Multiphoton multifocal microscopy exploiting a diffractive optical element

High-resolution multi-z confocal microscopy with a diffractive optical element

There has been recent interest in the development of fluorescence microscopes that provide high-speed volumetric imaging for life-science applications. For example, multi-z confocal microscopy enables simultaneous optically-sectioned imaging at multiple depths over relatively large fields of view. However, to date, multi-z microscopy has been hampered by limited spatial resolution owing to its initial design. Here we present a variant of multi-z microscopy that recovers the full spatial resolution of a conventional confocal microscope while retaining the simplicity and ease of use of our initial design. By introducing a diffractive optical element in the illumination path of our microscope, we engineer the excitation beam into multiple tightly focused spots that are conjugated to axially distributed confocal pinholes. We discuss the performance of this multi-z microscope in terms of resolution and detectability and demonstrate its versatility by performing in-vivo imaging of beating cardiomyocytes in engineered heart tissues and neuronal activity in c. elegans and zebrafish brains.

Design and Study of a Reflector-Separated Light Dispersion-Compensated 3D Microscopy System

The secondary-phase grating-based tomographic microscopy system, which is widely used in the biological and life sciences, can observe all the sample multilayer image information simultaneously because it has multifocal points. However, chromatic aberration exists in the grating diffraction, which seriously affects the observation of the image. To correct the chromatic aberration of the tomographic microscope system, this paper proposes a system that adopts blazed gratings and angle-variable reflectors as chromatic aberration correction devices according to the principle of dispersion compensation and Fourier phase-shift theory. A reflector-separated light dispersion-compensated 3D microscopy system is presented to achieve chromatic aberration correction while solving the problem of multilayer image overlap. The theoretical verification and optical design of the system were completed using ZEMAX software. The results show that the proposed system reduced the chromatic aberration of ordinary tomographic microscopy systems by more than 90%, retaining more wavelengths of light information. In addition, the system had a relatively wide range in the color difference compensation element installation position, reducing the difficulty of dispersion compensation element installation. Overall, the results indicate that the proposed system is effective in reducing chromatic aberration in grating diffraction.

Multi-Focal Laser Direct Writing through Spatial Light Modulation Guided by Scalable Vector Graphics

Multi-focal laser direct writing (LDW) based on phase-only spatial light modulation (SLM) can realize flexible and parallel nanofabrication with high-throughput potential. In this investigation, a novel approach of combining two-photon absorption, SLM, and vector path-guided by scalable vector graphics (SVGs), termed SVG-guided SLM LDW, was developed and preliminarily tested for fast, flexible, and parallel nanofabrication. Three laser focuses were independently controlled with different paths, which were optimized according to the SVG to improve fabrication and promote time efficiency. The minimum structure width could be as low as 81 nm. Accompanied by a translation stage, a carp structure of 18.10 μm × 24.56 μm was fabricated. This method shows the possibility of developing LDW techniques toward fully electrical systems, and provides a potential way to efficiently engrave complex structures on nanoscales.

Fast readout method for multidimensional optical data storage using interferometry-aided reflectance spectroscopy

The multiplex technique increases the capacity of optical data storage, but the current reading throughputs is limited by the single-bit reading. We propose a fast readout method of multidimensional optical data storage using interference-aided reflectance spectral measurement to readout multiple bits information simultaneously. The multidimensional data is recorded in the photoresist layer on the disc with dielectric multilayer substrate by laser direct writing. With the designed interference layer inside the disc, the relation of thickness of recording layer and the peak shift of the reflected spectra has been built up. With different writing depths representing different bit of data, 2 bits and 3 bits unit information have been recorded and successfully read out at one exposure. This fast readout method is not only suitable for optical data storage by engineering the optical path length for example Blu-ray disc but also for super resolution optical data storage.

Characterization of flat-fielding systems for quantitative microscopy

Optical flat-fielding systems, such as field-mapping or integration-based beam shapers, are used to transform nonuniform illumination into uniform illumination. Thus, flat-fielding paves the way for imaging that is independent of position within a field of view and enables more quantitative analysis. Here, we characterize and compare three systems for homogenizing both widefield and multifocal illumination. Our analysis includes two refractive field-mapping beam shapers: PiShaper and TopShape, as well as one integration-based: Köhler integrator. The comparison is based on figures of merit including ISO-standard values, such as the plateau uniformity and edge steepness, transmission efficiency, stability of the beams along propagation and multifocal intensity, pitch, and point width. By characterizing and comparing existing beam shapers, we facilitate the choice of the appropriate flat-fielding solution and increase their accessibility for different applications.

Homogeneous multifocal excitation for high-throughput super-resolution imaging

Super-resolution microscopies have become an established tool in biological research. However, imaging throughput remains a main bottleneck in acquiring large datasets required for quantitative biology. Here we describe multifocal flat illumination for field-independent imaging (mfFIFI). By integrating mfFIFI into an instant structured illumination microscope (iSIM), we extend the field of view (FOV) to >100 × 100 µm² while maintaining high-speed, multicolor, volumetric imaging at double the diffraction-limited resolution. We further extend the effective FOV by stitching adjacent images for fast live-cell super-resolution imaging of dozens of cells. Finally, we combine our flat-fielded iSIM with ultrastructure expansion microscopy to collect three-dimensional (3D) images of hundreds of centrioles in human cells, or thousands of purified Chlamydomonas reinhardtii centrioles, per hour at an effective resolution of ~35 nm. Classification and particle averaging of these large datasets enables 3D mapping of posttranslational modifications of centriolar microtubules, revealing differences in their coverage and positioning.

Azimuthal multiplexing 3D diffractive optics

Diffractive optics have increasingly caught the attention of the scientific community. Classical diffractive optics are 2D diffractive optical elements (DOEs) and computer-generated holograms (CGHs), which modulate optical waves on a solitary transverse plane. However, potential capabilities are missed by the inherent two-dimensional nature of these devices. Previous work has demonstrated that extending the modulation from planar (2D) to volumetric (3D) enables new functionalities, such as generating space-variant functions, multiplexing in the spatial or spectral domain, or enhancing information capacity. Unfortunately, despite significant progress fueled by recent interest in metasurface diffraction, 3D diffractive optics still remains relatively unexplored. Here, we introduce the concept of azimuthal multiplexing. We propose, design, and demonstrate 3D diffractive optics showing this multiplexing effect. According to this new phenomenon, multiple pages of information are encoded and can be read out across independent channels by rotating one or more diffractive layers with respect to the others. We implement the concept with multilayer diffractive optical elements. An iterative projection optimization algorithm helps solve the inverse design problem. The experimental realization using photolithographically fabricated multilevel phase layers demonstrates the predicted performance. We discuss the limitations and potential of azimuthal multiplexing 3D diffractive optics.

Flexible Multi-Beam Light-Sheet Fluorescence Microscope for Live Imaging Without Striping Artifacts

The development of light-sheet fluorescence microscopy (LSFM) has greatly expanded the experimental capabilities in many biological and biomedical research fields, enabling for example live studies of murine and zebrafish neural activity or of cell growth and division. The key feature of the method is the selective illumination of a sample single plane, providing an intrinsic optical sectioning and allowing direct 2D image recording. On the other hand, this excitation scheme is more affected by absorption or scattering artifacts in comparison to point scanning methods, leading to un-even illumination. We present here an easily implementable method, based on acousto-optical deflectors (AOD), to overcome this obstacle. We report the advantages provided by flexible and fast AODs in generating simultaneous angled multiple beams from a single laser beam and in fast light sheet pivoting and we demonstrate the suppression of illumination artifacts.

High speed functional imaging with source localized multifocal two-photon microscopy

Multifocal two-photon microscopy (MTPM) increases imaging speed over single-focus scanning by parallelizing fluorescence excitation. The imaged fluorescence's susceptibility to crosstalk, however, severely degrades contrast in scattering tissue. Here we present a source-localized MTPM scheme optimized for high speed functional fluorescence imaging in scattering mammalian brain tissue. A rastered line array of beamlets excites fluorescence imaged with a complementary metal-oxide-semiconductor (CMOS) camera. We mitigate scattering-induced crosstalk by temporally oversampling the rastered image, generating grouped images with structured illumination, and applying Richardson-Lucy deconvolution to reassign scattered photons. Single images are then retrieved with a maximum intensity projection through the deconvolved image groups. This method increased image contrast at depths up to 112 μm in scattering brain tissue and reduced functional crosstalk between pixels during neuronal calcium imaging. Source-localization did not affect signal-to-noise ratio (SNR) in densely labeled tissue under our experimental conditions. SNR decreased at low frame rates in sparsely labeled tissue, with no effect at frame rates above 50 Hz. Our non-descanned source-localized MTPM system enables high SNR, 100 Hz capture of fluorescence transients in scattering brain, increasing the scope of MTPM to faster and smaller functional signals.

Particle tracking by repetitive phase-shift interferometric super resolution microscopy

Accurate and rapid particle tracking is essential for addressing many research problems in single molecule and cellular biophysics and colloidal soft condensed matter physics. We developed a novel three-dimensional interferometric fluorescent particle tracking approach that does not require any sample scanning. By periodically shifting the interferometer phase, the information stored in the interference pattern of the emitted light allows localizing particles positions with nanometer resolution. This tracking protocol was demonstrated by measuring a known trajectory of a fluorescent bead with sub-5 nm axial localization error at 5 Hz. The interferometric microscopy was used to track the RecA protein in Bacillus subtilis bacteria to demonstrate its compatibility with biological systems.

Designing a large field-of-view two-photon microscope using optical invariant analysis

Conventional two-photon microscopy (TPM) is capable of imaging neural dynamics with subcellular resolution, but it is limited to a field-of-view (FOV) diameter [Formula: see text]. Although there has been recent progress in extending the FOV in TPM, a principled design approach for developing large FOV TPM (LF-TPM) with off-the-shelf components has yet to be established. Therefore, we present a design strategy that depends on analyzing the optical invariant of commercially available objectives, relay lenses, mirror scanners, and emission collection systems in isolation. Components are then selected to maximize the space-bandwidth product of the integrated microscope. In comparison with other LF-TPM systems, our strategy simplifies the sequence of design decisions and is applicable to extending the FOV in any microscope with an optical relay. The microscope we constructed with this design approach can image [Formula: see text] lateral and [Formula: see text] axial resolution over a 7-mm diameter FOV, which is a 100-fold increase in FOV compared with conventional TPM. As a demonstration of the potential that LF-TPM has on understanding the microarchitecture of the mouse brain across interhemispheric regions, we performed in vivo imaging of both the cerebral vasculature and microglia cell bodies over the mouse cortex.

Confocal multispot microscope for fast and deep imaging in semicleared tissues

Although perfectly transparent specimens are imaged faster with light-sheet microscopy, less transparent samples are often imaged with two-photon microscopy leveraging its robustness to scattering; however, at the price of increased acquisition times. Clearing methods that are capable of rendering strongly scattering samples such as brain tissue perfectly transparent specimens are often complex, costly, and time intensive, even though for many applications a slightly lower level of tissue transparency is sufficient and easily achieved with simpler and faster methods. Here, we present a microscope type that has been geared toward the imaging of semicleared tissue by combining multispot two-photon excitation with rolling shutter wide-field detection to image deep and fast inside semicleared mouse brain. We present a theoretical and experimental evaluation of the point spread function and contrast as a function of shutter size. Finally, we demonstrate microscope performance in fixed brain slices by imaging dendritic spines up to 400-μm deep.

Dynamic three-dimensional multifocal spots in high numerical-aperture objectives

Multifocal spots in high numerical-aperture (NA) objectives has emerged as a rapid, parallel, and multi-location method in a multitude of applications. However, the typical method used for forming three-dimensional (3D) multifocal spots based on iterative algorithms limits the potential applications. We demonstrate a non-iterative method using annular subzone phases (ASPs) that are composed of many annular subareas in which phase-only distributions with different 3D displacements are filled. The dynamic 3D multifocal spots with controllable position of each focal spot in the focal volume of the objective are created using the ASPs. The experimental results of such dynamic tunable 3D multifocal spots offer the possibility of versatile process in laser 3D fabrication, optical trapping, and fast focusing scanned microscopic imaging.

Correction of spherical aberration in multi-focal multiphoton microscopy with spatial light modulator

We demonstrate that high-quality images of the deep regions of a thick sample can be obtained from its surface by multi-focal multiphoton microscopy (MMM). The MMM system incorporates a spatial light modulator to separate the excitation beam into a multi-focal excitation beam and modulate the pre-distortion wavefront to correct spherical aberration (SA) caused by a refractive index mismatch between the immersion medium and the biological sample. When fluorescent beads in transparent epoxy resin were observed using four SA-corrected focal beams, the fluorescence signal of the obtained images was ~52 times higher than that obtained without SA correction until a depth of ~1100 μm, similar to the result for single-focal multiphoton microscopy (SMM). The MMM scanning time was four times less than that for SMM, and MMM showed an improved fluorescence intensity and depth resolution for an image of blood vessels in the brain of a mouse stained with a fluorescent dye.

High-speed multifocal array scanning using refractive window tilting

Confocal microscopy has several advantages over wide-field microscopy, such as out-of-focus light suppression, 3D sectioning, and compatibility with specialized detectors. While wide-field microscopy is a faster approach, multiplexed confocal schemes can be used to make confocal microscopy more suitable for high-throughput applications, such as high-content screening (HCS) commonly used in drug discovery. An increasingly powerful modality in HCS is fluorescence lifetime imaging microscopy (FLIM), which can be used to measure protein-protein interactions through Förster resonant energy transfer (FRET). FLIM-FRET for HCS combines the requirements of high throughput, high resolution and specialized time-resolving detectors, making it difficult to implement using wide-field and spinning disk confocal approaches. We developed a novel foci array scan method that can achieve uniform multiplex confocal acquisition using stationary lenslet arrays for high resolution and high throughput FLIM. Unlike traditional mirror galvanometers, which work in Fourier space between scan lenses, this scan method uses optical flats to steer a 2-dimension foci array through refraction. After integrating this scanning scheme in a multiplexing confocal FLIM system, we demonstrate it offers clear benefits over traditional mirror galvanometer scanners in scan linearity, uniformity, cost and complexity.

Non-descanned multifocal multiphoton microscopy with a multianode photomultiplier tube

Multifocal multiphoton microscopy (MMM) improves imaging speed over a point scanning approach by parallelizing the excitation process. Early versions of MMM relied on imaging detectors to record emission signals from multiple foci simultaneously. For many turbid biological specimens, the scattering of emission photons results in blurred images and degrades the signal-to-noise ratio (SNR). We have recently demonstrated that a multianode photomultiplier tube (MAPMT) placed in a descanned configuration can effectively collect scattered emission photons from each focus into their corresponding anodes significantly improving image SNR for highly scattering specimens. Unfortunately, a descanned MMM has a longer detection path resulting in substantial emission photon loss. Optical design constraints in a descanned geometry further results in significant optical aberrations especially for large field-of-view (FOV), high NA objectives. Here, we introduce a non-descanned MMM based on MAPMT that substantially overcomes most of these drawbacks. We show that we improve signal efficiency up to fourfold with limited image SNR degradation due to scattered emission photons. The excitation foci can also be spaced wider to cover the full FOV of the objective with minimal aberrations. The performance of this system is demonstrated by imaging interneuron morphological structures deep in the brains of living mice.

A pragmatic guide to multiphoton microscope design

Multiphoton microscopy has emerged as a ubiquitous tool for studying microscopic structure and function across a broad range of disciplines. As such, the intent of this paper is to present a comprehensive resource for the construction and performance evaluation of a multiphoton microscope that will be understandable to the broad range of scientific fields that presently exploit, or wish to begin exploiting, this powerful technology. With this in mind, we have developed a guide to aid in the design of a multiphoton microscope. We discuss source selection, optical management of dispersion, image-relay systems with scan optics, objective-lens selection, single-element light-collection theory, photon-counting detection, image rendering, and finally, an illustrated guide for building an example microscope.

Multispot multiphoton Ca²⁺ imaging in acute myocardial slices

Multiphoton microscopy has become essential for dynamic imaging in thick living tissues. High-rate, full-field image acquisition in multiphoton microscopy is achievable by parallelization of the excitation and detection pathways. We developed our approach via a diffractive optical element which splits a pulsed laser into 16 beamlets and exploits a descanned detection system consisting of an array of beamlet-associated photomultiplier tubes. The optical performance of the multiphoton multispot system (MCube) has been characterized in cardiac tissue sections and subsequently used for the first time for fluorescence imaging of cardiomyocyte Ca²⁺ dynamics in viable acute cardiac slices. Multispot multiphoton microscopy (MMM) has never been used before to monitor Ca²⁺ dynamics in thick, viable tissue samples. Acute heart slices are a powerful close-to-in vivo model of Ca²⁺ imaging allowing the simultaneous observation of several cells in their own tissue environment, exploiting the multiphoton excitation ability to penetrate scattering tissues. Moreover, we show that the concurrent high spatial and temporal resolutions afforded by the parallel scanning in MMM can be exploited to simultaneously assess subcellular Ca²⁺ dynamics in different cells in the tissue. We recorded local Ca²⁺ release events including macrosparks, travelling waves, and rotors.

Temporal focusing microscopy

Axial localization of multiphoton excitation to a single plane is achieved by temporal focusing of an ultrafast pulsed excitation. We take advantage of geometrical dispersion in an extremely simple experimental setup, where an ultrashort pulse is temporally stretched and hence its peak intensity is lowered outside the focal plane of the microscope. Using this strategy, out-of-focus multiphoton excitation is dramatically reduced, and the achieved axial resolution is comparable to line-scanning multiphoton microscopy for wide-field excitation and to point-scanning multiphoton microscopy for line excitation. In this introduction, we provide a detailed description of the considerations in choosing the experimental parameters, as well as the alignment of a temporal focusing add-on to a multiphoton microscope. We also review current advances and applications for this technique.

A high speed multifocal multiphoton fluorescence lifetime imaging microscope for live-cell FRET imaging

We demonstrate diffraction limited multiphoton imaging in a massively parallel, fully addressable time-resolved multi-beam multiphoton microscope capable of producing fluorescence lifetime images with sub-50ps temporal resolution. This imaging platform offers a significant improvement in acquisition speed over single-beam laser scanning FLIM by a factor of 64 without compromising in either the temporal or spatial resolutions of the system. We demonstrate FLIM acquisition at 500 ms with live cells expressing green fluorescent protein. The applicability of the technique to imaging protein-protein interactions in live cells is exemplified by observation of time-dependent FRET between the epidermal growth factor receptor (EGFR) and the adapter protein Grb2 following stimulation with the receptor ligand. Furthermore, ligand-dependent association of HER2-HER3 receptor tyrosine kinases was observed on a similar timescale and involved the internalisation and accumulation or receptor heterodimers within endosomes. These data demonstrate the broad applicability of this novel FLIM technique to the spatio-temporal dynamics of protein-protein interaction.

Rapid microstructure characterization of polymer thin films with 2D-array multifocus Raman microspectroscopy

Multifocus Raman imaging is one of the fast-imaging alternatives to the conventional single point mapping technique.

Stable and flexible multiple spot pattern generation using LCOS spatial light modulator

The LCOS spatial light modulator (LCOS-SLM) can generate desired multiple spot patterns (MSPs) via the application of suitable computer-generated-holograms (CGHs), but the MSP intensity distribution varies because ambient temperature affects the phase modulation characteristic and causes wavefront distortion. To generate high-optical-quality MSPs we use our hardware-compensated (with a Peltier system to even out phase modulation) and software-corrected (via multiplication of the CGH by temperature correction coefficients) LCOS-SLMs. Experimental results with a 14 × 14 MSP generation show that the hardware-compensated LCOS-SLM provides stable MSPs between 9 to 32 °C. The software-corrected LCOS-SLM provides uniform spots over twice the temperature range obtained with conventional SLM method. We confirm that our methods are highly efficient for use in two-photon excitation microscopy application such as multifocal mulitphoton microscopy.

Spectral-resolved multifocal multiphoton microscopy with multianode photomultiplier tubes

Multiphoton excitation fluorescence microscopy is the preferred method for in vivo deep tissue imaging. Many biological applications demand both high imaging speed and the ability to resolve multiple fluorophores. One of the successful methods to improve imaging speed in a highly turbid specimen is multifocal multiphoton microscopy (MMM) based on use of multi-anode photomultiplier tubes (MAPMT). This approach improves imaging speed by using multiple foci for parallelized excitation without sacrificing signal to noise ratio (SNR) due to the scattering of emission photons. In this work, we demonstrate that the MAPMT based MMM can be extended with spectral resolved imaging capability. Instead of generating multiple excitation foci in a 2D grid pattern, a linear array of foci is generated. This leaves one axis of the 2D MAPMT available for spectral dispersion and detection. The spectral-resolved MMM can detect several emission signals simultaneously with high imaging speed optimized for high-throughput, high-contents applications. The new procedure is illustrated using imaging data from the kidney, peripheral nerve regeneration and dendritic morphological data from the brain.

Three-dimensional shape-controllable focal spot array created by focusing vortex beams modulated by multi-value pure-phase grating

We propose a method for creating a three-dimensional (3D) shape-controllable focal spot array by combination of a two-dimensional (2D) pure-phase modulation grating and an additional axial shifting pure-phase modulation composed of four-quadrant phase distribution unit at the back aperture of a high numerical aperture (NA) objective. It is demonstrated that the one-dimensional (1D) grating designed by optimized algorithm of selected number of equally spaced arbitrary phase value in a single period could produce desired number of equally spaced diffraction spot with identical intensity. It is also shown that the 2D pure-phase grating designed with this method could generate 2D diffraction spot array. The number of the spots in the array along each of two dimensions depends solely on the number of divided area with different phase values of the dimension. We also show that, by combining the axial translation phase modulation at the back aperture, we can create 3D focal spot array at the focal volume of the high NA objective. Furthermore, the shape or intensity distribution of each focal spot in the 3D focal array can be manipulated by introducing spatially shifted multi vortex beams as the incident beam. These kinds of 3D shape-controllable focal spot array could be utilized in the fabrication of artificial metamaterials, in parallel optical micromanipulation and multifocal multiphoton microscopic imaging.

High-throughput nonlinear optical microscopy

High-resolution microscopy methods based on different nonlinear optical (NLO) contrast mechanisms are finding numerous applications in biology and medicine. While the basic implementations of these microscopy methods are relatively mature, an important direction of continuing technological innovation lies in improving the throughput of these systems. Throughput improvement is expected to be important for studying fast kinetic processes, for enabling clinical diagnosis and treatment, and for extending the field of image informatics. This review will provide an overview of the fundamental limitations on NLO microscopy throughput. We will further cover several important classes of high-throughput NLO microscope designs with discussions on their strengths and weaknesses and their key biomedical applications. Finally, this review will close with a perspective of potential future technological improvements in this field.

Reassignment of scattered emission photons in multifocal multiphoton microscopy

Multifocal multiphoton microscopy (MMM) achieves fast imaging by simultaneously scanning multiple foci across different regions of specimen. The use of imaging detectors in MMM, such as CCD or CMOS, results in degradation of image signal-to-noise-ratio (SNR) due to the scattering of emitted photons. SNR can be partly recovered using multianode photomultiplier tubes (MAPMT). In this design, however, emission photons scattered to neighbor anodes are encoded by the foci scan location resulting in ghost images. The crosstalk between different anodes is currently measured a priori, which is cumbersome as it depends specimen properties. Here, we present the photon reassignment method for MMM, established based on the maximum likelihood (ML) estimation, for quantification of crosstalk between the anodes of MAPMT without a priori measurement. The method provides the reassignment of the photons generated by the ghost images to the original spatial location thus increases the SNR of the final reconstructed image.

Multifocal spot array generated by fractional Talbot effect phase-only modulation

We propose an approach for generating a multifocal spot array (MSA) with a high numerical aperture (NA) objective. The MSA is generated by using a special designed phase-only modulation at the back aperture of an objective. Without using any iteration algorithm, the modulated phase pattern is directly obtained by the simple analytical expressions based on the fractional Talbot effect. It is shown that the number of the spots in the focal region depends solely on the fractional Talbot parameter. By engineering the phase pattern with a large fractional Talbot parameter, a large number of focal spots can be created. Furthermore, the intensity distribution of each focal spot can be manipulated by introducing a composite spatially shifted vortex beam (CSSVB) as the incident field, leading to creation of various kinds of specific shaped spots. Consequently, the MSA composed of multiple individual spots with specific shape is created by focusing the CSSVB combined with the multifocal phase-only modulation. These kinds of MSAs may be found applications in parallel optical micromanipulation, multifocal multiphoton microscopic imaging, and parallel laser printing nanofabrication.

Development of a doubly weighted Gerchberg-Saxton algorithm for use in multibeam imaging applications

We report on the development of a doubly weighted Gerchberg-Saxton algorithm (DWGS) to enable generation of uniform beamlet arrays with a spatial light modulator (SLM) for use in multiphoton multifocal imaging applications. The algorithm incorporates the WGS algorithm as well as feedback of fluorescence signals from the sample measured with a single-photon avalanche diode (SPAD) detector array. This technique compensates for issues associated with nonuniform illumination onto the SLM, the effects due to aberrations and the variability in gain between detectors within the SPAD array to generate a uniformly illuminated multiphoton fluorescence image. We demonstrate the use of the DWGS with a number of beamlet array patterns to image muscle fibers of a 5-day-old fixed zebrafish larvae.

An adaptive approach for uniform scanning in multifocal multiphoton microscopy with a spatial light modulator

We propose high-quality generation of uniform multiple fluorescence spots (MFS) with a spatial light modulator (SLM) and demonstrate uniform laser scanning in multifocal multiphoton microscopy (MMM). The MFS excitation method iteratively updates a computer-generated hologram (CGH) using correction coefficients to improve the fluorescence intensity distribution in a dye solution whose consistency is uniform. This simple correction method can be applied for calibration of the MMM before observation of living tissue. We experimentally demonstrate an improvement of the uniformity of a 10 × 10 grid of MFS by using a dye solution. After the calibration, we performed laser scanning with two-photon excitation to observe fluorescent polystyrene beads, as well as the gastric gland of a guinea pig specimen.

Parallel multiphoton microscopy with cylindrically polarized multifocal arrays

Diffraction-limited cylindrically polarized multifocal arrays are created in the focal region of a high numerical-aperture objective for multiphoton microscopy by applying the dynamic phase modulation on an incident light beam. We show that this kind of cylindrical-polarization multifocal multiphoton microscopy exhibits a parallel imaging capacity but also a dynamic switching-on or -off feature of individual focal spots. The parallel multiphoton microscopy results of the polarization-sensitive gold nanorods under the illumination of radially or azimuthally polarized multifocal arrays allow for the fast determination of the orientation of nanorods.

Multifocal multiphoton microscopy based on a spatial light modulator

We present a new multifocal multiphoton microscope that employs a programmable spatial light modulator to generate dynamic multifocus arrays which can be rapidly scanned by changing the incident angle of the laser beam using a pair of galvo scanners. Using this microscope, we can rapidly select the number and the spatial density of focal points in a multifocus array, as well as the locations and shapes of arrays according to the features of the areas of interest in the field of view without any change to the hardware.

Parallel nanometric 3D tracking of intracellular gold nanorods using multifocal two-photon microscopy

We report a novel technique for long-term parallel three dimensional (3D)-tracking of gold nanorods in live cells with nanometer resolution. Gold nanorods feature a strong plasmon-enhanced two-photon luminescence, can be easily functionalized, and have been shown to be nontoxic. These properties make gold nanorods very suitable for in vivo two-photon luminescence microscopy. By rapid multifocal scanning, we combine the advantages of 3D molecular tracking methods using wide-field imaging with the advantages of two-photon microscopy. Isolated gold nanorods can be localized with a resolution of 4 nm in the xy-plane and 8 nm in the z-direction. The polarization-dependence of the two-photon luminescence signal can be used to resolve the angular orientation, even when two gold nanorods are separated by less than the diffraction limit. Individual nanorods in live U2OS cells could be followed in 3 dimensions for over 30 min, with a photon noise limited accuracy, and a time resolution of 50 ms in 2D and 500 ms in 3D.

Dispersion management in two-photon microscopy by using diffractive optical elements

We demonstrate efficient generation of wide-field fluorescence signals in two-photon microscopy exploiting diffractive optical elements and short pulses by using a dispersion-compensated beam delivery optics module. Computer-generated holograms are codified onto a phase-only spatial light modulator, which allows for arbitrary single-shot patterning of the sample. Spatiotemporal shaping of the pulse is mandatory to overcome spatial chirp and pulse-front tilt effects that spread both in space and time the irradiance patterns, thus limiting not only the spatial resolution but also the signal-to-noise ratio in two-photon microscopy. By using a multipass amplifier delivering 30 fs, 0.8 mJ pulses at 1 kHz repetition rate, we experimentally demonstrated arbitrary single-shot fluorescence irradiance patterns in Rhodamine B.

Enhancement of lateral resolution and optical sectioning capability of two-photon fluorescence microscopy by combining temporal-focusing with structured illumination

We demonstrate super-resolution imaging with background fluorescence rejection by interferometric temporal focusing microscopy, in which temporal focusing is combined with structured illumination. The lateral resolution and the optical sectioning capability are simultaneously improved by factors of 1.6 and 1.4, respectively, compared to conventional temporal focusing microscopy. Fluorescent beads (200 nm diameter) that are difficult to distinguish from the background fluorescence in conventional temporal focusing microscopy, are clearly visualized by interferometric temporal focusing microscopy.

Addressable, large-field second harmonic generation microscopy based on 2D acousto-optical deflector and spatial light modulator

We present an addressable, large-field second harmonic generation microscope by combining a 2D acousto-optical deflector with a spatial light modulator. The SLM shapes an incoming mode-locked, near-infrared Ti:Sapphire laser beam into a multifocus array, which can be rapidly scanned by changing the incident angle of the laser beam using a 2D acousto-optical deflector. Compared to the single-beam-scan technique, the multifocus array scan can increase the scanning rate and the field-of-view size with the multi-region imaging ability.

RECENT PROGRESS IN MULTIFOCAL MULTIPHOTON MICROSCOPY

Multifocal multiphoton microscopy (MMM) has recently become an important tool in biomedicine for performing three-dimensional fast fluorescence imaging. Using various beamsplitting techniques, MMM splits the near-infrared laser beam into multiple beamlets and produces a multifocal array on the sample for parallel multiphoton excitation and then records fluorescence signal from all foci simultaneously with an area array detector, which significantly improves the imaging speed of multiphoton microscopy and allows for high efficiency in use of the excitation light. In this paper, we discuss the features of several MMM setups using different beamsplitting devices, including a Nipkow spinning disk, a microlens array, a set of beamsplitting mirrors, or a diffractive optical element (DOE). In particular, we present our recent work on the development of an MMM using a spatial light modulator (SLM).

Addressable multiregional and multifocal multiphoton microscopy based on a spatial light modulator

Through a combination of a deflective phase-only diffractive spatial light modulator (SLM) and galvo scanners, an addressable multiregional and multifocal multiphoton microscope (AM-MMM) is developed. The SLM shapes an incoming mode-locked, near-infrared Ti:sapphire laser beam into multiple beamlet arrays with addressable shapes and sizes that match the regions of interest on the sample. Compared with conventional multifocal multiphoton microscope (MMM), AM-MMM achieves the effective use of the laser power with an increase of imaging rate and a decrease of photodamage without sacrifice of resolution.

Addressable discrete-line-scanning multiphoton microscopy based on a spatial light modulator

We developed a novel addressable discrete-line-scanning multiphoton microscope with high lateral and axial resolutions based on a spatial light modulator. Our discrete-line-focus design eliminates the cross talk that occurs in conventional one-dimensional line-scanning multiphoton microscopies. Additionally, a phase-only spatial light modulator is able to scan only a sample's target area by generating a specific discrete line focus according to the shape and location of the target area. Compared with other multiphoton microscopies, this technique shortens scanning time and minimizes photodamage by concentrating scanning energy and dwell time on the area of interest.

Differential Multiphoton Laser Scanning Microscopy

Multifocal multiphoton microscopy (MMM) in the biological and medical sciences has become an important tool for obtaining high resolution images at video rates. While current implementations of MMM achieve very high frame rates, they are limited in their applicability to essentially those biological samples that exhibit little or no scattering. In this paper, we report on a method for MMM in which imaging detection is not necessary (single element point detection is implemented), and is therefore fully compatible for use in imaging through scattering media. Further, we demonstrate that this method leads to a new type of MMM wherein it is possible to simultaneously obtain multiple images and view differences in excitation parameters in a single shot.

Control of the multifocal properties of composite vector beams in tightly focusing systems

The numbers of the focal spots and the dominant field (i.e., whether the axial field or the transverse fields play dominant role in the focusing field) have significant effects on various applications. In this paper, we have derived the universal imaging model of the composite vector beam (CVB) composed of two orthogonally linearly polarized beams with inhomogeneous polarization modulation, which is also suitable for various polarized beams, such as linearly, circularly, radially, azimuthally, and vortex polarized beams. Moreover, the sin&cos amplitude modulation with arbitrary orders and the pupil filters with cylindrical symmetry are also involved in this imaging model. On the basis of this imaging model, the regulars to control the focal numbers and the dominant field are drawn. For the various applications, some important conclusions and constructive advices are given.

Multifocus with small size, uniform intensity, and nearly circular symmetry

We investigate in detail the focusing properties of the composite vector beam (CVB) composed of two orthogonally linearly polarized beams with inhomogeneous polarization modulation. By optimizing the modulation factor, a multifocus with excellent quality is obtained, where the sizes of each focus are fairly smaller than that of the focusing spot of a radially polarized beam, the uniformity in the intensity of the focal spots is as high as 1, and the distributions of each focal spot have nearly circular symmetry. In order to decrease the power loss of the incident beam, the CVB formed by an annular beam is demonstrated as the substitute for the optimized CVB formed by a Gaussian beam. This work is important for high-resolution and high-speed imaging in biology and micro-nanofabrication.

Dynamic generation of Debye diffraction-limited multifocal arrays for direct laser printing nanofabrication

We propose a Debye-theory-based iterative method to produce accurate phase patterns for generating highly uniform diffraction-limited multifocal arrays with a high-NA objective. It is shown that by using the Debye method, the uniformity of the diffraction-limited focal arrays can reach 99%, owing to the critical consideration of the depolarization effect associated with high-NA objectives. The generated phase patterns are implemented in fast dynamic laser printing nanofabrication for the generation of individually controlled high-quality microvoid arrays in a solid polymer material by a single exposure of a femtosecond laser beam. As a result of the high-quality multifocal arrays, functional three-dimensional photonic crystals possessing multiple stopgaps with suppression up to 80% in transmission spectra are demonstrated.

Remote focusing for programmable multi-layer differential multiphoton microscopy

We present the application of remote focusing to multiphoton laser scanning microscopy and utilize this technology to demonstrate simultaneous, programmable multi-layer imaging. Remote focusing is used to independently control the axial location of multiple focal planes that can be simultaneously imaged with single element detection. This facilitates volumetric multiphoton imaging in scattering specimens and can be practically scaled to a large number of focal planes. Further, it is demonstrated that the remote focusing control can be synchronized with the lateral scan directions, enabling imaging in orthogonal scan planes.

Remote focusing for programmable multi-layer differential multiphoton microscopy

We present the application of remote focusing to multiphoton laser scanning microscopy and utilize this technology to demonstrate simultaneous, programmable multi-layer imaging. Remote focusing is used to independently control the axial location of multiple focal planes that can be simultaneously imaged with single element detection. This facilitates volumetric multiphoton imaging in scattering specimens and can be practically scaled to a large number of focal planes. Further, it is demonstrated that the remote focusing control can be synchronized with the lateral scan directions, enabling imaging in orthogonal scan planes.

Two-photon microscopy with diffractive optical elements and spatial light modulators

Two-photon microscopy is often performed at slow frame rates due to the need to serially scan all points in a field of view with a single laser beam. To overcome this problem, we have developed two optical methods that split and multiplex a laser beam across the sample. In the first method a diffractive optical element (DOE) generates a fixed number of beamlets that are scanned in parallel resulting in a corresponding increase in speed or in signal-to-noise ratio in time-lapse measurements. The second method uses a computer-controlled spatial light modulator (SLM) to generate any arbitrary spatio-temporal light pattern. With an SLM one can image or photostimulate any predefined region of the image such as neurons or dendritic spines. In addition, SLMs can be used to mimic a large number of optical transfer functions including light path corrections as adaptive optics.

A simple scanless two-photon fluorescence microscope using selective plane illumination

We demonstrate a simple scanless two-photon (2p) excited fluorescence microscope based on selective plane illumination microscopy (SPIM). Optical sectioning capability is presented and depth-resolved imaging of cameleon protein in C. elegans pharyngeal muscle is implemented.

Invited review article: Imaging techniques for harmonic and multiphoton absorption fluorescence microscopy

We review the current state of multiphoton microscopy. In particular, the requirements and limitations associated with high-speed multiphoton imaging are considered. A description of the different scanning technologies such as line scan, multifoci approaches, multidepth microscopy, and novel detection techniques is given. The main nonlinear optical contrast mechanisms employed in microscopy are reviewed, namely, multiphoton excitation fluorescence, second harmonic generation, and third harmonic generation. Techniques for optimizing these nonlinear mechanisms through a careful measurement of the spatial and temporal characteristics of the focal volume are discussed, and a brief summary of photobleaching effects is provided. Finally, we consider three new applications of multiphoton microscopy: nonlinear imaging in microfluidics as applied to chemical analysis and the use of two-photon absorption and self-phase modulation as contrast mechanisms applied to imaging problems in the medical sciences.

Two-photon imaging with diffractive optical elements

Two-photon imaging has become a useful tool for optical monitoring of neural circuits, but it requires high laser power and serial scanning of each pixel in a sample. This results in slow imaging rates, limiting the measurements of fast signals such as neuronal activity. To improve the speed and signal-to-noise ratio of two-photon imaging, we introduce a simple modification of a two-photon microscope, using a diffractive optical element (DOE) which splits the laser beam into several beamlets that can simultaneously scan the sample. We demonstrate the advantages of DOE scanning by enhancing the speed and sensitivity of two-photon calcium imaging of action potentials in neurons from neocortical brain slices. DOE scanning can easily improve the detection of time-varying signals in two-photon and other non-linear microscopic techniques.