Progress in neuroscience relies on new techniques for investigating the complex dynamics of neuronal networks. An ongoing challenge is to achieve minimally invasive and high-resolution observations of neuronal activity in vivo inside deep brain areas. Recently introduced methods for holographic control of light propagation in complex media enable the use of a hair-thin multimode optical fibre as an ultranarrow imaging tool. Compared to endoscopes based on graded-index lenses or fibre bundles, this new approach offers a footprint reduction exceeding an order of magnitude, combined with a significant enhancement in resolution. We designed a compact and high-speed system for fluorescent imaging at the tip of a fibre, achieving a resolution of 1.18 ± 0.04 µm across a 50-µm field of view, yielding 7-kilopixel images at a rate of 3.5 frames/s. Furthermore, we demonstrate in vivo observations of cell bodies and processes of inhibitory neurons within deep layers of the visual cortex and hippocampus of anaesthetised mice. This study paves the way for modern microscopy to be applied deep inside tissues of living animal models while exerting a minimal impact on their structural and functional properties.

Holographic optical tweezers (HOT) holds great promise for many applications in modern biophotonics, allowing the creation and measurement of minuscule forces on biomolecules, molecular motors and cells. Optical geometries used in HOT currently make use of bulk optics, and their usage in-vivo is compromised by the optically turbid nature of living tissues-a limiting factor in any advanced high-resolution imaging method. We present an alternative HOT approach in which multiple three-dimensional optical traps are introduced through a high-numerical-aperture multimode optical fibre, thus enabling an equally versatile means of optical manipulation through channels having cross-section comparable to the size of a single cell. Our work demonstrates real-time manipulation of 3-D arrangements of micro-objects, as well as the possibility of manipulating inside otherwise inaccessible cavities. We show that the position of the optical traps can be controlled with nanometric resolution over fibre lengths exceeding 100 mm. The results provide the basis for exploitation of holographic manipulation and other high-numerical-aperture techniques, including advanced forms of microscopy, through single-core-fibre endoscopes deep inside living tissues and other complex environments.

Digital micro-mirror devices (DMDs) have recently emerged as practical spatial light modulators (SLMs) for applications in photonics, primarily due to their modulation rates, which exceed by several orders of magnitude those of the already well-established nematic liquid crystal (LC)-based SLMs. This, however, comes at the expense of limited modulation depth and diffraction efficiency. Here we compare the beam-shaping fidelity of both technologies when applied to light control in complex environments, including an aberrated optical system, a highly scattering layer and a multimode optical fibre. We show that, despite their binary amplitude-only modulation, DMDs are capable of higher beam-shaping fidelity compared to LC-SLMs in all considered regimes.

Holographic wavefront manipulation enables converting hair-thin multimode optical fibers into minimally invasive lensless imaging instruments conveying much higher information densities than conventional endoscopes. Their most prominent applications focus on accessing delicate environments, including deep brain compartments, and recording micrometer-scale resolution images of structures in close proximity to the distal end of the instrument. Here, we introduce an alternative “far-field” endoscope capable of imaging macroscopic objects across a large depth of field. The endoscope shaft with dimensions of 0.2 × 0.4 mm2 consists of two parallel optical fibers: one for illumination and the other for signal collection. The system is optimized for speed, power efficiency, and signal quality, taking into account specific features of light transport through step-index multimode fibers. The characteristics of imaging quality are studied at distances between 20 mm and 400 mm. As a proof-of-concept, we provide imaging inside the cavities of a sweet pepper commonly used as a phantom for biomedically relevant conditions. Furthermore, we test the performance on a functioning mechanical clock, thus verifying its applicability in dynamically changing environments. With the performance reaching the standard definition of video endoscopes, this work paves the way toward the exploitation of minimally invasive holographic micro-endoscopes in clinical and diagnostics applications.

Acoustic sensing is nowadays a very demanding field which plays an important role in modern society, with applications spanning from structural health monitoring to medical imaging. Fiber-optics can bring many advantages to this field, and fiber-optic acoustic sensors show already performance levels capable of competing with the standard sensors based on piezoelectric transducers. This review presents the recent advances in the field of fiber-optic dynamic strain sensing, particularly for acoustic detection. Three dominant technologies are identified -fiber Bragg gratings, interferometric Mach-Zehnder, and Fabry-Pérot configurations -and their recent developments are summarized.

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A palladium (Pd)-based optical metamaterial has been designed, fabricated and characterized for its application in hydrogen sensing. The metamaterial can replace Pd thin films in optical transmission schemes for sensing with performances far superior to those of conventional sensors. This artificial material consists of a palladium-alumina metamaterial fabricated using inexpensive and industrial-friendly bottom-up techniques. During the exposure to hydrogen, the system exhibits anomalous optical absorption when compared to the well-known response of Pd thin films, this phenomenon being the key factor for the sensor sensitivity. The exposure to hydrogen produces a large variation in the light transmission through the metamembrane (more than 30% with 4% in volume hydrogen-nitrogen gas mixture at room temperature and atmospheric pressure), thus avoiding the need for sophisticated optical detection systems. An optical homogenization model is proposed to explain the metamaterial response. These results contribute to the development of reliable and low-cost hydrogen sensors with potential applications in the hydrogen economy and industrial processes to name a few, and also open the door to optically study the hydrogen diffusion processes in Pd nanostructures.