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We report high efficiency, miniaturized, ultra-fast coherent beam combining with 3D printed micro-optics directly on the tip of a multicore fiber bundle. The highly compact device foot-print (180 micron diameter) facilitates its incorporation into a minimally invasive ultra-thin nonlinear endoscope to perform two-photon imaging, Comment: Published in Optics Letters

The field of 3D micro-optics is rapidly expanding, and essential advances in femtosecond laser direct-write 3D multi-photon lithography (MPL, also known as two-photon or multi-photon polymerization) are being made. Micro-optics realized via MPL emerged a decade ago and the field has exploded during the last five years. Impressive findings have revealed its potential for beam shaping, advanced imaging, optical sensing, integrated photonic circuits, and much more. This is supported by a game-changing and increasing industrial interest from key established companies in this field. In this review, the origin and the advancement of micro-optics fabrication with MPL are detailed by describing the chronology, distinguishing discrete application groups, providing generalized technical data on the processes and available materials, and discussing the foreseen near-future advances.

We report the use of 3D-printed microscale spiral phase plates to generate orbital angular momentum (OAM) carrying beams. We confirm that the generated beams have high purity, and we have successfully tested them to convey data signals with low bit error rates at the wavelength of 980 nm. This method will open new opportunities for generating OAM beams for many applications in optical communications, including free-space optics, as well as underwater, chip-tochip, and quantum communications.

Microstructured optical fibers (MOFs) have interesting properties such as high birefringence, high nonlinearity, and endlessly single-mode operation [1] . MOFs are optical fibers made with a single material, in which an array of micron-sized longitudinal holes enables light guidance. The design of the hole pattern allows controlling the optical properties of the MOFs.

Optical tweezers (OT) are non-destructive, contactless tools that use light to trap and manipulate microscopic objects. They have applications in diverse fields, particularly in biomedicine and physics [1] . Due to their unique applications and the ability to analyze sub-micron biological systems, the Nobel prize for Physics in 2018 was awarded to Arthur Ashkin, the inventor of OT. Conventional single-beam OT setups require the focusing of a laser beam with a high-Numerical Aperture (NA) objective. However, these setups are bulky and restrict the trapping capabilities to table-top arrangements. The miniaturization of OT to be implemented with a single optical fiber is vital as it opens up new possibilities such as trapping in turbid media or in-vivo. Several groups demonstrated single-fiber OT but the proposed approaches use non-standard fibers and grant only limited design flexibility in controlling the optical trap geometry [2] .

Bessel beams have gained prominence due to their diffraction-free propagation and self-healing properties [1] . The zeroth-order Bessel beam has maximum intensity at its center and has found many applications, including optical trapping and bio-imaging. On the other hand, higher-order Bessel beams have a node due to a phase singularity and carry orbital angular momentum (OAM). Over the past years, Bessel beams have been generated by using different approaches, namely by Fourier transforming a narrow circular beam with a lens, by using axicons and with holographic beam shaping techniques. The need to miniaturize optics has led to efforts towards the on-fiber generation of Bessel beams. The successful attempts include the fabrication of a customized annular-core fiber along with a polymer lens [2] , or modifying the end of the fiber to an axicon profile with chemical or lithographic methods [3] . These techniques have limited capability on generating Bessel beams with on-demand optical parameters and are only focused on creating zeroth-order Bessel beams from optical fibers.

We investigated terahertz resonant gold nanocones for out-of-plane field-driven photoemission and compare their performance with a standard non-resonant nanotip.

The miniaturization of optical fiber polarization beam splitters (PBS) is highly desired, especially in optical communications. Here, we demonstrate the first-ever PBS based on Photonic Crystal Fibers (PCFs). The device is enabled by high-resolution 3D printing based on two-photon lithography, a method that grants unprecedented design flexibility to realize PCF segments with arbitrary geometries. The PCF-PBS is composed of three segments, for a total length of 210 µm, and is directly printed in a single step on a single-mode fiber. The PCF-PBS has an extinction ratio greater than 10 dB on a 100 nm bandwidth centered at around 1550 nm.

Optical tweezers based on single optical fibers are typically fabricated with methods that grant only limited design flexibility in the trapping geometry. Here, we present a novel hybrid micro-optical structure 3D printed in a single step on a standard single-mode optical fiber, to create an ultra-compact fiber tweezer. The structure is made of stacked refractive, reflective, and waveguiding optical elements. Our approach allows an easy and independent tuning of trapping parameters as the numerical aperture and the working distance of the optical trap. We experimentally illustrate the 3D trapping capabilities of the proposed fiber optical tweezers.

We exploit the strong field enhancement offered by vertical gold nanocones resonating at 1 THz to induce THz field-driven electron emission. The nanocones are fabricated via an advanced 3D printing technique on a photopolymer and are successively gold coated. We demonstrate the clear advantage offered by nanocones featuring a monopolar resonance at THz frequencies with respect to traditional non-resonant tips via numerical modelling, THz far-field characterization, and the analysis of electron-induced argon gas fluorescence. Finally, we show that a further degree of optimization is enabled by tailoring the collective response of the nanocones when arranged in an array geometry.

Photonic crystal fibers (PCF) have revolutionized the field of optical fibers. Their unique properties, as endlessly single-mode operation, high birefringence, and hollow-core optical guidance, are enabled and controlled by their hole-array geometry. Thanks to these properties, optical waveguides based on geometrically unbound PCF designs could be exploited to realize miniaturized complex devices which implement advanced photonic operations. However, arbitrary PCF geometries are difficult or even impossible to produce using current PCF fabrication methods. Here, we show how to fabricate optical waveguide segments with PCF designs by 3D microprinting and how the combination of these segments can realize complex photonic devices.

We numerically and experimentally investigate out-of-plane gold nanostructures resonating at terahertz frequencies for field-driven photoemission and compare their performance with a traditional non-resonant nanotip geometry.

We present a fully 3D printed, on-fiber hybrid photonic structure to create customizable single-fiber optical tweezers. The structure contains waveguiding, reflecting, and refracting micro-optical elements stacked to generate a high-NA focal spot.

Bessel beams (BBs) have gained prominence thanks to their diffraction-free propagation and self-healing properties. These beams are conventionally generated using different approaches, namely by transforming a narrow circular beam with a lens, using axicons or holographic beam-shaping techniques. These methods involve space-consuming optics. To overcome this limitation, in the past, efforts have been made to create BBs directly from optical fibers. However, these solutions have limited capability to generate BBs with on-demand optical parameters and only focused on creating zeroth-order BBs. Here, we propose a photonic structure that uses stacked miniaturized optical elements 3D printed in a single step on the fiber facet. Our design allows for the generation of both zeroth- and high-order BBs and fully controllable tailoring of the beams’ parameters, such as their diffraction-free propagation distance or the width of their central peak or node. Remarkably, we report for the first time, to the best of our knowledge, the generation of high-order BBs from optical fibers. We expect our approach to be useful in applications such as optical and quantum communications, fiber-based sensors, microscopy, spectroscopy, and optical trapping.

Imaging neuronal activity with high and homogeneous spatial resolution across the field-of-view (FOV) and limited invasiveness in deep brain regions is fundamental for the progress of neuroscience, yet is a major technical challenge. We achieved this goal by correcting optical aberrations in gradient index lens-based ultrathin (≤ 500 µm) microendoscopes using aspheric microlenses generated through 3D-microprinting. Corrected microendoscopes had extended FOV (eFOV) with homogeneous spatial resolution for two-photon fluorescence imaging and required no modification of the optical set-up. Synthetic calcium imaging data showed that, compared to uncorrected endoscopes, eFOV-microendoscopes led to improved signal-to-noise ratio and more precise evaluation of correlated neuronal activity. We experimentally validated these predictions in awake head-fixed mice. Moreover, using eFOV-microendoscopes we demonstrated cell-specific encoding of behavioral state-dependent information in distributed functional subnetworks in a primary somatosensory thalamic nucleus. eFOV-microendoscopes are, therefore, small-cross-section ready-to-use tools for deep two-photon functional imaging with unprecedentedly high and homogeneous spatial resolution.

Imaging neuronal activity with high and homogeneous spatial resolution across the field-of-view (FOV) and limited invasiveness in deep brain regions is fundamental for the progress of neuroscience, yet is a major technical challenge. We achieved this goal by correcting optical aberrations in gradient index lens-based ultrathin (≤500 µm) microendoscopes using aspheric microlenses generated through 3D-microprinting. Corrected microendoscopes had extended FOV (eFOV) with homogeneous spatial resolution for two-photon fluorescence imaging and required no modification of the optical set-up. Synthetic calcium imaging data showed that, compared to uncorrected endoscopes, eFOV-microendoscopes led to improved signal-to-noise ratio and more precise evaluation of correlated neuronal activity. We experimentally validated these predictions in awake head-fixed mice. Moreover, using eFOV-microendoscopes we demonstrated cell-specific encoding of behavioral state-dependent information in distributed functional subnetworks in a primary somatosensory thalamic nucleus. eFOV-microendoscopes are, therefore, small-cross-section ready-to-use tools for deep two-photon functional imaging with unprecedentedly high and homogeneous spatial resolution.

In 2018 Arthur Ashkin shared half of the Nobel prize for physics "for the optical tweezers and their application to biological systems." Indeed, Optical Tweezers (OT) have found relevant applications in bioscience, thanks to the capability of trapping and manipulating small entities like viruses, bacteria, and DNA strands, in a non-destructive way. The balance of optical forces creating a stable all-optical trap is usually achieved by focusing a Gaussian beam with a high-NA microscope objective. Nevertheless, more efficient optical traps can be obtained by focusing a "donut" annular beam instead of a Gaussian beam [1], for particles larger than the focused beam size. Additionally, the need for bulky high-NA optics such as microscope objective may limit the use of OT in potential applications, like in-vivo trapping, and prompts the development of optical fiber-based OT. Here, we 3D printed a hybrid micro-optical structure on a single-mode optical fiber to create an ultra-compact annular beam fiber tweezer [2], with long working distance and a small probe diameter. A cascade of refractive, reflective, and waveguiding optical elements are microfabricated in a single step to transform the beam output from the fiber. The sequence is constituted by a gaussian to annular waveguide converter, a reflective annular beam expander, and a lensed prism deflector. The gaussian to annular waveguide converter transforms the single-mode fiber output into an annular beam with an inner radius of 4.5 µm and an outer radius of 9 µm. Then, a reflective inverted truncated cone expands the annular beam to an inner radius of 47 µm and an outer radius of 57 µm, by two total internal reflections. Afterward, a total internal reflection prism focuses the annular beam along its radius with a numerical aperture of 1 in water immersion. The output surface has a rounded profile with a radius of curvature of 7 µm to create a lens with a focal length of 50 µm that focuses the annular beam at the trapping point. All the total internal reflection surfaces are designed to work in water immersion, where the critical angle is around 60deg. The final result of this cascaded micro-optical structure is a sharply focused annular beam which forms an optical trap with a working distance of 30 µm. The microstructure is 275 µm long with a diameter of 116µm, smaller than the single-mode optical fiber which has a diameter of 125 µm. The structure has been 3D printed with a commercial Two-photon lithography system (Photonic Professional GT, Nanoscribe GmbH), by using the proprietary material Ip-Dip (Nanoscribe). We demonstrate the all-optical 3D trapping capabilities of this device by trapping polystyrene beads with different diameters. This device demonstrates the potential of 3D printing through two-photon lithography as a flexible tool to realize complexly integrated micro-optical systems. [1] O'Neil, A.T., Padgett, M.J., 2001. Axial and lateral trapping efficiency of Laguerre–Gaussian modes in inverted optical tweezers. Optics Communications, 193(1-6), pp.45-50. [2] Liberale, C. et al. Miniaturized all-fibre probe for three-dimensional optical trapping and manipulation. Nature Photonics, 1(12), p. 723.

We present a novel design for a Stimulated Raman Scattering (SRS) microscope based on a dual beam femtosecond laser in combination with spectral shaping through a fast and narrowband Acousto Optical Tunable Filter. This configuration allows the measurement of broad SRS spectra, all the way from fingerprint region to CH stretch region without any modification of the optical setup. High spectral resolution over a broad spectral region allows label-free quantitative imaging of biological samples. We will show the application of our SRS system to a quantitative study of lipid droplets in colon Cancer Stem Cells.

We show a dynamic counterpropagating optical trapping scheme based on a single low-NA objective and two right-angle prism mirrors µ-3D printed on a glass coverslip. Multiple beams are created with a Spatial Light Modulator and redirected to face each other by the mirrors. The key advantages of this approach are the simple alignment, the long working distance that allows trapping of large samples, the straightforward compatibility with other advanced microscopies and the intrinsic side-view of the trapped object. We demonstrate the viability of our approach by performing trapping and 3D manipulation of dielectric beads and cells.

We propose a dual step µ-3D printing (two-photon lithography) strategy, which is the 3D version of the classic lithography etching-mask strategy, to achieve selective gold metallization for the realization of reflective micro-optics. We apply this strategy to obtain right-angle prism micro-mirrors used to create dynamic counterpropagating optical traps for biological samples in a holographic optical trapping setup with a single low-NA objective. We print the prisms on standard glass coverslips to create integrated optical trapping chips. We show the automatization of alignment between prisms and masks by a computer vision software.

We report the development and application of a new approach to correct aberrations in GRIN microendoscopes using 3D printed polymer microlenses. Corrected microendoscopes are validated performing functional imaging experiments in the mouse brain in vivo.

High-resolution fabrication of three-dimensional polymeric structures with direct laser writing (DLW) holds a unique potential for the miniaturization of bulk optical elements. This has been so far mainly demonstrated for the fabrication of reflective, refractive, and phase-mask micro-optics. Here, we show the use of DLW for the fabrication of a miniaturized phase retarder element, a Fresnel Rhomb, which acts as a broadband quarter-wave plate. We show the integration of the Fresnel Rhomb onto a polarization-maintaining optical fiber, converting linearly polarized light from the fiber into circularly polarized light over a very broad spectral range.

In the last three decades, optical trapping techniques were heavily employed for contactless trapping and manipulation of biological samples. Dual-beam laser traps (DBLT) proved their convenience and became widely used as biophysical tool once a simplified experimental setup was proposed. This simplification was achieved by replacing the two objectives with optical fibers to deliver the two counter-propagating laser beams. However, fiber alignment can be inconvenient, time consuming and requires a lot of practice. Here, we present a novel way to overcome these issues by combining reconfigurable diffractive optical elements (DOE) and two photon lithography (2PL), using a single low NA objective. A single laser beam is divided into several beams by displaying a DOE on a spatial light modulator (SLM). This allows us to dynamically reconfigure the number of the beams, their shape, and relative 3D alignment. Furthermore, we use 3D printed micro-mirrors to direct the laser beams against each other and obtain a DBLT. The micro-mirrors were fabricated on top of a coverslip, by means of 2PL. Our preliminary results show the ability to trap dielectric and biological samples and their full 3D manipulation in a DBLT configuration. The ability to use DOEs to set the number of beams and their shape allow this technique to be coupled with novel forms of microscopy.

In optical microscopies which implement a forward detection scheme such as Coherent Anti-stokes Raman Scattering (CARS), Stimulated Raman Scattering (SRS) or Second — and Third-Harmonic Generation, efficient detection is usually achieved by collecting the signal with high Numerical Aperture (NA) objectives. While the use of high-end well-corrected high-NA microscope objectives for excitation is necessary to achieve high resolution, to efficiently induce non-linear effects and to have excellent imaging capabilities, the detection optics only requires the collimation of the highly divergent beam coming from the excitation path. Moreover, high NA objectives feature short working distances which render them incompatible with top stage incubators, which are the solution of choice for controlled long term experiments on live cells. Among the mentioned techniques, SRS strictly requires the collection of the full excitation beam NA to avoid spurious background signal caused by cross phase modulation (XPM) which can spoil the sensitivity of the system [1].

To avoid the unwanted non-resonant background contribution induced by cross-phase modulation (XPM) in Stimulated Raman Scattering (SRS) microscopy [1], the collection of forward transmitted signal is normally done with a microscope objective having a numerical aperture (NA) higher than the objective used for excitation. However, while high NA microscope objectives are usually bulky and expensive, because a complex design is needed to achieve good optical imaging performances, only a capability to collimate highly divergent beams is needed for forward detection of signal in SRS microscopy. Additionally, because high NA microscope objectives have a short working distance and are bulky, their use as forward collecting optical element is not compatible with tightly closed top-stage incubators, as used in live-cell experiments. Here we show the use of a high NA 3D printed ultra-thin optical lens, composed of micro-reflective and -refractive elements, to replace commercial high NA microscope objectives for forward collection of signal in Stimulated Raman Scattering microscopy. The lens is fabricated on a 170µm thick coverslip with direct laser writing based on Two-Photon Lithography with a commercial system (Nanoscribe) and using the proprietary IP-S photoresist. It has a thickness of 300 µm and a diameter of 1 cm. Thanks to its compactness, this optical element can easily fit inside top-stage microscope incubators. The resulting NA of this catadioptric condenser lens is 1.2 when working in water immersion. We show the complete removal of the non-resonant XPM contribution from SRS spectra of incubated cells. [1] Ji-Xin Cheng, Xiaoliang Sunney Xie, Coherent Raman scattering microscopy, CRC press, 2016.

Stimulated Raman scattering (SRS) microscopy is a label-free method generating images based on chemical contrast within samples, and has already shown its great potential for high-sensitivity and fast imaging of biological specimens. The capability of SRS to collect molecular vibrational signatures in bio-samples, coupled with the availability of powerful statistical analysis methods, allows quantitative chemical imaging of live cells with sub-cellular resolution. This application has substantially driven the development of new SRS microscopy platforms. Indeed, in recent years, there has been a constant effort on devising configurations able to rapidly collect Raman spectra from samples over a wide vibrational spectral range, as needed for quantitative analysis by using chemometric methods. In this paper, an SRS microscope which exploits spectral shaping by a narrowband and rapidly tunable acousto-optical tunable filter (AOTF) is presented. This microscope enables spectral scanning from the Raman fingerprint region to the Carbon-Hydrogen (CH)-stretch region without any modification of the optical setup. Moreover, it features also a high enough spectral resolution to allow resolving Raman peaks in the crowded fingerprint region. Finally, application of the developed SRS microscope to broadband hyperspectral imaging of biological samples over a large spectral range from 800 to 3600 cm-1 , is demonstrated.

Optical waveguide segments based on geometrically unbound photonic crystal fiber (PCF) designs could be exploited as building blocks to realize miniaturized complex devices that implement advanced photonic operations. Here, we show how to fabricate optical waveguide segments with PCF designs by direct high-resolution 3D printing and how the combination of these segments can realize complex photonic devices. We demonstrate the unprecedented precision and flexibility of our method by fabricating the first-ever fiber polarizing beam splitter based on PCFs. The device was directly printed in one step on the end-face of a standard single-mode fiber and was 210 µm long, offering broadband operation in the optical telecommunications C-band. Our approach harnesses the potential of high-resolution 3D printing and of PCF designs paving the way for the development of novel miniaturized complex photonic systems, which will positively impact and advance optical telecommunications, sensor technology, and biomedical devices.

We present a novel approach to correct optical aberrations in ultrathin gradient-index rod lens-based endoscopes using microfabricated aspherical lenses. Corrected microendoscopes have up to 9 folds larger field-of-view compared to uncorrected probes. Using extended field-of-view (eFOV) microendoscopes, we report two-photon imaging of GCaMP6 signals in the mouse hippocampus in vivo with unprecedented combination of high spatiotemporal resolution and minimal invasiveness.

A miniaturized and fiber-integrated Fresnel Rhomb has been 3D printed with Direct Laser Writing on a polarization-maintaining fiber to act as a broadband quarter waveplate, allowing generation of circularly polarized light.

Two-photon fluorescence microscopy provides high resolution information on the anatomy and function of cellular structures located several hundreds of microns deep within biological tissues. However, light scattering poses a fundamental limit to imaging of deeper areas (> 1.5 mm). Implantable microendoscopic probes based on graded index (GRIN) lenses are widely used tools to perform two-photon fluorescence microscopy in otherwise inaccessible regions[1], but the optical performances of with these probes are limited by intrinsic aberrations.

We report here on the design, fabrication and characterization of out-of-plane arrays of conical nanoantennas resonating in the terahertz frequency range.