arXiv subjects

Christoph Krafft

Publications and source records attributed to Christoph Krafft.

7 recordsLinked to original sources

arXiv2026

Millimeter-scale wide-field mid-infrared photothermal imaging enabled by a broadly tunable picosecond optical parametric oscillator

Wide-field mid-infrared photothermal (MIP) imaging enables chemically specific microscopy with submicron spatial resolution but remains fundamentally limited by the trade-off between field of view, mid-infrared pulse energy, and spectral tunability. As a result, current wide-field implementations are typically restricted to fields of view below 200 {\mu}m and to either the fingerprint or high-wavenumber spectral regions. Here, we overcome these limitations by developing a wide-field fluorescence-detected mid-infrared photothermal (F-MIP) imaging platform driven by a commercial picosecond optical parametric oscillator (OPO). The system provides pulse energies of up to 360 {\mu}J together with a broad tuning range from 625 to 4327 cm-1, enabling millimeter-scale wide-field imaging in the high-wavenumber regions. We demonstrate a field of view of approximately 1 mm in diameter for fluorescently labeled polystyrene beads while preserving spectral fidelity. Furthermore, the platform enables, to our knowledge, the first wide-field MIP imaging below 900 cm-1. To demonstrate its applicability to biomedical imaging, we performed large-area mosaic imaging of fluorescent tuberculosis-infected tissue sections, providing chemically resolved maps over millimeter-sized sample areas. These results establish broadly tunable OPO-driven F-MIP as a scalable platform for high-throughput vibrational imaging of large biological specimens and advanced materials.

physics.optics↗

arXiv2026

Free-electron laser-based extended wide-field mid-infrared photothermal imaging for biomedical and microplastic analysis

Wide-field mid-infrared photothermal (MIP) imaging offers rapid labelfree chemical contrast for biomedical and polymer analysis. However, its field of view (FOV) is limited by the pulse intensity of conventional infrared lasers. Here, we present a wide-field MIP microscope that uses a high-power free-electron laser (FEL) rather than a quantum cascade laser (QCL) as the pump source to achieve a substantially larger FOV. Both implementations use counter-propagating beam paths with a 450 nm LED as the probe source and a CMOS camera that records images using a virtual lock-in detection scheme. QCL nanojoule pulse energies enables FOV of around 45 micrometers for widefield MIP imaging with a sub-micrometer resolution for polystyrene beads, Mycobacterium tuberculosis infected fixed tissues, and laryngeal cancer cryosections. IR spectra in the range of 1000-1800 wavenumbers can be reconstructed by tuning the QCL. FEL pulse energies of up to microjoules expand the FOV by a factor of nearly 20 as demonstrated by wide-field MIP imaging of polystyrene beads, single cells, and murine brain tissue. We discuss current challenges and further improvements to implement high-power IR lasers for wide-field MIP imaging with even larger FOVs in the context of biomedical research and diagnostics.

physics.optics↗

arXiv2026

Denoising and Baseline Correction of Low-Scan FTIR Spectra: A Benchmark of Deep Learning Models Against Traditional Signal Processing

High-quality Fourier Transform Infrared (FTIR) imaging usually needs extensive signal averaging to reduce noise and drift which severely limits clinical speed. Deep learning can accelerate imaging by reconstructing spectra from rapid, single-scan inputs. However, separating noise and baseline drift simultaneously without ground truth is an ill-posed inverse problem. Standard black-box architectures often rely on statistical approximations that introduce spectral hallucinations or fail to generalize to unstable atmospheric conditions. To solve these issues we propose a physics-informed cascade Unet that separates denoising and baseline correction tasks using a new, deterministic Physics Bridge. This architecture forces the network to separate random noise from chemical signals using an embedded SNIP layer to enforce spectroscopic constraints instead of learning statistical approximations. We benchmarked this approach against a standard single Unet and a traditional Savitzky-Golay/SNIP workflow. We used a dataset of human hypopharyngeal carcinoma cells (FaDu). The cascade model outperformed all other methods, achieving a 51.3% reduction in RMSE compared to raw single-scan inputs, surpassing both the single Unet (40.2%) and the traditional workflow (33.7%). Peak-aware metrics show that the cascade architecture eliminates spectral hallucinations found in standard deep learning. It also preserves peak intensity with much higher fidelity than traditional smoothing. These results show that the cascade Unet is a robust solution for diagnostic-grade FTIR imaging. It enables imaging speeds 32 times faster than current methods.

eess.IV↗

arXiv2025

Effect of hybrid field coupling in nanostructured surfaces on anisotropic signal detection in nanoscale infrared spectroscopic imaging methods

Anisotropic intensity distributions on nanostructured surfaces and polarization-sensitive spectra have been observed in a number of nanoscale infrared spectroscopic imaging methods, including nano-FTIR [Bakir et al., Molecules, 2020, 25, 4295], photothermal induced resonance (PTIR) [Waeytens et al., Analyst, 2021, 146], tapping AFM-IR [Hondl et al., ACS Meas. Sci. Au, 2025, 5, 469; Luo et al., APL, 2022, 121, 23330], infrared photoinduced force microscopy (PiF-IR) [Anindo et al., JPCC, 2025, 129, 4517; Shcherbakov et al., Rev Methods Primers, 2025, 5, 1; Ali et al., Anal. Chem., 2025, 97, 23914] and peak force infrared microscopy (PFIR) [Xie et al., JPCC, 2022, 126, 8393; Anindo, JPCC, 2025]. A recent work combining modeling and experiment demonstrated that the hybrid field coupling of the IR illumination E0 with a polymer nanosphere and a metallic AFM probe is nearly as strong as the plasmonic coupling in case of a gold nanosphere [Anindo, JPCC, 2025]. For p-polarized illumination, this results in enhanced IR absorption on the surface perpendicular to the propagation of E0 which can explain the observed anisotropic intensity distribution. An additional anisotropy may be introduced by aligned surface molecules with oriented vibrational transition moments [Bakir et al., Molecules, 2020, 25, 4295; Luo, APL, 2022]. PiF-IR is strongly surface sensitive combining an unprecedented spatial resolution < 5 nm with high spectral resolution [Shcherbakov, Rev Methods Primers, 2025; Ali, Anal. Chem., 2025], which allows, for example, to visualize nanoscale chemical variation on the surface of bacteria cells affected by antimicrobial interaction [Ali, Anal. Chem., 2025]. We compare PiF-IR hyperspectra of aligned perylene Langmuir Blodgett monolayers on nanostructured and planar gold substrates and use quantum chemical calculations of the oriented vibrational oscillators to interpret the observations.

cond-mat.mes-hall↗

arXiv2025

Nano-chemical cell-surface evaluation in photothermal spectroscopic imaging of antimicrobial interaction in model system Bacillus subtilis & vancomycin

The power of photothermal spectroscopic imaging to visualize antimicrobial interaction on the surface of individual bacteria cells has been demonstrated on the model system Bacillus subtilis and vancomycin using mid-infrared photo-induced force microscopy (PiF-IR, also mid-IR PiFM). High-resolution PiF contrasts obtained by merging subsequent PiF-IR scans at two different illumination frequencies revealed chemical details of cell wall destruction after 30 and 60 min incubation with vancomycin with a spatial resolution of $\approx 5$ nm. This approach compensates local intensity variations induced by near-field coupling of the illuminating electric field with nanostructured surfaces, which appear in single-frequency contrasts in photothermal imaging methods, as shown by [Anindo et al., J. Phys. Chem C, 2025, 129, 4517]. Known spectral shifts associated with hydrogen bond formation between vancomycin and the N-acyl-D-Ala4-D-Ala5 termini in the peptidoglycan cell wall have been observed in chemometrics of PiF-IR spectra from treated and untreated Bacillus subtilis harvested after 30 min from the same experiment. Spectral signatures of the vancomyin interaction have been located in the piecrust of a progressing septum with $\approx 10$ nm resolution using PiF contrasts of three selected bands of a PiF-IR hyperspectral scan of an individual Bacillus subtilis cell harvested after 30 min incubation. Our results are complemented by a discussion of imaging artifacts and the influence of parameter settings supporting further development towards standardization in the application of PiF-IR for visualizing the chemical interaction of antibiotics on the surface of microbes with few nanometer resolution.

physics.bio-ph↗

arXiv2020

Wide Field Spectral Imaging with Shifted Excitation Raman Difference Spectroscopy Using the Nod and Shuffle Technique

Wide field Raman imaging using the integral field spectroscopy approach was used as a fast, one shot imaging method for the simultaneous collection of all spectra composing a Raman image. For the suppression of autofluorescence and background signals such as room light, shifted excitation Raman difference spectroscopy (SERDS) was applied to remove background artifacts in Raman spectra. To reduce acquisition times in wide field SERDS imaging, we adapted the nod and shuffle technique from astrophysics and implemented it into a wide field SERDS imaging setup. In our adapted version, the nod corresponds to the change in excitation wavelength, whereas the shuffle corresponds to the shifting of charges up and down on a Charge-Coupled Device (CCD) chip synchronous to the change in excitation wavelength. We coupled this improved wide field SERDS imaging setup to diode lasers with 784.4/785.5 and 457.7/458.9 nm excitation and applied it to samples such as paracetamol and aspirin tablets, polystyrene and polymethyl methacrylate beads, as well as pork meat using multiple accumulations with acquisition times in the range of 50 to 200 ms. The results tackle two main challenges of SERDS imaging: gradual photobleaching changes the autofluorescence background, and multiple readouts of CCD detector prolong the acquisition time.

physics.ins-det↗

arXiv2015

Sample Size Planning for Classification Models

In biospectroscopy, suitably annotated and statistically independent samples (e. g. patients, batches, etc.) for classifier training and testing are scarce and costly. Learning curves show the model performance as function of the training sample size and can help to determine the sample size needed to train good classifiers. However, building a good model is actually not enough: the performance must also be proven. We discuss learning curves for typical small sample size situations with 5 - 25 independent samples per class. Although the classification models achieve acceptable performance, the learning curve can be completely masked by the random testing uncertainty due to the equally limited test sample size. In consequence, we determine test sample sizes necessary to achieve reasonable precision in the validation and find that 75 - 100 samples will usually be needed to test a good but not perfect classifier. Such a data set will then allow refined sample size planning on the basis of the achieved performance. We also demonstrate how to calculate necessary sample sizes in order to show the superiority of one classifier over another: this often requires hundreds of statistically independent test samples or is even theoretically impossible. We demonstrate our findings with a data set of ca. 2550 Raman spectra of single cells (five classes: erythrocytes, leukocytes and three tumour cell lines BT-20, MCF-7 and OCI-AML3) as well as by an extensive simulation that allows precise determination of the actual performance of the models in question.

stat.AP↗