Acousto-optic lens with very fast focus scanning

Temperature impact on acoustic wave reflection in quasi-collinear acousto-optic devices

Quasi-collinear geometry is a special configuration of acousto-optic (AO) diffraction that applies the acoustic wave reflection from the AO cell input optical face and provides an extremely large interaction length for achieving abnormally high spectral resolution of AO tunable filters. As a result, it becomes possible to implement the multifrequency diffraction which has found important applications for laser pulse shaping. The operation of quasi-collinear AO devices in the multifrequency diffraction regimen is accompanied by the appearance of the longitudinal and transverse temperature gradients in the crystal, mainly due to the acoustic power absorption. Temperature changes the AO cell material stiffness moduli, affecting the characteristics of the incident and reflected acoustic waves (propagation velocities and walk-off angles), and the reflection condition in general. On the example of paratellurite crystal is shown that the AO cell heating near the reflecting facet leads to a deviation of the reflected acoustic beam propagation direction from that specified during the AO cell manufacturing. The deviation magnitude depends on the reflection geometry choice and, in the paratellurite, may exceed several degrees, which adversely affects the AO diffraction characteristics, reducing the AO interaction efficiency and distorting the transmission function shape. The reflected beam deviation may be compensated by means of choosing the angle between the AO cell reflecting face and the piezoelectric transducer face, taking into account the operating AO device thermal regimen.

Axial de-scanning using remote focusing in the detection arm of light-sheet microscopy

Rapid, high-resolution volumetric imaging without moving heavy objectives or disturbing delicate samples remains challenging. Pupil-matched remote focusing offers a promising solution for high NA systems, but the fluorescence signal's incoherent and unpolarized nature complicates its application. Thus, remote focusing is mainly used in the illumination arm with polarized laser light to improve optical coupling. Here, we introduce a novel optical design that can de-scan the axial focus movement in the detection arm of a microscope. Our method splits the fluorescence signal into S and P-polarized light, lets them pass through the remote focusing module separately, and combines them with the camera. This allows us to use only one focusing element to perform aberration-free, multi-color, volumetric imaging without (a) compromising the fluorescent signal and (b) needing to perform sample/detection-objective translation. We demonstrate the capabilities of this scheme by acquiring fast dual-color 4D (3D space + time) image stacks with an axial range of 70 μm and camera-limited acquisition speed. Owing to its general nature, we believe this technique will find its application in many other microscopy techniques that currently use an adjustable Z-stage to carry out volumetric imaging, such as confocal, 2-photon, and light sheet variants.

Mechanical-scan-free multicolor super-resolution imaging with diffractive spot array illumination

Point-scanning microscopy approaches are transforming super-resolution imaging. Despite achieving parallel high-speed imaging using multifocal techniques, efficient multicolor imaging methods with high-quality illumination are currently lacking. In this paper, we present for the first time Mechanical-scan-free multiColor Super-resolution Microscopy (MCoSM) with spot array illumination, which enables mechanical-scan-free super-resolution imaging with adjustable resolution and a good effective field-of-view based on spatial light modulators. Through 100-2,500 s super-resolution spot illumination with different effective fields of view for imaging, we demonstrate the adjustable capacity of MCoSM. MCoSM extends existing spectral imaging capabilities through a time-sharing process involving different color illumination with phase-shift scanning while retaining the spatial flexibility of super-resolution imaging with diffractive spot array illumination. To demonstrate the prospects of MCoSM, we perform four-color imaging of fluorescent beads at high resolution. MCoSM provides a versatile platform for studying molecular interactions in complex samples at the nanoscale level.

Axial de-scanning using remote focusing in the detection arm of light-sheet microscopy

The ability to image at high speeds is necessary for biological imaging to capture fast-moving or transient events or to efficiently image large samples. However, due to the lack of rigidity of biological specimens, carrying out fast, high-resolution volumetric imaging without moving and agitating the sample has been a challenging problem. Pupil-matched remote focusing has been promising for high NA imaging systems with their low aberrations and wavelength independence, making it suitable for multicolor imaging. However, owing to the incoherent and unpolarized nature of the fluorescence signal, manipulating this emission light through remote focusing is challenging. Therefore, remote focusing has been primarily limited to the illumination arm, using polarized laser light to facilitate coupling in and out of the remote focusing optics. Here, we introduce a novel optical design that can de-scan the axial focus movement in the detection arm of a microscope. Our method splits the fluorescence signal into S and P-polarized light, lets them pass through the remote focusing module separately, and combines them with the camera. This allows us to use only one focusing element to perform aberration-free, multi-color, volumetric imaging without (a) compromising the fluorescent signal and (b) needing to perform sample/detection-objective translation. We demonstrate the capabilities of this scheme by acquiring fast dual-color 4D (3D space + time) image stacks with an axial range of 70 μm and camera-limited acquisition speed. Owing to its general nature, we believe this technique will find its application in many other microscopy techniques that currently use an adjustable Z-stage to carry out volumetric imaging, such as confocal, 2-photon, and light sheet variants.

Acousto-optic holography for pseudo-two-dimensional dynamic light patterning

Optical systems use acousto-optic deflectors (AODs) mostly for fast angular scanning and spectral filtering of laser beams. However, AODs may transform laser light in much broader ways. When time-locked to the pulsing of low repetition rate laser amplifiers, AODs permit the holographic reconstruction of 1D and pseudo-two-dimensional (ps2D) intensity objects of rectangular shape by controlling the amplitude and phase of the light field at high (20-200 kHz) rates for microscopic light patterning. Using iterative Fourier transformations (IFTs), we searched for AOD-compatible holograms to reconstruct the given ps2D target patterns through either phase-only or complex light field modulation. We previously showed that phase-only holograms can adequately render grid-like patterns of diffraction-limited points with non-overlapping diffraction orders, while side lobes to the target pattern can be cured with an apodization mask. Dense target patterns, in contrast, are typically encumbered by apodization-resistant speckle noise. Here, we show the denoised rendering of dense ps2D objects by complex acousto-optic holograms deriving from simultaneous optimization of the amplitude and phase of the light field. Target patterns lacking ps2D symmetry, although not translatable into single holograms, were accessed by serial holography based on a segregation into ps2D-compatible components. The holograms retrieved under different regularizations were experimentally validated in an AOD random-access microscope. IFT regularizations characterized in this work extend the versatility of acousto-optic holography for fast dynamic light patterning.

Laser beam array spot steering customized trajectories using the acousto-optic effect

We demonstrate a proof of principle of a technique for array laser beam steering according to a generalized elliptical path, using the acousto-optic effect. We explore the dynamic trajectories of the diffraction orders of a Gaussian beam, diffracted by a dynamic 2D sinusoidal phase grating. The latter is generated by two crossed acoustic waves propagating in a transparent medium, giving rise to 2D dynamic sinusoidal refractive index variation. We particularly emphasize the case where the two crossed acoustic waves are modulated in frequency and have a phase difference. In such case, the resulting diffraction orders are dynamic and follow some particular trajectory's shapes, from linear and circular to generalized elliptical shapes. Inspired by Poincaré and Bloch spheres used to represent polarization and quantum states, we suggest the sphere volume trajectory shape to represent all trajectories' shapes of all diffraction orders.

Using an acousto-optic modulator as a fast spatial light modulator

High-speed spatial light modulators (SLM) are crucial components for free-space communication and structured illumination imaging. Current approaches for dynamical spatial mode generation, such as liquid crystal SLMs or digital micromirror devices, are limited to a maximum pattern refresh rate of 10 kHz and have a low damage threshold. We demonstrate that arbitrary spatial profiles in a laser pulse can be generated by mapping the temporal radio-frequency (RF) waveform sent to an acousto-optic modulator (AOM) onto the optical field. We find that the fidelity of the SLM performance can be improved through numerical optimization of the RF waveform to overcome the nonlinear effect of AOM. An AOM can thus be used as a 1-dimensional SLM, a technique we call acousto-optic spatial light modulator (AO-SLM), which has 50 µm pixel pitch, over 1 MHz update rate, and high damage threshold. We simulate the application of AO-SLM to single-pixel imaging, which can reconstruct a 32×32 pixel complex object at a rate of 11.6 kHz with 98% fidelity.

Ultrasound-Responsive Systems as Components for Smart Materials

Smart materials can respond to stimuli and adapt their responses based on external cues from their environments. Such behavior requires a way to transport energy efficiently and then convert it for use in applications such as actuation, sensing, or signaling. Ultrasound can carry energy safely and with low losses through complex and opaque media. It can be localized to small regions of space and couple to systems over a wide range of time scales. However, the same characteristics that allow ultrasound to propagate efficiently through materials make it difficult to convert acoustic energy into other useful forms. Recent work across diverse fields has begun to address this challenge, demonstrating ultrasonic effects that provide control over physical and chemical systems with surprisingly high specificity. Here, we review recent progress in ultrasound-matter interactions, focusing on effects that can be incorporated as components in smart materials. These techniques build on fundamental phenomena such as cavitation, microstreaming, scattering, and acoustic radiation forces to enable capabilities such as actuation, sensing, payload delivery, and the initiation of chemical or biological processes. The diversity of emerging techniques holds great promise for a wide range of smart capabilities supported by ultrasound and poses interesting questions for further investigations.

Fast optical recording of neuronal activity by three-dimensional custom-access serial holography

Optical recording of neuronal activity in three-dimensional (3D) brain circuits at cellular and millisecond resolution in vivo is essential for probing information flow in the brain. While random-access multiphoton microscopy permits fast optical access to neuronal targets in three dimensions, the method is challenged by motion artifacts when recording from behaving animals. Therefore, we developed three-dimensional custom-access serial holography (3D-CASH). Built on a fast acousto-optic light modulator, 3D-CASH performs serial sampling at 40 kHz from neurons at freely selectable 3D locations. Motion artifacts are eliminated by targeting each neuron with a size-optimized pattern of excitation light covering the cell body and its anticipated displacement field. Spike rates inferred from GCaMP6f recordings in visual cortex of awake mice tracked the phase of a moving bar stimulus with higher spike correlation between intra compared to interlaminar neuron pairs. 3D-CASH offers access to the millisecond correlation structure of in vivo neuronal activity in 3D microcircuits.

Neurophotonic tools for microscopic measurements and manipulation: status report

Neurophotonics was launched in 2014 coinciding with the launch of the BRAIN Initiative focused on development of technologies for advancement of neuroscience. For the last seven years, Neurophotonics' agenda has been well aligned with this focus on neurotechnologies featuring new optical methods and tools applicable to brain studies. While the BRAIN Initiative 2.0 is pivoting towards applications of these novel tools in the quest to understand the brain, this status report reviews an extensive and diverse toolkit of novel methods to explore brain function that have emerged from the BRAIN Initiative and related large-scale efforts for measurement and manipulation of brain structure and function. Here, we focus on neurophotonic tools mostly applicable to animal studies. A companion report, scheduled to appear later this year, will cover diffuse optical imaging methods applicable to noninvasive human studies. For each domain, we outline the current state-of-the-art of the respective technologies, identify the areas where innovation is needed, and provide an outlook for the future directions.

Adaptive optics enables aberration-free single-objective remote focusing for two-photon fluorescence microscopy

Two-photon fluorescence microscopy has been widely applied to three-dimensional (3D) imaging of complex samples. Remote focusing by controlling the divergence of excitation light is a common approach to scanning the focus axially. However, microscope objectives induce distortion to the wavefront of non-collimated excitation beams, leading to degraded imaging quality away from the natural focal plane. In this paper, using a liquid-crystal spatial light modulator to control the divergence of the excitation beam through a single objective, we systematically characterized the aberrations introduced by divergence control through microscope objectives of NA 0.45, 0.8, and 1.05. We used adaptive optics to correct the divergence-induced-aberrations and maintain diffraction-limited focal quality over up to 800-µm axial range. We further demonstrated aberration-free remote focusing for in vivo imaging of neurites and synapses in the mouse brain.

High-NA two-photon single cell imaging with remote focusing using a diffractive tunable lens

Fast, volumetric structural and functional imaging of cellular and sub-cellular dynamics inside the living brain is one of the most desired capabilities in the neurosciences, but still faces serious challenges. Specifically, while few solutions for rapid 3D scanning exist, it is generally much easier to facilitate fast in-plane scanning than it is to scan axially at high speeds. Remote focusing in which the imaging plane is shifted along the optical axis by a tunable lens while maintaining the position of the sample and objective is a promising approach to increase the axial scan speed, but existing techniques often introduce severe optical aberrations in high-NA imaging systems, eliminating the possibility of diffraction-limited single-cell imaging. Here, we demonstrate near diffraction-limited, volumetric two-photon fluorescence microscopy in which we resolve the deep sub-micron structures of single microglia cells with axial scanning performed using a novel high-NA remote focusing method. Image contrast is maintained to within 7% compared to mechanical sample stepping and the focal volume remains nearly diffraction-limited over an axial range greater than 86 µm.

Reproductive and endocrinological effects of Benign Prostatic Hyperplasia and finasteride therapy in dogs

Benign prostatic hyperplasia (BPH) is one of the most important reproductive disorders in aging dogs. Therapeutic measures include orchiectomy and pharmacological treatment, leading to reduction of prostate volume and clinical signs. One of the most common drugs used in BPH treatment is finasteride, but data regarding its possible side effects are scarce. Thus, the aim of this study was to evaluate the effects of BPH and short-term (2 months) finasteride therapy on clinical, endocrinological, and reproductive parameters in dogs. Dogs were allocated into four experimental groups: Non-affected (n = 5), BPH (n = 5), Non-Affected-Finasteride (n = 5) and BPH-Finasteride (n = 5) groups. Dogs were evaluated monthly during 2 months by a complete breeding soundness examination, B-mode ultrasound and Doppler ultrasonography of the testicular artery, hormonal profile (testosterone, estrogen and dihydrotestosterone) and oxidative profile of the prostatic fluid. After 2 months, dogs were gonadectomized and testicles were subjected to histologic analysis. Finasteride treatment reduced dihydrotestosterone concentrations, without negative influence on semen quality and also reverted testicular hemodynamics changes of BPH. On the other hand, BPH was accompanied by significant changes in testosterone and estrogen concentrations and semen quality, mainly related to sperm kinetics alterations. In conclusion, BPH dogs have important hormonal and sperm alterations, however, short-term finasteride treatment (2 months) was able to reduce overall effects of BPH, thus representing a method of therapy for BPH treatment.

Diffractive tunable lens for remote focusing in high-NA optical systems

Remote focusing means to translate the focus position of an imaging system along the optical axis without moving the objective lens. The concept gains increasing importance as it allows for quick 3D focus steering in scanning microscopes, leaves the sample region unperturbed and is compatible with conjugated adaptive optics. Here we present a novel remote focusing approach that can be used in conjunction with high numerical aperture optics. Our method is based on a pair of diffractive elements, which jointly act as a tunable auxiliary lens. By changing the mutual rotation angle between the two elements, we demonstrate an axial translation of the focal spot produced by a NA = 0.95 air objective (corresponding to NA = 1.44 for an oil immersion lens) over more than 140 µm with largely maintained focus quality. We experimentally show that for the task of focus shifting, the wavefront produced by the high-NA design is superior to those produced by a parabolic lens design or a regular achromatic lens doublet.

Optogenetic stimulation of complex spatio-temporal activity patterns by acousto-optic light steering probes cerebellar granular layer integrative properties

Optogenetics provides tools to control afferent activity in brain microcircuits. However, this requires optical methods that can evoke asynchronous and coordinated activity within neuronal ensembles in a spatio-temporally precise way. Here we describe a light patterning method, which combines MHz acousto-optic beam steering and adjustable low numerical aperture Gaussian beams, to achieve fast 2D targeting in scattering tissue. Using mossy fiber afferents to the cerebellar cortex as a testbed, we demonstrate single fiber optogenetic stimulation with micron-scale lateral resolution, >100 µm depth-penetration and 0.1 ms spiking precision. Protracted spatio-temporal patterns of light delivered by our illumination system evoked sustained asynchronous mossy fiber activity with excellent repeatability. Combining optical and electrical stimulations, we show that the cerebellar granular layer performs nonlinear integration, whereby sustained mossy fiber activity provides a permissive context for the transmission of salient inputs, enriching combinatorial views on mossy fiber pattern separation.

Fidelity test for through-focus or volumetric type of optical imaging methods

Rapid increase in interest and applications of through-focus (TF) or volumetric type of optical imaging in biology and other areas has resulted in the development of several TF image collection methods. Achieving quantitative results from images requires standardization and optimization of image acquisition protocols. Several standardization protocols are available for conventional optical microscopy where a best-focus image is used, but to date, rigorous testing protocols do not exist for TF optical imaging. In this paper, we present a method to determine the fidelity of the TF optical data using the TF scanning optical microscopy images.

Through-focus or volumetric type of optical imaging methods: a review

In recent years, the use of through-focus (TF) or volumetric type of optical imaging has gained momentum in several areas such as biological imaging, microscopy, adaptive optics, material processing, optical data storage, and optical inspection. We provide a review of basic TF optical methods highlighting their design, major unique characteristics, and application space.

Remote focusing in confocal microscopy by means of a modified Alvarez lens

Alvarez lenses are actuated lens-pairs which allow one to tune the optical power by mechanical displacement of subelements. Here, we show that a recently realized modified Alvarez lens design which does not require mechanical actuation can be integrated into a confocal microscope. Instead of mechanically moving them, the sublenses are imaged onto each other in a 4f-configuration, where the lateral image shift leading to a change in optical power is created by a galvo-mirror. The avoidance of mechanical lens shifts leads to a large speed gain for axial (and hence also 3D) image scans compared to classical Alvarez lenses. We demonstrate that the suggested operation principle is compatible with confocal microscopy. In order to optimize the system, we have drawn advantage of the flexibility a liquid-crystal spatial light modulator offers for the implementation. For given specifications, dedicated diffractive optical elements or freeform elements can be used in combination with resonant galvo-scanners or acousto-optic beam deflectors, to achieve even faster z-scans than reported here, reaching video rate.

Rapid acousto-optic focus tuning for improvement of imaging performance in confocal microscopy [Invited]

We demonstrate the application of focus-tunable acousto-optic lens technology in confocal microscopy for a high-speed axial scanning of the object. The advantages of the proposed approach include high axial scan rate, no mechanical sample movement, no additional non-symmetric aberrations, and the control of the effective depth of focus. The acousto-optic lens operating at the focus tuning rate of 300 kHz is developed and implemented in scanning laser confocal microscopy. The performance of the instrumentation is presented using test targets. Rapid focus tuning may enhance in vivo three-dimensional imaging in confocal microscopy.

Modified Alvarez lens for high-speed focusing

We present a modified configuration of a tunable Alvarez lens with a refocusing frequency of 1 kHz or more. In contrast to the classic Alvarez lens, the approach does not utilize a translational motion of two sub-lenses with respect to each other, but uses a 4f-setup to image two diffractive sub-lenses onto each other. Hereby focus tuning is achieved by rotating a galvo-mirror which affects the overlap of the two sub-lenses which together form an effective lens of refractive power which depends on the rotation angle of the galvo-mirror. We have demonstrated tuning of the optical power in a system where the diffractive Alvarez lens is realized by an LCOS-SLM. We consider our Alvarez setup especially suitable for applications where high refocusing rates are important, as for example in 3D life cell monitoring or tracking.

Continuous axial scanning of a Gaussian beam via beam steering

We propose and demonstrate experimentally the transfer of one spatial degree of freedom of a laser beam onto another one. Using a multi-plane light conversion device (MPLC) and a modal analysis, we designed a passive setup with immediate response which couples a displacement and tilt in the transverse plane to a longitudinal shift of the focus point of a beam. With this design, we demonstrated a shift of the focal point of the output beam by 4 zR along the propagation axis.

Technologies for imaging neural activity in large volumes

Neural circuitry has evolved to form distributed networks that act dynamically across large volumes. Conventional microscopy collects data from individual planes and cannot sample circuitry across large volumes at the temporal resolution relevant to neural circuit function and behaviors. Here we review emerging technologies for rapid volume imaging of neural circuitry. We focus on two critical challenges: the inertia of optical systems, which limits image speed, and aberrations, which restrict the image volume. Optical sampling time must be long enough to ensure high-fidelity measurements, but optimized sampling strategies and point-spread function engineering can facilitate rapid volume imaging of neural activity within this constraint. We also discuss new computational strategies for processing and analyzing volume imaging data of increasing size and complexity. Together, optical and computational advances are providing a broader view of neural circuit dynamics and helping elucidate how brain regions work in concert to support behavior.

In vivo imaging of neural activity

Since the introduction of calcium imaging to monitor neuronal activity with single-cell resolution, optical imaging methods have revolutionized neuroscience by enabling systematic recordings of neuronal circuits in living animals. The plethora of methods for functional neural imaging can be daunting to the nonexpert to navigate. Here we review advanced microscopy techniques for in vivo functional imaging and offer guidelines for which technologies are best suited for particular applications.

Imaging and Optically Manipulating Neuronal Ensembles

The neural code that relates the firing of neurons to the generation of behavior and mental states must be implemented by spatiotemporal patterns of activity across neuronal populations. These patterns engage selective groups of neurons, called neuronal ensembles, which are emergent building blocks of neural circuits. We review optical and computational methods, based on two-photon calcium imaging and two-photon optogenetics, to detect, characterize, and manipulate neuronal ensembles in three dimensions. We review data using these methods in the mammalian cortex that demonstrate the existence of neuronal ensembles in the spontaneous and evoked cortical activity in vitro and in vivo. Moreover, two-photon optogenetics enable the possibility of artificially imprinting neuronal ensembles into awake, behaving animals and of later recalling those ensembles selectively by stimulating individual cells. These methods could enable deciphering the neural code and also be used to understand the pathophysiology of and design novel therapies for neurological and mental diseases.

Random-access scanning microscopy for 3D imaging in awake behaving animals

Understanding how neural circuits process information requires rapid measurements of activity from identified neurons distributed in 3D space. Here we describe an acousto-optic lens two-photon microscope that performs high-speed focusing and line scanning within a volume spanning hundreds of micrometers. We demonstrate its random-access functionality by selectively imaging cerebellar interneurons sparsely distributed in 3D space and by simultaneously recording from the soma, proximal and distal dendrites of neocortical pyramidal cells in awake behaving mice.

Generation of Aspherical Optical Lenses via Arrested Spreading and Pinching of a Cross-Linkable Liquid

Aspherical optical lenses with spatially varying curvature are desired for capturing high quality, aberration free images in numerous optical applications. Conventionally such lenses are prepared by multistep top-down processes which are expensive, time-consuming, and prone to high failure rate. In this context, an alternate method is presented here based on arrested spreading of a sessile drop of a transparent, cross-linkable polymeric liquid on a solid substrate heated to an elevated temperature. Whereas surface tension driven flow tends to render it spherical, rapid cross-linking arrests such flow so that nonequilibrium aspherical shapes are attained. It is possible to tune also the initial state of the drop via delayed pinching of a liquid cylinder which precedes its release on the substrate. This method has led to the generation of a wide variety of optical lenses, ranging from spherical plano convex to superspherical solid immersion to exotic lenses not achieved via conventional methods.

Fast volumetric imaging with patterned illumination via digital micro-mirror device-based temporal focusing multiphoton microscopy

Temporal focusing multiphoton microscopy (TFMPM) has the advantage of area excitation in an axial confinement of only a few microns; hence, it can offer fast three-dimensional (3D) multiphoton imaging. Herein, fast volumetric imaging via a developed digital micromirror device (DMD)-based TFMPM has been realized through the synchronization of an electron multiplying charge-coupled device (EMCCD) with a dynamic piezoelectric stage for axial scanning. The volumetric imaging rate can achieve 30 volumes per second according to the EMCCD frame rate of more than 400 frames per second, which allows for the 3D Brownian motion of one-micron fluorescent beads to be spatially observed. Furthermore, it is demonstrated that the dynamic HiLo structural multiphoton microscope can reject background noise by way of the fast volumetric imaging with high-speed DMD patterned illumination.

Dynamic wavefront shaping with an acousto-optic lens for laser scanning microscopy

Acousto-optic deflectors (AODs) arranged in series and driven with linearly chirped frequencies can rapidly focus and tilt optical wavefronts, enabling high-speed 3D random access microscopy. Non-linearly chirped acoustic drive frequencies can also be used to shape the optical wavefront allowing a range of higher-order aberrations to be generated. However, to date, wavefront shaping with AODs has been achieved by using single laser pulses for strobed illumination to 'freeze' the moving acoustic wavefront, limiting voxel acquisition rates. Here we show that dynamic wavefront shaping can be achieved by applying non-linear drive frequencies to a pair of AODs with counter-propagating acoustic waves, which comprise a cylindrical acousto-optic lens (AOL). Using a cylindrical AOL we demonstrate high-speed continuous axial line scanning and the first experimental AOL-based correction of a cylindrical lens aberration at 30 kHz, accurate to 1/35^th of a wave at 800 nm. Furthermore, we develop a model to show how spherical aberration, which is the major aberration in AOL-based remote-focusing systems, can be partially or fully corrected with AOLs consisting of four or six AODs, respectively.

Fast spatial beam shaping by acousto-optic diffraction for 3D non-linear microscopy

Acousto-optic deflection (AOD) devices offer unprecedented fast control of the entire spatial structure of light beams, most notably their phase. AOD light modulation of ultra-short laser pulses, however, is not straightforward to implement because of intrinsic chromatic dispersion and non-stationarity of acousto-optic diffraction. While schemes exist to compensate chromatic dispersion, non-stationarity remains an obstacle. In this work we demonstrate an efficient AOD light modulator for stable phase modulation using time-locked generation of frequency-modulated acoustic waves at the full repetition rate of a high power laser pulse amplifier of 80 kHz. We establish the non-local relationship between the optical phase and the generating acoustic frequency function and verify the system for temporal stability, phase accuracy and generation of non-linear two-dimensional phase functions.

Fluorescence micro-optical sectioning tomography using acousto-optical deflector-based confocal scheme

Fluorescent labeling has opened up the possibility of clarifying the complex distribution and circuit wiring of specific neural circuits for particular functions. To acquire the brain-wide fluorescently labeled neural wiring, we have previously developed the fluorescence micro-optical sectioning tomography imaging system. This employs simultaneous mechanical sectioning and confocal imaging of the slices, and is capable of acquiring the image dataset of a centimeter-sized whole-mouse brain at a voxel resolution of [Formula: see text]. We analyze the key optical considerations for the use of an acousto-optical deflector (AOD) scanner-based confocal detection scheme in this system. As a result, the influence of confocal detection, the imaging site during sectioning, and AOD fast scan mode on signal-to-background noise ratio are described. It is shown that mechanical sectioning to separate the slice and optical sectioning by confocal detection should be combined to maximize background suppression in simultaneous fast scan imaging while sectioning system setup.

Development and application of a ray-based model of light propagation through a spherical acousto-optic lens

A spherical acousto-optic lens (AOL) consists of four acousto-optic deflectors (AODs) that can rapidly and precisely control the focal position of an optical beam in 3D space. Development and application of AOLs has increased the speed at which 3D random access point measurements can be performed with a two-photon microscope. This has been particularly useful for measuring brain activity with fluorescent reporter dyes because neuronal signalling is rapid and sparsely distributed in 3D space. However, a theoretical description of light propagation through AOLs has lagged behind their development, resulting in only a handful of simplified principles to guide AOL design and optimization. To address this we have developed a ray-based computer model of an AOL incorporating acousto-optic diffraction and refraction by anisotropic media. We extended an existing model of a single AOD with constant drive frequency to model a spherical AOL: four AODs in series driven with linear chirps. AOL model predictions of the relationship between optical transmission efficiency and acoustic drive frequency including second order diffraction effects closely matched experimental measurements from a 3D two-photon AOL microscope. Moreover, exploration of different AOL drive configurations identified a new simple rule for maximizing the field of view of our compact AOL design. By providing a theoretical basis for understanding optical transmission through spherical AOLs, our open source model is likely to be useful for comparing and improving different AOL designs, as well as identifying the acoustic drive configurations that provide the best transmission performance over the 3D focal region.

Modeling of in vivo acousto-optic two-photon imaging of the retina in the human eye

Our aim is to establish a novel combined acousto-optical method for in vivo imaging of the human retina with the two-photon microscope. In this paper we present modeling results based on eye model samples constructed with parameters measured on patients. We used effectively the potential of the electronic compensation offered by the acousto-optic lenses to avoid the use of adaptive optical correction. Simulation predicted lateral resolution between 1.6 µm and 3 µm on the retina. This technology allows the visualization of single cells and promises real time measuring of neural activity in individual neurons, neural segments and cell assemblies with 30-100 µs temporal and subcellular spatial resolution.

Beam deformation within an acousto-optic lens

The acousto-optic lens (AOL) is becoming a popular tool in the neuroscience field. Here we analyzed the deformation of the diffraction beam after passage through an AOL consisting of a pair of acousto-optic deflectors using both theoretical and experimental data. The results showed that, because of the high sensitivity of optical spatial frequencies of acousto-optic deflectors, the boundary strength of the diffraction beam of the AOL decreases significantly. When the focal length of AOL diminishes, the deformation of the diffraction beam becomes more serious with a smaller beam size. This deformation of the diffraction beam finally leads to a decreased illuminative numerical aperture, which worsens the image's spatial resolution.

Monitoring synaptic and neuronal activity in 3D with synthetic and genetic indicators using a compact acousto-optic lens two-photon microscope

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We expand the utility of acousto-optic lens (AOL) 3D 2P microscopy.

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We show rapid, simultaneous monitoring of synaptic inputs distributed in 3D.

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First use of genetically encoded indicators with AOL 3D functional imaging.

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Measurement of sensory-evoked neuronal population activity in 3D in vivo.

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Strategies for improving the measurement of the timing of neuronal signals.

Background

Two-photon microscopy is widely used to study brain function, but conventional microscopes are too slow to capture the timing of neuronal signalling and imaging is restricted to one plane. Recent development of acousto-optic-deflector-based random access functional imaging has improved the temporal resolution, but the utility of these technologies for mapping 3D synaptic activity patterns and their performance at the excitation wavelengths required to image genetically encoded indicators have not been investigated.

New method

Here, we have used a compact acousto-optic lens (AOL) two-photon microscope to make high speed [Ca²⁺] measurements from spines and dendrites distributed in 3D with different excitation wavelengths (800–920 nm).

Results

We show simultaneous monitoring of activity from many synaptic inputs distributed over the 3D arborisation of a neuronal dendrite using both synthetic as well as genetically encoded indicators. We confirm the utility of AOL-based imaging for fast in vivo recordings by measuring, simultaneously, visually evoked responses in 100 neurons distributed over a 150 μm focal depth range. Moreover, we explore ways to improve the measurement of timing of neuronal activation by choosing specific regions within the cell soma.

Comparison with existing methods

These results establish that AOL-based 3D random access two-photon microscopy has a wider range of neuroscience applications than previously shown.

Conclusions

Our findings show that the compact AOL microscope design has the speed, spatial resolution, sensitivity and wavelength flexibility to measure 3D patterns of synaptic and neuronal activity on individual trials.

Beam shaping and high-speed, cylinder-lens-free beam guiding using acousto-optical deflectors without additional compensation optics

Using acousto-optical deflectors at high deflection speeds via acoustical frequency chirping induces astigmatism, deforming the laser beam in an unfavorable way. Within the paper, we present a method to prevent this effect for an ultrashort pulsed laser beam via acoustical frequency jumps synchronized to the pulse-to-pulse pause. We also demonstrate and give a method to calculate beam shaping capability of acousto-optical deflectors via arbitrary spatial frequency developments during ultrashort laser pulse transit through the deflector. Cylinder-lens-free deflection at >2000 rad/s and beam shaping capability is demonstrated experimentally. In our experiments the switching time between two beam shapes is 1 µs.

Visualization of brain circuits using two-photon fluorescence micro-optical sectioning tomography

Neural circuits are fundamental for brain functions. However, obtaining long range continuous projections of neurons in the entire brain is still challenging. Here a two-photon fluorescence micro-optical sectioning tomography (2p-fMOST) method is developed for high-throughput, high-resolution visualization of the brain circuits. Two-photon imaging technology is used to obtain high resolution, and acoustical optical deflector (AOD), an inertia-free beam scanner is used to realize fast and prolonged stable imaging. The combination of these techniques with imaging and then sectioning method of a plastic-embedded mouse brain facilitated the acquisition of a three-dimensional data set of a fluorescent mouse brain with a resolution adequate to resolve the spines. In addition, the brain circuit tracing ability is showed by several neurons projecting across different brain regions. Besides brain imaging, 2p-fMOST could be used in many studies that requires sub-micro resolution or micro resolution imaging of a large sample.

Continuously tracing brain-wide long-distance axonal projections in mice at a one-micron voxel resolution

Revealing neural circuit mechanisms is critical for understanding brain functions. Significant progress in dissecting neural connections has been made using optical imaging with fluorescence labels, especially in dissecting local connections. However, acquiring and tracing brain-wide, long-distance neural circuits at the neurite level remains a substantial challenge. Here, we describe a whole-brain approach to systematically obtaining continuous neuronal pathways in a fluorescent protein transgenic mouse at a one-micron voxel resolution. This goal is achieved by combining a novel resin-embedding method for maintaining fluorescence, an automated fluorescence micro-optical sectioning tomography system for long-term stable imaging, and a digital reconstruction-registration-annotation pipeline for tracing the axonal pathways in the mouse brain. With the unprecedented ability to image a whole mouse brain at a one-micron voxel resolution, the long-distance pathways were traced minutely and without interruption for the first time. With advancing labeling techniques, our method is believed to open an avenue to exploring both local and long-distance neural circuits that are related to brain functions and brain diseases down to the neurite level.

Fast two-photon in vivo imaging with three-dimensional random-access scanning in large tissue volumes

The understanding of brain computations requires methods that read out neural activity on different spatial and temporal scales. Following signal propagation and integration across a neuron and recording the concerted activity of hundreds of neurons pose distinct challenges, and the design of imaging systems has been mostly focused on tackling one of the two operations. We developed a high-resolution, acousto-optic two-photon microscope with continuous three-dimensional (3D) trajectory and random-access scanning modes that reaches near-cubic-millimeter scan range and can be adapted to imaging different spatial scales. We performed 3D calcium imaging of action potential backpropagation and dendritic spike forward propagation at sub-millisecond temporal resolution in mouse brain slices. We also performed volumetric random-access scanning calcium imaging of spontaneous and visual stimulation-evoked activity in hundreds of neurons of the mouse visual cortex in vivo. These experiments demonstrate the subcellular and network-scale imaging capabilities of our system.

A compact Acousto-Optic Lens for 2D and 3D femtosecond based 2-photon microscopy

We describe a high speed 3D Acousto-Optic Lens Microscope (AOLM) for femtosecond 2-photon imaging. By optimizing the design of the 4 AO Deflectors (AODs) and by deriving new control algorithms, we have developed a compact spherical AOL with a low temporal dispersion that enables 2-photon imaging at 10-fold lower power than previously reported. We show that the AOLM can perform high speed 2D raster-scan imaging (>150 Hz) without scan rate dependent astigmatism. It can deflect and focus a laser beam in a 3D random access sequence at 30 kHz and has an extended focusing range (>137 mum; 40X 0.8NA objective). These features are likely to make the AOLM a useful tool for studying fast physiological processes distributed in 3D space.

Two-photon microscope for multisite microphotolysis of caged neurotransmitters in acute brain slices

We developed a two-photon microscope optimized for physiologically manipulating single neurons through their postsynaptic receptors. The optical layout fulfills the stringent design criteria required for high-speed, high-resolution imaging in scattering brain tissue with minimal photodamage. We detail the practical compensation of spectral and temporal dispersion inherent in fast laser beam scanning with acousto-optic deflectors, as well as a set of biological protocols for visualizing nearly diffraction-limited structures and delivering physiological synaptic stimuli. The microscope clearly resolves dendritic spines and evokes electrophysiological transients in single neurons that are similar to endogenous responses. This system enables the study of multisynaptic integration and will assist our understanding of single neuron function and dendritic computation.

Realization and performance evaluation of high speed autofocusing for direct laser lithography

The autofocusing is one of the important parts in the automated vision inspection or measurement using optical microscopes. Moreover, laser micromachining or laser lithography requires a high speed and precision autofocusing. In this paper, we propose and realize an autofocusing system using two cylindrical lenses, which is the enhanced version of the previous astigmatism method. It shows very good performances, especially very high speed and the largest range in comparison with the previous astigmatic methods. The performance of our autofocusing system was evaluated by tracing the linear stage whose position was monitored by a commercial laser interferometer. Then we applied the autofocusing to the direct laser lithographic system, and successfully fabricated circular symmetry patterns on a 300 mm diameter surface with the resolution of less than 1 microm within the defocusing range of +/-50 microm. The speed of the autofocusing was 150 Hz.

Fast mode-hop-free acousto-optically tuned laser: theoretical and experimental investigations

We developed a theory that describes fast mode-hop-free tuning of an external cavity diode laser in Littrow configuration with two acousto-optic modulators (AOMs) inside the laser cavity. The theory is based on synchronous shifting of the external cavity modes and the Littrow grating selectivity. It allows calculating the driving signals of both AOMs in order to reach a desired temporal variation of the laser frequency, including particularly fast tuning as well as an arbitrary shape of the tuning function. Furthermore, we present a laser setup for which the needed signals for both AOMs are generated by two direct digital synthesizer circuits. Thereby we were able to verify the theoretical predictions experimentally, achieving, e.g., sinusoidal single-mode tuning of the laser frequency over 40 GHz at a repetition rate of 10 kHz and over 12 GHz at 25 kHz. Finally, the limitations of the theory are discussed.

High-speed varifocal imaging with a tunable acoustic gradient index of refraction lens

Fluidic lenses allow for varifocal optical elements, but current approaches are limited by the speed at which focal length can be changed. Here we demonstrate the use of a tunable acoustic gradient (TAG) index of refraction lens as a fast varifocal element. The optical power of the TAG lens varies continuously, allowing for rapid selection and modification of the effective focal length at time scales of 1 mus and shorter. The wavefront curvature applied to the incident light is experimentally quantified as a function of time, and single-frame imaging is demonstrated. Results indicate that the TAG lens can successfully be employed to perform high-rate imaging at multiple locations.

Three-dimensional random access multiphoton microscopy for functional imaging of neuronal activity

The dynamic ability of neuronal dendrites to shape and integrate synaptic responses is the hallmark of information processing in the brain. Effectively studying this phenomenon requires concurrent measurements at multiple sites on live neurons. Substantial progress has been made by optical imaging systems that combine confocal and multiphoton microscopy with inertia-free laser scanning. However, all of the systems developed so far restrict fast imaging to two dimensions. This severely limits the extent to which neurons can be studied, as they represent complex three-dimensional structures. Here we present a new imaging system that utilizes a unique arrangement of acousto-optic deflectors to steer a focused, ultra-fast laser beam to arbitrary locations in three-dimensional space without moving the objective lens. As we demonstrate, this highly versatile random-access multiphoton microscope supports functional imaging of complex three-dimensional cellular structures such as neuronal dendrites or neural populations at acquisition rates on the order of tens of kilohertz.

Ray equation for a spatially variable uniaxial crystal and its use in the optical design of liquid-crystal lenses

To be able to design optical systems (e.g., variable focus or zoom lenses) made from liquid-crystal devices, it is necessary to be able to ray trace in a birefringent medium where the angle of the optical axis is a function of position in the device. To our knowledge, the theory required to achieve this has not previously been published, and we derive a suitable algorithm and give some examples of its use.

Principles of Two-Photon Excitation Microscopy and Its Applications to Neuroscience

The brain is complex and dynamic. The spatial scales of interest to the neurobiologist range from individual synapses (approximately 1 microm) to neural circuits (centimeters); the timescales range from the flickering of channels (less than a millisecond) to long-term memory (years). Remarkably, fluorescence microscopy has the potential to revolutionize research on all of these spatial and temporal scales. Two-photon excitation (2PE) laser scanning microscopy allows high-resolution and high-sensitivity fluorescence microscopy in intact neural tissue, which is hostile to traditional forms of microscopy. Over the last 10 years, applications of 2PE, including microscopy and photostimulation, have contributed to our understanding of a broad array of neurobiological phenomena, including the dynamics of single channels in individual synapses and the functional organization of cortical maps. Here we review the principles of 2PE microscopy, highlight recent applications, discuss its limitations, and point to areas for future research and development.

Acousto-optic lens for pulsed lasers

Theoretical predictions of the acousto-optic interaction under Raman-Nath conditions of ultrasound and laser light with a beam width w comparable to or smaller than the ultrasonic wavelength lambda are experimentally investigated, and good agreement between experiment and prediction is found. The experimental setup consists of a pulsed laser, emitting pulses that are synchronized with the sound frequency by means of a controllable delay time. When w approximately lambda, strong focusing is obtained for most delay times. The lensing properties of this configuration are carefully investigated.