arXiv subjects

Dennis van de Lockand

Publications and source records attributed to Dennis van de Lockand.

2 recordsLinked to original sources

arXiv2026

Bidirectional phase sensitivity in holographic phototransient microscopy

Mid-infrared photothermal microscopy combines the chemical specificity of infrared absorption with the spatial resolution of visible-light detection, but practical implementations face a persistent trade-off between forward-scattering (FWS) and backward-scattering (BWS) detection geometries. FWS provides quantitative, shape-independent phase contrast but requires two-sided optical access that is difficult to achieve in aqueous or thick samples. BWS offers convenient single-sided access, but its signals are strongly distorted by depth-dependent interference for micron-scale objects. Here we present a bidirectional femtosecond mid-infrared pump-probe holographic microscope capable of switching between FWS and BWS geometries within a single instrument, and use it to introduce and validate a new imaging modality, internal forward scattering (IFS). IFS exploits the back-reflection generated at the top surface of the mid-infrared-transparent sample substrate as an internally generated forward-scattering illumination wave, isolated from the directly backscattered field via temporal coherence gating. Using polystyrene beads on CaF2 substrates in air, water, and a refractive-index-matched glycerol-water mixture, we show that IFS reproduces the signal magnitudes and temporal dynamics of true FWS measurements while retaining the mechanical simplicity and single-sided accessibility of BWS. These results establish IFS as a practical, quantitative alternative to conventional FWS and BWS geometries for photothermal, and more broadly quantitative phase, imaging, with direct relevance to single-sided imaging of biological or solvent-contained specimens.

physics.optics↗

arXiv2026

Time-resolved infrared photothermal imaging: From transient observations towards the steady-state

Mid-infrared photothermal microscopy is a highly promising imaging technique that enables spatially resolved vibrational fingerprinting. The combination of infrared induced heating with optical readout at visible wavelengths provides excellent spatial resolution while retaining the spectral observations of conventional infrared imaging. Most current implementations rely on long-duration illumination periods, to ensure sufficient heating and hence large signals. However, undesirable processes such as heat-diffusion degrade spatial resolution and the interplay between heat-induced refractive index changes and sample expansion adds additional uncertainties. Fundamentally, these issues stem from the difficulties associated with separating non-equilibrium and photoacoustic contributions from purely thermal signals. Highly time-resolved observations hold great promise for addressing these issues and are imperative for enabling future imaging modalities in this exciting temporal window. Here, we provide this much needed insight by employing widefield phototransient holography to phase-resolve optical responses from pico- to tens of nanoseconds following vibrational overtone excitation. We observe rapid transient-induced phase shifts, followed by heat-induced coherent expansion and thermalisation dynamics. Our observations provide a direct link between the photoacoustic and photothermal realms, thus not only offering much-needed insight for rationally optimising these exciting technologies but also avenues towards future all-optical stiffness and super-resolution imaging modalities.

physics.optics↗