High-resolution in vivo imaging of blood vessels without labeling

Quantitative Optical Imaging of Oxygen in Brain Vasculature

The intimate relationship between neuronal activity and cerebral oxygenation underpins fundamental brain functions like cognition, sensation, and motor control. Optical imaging offers a noninvasive approach to assess brain oxygenation and often serves as an indirect proxy for neuronal activity. However, deciphering neurovascular coupling─the intricate interplay between neuronal activity, blood flow, and oxygen delivery─necessitates independent, high spatial resolution, and high temporal resolution measurements of both microvasculature oxygenation and neuronal activation. This Perspective examines the established optical techniques employed for brain oxygen imaging, specifically functional near-infrared spectroscopy, photoacoustic imaging, optical coherence tomography, and two-photon phosphorescent lifetime microscopy, highlighting their fundamental principles, strengths, and limitations. Several other emerging optical techniques are also introduced. Finally, we discuss key technological challenges and future directions for quantitative optical oxygen imaging, paving the way for a deeper understanding of oxygen metabolism in the brain.

Progress and Prospects in Optical Ultrafast Microscopy in the Visible Spectral Region: Transient Absorption and Two-Dimensional Microscopy

Ultrafast optical microscopy, generally employed by incorporating ultrafast laser pulses into microscopes, can provide spatially resolved mechanistic insight into scientific problems ranging from hot carrier dynamics to biological imaging. This Review discusses the progress in different ultrafast microscopy techniques, with a focus on transient absorption and two-dimensional microscopy. We review the underlying principles of these techniques and discuss their respective advantages and applicability to different scientific questions. We also examine in detail how instrument parameters such as sensitivity, laser power, and temporal and spatial resolution must be addressed. Finally, we comment on future developments and emerging opportunities in the field of ultrafast microscopy.

Optical characteristics of the skin with dark circles using pump-probe imaging

Pump-probe imaging was first used for quantitative analysis of melanin in dark circles' skin to improve the ability to diagnose and treat dark circles on human skin. This study aimed to compare the distribution characteristics in melanin of lower eyelid skin tissues and to determine whether pump-probe imaging has potential for the classification of dark circles in vivo. Specimens obtained from 15 patients undergoing blepharoplasty were examined using pump-probe imaging. Furthermore, adjacent slices were respectively treated with hematoxylin-eosin (HE) and ferrous sulfate (FeSO₄) staining for cross-references. Subsequently, the melanin content index (MCI) and mean fluorescence intensity (MFI) were quantitatively analyzed by the pump-probe imaging. The distribution of melanin granules in the pump-probe image and FeSO₄ staining was consistent. Meanwhile, the tissues of the skin with dark circles and normal skin demonstrated significant differences in MCI and MFI. These differences can be used to distinguish the skin with dark circles from the normal skin. Pump-probe imaging could be used for the analysis of the microstructure and spectral characteristics of melanin granules in skin with dark circles. Significant differences were noted between the pigmented type of dark circles and the other two groups (normal skin and the vascular type of dark circles), while no significant differences were found between normal skin and the vascular type of dark circles.

Label-Free Delineation of Human Uveal Melanoma Infiltration With Pump–Probe Microscopy

Uveal melanoma (UM) is the most frequent primary intraocular malignancy in adults, characterized by melanin depositions in melanocytes located in the uveal tract in the eyes. Differentiation of melanin species (eumelanin and pheomelanin) is crucial in the diagnosis and management of UM, yet it remains inaccessible for conventional histology. Here, we report that femtosecond time-resolved pump-probe microscopy could provide label-free and chemical-specific detection of melanin species in human UM based on their distinct transient relaxation dynamics at the subpicosecond timescale. The method is capable of delineating the interface between melanoma and paracancerous regions on various tissue conditions, including frozen sections, paraffin sections, and fresh tissues. Moreover, transcriptome sequencing was conducted to confirm the active eumelanin synthesis in UM. Our results may hold potential for sensitive detection of tumor boundaries and biomedical research on melanin metabolism in UM.

Label-Free Super-Resolution Imaging Techniques

Biological and material samples contain nanoscale heterogeneities that are unresolvable with conventional microscopy techniques. Super-resolution fluorescence methods can break the optical diffraction limit to observe these features, but they require samples to be fluorescently labeled. Over the past decade, progress has been made toward developing super-resolution techniques that do not require the use of labels. These label-free techniques span a variety of different approaches, including structured illumination, transient absorption, infrared absorption, and coherent Raman spectroscopies. Many draw inspiration from widely successful fluorescence-based techniques such as stimulated emission depletion (STED) microscopy, photoactivated localization microscopy (PALM), and stochastic optical reconstruction microscopy (STORM). In this review, we discuss the progress made in these fields along with the current challenges and prospects in reaching resolutions comparable to those achieved with fluorescence-based methods.

Transient absorption spectroscopy and imaging of redox in muscle mitochondria

Mitochondrial redox is an important indicator of cell metabolism and health, with implications in cancer, diabetes, aging, neurodegenerative diseases, and mitochondrial disease. The most common method to observe redox of individual cells and mitochondria is through fluorescence of NADH and FAD+, endogenous cofactors serve as electron transport inputs to the mitochondrial respiratory chain. Yet this leaves out redox within the respiratory chain itself. To a degree, the missing information can be filled in by exogenous fluorophores, but at the risk of disturbed mitochondrial permeability and respiration. Here we show that variations in respiratory chain redox can be detected up by visible-wavelength transient absorption microscopy (TAM). In TAM, the selection of pump and probe wavelengths can provide multiphoton imaging contrast between non-fluorescent molecules. Here, we applied TAM with a pump at 520nm and probe at 450nm, 490nm, and 620nm to elicit redox contrast from mitochondrial respiratory chain hemeproteins. Experiments were performed with reduced and oxidized preparations of isolated mitochondria and whole muscle fibers, using mitochondrial fuels (malate, pyruvate, and succinate) to set up physiologically relevant oxidation levels. TAM images of muscle fibers were analyzed with multivariate curve resolution (MCR), revealing that the response at 620nm probe provides the best redox contrast and the most consistent response between whole cells and isolated mitochondria.

Using saturated absorption for superresolution laser scanning transmission microscopy

We improved the three-dimensional spatial resolution of laser scanning transmission microscopy by exploiting the saturated absorption of dye molecules. The saturated absorption is induced by the high-intensity light irradiation and localises the signal within the centre of the focal spot. Our numerical calculation indicates that the spatial resolution in transmission imaging is significantly improved for both lateral and axial directions using nonlinear transmitted signals induced by saturated absorption. We experimentally demonstrated the improvement of the three-dimensional resolution by observing fine structures of stained rat kidney tissues, which were not able to be visualised by conventional laser scanning transmission microscopy.

Melanin-Like Nanomedicine in Photothermal Therapy Applications

Photothermal therapy (PTT) mediated by nanomaterial has become an attractive tumor treatment method due to its obvious advantages. Among various nanomaterials, melanin-like nanoparticles with nature biocompatibility and photothermal conversion properties have attracted more and more attention. Melanin is a natural biological macromolecule widely distributed in the body and displays many fascinating physicochemical properties such as excellent biocompatibility and prominent photothermal conversion ability. Due to the similar properties, Melanin-like nanoparticles have been extensively studied and become promising candidates for clinical application. In this review, we give a comprehensive introduction to the recent advancements of melanin-like nanoparticles in the field of photothermal therapy in the past decade. In this review, the synthesis pathway, internal mechanism and basic physical and chemical properties of melanin-like nanomaterials are systematically classified and evaluated. It also summarizes the application of melanin-like nanoparticles in bioimaging and tumor photothermal therapy (PTT)in detail and discussed the challenges they faced in clinical translation rationally. Overall, melanin-like nanoparticles still have significant room for development in the field of biomedicine and are expected to applied in clinical PTT in the future.

Stimulated Raman microspectroscopy as a new method to classify microfibers from environmental samples

Microfibers are reported as the most abundant microparticle type in the environment. Their small size and light weight allow easy and fast distribution, but also make it challenging to determine their chemical composition. Vibrational microspectroscopy methods as infrared and spontaneous Raman microscopy have been widely used for the identification of environmental microparticles. However, only few studies report on the identification of microfibers, mainly due to difficulties caused by their small diameter. Here we present the use of Stimulated Raman Scattering (SRS) microscopy for fast and reliable classification of microfibers from environmental samples. SRS microscopy features high sensitivity and has the potential to be faster than other vibrational microspectroscopy methods. As a proof of principle, we analyzed fibers extracted from the fish gastrointestinal (GIT) tract, deep-sea and coastal sediments, surface seawater and drinking water. Challenges were faced while measuring fibers from the fish GIT, due to the acidic degradation they undergo. However, the main vibrational peaks were still recognizable and sufficient to determine the natural or synthetic origin of the fibers. Notably, our results are in accordance to other recent studies showing that the majority of the analyzed environmental fibers has a natural origin. Our findings suggest that advanced spectroscopic methods must be used for estimation of the plastic fibers concentration in the environment.

Direct Quantification of Single Red Blood Cell Hemoglobin Concentration with Multiphoton Microscopy

Blood disorders, diseases, and infections often affect the shape, number, and content of red blood cells (RBCs) dramatically. To combat these pathologies, many therapies target RBCs and their contents directly. Mean corpuscular hemoglobin concentration (MCHC) is an important pathological metric in both identification and treatment. However, current methods for RBC analysis and MCHC quantification rely on bulk measurements. Single RBC measurements could provide necessary insight into the heterogeneity of RBC health and improve therapeutic efficacy. In this study, we present a novel multimodal multiphoton approach for quantifying hemoglobin concentration at single RBC resolution. We achieve this by collecting two images simultaneously that allows us to excite water with stimulated Raman scattering and hemoglobin with transient absorption. This multimodal imaging is enabled by a newly designed orthogonal modulation theme for dual-channel lock-in detection. By leveraging water as an internal standard, we quantify MCHC of healthy RBCs and RBCs infected with Plasmodium yoelii, a commonly studied rodent parasite model.

Low cost blood vein detection system based on near-infrared LEDs and image-processing techniques

Drawing blood and injecting drugs are common medical procedures, for which accurate identification of veins is needed to avoid causing unnecessary pain. In this paper, we propose a low-cost system for the detection of veins. The system emits near-infrared radiation from four light-emitting diodes (LEDs), with a charge-coupled device (CCD) camera located in the middle of the LEDs. The camera captures an image of the palm of the hand. A series of digital image-processing techniques, ranging from image enhancement and increased contrast to isolation using a threshold limit based on statistical properties, are applied to effectively isolate the veins from the rest of the image.

Background-suppressed SRS fingerprint imaging with a fully integrated system using a single optical parametric oscillator

Stimulated Raman Scattering (SRS) imaging can be hampered by non-resonant parasitic signals that lead to imaging artifacts and eventually overwhelm the Raman signal of interest. Stimulated Raman gain opposite loss detection (SRGOLD) is a three-beam excitation scheme capable of suppressing this nonlinear background while enhancing the resonant Raman signal. We present here a compact electro-optical system for SRGOLD excitation which conveniently exploits the idler beam generated by an optical parametric oscillator (OPO). We demonstrate its successful application for background suppressed SRS imaging in the fingerprint region. This system constitutes a simple and valuable add-on for standard coherent Raman laser sources since it enables flexible excitation and background suppression in SRS imaging.

Total Internal Reflection Transient Absorption Microscopy: An Online Detection Method for Microfluidics

Microreactors have garnered widespread attention for their tunability and precise control of synthetic parameters to efficiently produce target species. Despite associated advances, a lack of online detection and optimization methods has stalled the progression of microfluidic reactors. Here we employ and characterize a total internal reflection transient absorption microscopy (TIRTAM) instrument to image excited state dynamics on a continuous flow device. The experiments presented demonstrate the capability to discriminate between different chromophores as well as in differentiating the effects of local chemical environments that a chromophore experiences. This work presents the first such online transient absorption measurements and provides a new direction for the advancement and optimization of chemical reactions in microfluidic devices.

Transient absorption microscopy: Technological innovations and applications in materials science and life science

Transient absorption (TA) spectroscopy has been extensively used in the study of excited state dynamics of various materials and molecules. The transition from TA spectroscopy to TA microscopy, which enables the space-resolved measurement of TA, is opening new investigations toward a more complete picture of excited state dynamics in functional materials, as well as the mapping of crucial biopigments for precision diagnosis. Here, we review the recent instrumental advancement that is pushing the limit of spatial resolution, detection sensitivity, and imaging speed. We further highlight the emerging application in materials science and life science.

Frequency Modulation Stimulated Raman Scattering Microscopy through Polarization Encoding

Stimulated Raman scattering (SRS) microscopy is a powerful method for imaging molecular distributions based on their intrinsic vibrational contrast. However, despite a growing list of biological applications, SRS is frequently hindered by a parasitic background signal which both overpowers the signal in low-signal applications and makes the extraction of quantitative information from images challenging. Frequency modulation (FM) has been used to suppress this parasitic background. However, many FM-SRS methods require either the acquisition of multiple images or the addition of multiple optomechanical components and an extensive realignment procedure. Herein, we report a new procedure for alignment-free FM-SRS utilizing polarization encoding. We demonstrate the efficacy of this approach, along with parabolic amplification of the Stokes pulse, at removing parasitic background signals in SRS microscopy applications. We further highlight how this technique can be used to suppress Raman signals from major molecular species to unveil spectral signatures from nucleic acids in both murine brain tissue and whole blood. Due to its ease of use and demonstrated experimental capabilities, we expect this technique to see broad use in the SRS microscopy community.

Label-Free Imaging of Blood Vessels in Human Normal Breast and Breast Tumor Tissue Using Multiphoton Microscopy

Blood vessels are the important components of the circulatory systems that transport blood throughout the human body and maintain the homeostasis of physiological tissues. Pathologically, blood vessels are often affected by diseases, leading to the formation of unstable, irregular, and hyperpermeable blood vessels. In the tumor microenvironment, abnormal leakage of tumor blood vessels is related to the histological grade and malignant potential of tumors and may also facilitate metastasis of cancer. Visual diagnosis of blood vessels is very important for us to understand the occurrence and development of diseases. Multiphoton microscopy (MPM) is a potential label-free diagnostic tool based on second harmonic generation (SHG) and two-photon excited fluorescence (TPEF). MPM can effectively observe the morphological changes of biological tissues at the molecular and cellular levels. In this work, we demonstrate that label-free MPM can be used to visualize the microstructure of blood vessels in human normal breast and breast tumor tissue. Moreover, MPM can monitor the changes of blood vessels in tumor microenvironment. These results show that the MPM will become a promising technique for clinicians to study the properties of the microstructure of the blood vessels.

In Vitro Quantification of Single Red Blood Cell Oxygen Saturation by Femtosecond Transient Absorption Microscopy

Hemoglobin, the oxygen carrying protein, ferries nearly all bodily oxygen from the lungs to cells and tissues in need. Blood oxygen saturation (sO₂) thus plays an important role in maintaining energy homeostasis throughout the body. Clinical and research tools have been developed to monitor sO₂ at a wide range of temporal and spatial scales. However, real-time quantification of sO₂ at single red blood cell (RBC) resolution remains challenging. Such capability is critically important to study energy metabolism in heterogeneous tissues including brain and tumor tissue. In this work, we develop a ratiometric transient absorption microscopy technique to image hemoglobin sO₂. By exploiting differences in transient lifetime kinetics between  oxyhemoglobin and deoxyhemoglobin, we directly quantified the sO₂ of single RBCs in real-time without the need for injection of exogenous agents. This simple and high-speed nonlinear optical imaging technique is well suited for in vitro and in vivo quantification of sO₂.

Label-free imaging of hemoglobin degradation and hemosiderin formation in brain tissues with femtosecond pump-probe microscopy

The degradation of hemoglobin in brain tissues results in the deposition of hemosiderin, which is a major form of iron-storage protein and closely related to neurological disorders such as epilepsy. Optical detection of hemosiderin is vitally important yet challenging for the understanding of disease mechanisms, as well as improving surgical resection of brain lesions. Here, we provide the first label-free microscopy study of sensitive hemosiderin detection in both an animal model and human brain tissues. Methods: We applied spectrally and temporally resolved femtosecond pump-probe microscopy, including transient absorption (TA) and stimulated Raman scattering (SRS) techniques, to differentiate hemoglobin and hemosiderin in brain tissues. The label-free imaging results were compared with Perls' staining to evaluate our method for hemosiderin detection. Results: Significant differences between hemoglobin and hemosiderin transient spectra were discovered. While a strong ground-state bleaching feature of hemoglobin appears in the near-infrared region, hemosiderin demonstrates pure excited-state absorption dynamics, which could be explained by our proposed kinetic model. Furthermore, simultaneous imaging of hemoglobin and hemosiderin can be rapidly achieved in both an intracerebral hemorrhage (ICH) rat model and human brain surgical specimens, with perfect correlation with Perls' staining. Conclusion: Our results suggest that rapid, label-free detection of hemosiderin in brain tissues could be realized by femtosecond pump-probe microscopy. Our method holds great potential in providing a new tool for intraoperative detection of hemosiderin during brain surgeries.

Compressed supercontinuum probe for transient absorption microscopy

Here, we combine three optical advancements to transient absorption microscopy in order to access the photodynamics in systems requiring stringent spatial and temporal resolution criteria. First, a broadband visible probe is generated by a commercial photonic crystal fiber. Second, a spatial light modulator-based pulse shaper is incorporated to reduce the pulse dispersion and improve temporal resolution. Third, 1.4 numerical aperture objectives for excitation and light collection provide optimal spatial resolution. The result of these improvements is a probe beam that spans 115 nm across the visible region yet maintains a ∼100  fs instrument response at the sample position. We demonstrate the capabilities of this microscope by imaging polystyrene beads in a solution of IR-144 dye, revealing aggregated species at the bead surfaces.

Transient absorption imaging of hemes with 2-color, independently tunable visible-wavelength ultrafast source

Pump probe microscopy is a time-resolved multiphoton imaging technique capable of generating contrast between non-fluorescent pigments based on differences in excited-state lifetimes. Here we describe a fiber-based ultrafast system designed for imaging heme proteins with an independently-tunable pulse pair in the visible-wavelength regime. Starting with a 1060 nm fiber amplifier (1.3 W at 63 MHz, 140 fs pulses), visible pulses were produced in the vicinity of 488 nm and 532 nm by doubling the output of a short photonic crystal fiber with a pair of periodically-poled lithium niobate crystals, providing 5-20 mW power in each beam. This was sufficient for acquiring transient absorption images from unstained cryosectioned tissue.

Simultaneous two-color stimulated Raman scattering microscopy by adding a fiber amplifier to a 2 ps OPO-based SRS microscope

Stimulated Raman scattering (SRS) microscopy is a label-free chemical imaging technique. Two-color imaging is often necessary to determine the distribution of chemical species in SRS microscopy. Current multi-color SRS imaging methods involve complicated instrumentation or longer data acquisition time or are limited to transmission imaging. In this Letter, we show that by adding a simple fiber amplifier to a 2 ps laser source and optical-parametric-oscillator-based SRS setup, one can achieve simultaneous two-color or frequency modulation SRS microscopy. The fiber amplifier can generate a wavelength tunable laser of ±10  nm around the Stokes laser wavelength at 1031 nm with average power greater than 200 mW. In vivo and ex vivo lipid-protein imaging of mouse brain and skin is demonstrated. To further demonstrate the potential of this technique in high-speed in vivo imaging, white blood cells in a blood stream are imaged in a live mouse.

Multiphoton excited hemoglobin fluorescence and third harmonic generation for non-invasive microscopy of stored blood

Red blood cells (RBC) in two-photon excited fluorescence (TPEF) microscopy usually appear as dark disks because of their low fluorescent signal. Here we use 15fs 800nm pulses for TPEF, 45fs 1060nm pulses for three-photon excited fluorescence, and third harmonic generation (THG) imaging. We find sufficient fluorescent signal that we attribute to hemoglobin fluorescence after comparing time and wavelength resolved spectra of other expected RBC endogenous fluorophores: NADH, FAD, biliverdin, and bilirubin. We find that both TPEF and THG microscopy can be used to examine erythrocyte morphology non-invasively without breaching a blood storage bag.

Stain-free histopathology by programmable supercontinuum pulses

The preparation, staining, visualization, and interpretation of histological images of tissue is well-accepted as the gold standard process for the diagnosis of disease. These methods were developed historically, and are used ubiquitously in pathology, despite being highly time and labor intensive. Here we introduce a unique optical imaging platform and methodology for label-free multimodal multiphoton microscopy that uses a novel photonic crystal fiber source to generate tailored chemical contrast based on programmable supercontinuum pulses. We demonstrate collection of optical signatures of the tumor microenvironment, including evidence of mesoscopic biological organization, tumor cell migration, and (lymph-)angiogenesis collected directly from fresh ex vivo mammary tissue. Acquisition of these optical signatures and other cellular or extracellular features, which are largely absent from histologically processed and stained tissue, combined with an adaptable platform for optical alignment-free programmable-contrast imaging, offers the potential to translate stain-free molecular histopathology into routine clinical use.

Contributed Review: A new synchronized source solution for coherent Raman scattering microscopy

Based on vibrational spectroscopy, coherent Raman Scattering (CRS) microscopy allows label-free imaging of biological and chemical samples with endogenous image contrast. Two-color, synchronized picosecond pulses are typically used for high spectral resolution imaging, which in turn constitutes a dramatic laser source challenge for CRS microscopy. Recently, synchronized time-lens source, inspired from ultrafast optical signal processing, has emerged as a promising laser source solution and has found application in various modalities of CRS microscopy. Time-lens is based on space-time analogy, which uses a "lens" in the time domain to compress long optical pulses or even continuous waves to ultrashort pulses, mimicking a lens in the space domain. Phase and intensity modulators driven with electrical signals are used in the time-lens source for picosecond pulse generation. As a result, the time-lens source is highly versatile and naturally compatible with modulation capabilities. More importantly, if the electrical signals used to drive the time-lens source are derived from other laser sources, such as mode-locked lasers, then synchronization between them can be realized, underlying the physics of a synchronized time-lens source. In this paper, we review recent progress on the basic principle, design of the synchronized time-lens source, and its applications to CRS microscopy of both biological and chemical samples.

Invited Review Article: Pump-probe microscopy

Multiphoton microscopy has rapidly gained popularity in biomedical imaging and materials science because of its ability to provide three-dimensional images at high spatial and temporal resolution even in optically scattering environments. Currently the majority of commercial and home-built devices are based on two-photon fluorescence and harmonic generation contrast. These two contrast mechanisms are relatively easy to measure but can access only a limited range of endogenous targets. Recent developments in fast laser pulse generation, pulse shaping, and detection technology have made accessible a wide range of optical contrasts that utilize multiple pulses of different colors. Molecular excitation with multiple pulses offers a large number of adjustable parameters. For example, in two-pulse pump-probe microscopy, one can vary the wavelength of each excitation pulse, the detection wavelength, the timing between the excitation pulses, and the detection gating window after excitation. Such a large parameter space can provide much greater molecular specificity than existing single-color techniques and allow for structural and functional imaging without the need for exogenous dyes and labels, which might interfere with the system under study. In this review, we provide a tutorial overview, covering principles of pump-probe microscopy and experimental setup, challenges associated with signal detection and data processing, and an overview of applications.

Nonlinear femtosecond pump-probe spectroscopy using a power-encoded soliton delay line

We show femtosecond time-resolved nonlinear pump-probe spectroscopy using a fiber soliton as the probe pulse. Furthermore, we exploit soliton dynamics to record an entire transient trace with a power-encoded delay sweep. The power-encoded delay line takes advantage of the dependency of the soliton trajectory in the (λ,z) space upon input power; the difference in accumulated group delay between trajectories converts a fast power sweep into a fast delay sweep. We demonstrate the concept by performing transient absorption spectroscopy in a test sample and validate it against a conventional pump-probe setup.

Pulse width reduction due to RF filtering in synchronized time-lens source

Synchronized time-lens source has emerged as a promising solution for two-color synchronized ultrashort laser source. Various experiments have demonstrated that the synchronized time-lens source is especially suitable for applications in coherent Raman scattering (CRS) imaging and spectroscopy. In a synchronized time-lens source, phase modulators are typically driven by a filtered high-order harmonic (sine wave) of the fundamental repetition rate of the mode-locked laser. The bandwidth of the narrowband RF filter for this high-order harmonic is thus a programmable parameter for optimizing the performance of the time-lens source. In this paper, through both analytical and numerical investigations we demonstrate that, manipulating the bandwidth of RF filtering can reduce the pulse with of the synchronized time-lens source. This is potentially helpful for reducing the overall system complexity or extending the synchronized time-lens source to the femtosecond level.

Amplitude and polarization modulated hyperspectral Stimulated Raman Scattering Microscopy

We present a simple hyperspectral Stimulated Raman Scattering (SRS) microscopy method based on spectral focusing of chirped femtosecond pulses, combined with amplitude (AM) and polarization (PM) modulation. This approach permits the imaging of low concentration components with reduced background signals, combined with good hyperspectral resolution and rapid spectral scanning. We demonstrate, using PM-SRS in a Raman loss configuration, the spectrally resolved detection of deuterated dimethyl sulfoxide (DMSO-d6) at concentrations as low as 0.039 % (5.5 mM). In general, background signals due to cross-phase modulation (XPM), two-photon absorption (TPA) and thermal lensing (TL) can reduce the contrast in SRS microscopy. We show that the nonresonant background signal contributing to the SRS signal is, in our case, largely due to XPM. Polarization modulation of the Stokes beam eliminates the nonresonant XPM background, yielding high quality hyperspectral scans at low analyte concentration. The flexibility of our combined AM-PM approach, together with the use of variable modulation frequency and lock-in phase, should allow for optimization of SRS imaging in more complex samples.

Timing jitter in synchronized time-lens source for coherent Raman scattering imaging

Synchronized time-lens source is suitable for various applications in coherent Raman scattering (CRS) imaging up to video-rate. Timing jitter between the time-lens source and the mode-locked laser is a crucial parameter for estimating the synchronization performance. Although it has been measured experimentally, there is a lack of theoretical investigation of this parameter. Here we demonstrate numerical simulation results of timing jitter in a synchronized time-lens system, with parameters similar to those in real experiments. Our results show that due to the optical delay between the time-lens source and the mode-locked laser, the timing jitter is close to the intrinsic timing jitter of the mode-locked laser. Further reduction in timing jitter can be achieved by matching the optical delays between the time-lens source and the mode-locked laser.

Frequency domain approach for time-resolved pump-probe microscopy using intensity modulated laser diodes

We present a scheme for time-resolved pump-probe microscopy using intensity modulated laser diodes. The modulation frequencies of the pump and probe beams are varied up to 500 MHz with fixed frequency detuning typically set at 15 kHz. The frequency response of the pump-probe signal is detected using a lock-in amplifier referenced at the beat frequency. This frequency domain method is capable of characterizing the nanosecond to picosecond relaxation dynamics of sample species without the use of a high speed detector or a high frequency lock-in amplifier. Furthermore, as the pump-probe signal is based on the nonlinear interaction between the two laser beams and the sample, our scheme provides better spatial resolution than the conventional diffraction-limited optical microscopes. Time-resolved pump-probe imaging of fluorescence beads and aggregates of quantum dots demonstrates that this method is useful for the microscopic analysis of optoelectronic devices. The system is implemented using compact and low-cost laser diodes, and thus has a broad range of applications in the fields of photochemistry, optical physics, and biological imaging.

Resolution improvement in laser diode-based pump-probe microscopy with an annular pupil filter

We experimentally demonstrate the use of annular beams to improve lateral resolution in laser-diode-based pump-probe microscopy. We found that the width of the point-spreading function in the case of the annular pump-probe beams is 162 nm, which is 30% smaller than that of the circular beams (232 nm). Furthermore, side lobes were efficiently suppressed at the focal plane since the pump-probe signal is proportional to the product of the two beam intensities. This scheme is demonstrated for the photothermal signal of nonfluorescent gold nanoparticles and the stimulated emission signal of fluorescence beads.

Time-resolved coherent Raman spectroscopy by high-speed pump-probe delay scanning

Using a spinning window pump-probe delay scanner, we demonstrate a means of acquiring time-resolved vibrational spectra at rates up to 700 Hz. The time-dependent phase shift accumulated by the probe pulse in the presence of a coherently vibrating sample gives rise to a Raman-induced frequency shifting readily detectable in a balanced detector. This rapid delay scanning system represents a 23-fold increase in averaging speed and is >10× faster than state-of-the-art voice coil delay lines. These advancements make pump-probe spectroscopy a more practical means of imaging complex media.

Sub-diffraction resolution pump-probe microscopy with shot-noise limited sensitivity using laser diodes

We demonstrate the use of intensity-modulated laser diodes to implement pump-probe microscopy and achieved sub-diffraction resolution imaging with shot-noise limited sensitivity with a scheme of balanced detection. This technique has several applications for various types of induced transmission change, including excited-state absorption, ground state absorption bleaching and stimulated emission. By using our technique, biological imaging of mouse T cells and the axons of neurons in the cerebral cortex was demonstrated.

Femtosecond pump-probe microscopy generates virtual cross-sections in historic artwork

The layering structure of a painting contains a wealth of information about the artist's choice of materials and working methods, but currently, no 3D noninvasive method exists to replace the taking of small paint samples in the study of the stratigraphy. Here, we adapt femtosecond pump-probe imaging, previously shown in tissue, to the case of the color palette in paintings, where chromophores have much greater variety. We show that combining the contrasts of multispectral and multidelay pump-probe spectroscopy permits nondestructive 3D imaging of paintings with molecular and structural contrast, even for pigments with linear absorption spectra that are broad and relatively featureless. We show virtual cross-sectioning capabilities in mockup paintings, with pigment separation and nondestructive imaging on an intact 14th century painting (The crucifixion by Puccio Capanna). Our approach makes it possible to extract microscopic information for a broad range of applications to cultural heritage.

Imaging microscopic pigment chemistry in conjunctival melanocytic lesions using pump-probe laser microscopy

PURPOSE

To report the application of a novel imaging technique, pump-probe microscopy, to analyze patterns of pigment chemistry of conjunctival melanocytic lesion biopsies.

METHODS

Histopathologic specimens of eight previously excised conjunctival melanocytic lesions were analyzed with pump-probe microscopy. The technique uses a laser scanning microscope with a two-color pulsed laser source to distinguish hemoglobin, eumelanin, and pheomelanin pigment based on differences in transient excited state and ground state photodynamics. The pump-probe signatures of conjunctival melanins were compared with cutaneous melanins. The distributions of hemoglobin, eumelanin, and pheomelanin were analyzed, and pump-probe images were correlated with adjacent hematoxylin and eosin (H&E)-stained sections.

RESULTS

The pump-probe signatures of conjunctival melanins are similar, but not identical to cutaneous melanins. In addition, there are qualitative and quantitative differences in the structure and pigment chemistry of conjunctival benign nevi, primary acquired melanosis of the conjunctiva (PAM), and conjunctival melanomas. The pump-probe images correlated well with histopathologic features observed in the adjacent H&E-stained sections, and provided a label-free means of discerning conjunctival anatomic features and pathologic benign or malignant tissue.

CONCLUSIONS

Pump-probe laser microscopy shows promise as an adjuvant diagnostic tool in evaluation of ocular melanocytic lesions based on morphologic correlation with the histopathology results and pigment chemistry. This initial study suggests systematic differences in pigmentation patterns among conjunctival benign nevi, primary acquired melanosis, and melanomas. In addition, pump-probe microscopy has the potential for use as a noninvasive "in vivo" optical biopsy technique to aid clinical and surgical management of conjunctival melanocytic lesions.

Chemically specific imaging and in-situ chemical analysis of articular cartilage with stimulated Raman scattering

Stimulated Raman scattering (SRS) has been applied to unstained samples of articular cartilage enabling the investigation of living cells within fresh tissue. Hyperspectral SRS measurements over the CH vibrational region showed variations in protein and lipid content within the cells, pericellular matrix and interterritorial matrix. Changes in the cells and pericellular matrix were investigated as a function of depth into the cartilage. Lipid was detected in the pericellular matrix of superficial zone chondrocytes. The spectral profile of lipid droplets within the chondrocytes indicated that they contained predominantly unsaturated lipids. The mineral content has been imaged by using the PO₄³⁻ vibration at 959 cm⁻¹ and the CO₃²⁻ vibration at 1070 cm⁻¹. Both changes in cells and mineralization are known to be important factors in the progression of osteoarthritis. SRS enables these to be visualized in fresh unstained tissue and consequently should benefit osteoarthiritis research.

Simultaneous imaging of two-photon absorption and stimulated Raman scattering by spatial overlap modulation nonlinear optical microscopy

Imaging of simultaneous two-photon absorption and stimulated Raman scattering is accomplished by detecting the intensity changes of the two-color pulses simultaneously and the mathematical operations of addition and subtraction. The stimulated Raman scattering is quantitatively separated from the two-photon absorption, generated in a mixed solution in which a glycerin solution is miscible in various proportions with a quantum dot solution. Our technique is applied to simultaneous two-photon absorption and stimulated Raman scattering imaging.

Near shot-noise limited hyperspectral stimulated Raman scattering spectroscopy using low energy lasers and a fast CMOS array

We demonstrate near shot-noise limited hyperspectral stimulated Raman scattering (SRS) spectroscopy using oscillator-only excitation conditions. Using a fast CMOS camera synchronized to an acousto-optic modulator and subtracting subsequent frames acquired at up to 1 MHz frame rates, we demonstrate demodulation and recovery of the SRS spectrum. Surprisingly, we observe that the signal-to-noise of SRS spectra is invariant at modulation frequencies down to 2.5 kHz. Our approach allows for a direct comparison of SRS with coherent anti-Stokes Raman scattering (CARS) spectroscopy under identical experimental conditions. Our findings suggest that hyperspectral SRS imaging with shot-noise limited performance at biologically compatible excitation energies is feasible after minor modifications to fast frame-rate CMOS array technology.

Optical imaging of non-fluorescent nanodiamonds in live cells using transient absorption microscopy

We directly observe non-fluorescent nanodiamonds in living cells using transient absorption microscopy. This label-free technology provides a novel modality to study the dynamic behavior of nanodiamonds inside the cells with intrinsic three-dimensional imaging capability. We apply this method to capture the cellular uptake of nanodiamonds under various conditions, confirming the endocytosis mechanism.

Spectrally modulated stimulated Raman scattering imaging with an angle-to-wavelength pulse shaper

The stimulated Raman scattering signal is often accompanied by unwanted background arising from other pump-probe modalities. We demonstrate an approach to overcome this challenge based on spectral domain modulation, enabled by a compact, cost-effective angle-to-wavelength pulse shaper. The pulse shaper switches between two spectrally narrow windows, which are cut out of a broadband femtosecond pulse and selected for on- and off- Raman resonance excitation, at 2.1 MHz frequency for detection of stimulated Raman scattering signal. Such spectral modulation reduced the unwanted pump-probe signals by up to 20 times and enabled stimulated Raman scattering imaging of molecules in a pigmented environment.

Hyperspectral imaging with stimulated Raman scattering by chirped femtosecond lasers

Raman microscopy is a quantitative, label-free, and noninvasive optical imaging technique for studying inhomogeneous systems. However, the feebleness of Raman scattering significantly limits the use of Raman microscopy to low time resolutions and primarily static samples. Recent developments in narrowband stimulated Raman scattering (SRS) microscopy have significantly increased the acquisition speed of Raman based label-free imaging by a few orders of magnitude, at the expense of reduced spectroscopic information. On the basis of a spectral focusing approach, we present a fast SRS hyperspectral imaging system using chirped femtosecond lasers to achieve rapid Raman spectra acquisition while retaining the full speed and image quality of narrowband SRS imaging. We demonstrate that quantitative concentration determination of cholesterol in the presence of interfering chemical species can be achieved with sensitivity down to 4 mM. For imaging purposes, hyperspectral imaging data in the C-H stretching region is obtained within a minute. We show that mammalian cell SRS hyperspectral imaging reveals the spatially inhomogeneous distribution of saturated lipids, unsaturated lipids, cholesterol, and protein. The combination of fast spectroscopy and label-free chemical imaging will enable new applications in studying biological systems and material systems.

Pump-probe nonlinear phase dispersion spectroscopy

Pump-probe microscopy is an imaging technique that delivers molecular contrast of pigmented samples. Here, we introduce pump-probe nonlinear phase dispersion spectroscopy (PP-NLDS), a method that leverages pump-probe microscopy and spectral-domain interferometry to ascertain information from dispersive and resonant nonlinear effects. PP-NLDS extends the information content to four dimensions (phase, amplitude, wavelength, and pump-probe time-delay) that yield unique insight into a wider range of nonlinear interactions compared to conventional methods. This results in the ability to provide highly specific molecular contrast of pigmented and non-pigmented samples. A theoretical framework is described, and experimental results and simulations illustrate the potential of this method. Implications for biomedical imaging are discussed.

Implementation of spatial overlap modulation nonlinear optical microscopy using an electro-optic deflector

A spatial overlap modulation (SPOM) technique is a nonlinear optical microscopy technique which enhances the three-dimensional spatial resolution and rejects the out-of-focus background limiting the imaging depth inside a highly scattering sample. Here, we report on the implementation of SPOM in which beam pointing modulation is achieved by an electro-optic deflector. The modulation and demodulation frequencies are enhanced to 200 kHz and 400 kHz, respectively, resulting in a 200-fold enhancement compared with the previously reported system. The resolution enhancement and suppression of the out-of-focus background are demonstrated by sum-frequency-generation imaging of pounded granulated sugar and deep imaging of fluorescent beads in a tissue-like phantom, respectively.

Multicolored stain-free histopathology with coherent Raman imaging

Conventional histopathology with hematoxylin & eosin (H&E) has been the gold standard for histopathological diagnosis of a wide range of diseases. However, it is not performed in vivo and requires thin tissue sections obtained after tissue biopsy, which carries risk, particularly in the central nervous system. Here we describe the development of an alternative, multicolored way to visualize tissue in real-time through the use of coherent Raman imaging (CRI), without the use of dyes. CRI relies on intrinsic chemical contrast based on vibrational properties of molecules and intrinsic optical sectioning by nonlinear excitation. We demonstrate that multicolor images originating from CH(2) and CH(3) vibrations of lipids and protein, as well as two-photon absorption of hemoglobin, can be obtained with subcellular resolution from fresh tissue. These stain-free histopathological images show resolutions similar to those obtained by conventional techniques, but do not require tissue fixation, sectioning or staining of the tissue analyzed.

Phasor analysis for nonlinear pump-probe microscopy

Pump-probe microscopy provides molecular information by probing transient, excited state dynamic properties of pigmented samples. Analysis of the transient response is typically conducted using principal component analysis or multi-exponential fitting, however these methods are not always practical or feasible. Here, we show an adaptation of phasor analysis to provide an intuitive, robust, and efficient method for analyzing and displaying pump-probe images, thereby alleviating some of the challenges associated with differentiating multiple pigments. A theoretical treatment is given to understand how the complex transient signals map onto the phasor plot. Analyses of cutaneous and ocular pigmented tissue samples, as well as historical pigments in art demonstrate the utility of this approach.

Pump-probe optical microscopy for imaging nonfluorescent chromophores

Many chromophores absorb light intensely but have undetectable fluorescence. Hence microscopy techniques other than fluorescence are highly desirable for imaging these chromophores inside live cells, tissues, and organisms. The recently developed pump-probe optical microscopy techniques provide fluorescence-free contrast mechanisms by employing several fundamental light-molecule interactions including excited state absorption, stimulated emission, ground state depletion, and the photothermal effect. By using the pump pulse to excite molecules and the subsequent probe pulse to interrogate the created transient states on a laser scanning microscope, pump-probe microscopy offers imaging capability with high sensitivity and specificity toward nonfluorescent chromophores. Single-molecule sensitivity has even been demonstrated. Here we review and summarize the underlying principles of this emerging class of molecular imaging techniques.