Graded arc beam in light needle microscopy for axially resolved, rapid volumetric imaging without nonlinear processes

High-speed three-dimensional (3D) imaging is essential for revealing the structure and functions of biological specimens. Confocal laser scanning microscopy has been widely employed for this purpose. However, it requires a time-consuming image-stacking procedure. As a solution, we previously developed light needle microscopy using a Bessel beam with a wavefront-engineered approach [Biomed. Opt. Express13, 1702 (2022)10.1364/BOE.449329]. However, this method applies only to multiphoton excitation microscopy because of the requirement to reduce the sidelobes of the Bessel beam. Here, we introduce a beam that produces a needle spot while eluding the intractable artifacts due to the sidelobes. This beam can be adopted even in one-photon excitation fluorescence 3D imaging. The proposed method can achieve real-time, rapid 3D observation of 200-nm particles in water at a rate of over 50 volumes per second. In addition, fine structures, such as the spines of neurons in fixed mouse brain tissue, can be visualized in 3D from a single raster scan of the needle spot. The proposed method can be applied to various modalities in biological imaging, enabling rapid 3D image acquisition.

Wavefront engineered light needle microscopy for axially resolved rapid volumetric imaging

Increasing the acquisition speed of three-dimensional volumetric images is important-particularly in biological imaging-to unveil the structural dynamics and functionalities of specimens in detail. In conventional laser scanning fluorescence microscopy, volumetric images are constructed from optical sectioning images sequentially acquired by changing the observation plane, limiting the acquisition speed. Here, we present a novel method to realize volumetric imaging from two-dimensional raster scanning of a light needle spot without sectioning, even in the traditional framework of laser scanning microscopy. Information from multiple axial planes is simultaneously captured using wavefront engineering for fluorescence signals, allowing us to readily survey the entire depth range while maintaining spatial resolution. This technique is applied to real-time and video-rate three-dimensional tracking of micrometer-sized particles, as well as the prompt visualization of thick fixed biological specimens, offering substantially faster volumetric imaging.

Image scanning microscopy with a long depth of focus generated by an annular radially polarized beam

Image scanning microscopy (ISM) is a promising tool for bioimaging owing to its integration of signal to noise ratio (SNR) and super resolution superior to that obtained in confocal scanning microscopy. In this paper, we introduce the annular radially polarized beam to the ISM, which yields an axially extended excitation focus and enhanced resolution, providing a new possibility to obtain the whole information of thick specimen with a single scan. We present the basic principle and a rigorous theoretical model for ISM with annular radially polarized beam (ISM-aRP). Results show that the resolution of ISM-aRP can be enhanced by 4% compared with that in conventional ISM, and the axial extent of the focus is longer than 6λ. The projected view of the simulated fluorescent beads suspension specimen demonstrates the validity of ISM-aRP to obtain the whole information of volume sample. Moreover, this simple method can be easily integrated into the commercial laser scanning microscopy systems.

Imaging with a longitudinal electric field in confocal laser scanning microscopy to enhance spatial resolution

The longitudinal electric field produced by focusing a radially polarized beam is applied in confocal laser scanning microscopy by introducing a higher-order transverse mode, combined with a technique of polarization conversion for signal detection. This technique improves signal detection corresponding to the longitudinally polarized field under a small confocal pinhole, enabling full utilization of the small focal spot characteristic of the longitudinal field. Detailed numerical and experimental studies demonstrate the enhanced spatial resolution in confocal imaging that detects a scattering signal using a higher-order radially polarized beam. Our method can be widely applied in various imaging techniques that detect coherent signals such as second-harmonic generation microscopy.

Light needle microscopy with spatially transposed detection for axially resolved volumetric imaging

The demand for rapid three-dimensional volumetric imaging is increasing in various fields, including life science. Laser scanning fluorescence microscopy has been widely employed for this purpose; however, a volumetric image is constructed by two-dimensional image stacking with a varying observation plane, ultimately limiting the acquisition speed. Here we propose a method enabling axially resolved volumetric imaging without a moving observation plane in the framework of laser scanning microscopy. A scanning light needle spot with an extended focal depth provides excitation, which normally produces a deep focus image with a loss of depth information. In our method, the depth information is retrieved from transposed lateral information on an array detector by utilising non-diffracting and self-bending characteristics imposed on fluorescent signals. This technique, implemented in two-photon microscopy, achieves truly volumetric images constructed from a single raster scan of a light needle, which has the capability to significantly reduce the acquisition time.

Resolution enhancement in laser scanning microscopy with deconvolution switching laser modes (D-SLAM)

Laser scanning microscopy is limited in lateral resolution by the diffraction of light. Superresolution methods have been developed since the 90s to overcome this limitation. However superresolution is generally achieved at the expense of a greater complexity (high power lasers, very long acquisition times, specific fluorophores) and limitations on the observable samples. In this paper we propose a method to improve the resolution of confocal microscopy by combining different laser modes and deconvolution. Two images of the same field are acquired with the confocal microscope using different laser modes and used as inputs to a deconvolution algorithm. The two laser modes have different Point Spread Functions and thus provide complementary information leading to an image with enhanced resolution compared to using a single confocal image as input to the same deconvolution algorithm. By changing the laser modes to Bessel-Gauss beams we were able to further improve the efficiency of the deconvolution algorithm and obtain images with a residual Point Spread Function having a width of 0.14 λ (72 nm at a wavelength of 532 nm). This method only requires a laser scanning microscope and is not dependent on certain specific properties of fluorescent proteins. The proposed method requires only a few add-ons to classical confocal or two-photon microscopes and can easily be retrofitted into an existing commercial laser scanning microscope.

Advanced easySTED microscopy based on two-photon excitation by electrical modulations of light pulse wavefronts

We developed a compact stimulated emission depletion (STED) two-photon excitation microscopy that utilized electrically controllable components. Transmissive liquid crystal devices inserted directly in front of the objective lens converted the STED light into an optical vortex while leaving the excitation light unaffected. Light pulses of two different colors, 1.06 and 0.64 μm, were generated by laser diode-based light sources, and the delay between the two pulses was flexibly controlled so as to maximize the fluorescence suppression ratio. In our experiments, the spatial resolution of this system was up to three times higher than that obtained without STED light irradiation, and we successfully visualize the fine microtubule network structures in fixed mammalian cells without causing significant photo-damage.

Recent research on stimulated emission depletion microscopy for reducing photobleaching

Stimulated emission depletion (STED) microscopy is a useful tool in investigation for super-resolution realm. By silencing the peripheral fluorophores of the excited spot, leaving only the very centre zone vigorous for fluorescence, the effective point spread function (PSF) could be immensely squeezed and subcellular structures, such as organelles, become discernable. Nevertheless, because of the low cross-section of stimulated emission and the short fluorescence lifetime, the depletion power density has to be extremely higher than the excitation power density and molecules are exposed in high risk of photobleaching. The existence of photobleaching greatly limits the research of STED in achieving higher resolution and more delicate imaging quality, as well as long-term and dynamic observation. Since the first experimental implementation of STED microscopy, researchers have lift out variety of methods and techniques to alleviate the problem. This paper would present some researches via conventional methods which have been explored and utilised relatively thoroughly, such as fast scanning, time-gating, two-photon excitation (TPE), triplet relaxation (T-Rex) and background suppression. Alternatively, several up-to-date techniques, especially adaptive illumination, would also be unveiled for discussion in this paper. The contrast and discussion of these modalities would play an important role in ameliorating the research of STED microscopy.

Fluorescence emission difference with surface plasmon-coupled emission applied in confocal microscopy

We combined confocal surface plasmon coupled emission microscopy (C-SPCEM) together with fluorescence emission difference (FED) technique to pursuit super-resolution fluorescent image. Solid or hollow point spread function (PSF) for C-SPCEM is achieved with radially-polarized or circularly-polarized illumination. The reason why PSF can be manipulated by the polarization of illumination light is corroborated by the interaction of fluorescent emitter with vector focal field on the plasmonic substrate. After introduction of FED technique, PSF for C-SPECM can shrunk to around λ/4 in full-width half-maximum, which is unambiguously beyond Rayleigh's diffraction limit. The super-resolution capability of C-SPCEM with FED technique is experimentally demonstrated by imaging aggregated fluorescent beads with 150 nm in diameter.

Generalized vector wave theory for ultrahigh resolution confocal optical microscopy

Polarization modulation of a tightly focused beam in a confocal imaging scheme is considered for incident and collected light fields. Rigorous vector wave theory of a confocal optical microscopy is developed, which provides clear physical pictures without the requirement for fragmentary calculations. Multiple spatial modulations on polarization, phase, or amplitude of the illuminating and the detected beams can be mathematically described by a uniform expression. Linear and nonlinear excitation schemes are derived with tailored excitation and detection fields within this generalized theory, whose results show that the ultimate resolution achieved with the linear excitation can reach one-fifth of the excitation wavelength (or λ/5), while the nonlinear excitation scheme gives rise to a resolution better than λ/12 for two-photon fluorescence excitation and λ/20 for three-photon fluorescence excitation. Hence the resolution of optical microscopy with a near-infrared excitation can routinely reach sub-60 nm. In addition, simulations for confocal laser scanning microscopy are carried out with the linear excitation scheme and the fluorescent one, respectively.

Transmissive liquid-crystal device for correcting primary coma aberration and astigmatism in biospecimen in two-photon excitation laser scanning microscopy

All aberrations produced inside a biospecimen can degrade the quality of a three-dimensional image in two-photon excitation laser scanning microscopy. Previously, we developed a transmissive liquid-crystal device to correct spherical aberrations that improved the image quality of a fixed-mouse-brain slice treated with an optical clearing reagent. In this study, we developed a transmissive device that corrects primary coma aberration and astigmatism. The motivation for this study is that asymmetric aberration can be induced by the shape of a biospecimen and/or by a complicated refractive-index distribution in a sample; this can considerably degrade optical performance even near the sample surface. The device’s performance was evaluated by observing fluorescence beads. The device was inserted between the objective lens and microscope revolver and succeeded in improving the spatial resolution and fluorescence signal of a bead image that was originally degraded by asymmetric aberration. Finally, we implemented the device for observing a fixed whole mouse brain with a sloping surface shape and complicated internal refractive-index distribution. The correction with the device improved the spatial resolution and increased the fluorescence signal by ?2.4×. The device can provide a simple approach to acquiring higher-quality images of biospecimens.

Development of novel two-photon microscopy for living brain and neuron

"In vivo" two-photon microscopy (TPLSM) has revealed vital information on neural activity for brain function, even in light of its limitation in imaging events at depths greater than a several hundred micrometers from the brain surface. To break the limit of this penetration depth, we introduced a novel light source based on a semiconductor laser [1]. The light source successfully visualized not only cortex layer V pyramidal neurons spreading to all cortex layers at a superior S/N ratio, but visualize hippocampal CA1 neurons in young adult mice [2]. These results indicate that the penetration depth of this laser was ∼1.4 mm. In vivo TPLSM with a laser emitting a longer wavelength might give us insights on activities of neurons in the cortex or the hippocampus. This deep imaging method could be applicable to other living organs including tumor tissues. In addition, we developed liquid crystal devices to convert linearly polarized beams (LP) to vector beams [3]. A liquid device generated a vector beam called higher-order radially polarized (HRP) beam, which enabled that each of the aggregated 0.17 m beads was distinguished individually, whereas in conventional confocal microscopy or TPLSM they could not. We also visualized the finer structures of networks of filamentous cytoskeleton microtubule fluorescently-labeled in the COS-7, and primary culture of mouse neurons. Moreover, by taking an advantage of the LCDs that can utilize various wavelengths including near-infrared, we could employ an HRP beam for improving TPLSM. An HRP beam visualized fine intracellular structures not only in fixed cells stained with various dyes, but also in living cells expressing a fluorescent protein [4]. HRP beam also visualized finer structures of microtubules in fixed cells. Here, we will discuss these improvements and future application on the basis of our recent data.jmicro;63/suppl_1/i7/DFU087F1F1DFU087F1Fig. 1."in vivo" imaging of living mouse brain (H-line).

Correcting spherical aberrations in a biospecimen using a transmissive liquid crystal device in two-photon excitation laser scanning microscopy

Two-photon excitation laser scanning microscopy has enabled the visualization of deep regions in a biospecimen. However, refractive-index mismatches in the optical path cause spherical aberrations that degrade spatial resolution and the fluorescence signal, especially during observation at deeper regions. Recently, we developed transmissive liquid-crystal devices for correcting spherical aberration without changing the basic design of the optical path in a conventional laser scanning microscope. In this study, the device was inserted in front of the objective lens and supplied with the appropriate voltage according to the observation depth. First, we evaluated the device by observing fluorescent beads in single- and two-photon excitation laser scanning microscopes. Using a 25× water-immersion objective lens with a numerical aperture of 1.1 and a sample with a refractive index of 1.38, the device recovered the spatial resolution and the fluorescence signal degraded within a depth of 0.6 mm. Finally, we implemented the device for observation of a mouse brain slice in a two-photon excitation laser scanning microscope. An optical clearing reagent with a refractive index of 1.42 rendered the fixed mouse brain transparent. The device improved the spatial resolution and the yellow fluorescent protein signal within a depth of 0-0.54 mm.

Numerical analysis of resolution enhancement in laser scanning microscopy using a radially polarized beam

The spatial resolution characteristics in confocal laser scanning microscopy (LSM) and two-photon LSM utilizing a higher-order radially polarized Laguerre-Gaussian (RP-LG) beam are numerically analyzed. The size of the point spread function (PSF) and its dependence on the confocal pinhole size are compared with practical LSM using a circularly polarized Gaussian beam on the basis of vector diffraction theory. The spatial frequency response in terms of the optical transfer function (OTF) is also evaluated for LSM using the RP-LG beam. The smaller focal spot characteristics of higher-order RP-LG beams contribute to a dramatic enhancement of the lateral spatial resolution in confocal LSM and two-photon LSM.

Two-photon excitation STED microscopy by utilizing transmissive liquid crystal devices

Transmissive liquid crystal devices (tLCDs) enable the modification of optical properties, such as phase, polarization, and laser light intensity, over a wide wavelength region at a high conversion efficiency. By utilizing tLCDs, we developed a new two-photon excitation stimulated emission depletion microscopy technique based on a conventional two-photon microscope. Spatial resolution was improved by compensating for phase shifts distributed in the optical path. Using this technique, we observed the fine structures of microtubule networks in fixed biological cells.