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.

Laser nanoprocessing via an enhanced longitudinal electric field of a radially polarized beam.

Single-shot laser ablation is performed on the surface of a transparent glass material using a radially polarized femtosecond beam. Theoretical and experimental investigation revealed the significant role of the material interface under high-numerical-aperture conditions. The longitudinal electric field at the focus was remarkably enhanced due to the total reflection on the interface when a radially polarized beam was focused on the back surface of the glass from the inside using an immersion lens. This focusing condition enabled the fabrication of a small ablation hole sized 67 nm. This study offers a novel, to the best of our knowledge, approach to realize laser nanoprocessing with radially polarized beams.

Nanoprocessing of Self-Suspended Monolayer Graphene and Defect Formation by Femtosecond-Laser Irradiation.

We demonstrate the femtosecond-laser processing of self-suspended monolayer graphene grown by chemical vapor deposition, resulting in multipoint drilling with holes having a diameter of <100 nm. Scanning transmission electron microscopy revealed the formation of many nanopores on the laser-irradiated graphene. Furthermore, atomic-level defects as well as nanopores were found in the graphene membrane by high-resolution transmission electron microscopy, while the overall crystal structure remained intact. Raman spectroscopy showed an increase in the defect density with an increase in the number of laser shots, suggesting that the nanopore formation triggered the creation of the <100 nm holes. The approach presented herein can offer an experimental insight into the simulation of atomic dynamics in graphene under femtosecond-laser irradiation. The thorough examination of the atomic defect formation and secondary effect of surface cleaning observed in this study would help develop engineering methods for graphene and other two-dimensional materials in the future.

Single-path single-shot phase-shifting digital holographic microscopy without a laser light source.

We propose single-path single-shot phase-shifting digital holographic microscopy (SSP-DHM) in which the quantitative phase information of an object wave is acquired without a laser light source. Multiple phase-shifted holograms are simultaneously obtained using a linear polarizer, a liquid crystal on a silicon spatial light modulator (LCoS-SLM), and a polarization-imaging camera. Complex amplitude imaging of a USAF1951 test target and phase imaging of transparent HeLa cells are performed to show its quantitative phase-imaging ability. We also conduct an experiment for the motion-picture imaging of transparent particles to highlight the single-shot imaging ability of SSP-DHM.

Two-step phase-shifting interferometry for self-interference digital holography.

We propose a phase-shifting interferometry technique using only two in-line phase-shifted self-interference holograms. There is no requirement for additional recording or estimation in the measurement. The proposed technique adopts a mathematical model for self-interference digital holography. The effectiveness of the proposed technique is demonstrated by experiments on incoherent digital holographic microscopy and color-multiplexed fluorescence digital holography with computational coherent superposition. Two-color-multiplexed four-step phase-shifting incoherent digital holography is realized for the first time, to the best of our knowledge, using the proposed technique.

Ultrafast laser ablation of 10-nm self-supporting membranes by two-beam interference processing.

Ultrafast laser ablation was applied to process 10-nm self-supporting membranes. The membranes were processed over tens of square micrometers by single-shot irradiation of two visible laser pulses, followed by the realization of periodic sub-microstructures. The fabricated geometry is dependent on the intensity distribution of the superposed input pulses, providing flexibility and facilitating practical micro- and nanoengineering. Ease of designing the processing parameters and speed of processing are the significant advantages of this method compared to focused ion beam (FIB) milling.

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.

Direct generation of the lowest-order vortex beam using a spot defect mirror in the ultraviolet region.

The creation of ultraviolet optical vortex beams with the topological charge of $ \vert l \vert = 1 $|l|=1 at the wavelength of 325 nm was demonstrated from a He-Cd metal vapor laser with a spot defect mirror. The measured $ {{\rm M}^2} $M2 factor was close to the theoretical value of two of the $ {{\rm LG}_{01}} $LG01 Laguerre-Gaussian mode. Some interference experiments showed that the obtained vortex beams were stable enough for practical applications such as holographic lithography.

Laser microprocessing of metal surfaces using a tightly focused radially polarized beam.

Tight focusing of a radially polarized beam is used for single-shot laser ablation of metals. The strong longitudinal field is generated at the focus, and its contribution to the ablation process is comprehensively examined for various metal materials. In the presence of the longitudinal field at the focus, a fabricated crater at the surface exhibits either a spot shape or a doughnut shape, depending on the material. The experimental results indicate that the strong longitudinal electric field on metal surfaces is capable of promoting material removal, which may provide a novel processing scheme in ultrafast laser microprocessing with enhanced spatial resolution.

Subtraction imaging by the combination of higher-order vector beams for enhanced spatial resolution.

We demonstrate that the spatial resolution in confocal laser scanning microscopy is remarkably improved by simple image subtraction between images acquired by higher-order transverse modes of a cylindrical vector beam, referred to as radially and azimuthally polarized Laguerre-Gaussian mode beams. Two types of combinations of vector mode beams suitable for subtraction imaging are derived by a systematical study based on numerical calculations. The spatial resolution of about 100 nm is experimentally achieved without the degradation of image quality.

Micro-hole drilling by tightly focused vector beams.

We demonstrated that azimuthally polarized beams offer high-speed laser micro-hole drilling compared with radially and linearly polarized beams under a tight focusing condition. The speed was evaluated by piercing time by vector beams for several objective lenses with different numerical apertures (NAs). As a result, in the case of NA=0.63, the piercing time of an azimuthally polarized beam was 2.4 to 3.2 times shorter than that of a linearly polarized beam for all materials tested. Surprisingly, for NA=0.85, the difference is expanded to about 7.7 times for copper. This indicates that the number of reflections on the side wall of the hole played a significant role to give rise to the difference in the piercing time, depending on the polarization of the light beam.

Non-diffracting linear-shift point-spread function by focus-multiplexed computer-generated hologram.

An Airy beam can be used to implement a non-diffracting self-bending point-spread function (PSF), which can be utilized for computational 3D imaging. However, the parabolic depth-dependent spot trajectory limits the range and resolution in rangefinding. In this Letter, we propose a novel pupil-phase-modulation method to realize a non-diffracting linear-shift PSF. For the modulation, we use a focus-multiplexed computer-generated hologram, which is calculated by multiplexing multiple lens-function holograms with 2D sweeping of the foci. With this method, the depth-dependent trajectory of the non-diffracting spot is straightened, which improves the range and resolution in rangefinding. The proposed method was verified by numerical simulations and optical experiments. The method can be applied to laser-based microscopy, time-of-flight rangefinding, and so on.

Vector beam generation from vertical cavity surface emitting lasers.

Radially and azimuthally polarized beams in a single transverse mode are generated from a commercially available vertical cavity surface emitting laser (VCSEL) in an external cavity with a birefringent rutile lens, of which the c axis is parallel to the optical axis of the cavity, to select favorable polarization. Additionally, a vector Bessel-Gaussian beam is generated from a VCSEL, which is fabricated to oscillate with a linear polarization in a fixed direction in free running, in the same way. These results clearly show the potential ability of VCSELs to generate vector beams, which will be essential to space-division multiplexing in the future optical communication.

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.

Creation of polarization gradients from superposition of counter propagating vector LG beams.

We present a detailed theoretical analysis of the formation of standing waves using cylindrically polarized vector Laguerre-Gaussian (LG) beams. It is shown that complex interplay between the radial and azimuthal polarization state can be used to realize different kinds of polarization gradients with cylindrically symmetric polarization distribution. Expressions for four different cases are presented and local dynamics of spatial polarization distribution is studied. We show cylindrically symmetric Sisyphus and corkscrew type polarization gradients can be obtained from vector LG beams. The optical landscape presented here with spatially periodic polarization patterns may find important applications in the field of atom optics, atom interferometry, atom lithography, and optical trapping.

Transverse-mode selective laser operation by unicursal fast-scanning pumping.

We demonstrate that higher order transverse-mode beams can be selectively operated by creating their suitable gain distributions in a laser gain medium. By adjusting the scanning angles and the phase difference between two resonant scanning mirrors used for the manipulation of pump beam patterns, a wide variety of gain distributions such as linear, elliptical, and circular patterns were generated. As a result, single transverse-mode operations of many higher order transverse modes of Hermite-Gaussian, Laguerre-Gaussian, and Ince-Gaussian beams were realized. The switching of these modes was readily and quickly accomplished without further mechanical alignment and any optical elements.

Super-resolution imaging of lateral distribution for the blue-light emission of an InGaN single-quantum-well structure utilizing the stimulated emission depletion effect.

We have observed a remarkable decrease in photoluminescence (PL) from a blue-light emitting InGaN single-quantum-well (SQW) structure under the radiation of a green laser due to the stimulated emission depletion (STED) phenomenon. By extending the observed STED effect, super-resolution imaging of the blue-light emission lateral distribution was demonstrated for the InGaN-SQW structure through co-irradiation using a doughnut-shaped green light beam and a Gaussian-shaped violet excitation light beam. We measured point-spread functions (PSFs) to evaluate the spatial resolution of the system by imaging a small emission area. A lateral PSF size of ~150 nm was confirmed, which was approximately 40% smaller than that without the STED beam. This demonstrates that the STED technique is applicable for PL imaging of semiconductor quantum structures. The present approach may make possible a new strategy for characterizing and investigating the spatial inhomogeneity of emission properties and carrier dynamics in InGaN-based quantum wells, as well as in other semiconductor materials exhibiting quantum confinement effects.

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.

7-ps optical pulse generation from a 1064-nm gain-switched laser diode and its application for two-photon microscopy.

In this study, we investigated the picosecond optical pulse generation from a 1064-nm distributed feedback laser diode under strong gain switching. The spectrum of the generated optical pulses was manipulated in two different ways: (i) by extracting the short-wavelength components of the optical pulse spectrum and (ii) by compensating for spectral chirping in the extracted mid-spectral region. Both of these methods shortened the optical pulse duration to approximately 7 ps. These optical pulses were amplified to over 20-kW peak power for two-photon microscopy. We obtained clear two-photon images of neurons in a fixed brain slice of H-line mouse expressing enhanced yellow fluorescent protein. Furthermore, a successful experiment was also confirmed for in vivo deep region H-line mouse brain neuron imaging.

Generation of radially polarized Bessel-Gaussian beams from c-cut Nd:YVO4 laser.

We experimentally demonstrate the generation of radially polarized Bessel-Gaussian beams from a c-cut Nd:YVO4 laser with a hemispherical cavity configuration by proper mode control. The output beam has an annular-shaped intensity distribution with radial polarization. When the beam is focused, the intensity pattern changes to a multi-ring, which is a typical characteristic of the lowest transverse mode of vector Bessel-Gaussian beam. Higher-order modes of vector Bessel-Gaussian beam are also observed from the same cavity by slightly changing the cavity alignment. The experimental results show a good agreement with the simulation results for both focal and far fields. The present method is a simple and direct way for generating vector Bessel-Gaussian beams.