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Takuro Ideguchi

Publications and source records attributed to Takuro Ideguchi.

At least 19 recordsLinked to original sources
arXiv2026

Ghost infrared spectroscopy with bright twin beams

Frequency-correlated light offers a route to mid-infrared (MIR) spectroscopy without direct spectral detection in the MIR. Previous MIR ghost spectroscopy has mainly relied on low-gain spontaneous parametric down-conversion (SPDC) and photon-pair coincidence measurements, where the limited photon flux has restricted acquisition times to longer than one minute. Here, we demonstrate ghost infrared spectroscopy using bright twin beams generated by high-gain parametric down-conversion (PDC). High-gain PDC amplifies vacuum fluctuations, producing a different pair of frequency-correlated random spectra in each pump pulse. This pulse-resolved stochastic emission is naturally matched to time-stretch detection, which records the spectrum of the correlated near-infrared telecom signal for every pulse, while the MIR idler is measured by bucket detection. Consequently, each pump pulse yields one paired projection measurement comprising a spectrally resolved reference and the corresponding bucket value. We reconstruct the transmission spectrum of a structured optical filter and the molecular vibrational absorption spectrum of liquid benzene near 3.3 um with millisecond-scale acquisition, in good agreement with Fourier-transform infrared spectroscopy. This reduces the acquisition time by four to five orders of magnitude compared with previous MIR ghost spectroscopy demonstrations. These results transform ghost spectroscopy from coincidence-based photon counting to high-flux analog correlation spectroscopy, establishing a practical architecture for high-speed computational infrared spectroscopy driven by a narrowband semiconductor laser.

physics.optics↗
arXiv2026

Femtosecond-to-millisecond holographic imaging of laser ablation dynamics

Femtosecond laser ablation redistributes optically deposited energy across electronic, structural, mechanical, and thermal degrees of freedom over timescales from femtoseconds to milliseconds. However, these coupled processes are usually measured in separate temporal ranges and through different observables, limiting quantitative comparison between early transient dynamics, residual heating, and final morphology. Here we introduce pump-probe holographic imaging that reconstructs amplitude- and phase-resolved optical fields across this full temporal range under matched imaging conditions. Applied to deep-ultraviolet femtosecond ablation of BK7 glass, the method captures the transition from early excitation and removal-stage dynamics to residual substrate heating and permanent modification. Differential phase analysis isolates sub-nanosecond evolution of the transient ablating layer and microsecond residual heating after material removal. Above the ablation threshold, crater depth increases with fluence, whereas the residual thermal signal saturates, indicating that additional absorbed energy is preferentially partitioned into material removal and ablation-related processes rather than retained as substrate heat. These results identify fluence-dependent energy partitioning as a dynamical basis of low-heat-affected femtosecond processing and establish holographic imaging as a route to tracking laser-driven nonequilibrium material transformations.

physics.optics↗
arXiv2026

Pulse-to-pulse spectral phase characterization of mid-infrared pulses at megahertz rates

Pulse-resolved spectral phase measurement of mid-infrared (MIR) pulses is essential for many applications, from precise waveform control to ultrafast quantum optics. However, conventional MIR pulse characterization techniques are typically limited to sub-kHz-rate operation, leaving a substantial speed mismatch with MIR sources operating at kHz or MHz rates. Here, we introduce time-stretch upconversion-based mid-infrared pulse evaluation (TSUBAME), a technique that enables pulse-to-pulse spectral phase characterization of ultrashort MIR pulses at the laser repetition rate. TSUBAME combines MIR-to-NIR (near-infrared) upconversion, time-stretch, and spectral interferometry to achieve scan-free high-speed spectral phase measurements. We validated the technique by measuring MIR pulses spanning 4.98-5.30 um while introducing well-defined dispersion, obtaining excellent agreement with theoretical predictions. Operating at a measurement rate of 1 MHz, TSUBAME achieves the fastest single-pulse-resolved spectral phase characterization of MIR pulses reported to date. As a further demonstration, we captured dynamic spectral phase variations on a microsecond timescale. TSUBAME provides a powerful tool for real-time monitoring and optimization of high-repetition-rate MIR pulses, with potential applications in strong-field physics, high-harmonic generation, and coherent molecular control.

physics.optics↗
arXiv2026

A unified framework for sensitivity comparison across stimulated Raman, stimulated Raman photothermal, and mid-infrared photothermal microscopy

Label-free vibrational microscopy based on Raman scattering and mid-infrared (MIR) absorption has advanced biological imaging through the development of diverse high-sensitivity modalities. However, rigorous sensitivity comparisons across these techniques remain lacking. Here, we establish a unified theoretical framework for quantitatively comparing the shot-noise-limited sensitivities of stimulated Raman scattering (SRS), MIR photothermal (MIP), and stimulated Raman photothermal (SRP) microscopy. The framework is built on two quantities: an effective absorption coefficient, which places SRS on the same footing as linear absorption, and a photothermal (PT) factor, which isolates the contribution specific to PT readout. Using experimentally realistic parameters, we show that MIR absorption coefficients for representative polar vibrational modes are typically about two orders of magnitude larger than the effective absorption coefficients of SRS, even for strong off-resonant Raman bands. In MIP, however, this intrinsic advantage is partly offset by a small PT factor, limiting the net sensitivity gain over SRS to several-fold and, at most, about one order of magnitude. In SRP, by contrast, burst-pulse thermal accumulation substantially increases the PT factor. Under ideal shot-noise-limited conditions with matched per-pulse excitation-energy conditions in water, SRP is predicted to provide sensitivities several-fold higher than those of SRS and to approach those of MIP. This framework provides a common basis for sensitivity assessment across vibrational imaging modalities and clarifies how absorption strength, photothermal accumulation, and readout conditions determine the practical advantages of each modality.

physics.optics↗
arXiv2026

Mid-infrared photothermal imaging flow cytometry

Imaging flow cytometry (IFC) enables high-throughput single-cell analysis but largely relies on fluorescence labeling to obtain molecular specificity. Label-free vibrational imaging can provide intrinsic chemical contrast, yet coherent Raman-based methods interrogate only a limited axial volume, which restricts quantitative whole-cell analysis under flow. Mid-infrared photothermal (MIP) microscopy offers a promising route to overcome this limitation by combining linear mid-infrared (MIR) absorption-based chemical contrast with visible-light detection, allowing chemical imaging of a broader axial volume of each cell in a wide-field configuration. However, applying MIP microscopy to rapidly flowing cells has been difficult because conventional frame-sequential acquisition of MIR-ON and MIR-OFF images is highly susceptible to motion-induced subtraction artifacts. Here we demonstrate MIP-IFC, a label-free imaging flow cytometry platform based on single-shot nanosecond-dual-pulse MIP (SNAP-MIP) microscopy. SNAP-MIP encodes the MIR-ON and MIR-OFF states into separate holographic channels within a single camera exposure, reducing their temporal separation to 20 ns. This single-shot acquisition suppresses motion artifacts and increases the allowable sample velocity for artifact-free MIP imaging by five orders of magnitude compared with conventional frame-sequential MIP imaging. Leveraging this capability, MIP-IFC acquired chemical images at 500 frames per second and achieved a cellular event rate up to ~70 events s^-1. We demonstrate quantitative chemical discrimination of flowing microbeads and apply MIP-IFC to single-cell profiling of oleic-acid-induced lipid accumulation, adipocyte differentiation, and confluence-dependent cellular heterogeneity. These results establish MIP-IFC as a high-throughput, quantitative, label-free chemical imaging platform for single-cell phenotyping under flow.

physics.optics↗
arXiv2026

Label-free mid-infrared photothermal microscopy revisits intracellular thermal dynamics: what do fluorescent nanothermometers measure?

Fluorescent nanothermometry has revealed pronounced intracellular temperature heterogeneity, establishing the field of single-cell thermal biology. However, these observations have sparked a controversy known as the "10^5 gap issue", because heat conduction calculations in aqueous environments predict that such large temperature distributions cannot be sustained within cells. Here, we address this issue using label-free mid-infrared photothermal microscopy. This technique quantifies heat-induced temperature changes under local thermal equilibrium (LTE), in accordance with the conventional thermodynamic and statistical-mechanical definition of temperature, by detecting refractive index variations. From transient thermal decay measurements, we determined that intracellular thermal diffusivity corresponds to 93-94% that of water. This result indicates that intracellular heat conduction is essentially water-like and rules out the hypothesis that anomalously slow intracellular heat conduction underlies the 10^5 gap discrepancy. We then directly compared fluorescent nanothermometry with our label-free thermometry. Under seconds-long heating, the label-free method exhibited a rapid temperature response consistent with water-like heat conduction. In contrast, fluorescent nanothermometers showed not only a similarly fast response but also an additional slow variation that was absent in the label-free readout. This slow component cannot be explained by temperature changes defined under LTE, but instead likely reflects slower intracellular processes not governed by conductive heat transfer. These results suggest that the "10^5 gap issue" stems from comparing two fundamentally distinct physical quantities: the LTE-defined temperature and a slowly-varying, long-lived non-conductive signal detected by fluorescent nanothermometers.

physics.bio-ph↗
arXiv2026

Video-rate volumetric chemical imaging via mid-infrared photothermal optical diffraction tomography

Label-free vibrational microscopy provides chemically specific access to cellular structure, yet quantitative volumetric chemical dynamics in living cells remain largely inaccessible, particularly on subsecond timescales relevant to intracellular transport and structural reorganization. This limitation arises because most high-speed vibrational techniques rely on raster scanning, which constrains volumetric throughput to approximately one volume per second (vps). Although mid-infrared photothermal (MIP) imaging offers a pathway toward spatially parallel chemical detection, existing implementations have remained far below video-rate volumetric operation, reflecting a fundamental trade-off between imaging speed and signal-to-noise ratio. Here, we overcome this trade-off in MIP tomography and realize video-rate volumetric chemical imaging using mid-infrared photothermal optical diffraction tomography (MIP-ODT), achieving high photothermal sensitivity while maintaining quantitative measurement fidelity. High per-angle detectability supports volumetric reconstruction without temporal averaging, yielding a signal-to-noise ratio exceeding 70 under video-rate acquisition conditions. Consequently, volumetric imaging at 19.2 vps is achieved, representing a nearly 400-fold improvement over prior implementations. Using this capability, we performed video-rate three-dimensional tracking of lipid droplets in living cells and quantified anomalous diffusion from full volumetric trajectories, revealing heterogeneous intracellular transport behaviors that are obscured in two-dimensional measurements. We further demonstrate high-speed hyperspectral volumetric chemical imaging across a 300 cm-1 spectral window within 1 s through rapid MIR wavenumber sweeping, paving the way for real-time three-dimensional organelle-specific chemical phenotyping.

physics.optics↗
arXiv2025

Bidirectional quantitative scattering microscopy

Quantitative phase microscopy (QPM) and interferometric scattering (iSCAT) microscopy are powerful label-free imaging techniques and are widely used for biomedical applications. Each method, however, possesses distinct limitations: QPM, which measures forward scattering (FS), excels at imaging microscale structures but struggles with rapidly moving nanoscale objects, while iSCAT, based on backward scattering (BS), is highly sensitive to nanoscale dynamics but lacks the ability to image microscale structures comprehensively. Here, we introduce bidirectional quantitative scattering microscopy (BiQSM), an innovative approach that integrates FS and BS detection using off-axis digital holography with bidirectional illumination and spatial-frequency multiplexing. BiQSM achieves spatiotemporal consistency and a dynamic range 14 times wider than QPM, enabling simultaneous imaging of nanoscale and microscale cellular components. We demonstrate BiQSM's ability to reveal spatiotemporal behaviors of intracellular structures, with FS-BS correlation analysis providing insights into proteins, lipids, and membranes. Time-lapse imaging of dying cells further highlights BiQSM's potential as a label-free tool for monitoring cellular vital states through structural and motion-related changes. By bridging the strengths of QPM and iSCAT, BiQSM advances quantitative cellular imaging and opens new avenues for studying dynamic biological processes.

physics.optics↗
arXiv2025

Ludwig-Soret microscopy with vibrational photothermal effect

Vibrational microscopy provides label-free, bond-selective chemical contrast by detecting molecular vibrations, making it invaluable for biomedical research. While conventional methods rely on the direct detection of Raman scattering or infrared absorption, recently developed vibrational photothermal (ViP) microscopy achieves chemical contrast indirectly through refractive index (RI) changes. This indirect approach enables unique imaging capabilities beyond traditional chemical imaging. Here, we introduce a novel application of ViP microscopy: label-free intracellular thermophoretic (Soret) imaging, which visualizes biomolecular transport driven by temperature gradients. ViP-induced Soret (ViPS) imaging leverages a steady-state temperature distribution generated by optical heating through vibrational photothermal effect, combined with time-resolved RI imaging via optical diffraction tomography (ODT). Using ViPS imaging, we measured thermophoretic behavior in living COS7 cells, determining intracellular diffusion and Soret coefficients. Notably, we observed a reversed direction of molecular transport (negative Soret effect) in the cytoplasm compared to the nucleus, possibly driven by thermophoresis-induced diffusiophoresis. Furthermore, time-lapse imaging under CO2-depleted conditions revealed a remarkable reduction in thermophoretic activity, suggesting glass formation during the dying process, likely due to polymer aggregation. ViPS imaging represents a new frontier in intracellular thermophoretic studies, expanding the capabilities of vibrational microscopy.

physics.optics↗
arXiv2025

Phase retrieval via Zernike phase contrast microscopy with an untrained neural network

Zernike's phase contrast microscopy (PCM) is among the most widely used techniques for observing phase objects, but it lacks quantitative nature, as it cannot directly provide phase information. Current methods for computationally extracting phase distributions from PCM images, however, rely heavily on empirical regularization parameter tuning. In this paper we extend an existing approach by employing an untrained neural network as an image prior, removing the need for manual regularization. We quantitatively demonstrate improved accuracy and robustness in phase retrieval compared to existing methods, using numerical and experimental PCM images. Our results confirm the feasibility of applying deep priors for phase retrieval in incoherent illumination setups.

physics.optics↗
arXiv2025

Fiber-based mid-infrared frequency-swept laser at 50 MScans/s via frequency down-conversion of time-stretched pulses

Increasing the sweep rate of mid-infrared (MIR) frequency-swept sources offers significant potential for various high-speed spectroscopy-based applications. While continuous-wave frequency-swept lasers have achieved sweep rates up to 1 MHz, a recently demonstrated time-stretched ultrashort pulsed laser has reached a significantly higher sweep rate, up to tens of MHz. However, the previous system relied on a bulky femtosecond optical parametric oscillator and produced only ~30 discrete spectral elements due to the use of a free-space time stretcher. In this work, we present a fiber-based frequency-swept MIR source that utilizes the frequency down-conversion of time-stretched near-infrared pulses, employing a compact mode-locked fiber laser and telecommunication fiber. As a proof-of-concept demonstration, we performed MIR spectroscopy of methane gas around 3.4 um at a rate of 50 MSpectra/s, capturing 220 spectral elements over a range of 19.0 cm-1. This compact and robust high-speed MIR frequency-swept laser system holds the potential for deployment in field applications.

physics.optics↗
arXiv2023

Mid-infrared optical coherence tomography with MHz axial line rate for real-time non-destructive testing

Non-destructive testing (NDT) is crucial for ensuring product quality and safety across various industries. Conventional methods such as ultrasonic, terahertz, and X-ray imaging have limitations in terms of probe-contact requirement, depth resolution, or radiation risks. Optical coherence tomography (OCT) is a promising alternative to solve these limitations, but it suffers from strong scattering, limiting its penetration depth. Recently, OCT in the mid-infrared (MIR) spectral region has attracted attention with a significantly lower scattering rate than in the near-infrared region. However, the highest reported A-scan rate of MIR-OCT has been 3 kHz, which requires long data acquisition time to take an image, unsatisfying industrial demands for real-time diagnosis. Here, we present a high-speed MIR-OCT system operating in the 3-4 um region that employs the swept-source OCT technique based on time-stretch infrared spectroscopy. By integrating a broadband femtosecond MIR pulsed laser operating at a repetition rate of 50 MHz, we achieved an A-scan rate of 1 MHz with an axial resolution of 11.6 um and a sensitivity of 55 dB. As a proof-of-concept demonstration, we imaged the surface of substrates covered by highly scattering paint coatings. The demonstrated A-scan rate surpasses previous state-of-the-art by more than two orders of magnitude, paving the way for real-time NDT of industrial products, cultural assets, and structures.

physics.optics↗
arXiv2023

Mid-infrared wide-field nanoscopy

Mid-infrared (MIR) spectroscopy is widely recognized as a powerful, non-distractive method for chemical analysis. However, its utility is constrained by a micrometer-scale spatial resolution imposed by the long-wavelength MIR diffraction limit. This limitation has been recently overcome by MIR photothermal (MIP) imaging, which detects photothermal effects induced in the vicinity of MIR absorbers using a visible-light microscope. Despite its promise, the full potential of its spatial resolving power has not been realized. Here, we present an optimal implementation of wide-field MIP imaging to achieve high spatial resolution. This is accomplished by employing single-objective synthetic-aperture quantitative phase imaging (SOSA-QPI) with synchronized sub-nanosecond MIR and visible light sources, effectively suppressing the resolution-degradation effect caused by photothermal heat diffusion. We demonstrate far-field MIR spectroscopic imaging with a spatial resolution limited by the visible diffraction, down to 125 nm, in the MIR region of 3.12-3.85 um (2,600-3,200 cm-1). This technique, through the use of a shorter visible wavelength and/or a higher objective numerical aperture, holds the potential to achieve a spatial resolution of less than 100 nm, thus paving the way for MIR wide-field nanoscopy.

physics.optics↗
arXiv2023

Quantitative phase retrieval for Zernike phase-contrast microscopy

We present a single-image numerical phase retrieval method for Zernike phase-contrast microscopy (ZPM) that addresses halo and shade-off artifacts, as well as the weak phase condition, without requiring hardware modifications. By employing a rigorous physical model of ZPM and a gradient descent algorithm for its inversion, we achieve quantitative ZPM imaging. Our approach is experimentally validated using biological cells and its quantitative nature is confirmed through comparisons with digital holography observations.

physics.optics↗
arXiv2023

Broadband coherent Raman scattering spectroscopy at 50,000,000 spectra/s

Raman scattering spectroscopy is widely used as an analytical technique in various fields, but its measurement process tends to be slow due to the low scattering cross-section. In the last decade, various broadband coherent Raman scattering spectroscopy techniques have been developed to address this limitation, achieving a measurement rate of about 100 kSpectra/s. Here, we present a significantly increased measurement rate of 50 MSpectra/s, which is 500 times higher than the previous state-of-the-art, by developing time-stretch coherent Raman scattering spectroscopy. Our newly-developed system, based on a mode-locked Yb fiber laser, enables highly-efficient broadband excitation of molecular vibrations via impulsive stimulated Raman scattering with an ultrashort femtosecond pulse and sensitive time-stretch detection with a picosecond probe pulse at a high repetition rate of the laser. As a proof-of-concept demonstration, we measure broadband coherent Stokes Raman scattering spectra of organic compounds covering the molecular fingerprint region from 200 to 1,200 cm-1. This high-speed broadband vibrational spectroscopy technique holds promise for unprecedented measurements of sub-microsecond dynamics of irreversible phenomena and extremely high-throughput measurements.

physics.optics↗
arXiv2022

Upconversion time-stretch infrared spectroscopy

High-speed measurement confronts the extreme speed limit when the signal becomes comparable to the noise level. In the context of broadband mid-infrared spectroscopy, state-of-the-art ultrafast Fourier-transform infrared spectrometers, in particular dual-comb spectrometers, have improved the measurement rate up to a few Mspectra/s, which is limited by the signal-to-noise ratio. Time-stretch infrared spectroscopy, an emerging ultrafast frequency-swept mid-infrared spectroscopy technique, has shown a record-high rate of 80 Mspectra/s with an intrinsically higher signal-to-noise ratio than Fourier-transform spectroscopy by more than the square-root of the number of spectral elements. However, it can measure no more than ~30 spectral elements with a low resolution of several cm-1. Here, we significantly increase the measurable number of spectral elements to more than 1,000 by incorporating a nonlinear upconversion process. The one-to-one mapping of a broadband spectrum from the mid-infrared to the near-infrared telecommunication region enables low-loss time-stretching with a single-mode optical fiber and low-noise signal detection with a high-bandwidth photoreceiver. We demonstrate high-resolution mid-infrared spectroscopy of gas-phase methane molecules with a high resolution of 0.017 cm-1. This unprecedentedly high-speed vibrational spectroscopy technique would satisfy various unmet needs in experimental molecular science, e.g., measuring ultrafast dynamics of irreversible phenomena, statistically analyzing a large amount of heterogeneous spectral data, or taking broadband hyperspectral images at a high frame rate.

physics.optics↗
arXiv2022

Mid-infrared photothermal single-live-cell imaging beyond video rate

Advancement in mid-infrared (MIR) technology has led to promising biomedical applications of MIR spectroscopy, such as liquid biopsy or breath diagnosis. On the contrary, MIR microscopy has been rarely used for live biological samples in an aqueous environment due to the lack of spatial resolution and the large water absorption background. Recently, mid-infrared photothermal (MIP) imaging has proven to be applicable to 2D and 3D single-cell imaging with high spatial resolution inherited from visible light. However, the maximum measurement rate has been limited to several frames/s, limiting its range of use. Here, we develop a significantly-improved wide-field MIP quantitative phase microscope with two orders-of-magnitude higher signal-to-noise ratio than previous MIP imaging techniques and demonstrate single-live-cell imaging beyond video rate. We first derive optimal system design by numerically simulating thermal conduction following the photothermal effect. Then, we develop the designed system with a homemade nanosecond MIR optical parametric oscillator and a high full-well-capacity image sensor. Our high-speed and high-spatial-resolution MIR microscope has great potential to become a new tool for life science, in particular for single-live-cell analysis.

physics.optics↗
arXiv2021

A simple approach of broadband mid-infrared pulse generation with a mode-locked Yb-doped fiber laser

Broadband mid-infrared (MIR) molecular spectroscopy demands a bright and broadband light source in the molecular fingerprint region. To this end, intra-pulse difference frequency generation (IDFG) has shown excellent properties among various techniques. However, previous IDFG systems have mainly used unconventional long-wavelength 2-$μ$m ultrashort pulsed lasers. A few systems have been demonstrated with 1-$μ$m lasers, but they use bulky 100-W-class high-power Yb thin-disk lasers. In this work, we demonstrate a simple and robust approach of 1-$μ$m-pumped broadband IDFG with a conventional mode-locked Yb-doped fiber laser. We first generate 3.3-W, 12.1-fs ultrashort pulses at 50 MHz by a simple combination of spectral broadening with a short single-mode fiber and pulse compression with chirped mirrors. Then, we use them for pumping a thin orientation-patterned gallium phosphide (OP-GaP) crystal, generating 1.2-mW broadband MIR pulses with the -20-dB bandwidth of 480 cm$^{-1}$ in the fingerprint region (760-1240 cm$^{-1}$, 8.1-13.1 $μ$m). The 1-$μ$m-based IDFG system allows for simultaneous generation of ultrashort pulses in the ultraviolet and visible regions, enabling, for example, 100-MHz-level high-repetition-rate vibrational sum-frequency generation spectroscopy or pump-probe spectroscopy.

physics.optics↗