- The paper demonstrates that internal forward scattering (IFS) recovers FWS-like quantitative phase sensitivity under BWS conditions via temporal coherence gating.
- It employs an ultrafast off-axis holographic setup with rapid geometry switching to capture time-resolved phase dynamics in diverse experimental environments.
- Experimental results confirm that IFS yields consistent, interpretable phase images even in complex, solvent-immersed settings, overcoming traditional BWS limitations.
Bidirectional Phase Sensitivity in Holographic Phototransient Microscopy: IFS as an Experimental and Quantitative Advance
Introduction and Motivation
Mid-infrared photothermal (MIP) microscopy enables chemically selective, label-free imaging with submicron spatial resolution, translating vibrational absorption in the MIR into quantifiable refractive index or volumetric changes that are read out with visible probe beams. Despite conceptual advantages—most notably, compatibility with aqueous and even thick biological environments—the experimental literature remains limited by technical constraints, particularly the required integration of MIR excitation and visible-light detection. The geometric configuration of excitation and detection is critical: forward-scattering (FWS) geometries provide quantitative, shape-independent phase contrast for larger or heterogeneous samples but demand two-sided optical access, which is impractical in many applications. Backward-scattering (BWS) offers single-sided access, facilitating compatibility with thick or in vivo specimens, but the acquired signal is subject to significant spatial distortions and interpretability issues, especially in the Mie scattering regime due to depth-dependent destructive interference.
The work introduces a bidirectional ultrafast holographic pump-probe microscope capable of temporally and spatially switching between FWS and BWS geometries. Building on this, the paper establishes a new imaging modality termed internal forward scattering (IFS), which leverages substrate-mediated internal reflection to generate an FWS-like signal under BWS illumination conditions. IFS combines the quantitative interpretability of FWS with the practical, single-sided accessibility of BWS, and the study systematically evaluates the phase sensitivity and imaging properties across these configurations.
Experimental Implementation
The microscope architecture is an off-axis holographic system employing 515 nm femtosecond visible probe light and spectrally tunable MIR pump pulses generated by a multi-stage noncollinear OPA system. The core innovation lies in three instrumental features:
- Rapid geometry switching: A flip mount enables seamless transition between FWS and BWS/IFS configurations without disturbing MIR pump alignment.
- Temporal coherence gating: Adjustable time delays in the reference and probe enable selective access to a specific scattered component (FWS, BWS, or IFS) via temporal coherence control.
- High-frequency acquisition: Holographic frames are obtained at 1 kHz, with pump modulation at 500 Hz for rapid-on/off differential phase imaging.
Signal retrieval involves Fourier filtering of the off-axis hologram and extraction of differential phase by computing the complex ratio between pump-on and pump-off fields. This pipeline delivers time-resolved, quantitative phase contrasts for all three geometries.
Comparative Evaluation: FWS, BWS, and IFS
Systematic measurements were conducted on 5 µm polystyrene beads immobilized on CaF₂ substrates under air, water, and refractive-index-matched environments. The principal findings are:
- FWS: Delivers quantitatively interpretable, shape-accurate phase images. Time-resolved measurements reveal oscillatory phase dynamics consistent with vibrationally-specific energy deposition and subsequent thermoelastic expansion.
- BWS: Yields higher absolute signal amplitudes, but phase images show severe spatial distortions and loss of geometric fidelity, corroborating theoretical predictions of detrimental Mie scattering and interference effects. BWS fails to provide equivalent quantitative information for micron-scale and heterogeneous objects.
- IFS: Achieves FWS-equivalent phase signals and temporal profiles, as evidenced by nearly identical amplitude and dynamics in differential phase images, but crucially retains single-sided access, substantially simplifying experimental design.
In index-matched (quasi-3D) settings, IFS successfully recovers both localized bead and extended solvent background signals, whereas BWS captures only highly spatially modulated bead responses. This demonstrates that IFS maintains phase sensitivity in optically complex or interface-free scenarios, whereas BWS loses quantitative interpretability.
Technical and Practical Implications
The analysis confirms that IFS reconstructs the advantages of FWS—quantitative, shape-independent phase contrast—within a BWS-accessible geometry. The crucial technical element is coherence gating, which discriminates the internally reflected probe (acting as an effective forward-illumination wave) from directly backscattered fields. The configuration is robust against substrate and solvent background variations and can be implemented with standard diode lasers due to the coherent gate lengths matching or exceeding relevant optical path differences.
Notable strong claims demonstrated by numerical results and experimental data:
- IFS phase and temporal signal fidelity match those from canonical FWS for all geometries and sample environments tested.
- BWS, despite higher signal amplitude, is fundamentally less suitable for quantitative phase imaging of micrometer-sized or heterogeneous samples due to the breakdown of phase preservation.
- In solvent-immersed or refractive-index-matched cases, IFS exposes both sample and environment-induced phase contributions, enabling quantitative separation and analysis.
Broader Impact and Future Directions
These findings establish IFS as a highly versatile modality for MIR photothermal and quantitative holographic imaging, principally in scenarios constrained by optical access (e.g., microfluidic chambers, single-use samples, and biological tissues). IFS is a clear candidate for high-resolution imaging in live cell and tissue studies, where single-sided access, mechanical simplicity, and environmental control are required.
Potential future developments include:
- Integration of high-NA objectives for improved spatial resolution
- Application to dynamically varying, live biological systems with complex refractive index landscapes
- Extension of coherence gating and IFS principles to other multi-modal or phase-sensitive pump-probe platforms
The BWS geometry remains preferable for ultrasmall (Rayleigh regime) nanoscopy targets, where signal enhancement and attenuation properties are advantageous. For all other applications requiring quantitative, interpretable phase information, IFS provides a practical and effective solution that can be readily adopted using commercially available components.
Conclusion
This work demonstrates that internal forward scattering (IFS), enabled by temporal coherence gating in a bidirectional holographic MIP microscope, offers a robust, quantitative imaging modality that combines the interpretability of FWS with the accessibility of BWS. IFS addresses longstanding geometric and technical limitations in MIR photothermal microscopy, facilitating its application to a wider range of scientific and biomedical imaging problems. The validation of IFS sets the stage for its widespread adoption in label-free chemical imaging and expands the experimental toolkit for researchers requiring quantitative phase information under challenging sample and optical environments.