidating strain-induced degradation origins. Volume changes and lithium gradients in cathode materials are mapped in 3D, with front velocities (2–6 nm/s) and inter-particle heterogeneities tied to device instability. Multimodal imaging in perovskite solar cells couples hyperspectral photoluminescence with nanoscale XRF and voltage-dependent mapping to uniquely attribute stability to low initial spatial disorder (Frohna et al., 25 Mar 2024).

5. Method Integration: Manipulation, Spectrochemical Analysis, and High-Throughput Imaging

Optical tweezers, spatial light modulator–engineered PSFs, and fast scanning architectures (e.g. Nipkow disk, piezo z-stepping) are integrated for simultaneous manipulation and visualization of soft matter components in operando. Combined microscopy–spectroscopy platforms (e.g., near ambient pressure scanning photoelectron microscopy, NAP-SPM) deliver correlated chemical state, elemental mapping, and dynamic morphology under practical pressures and temperatures (Amati et al., 2021). The use of both spatial and spectroscopic data enables high-throughput device screening and mechanistic elucidation—whether assessing battery electrode aging, solar cell recombination losses, or sensing performance at active interfaces.

6. Challenges, Limitations, and Future Directions

Operando optical methods face limitations in non-transparent systems, rapid photobleaching, refractive index heterogeneity, and computational throughput demands. Aberration corrections and calibration for refractive index mismatch are essential for quantitative analysis. Advanced machine learning and deep learning approaches are recommended for data postprocessing and object recognition in large datasets (Lemineur et al., 2021, Kanoufi, 2021). Multimodal integration with complementary techniques (TEM, SECCM), development of novel optically transparent electrodes, and extension to quantum microscopy (via geometric phase differentiators such as RVB and PB phase elements (Shou et al., 2023)) constitute active research frontiers.

7. Impact on Scientific Understanding and Device Optimization

Operando optical microscopy is transformative in elucidating dynamic processes—aggregation, self-assembly, redox kinetics, phase transition, interface dynamics, and defect formation—across material classes. It enables direct mapping of chemical, structural, and electrical performance under operational loads. The technique is widely adopted in soft matter physics (Lee et al., 2011), electrochemical energy storage and conversion (Lemineur et al., 2021, Merryweather et al., 2020, Merryweather et al., 2021, Grant et al., 2022, Lonergan et al., 2023, 2207.13073), real-world device engineering (sensors, memristors, solar cells (Frohna et al., 25 Mar 2024, Fernández et al., 2023)), and emerging quantum and hybrid imaging platforms.

Operando optical microscopy’s convergence of spatial resolution, temporal fidelity, chemical specificity, and manipulative capability continues to expand the reach of experimental science, yielding new mechanistic insights and catalyzing the rational design of advanced materials and devices.