Neuroscience research related to functionality, connectivity and metabolism of neuronal circuits, individual neuronal cells and sub-cellular structures, nowadays, experiences a burgeoning need to develop techniques for the detailed investigation inside the complexity of the living matter. Particularly, high-resolution observations combined with an extended depth of penetration in tissue represents an ongoing challenge.
Holographic control of light propagation in complex media opens a promising way to overcome this technological barrier via exploiting multimode fibres as hair-thin, minimally-invasive endoscopes. This concept allows for more than one order of magnitude reduction of the instrument’s footprint and a significant enhancement of imaging resolution, compared with current minimally invasive endoscopes.
Here, we demonstrate a compact and high-speed system for fluorescent imaging at the tip of a fibre. The instrument’s performance reaches micron-scale resolution across a field of view 50 micrometres, yielding 7-kilopixel image information at a rate of 3.5 frames per second. The resolution limit is dictated only by the numerical aperture of the fibre probe, and the contrast/pureness of the focal points, utilised for raster-scanning regime, approach the theoretical limits for phase-only holographic wavefront shaping.
The achieved performance allowed for in-vivo observations of neuronal somata and processes, residing deep inside the visual cortex and hippocampus of an animal model with minimal damage to the tissue surrounding the fibre penetration area.
We believe that this demonstration represents an important step towards implementations of various advanced forms of imaging through multimode fibre based endoscopes to address numerous key challenges in neuroscience.
We have developed an all-solid, step-index multimode fibre based on compound "soft-glasses" yielding a very-high NA reaching 0.96 at 1064nm. By further extending the methods of holographic control of light propagation in multimode fibres, we were able to mitigate the adverse effect of mode-dependent loss affecting the new fibre type. This enabled harnessing the full available NA almost completely, and demonstrating high-resolution focussing with output NAs up to 0.91 through lensless fibres. Further, we show that the NA and pureness of such foci allow stable three-dimensional optical confinement of micrometre-sized dielectric objects. Being inherently holographic, this technique is capable of generating an arbitrary number of optical tweezers, as well as precisely repositioning them independently in all directions. The versatility of the new instrument is demonstrated by simultaneous and dynamic 3D manipulation of large assemblies of dielectric microparticles, as well as manipulation of micro-objects inside optically inaccessible environments such a turbid cavity through an opening as small as 0.1mm.
Moreover, the possibility of generating aberration-free foci with NA approaching 0.9 across the fibre core opens new perspectives for high-resolution holographic micro-endoscopy, paving the way for the delivery of advanced microscopy techniques through hair-thin fibre-optic probes.
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 amplitudeonly modulation, DMDs are capable of higher beam-shaping fidelity compared to LC-SLMs in all considered regimes.
Using spatial light modulators(SLM) to control light propagation through scattering media is a critical topic for various applications in biomedical imaging, optical micromanipulation, and fibre endoscopy.
Having limited switching rate, typically 10-100Hz, current liquid-crystal SLM can no longer meet the growing demands of high-speed imaging. A new way based on binary-amplitude holography implemented on digital micromirror devices(DMD) has been introduced recently, allowing to reach refreshing rates of 30kHz.
Here, we summarise the advantages and limitations in speed, efficiency, scattering noise, and pixel cross-talk for each device in ballistic and diffusive regimes, paving the way for high-speed imaging through multimode fibres.
Optical fiber optrodes are attractive sensing devices due to their ability to perform point measurement in remote locations. Mostly, they are oriented to biochemical sensing, quite often relying on fluorescent and spectroscopic techniques, but with the refractometric approach being also considered when the objective is high measurement performance, particularly when focusing on measurand resolution. In this work, we address this subject proposing and theoretically analyzing the characteristics of a fiber optic optrode relying on plasmonic interaction. The optrode structure is a fiber optic tapered tip layout incorporating a lateral bimetallic layer (silver + gold) and operating in reflection.
Optical fiber sensors based on the phenomenon of plasmonic resonance can be interrogated applying different methods, the most common one being the spectral approach where the measurand information is derived from the reading of the wavelength resonance dip. In principle, a far better performance can be achieved considering the reading of the phase of the light at a specific wavelength located within the spectral plasmonic resonance. This approach is investigated in this work for surface plasmon based fiber optic sensors with overlays which are combinations of bimetallic layers, permitting not only to tune the wavelength of the plasmon resonance but also the sensitivity associated with the phase interrogation of the sensors. The metals considered for the present analysis are silver, gold, copper, and aluminum.
Optical fiber sensors based on the phenomenon of plasmonic resonance can be interrogated applying different methods, the most common one being the spectral approach where the measurand information is derived from the reading of the wavelength resonance dip. In principle, a far better performance can be achieved considering the reading of the phase of the light at a specific wavelength located within the spectral plasmonic resonance. This approach is investigated in this work for fiber optic SPR sensors with overlays which are combinations of metallic and dielectric thin films, permitting not only to tune the wavelength of the SPR resonance but also the sensitivity associated with the phase interrogation of the sensors.
This paper investigates numerically the performance of a design for an optical sensor of the refractive index of gases and liquids based on smart or functional metamaterial films (smart optical metamembranes).
In this work we show numerically the potential of using a metamaterial constituted by ordered arrays of silver nanowires as a sensor for refractive index changes of a surrounding dielectric medium. The results show a strong dependence of the reflectance spectrum on the refractive index of the dielectric medium.
One of the major issues in the modeling of subwavelength optical materials resides in how to compute the effective properties of such media. An efficient technique must be able to describe appropriately the electromagnetic response of the overall structure. Within this context, this work is focused on the calculation of effective parameters of metallic silver nanowires embedded in alumina background. An algorithm based on modal propagation is considered in order to estimate the refractive index at the visible spectrum. The resonances obtained in the computing model are compared to the predictions of analytical Bruggeman and Maxell-Garnett theories and analyzed by regarding excitation of surface modes at the metal-dielectric interface.
In this work, we address a study of the spectral reflectance of silver nanowire metamaterials in the visible and near-infrared regions. To this end, several samples were fabricated with different fill-ratios and lattice constants, and their respective optical responses characterized in terms of these parameters. We perform a direct comparison between the collected experimental data with the values predicted by different analytical homogenization models to provide a better understanding of the effective optical behavior of this kind of metamaterials.
An analytical model based on geometrical optics and multilayer transfer matrix method is applied to determine the sensing properties of tapered optical fiber based SPR sensors incorporating bimetallic (Gold and Silver) layers, particularly when phase interrogation is considered. Phase interrogation is studied as a methodology to attain enhanced sensitivities. The performance of the sensing heads as function of the bimetallic layers and taper parameters is analyzed. It is shown the bimetallic combination is capable to provide larger values of sensitivity compared with the single layer approach. The results derived from this study are guiding the experimental study of these structures.
An analytical model based on geometrical optics and multilayer transfer matrix method is applied to the surface plasmonic resonance supported by fibre taper structures in the context of optical sensing applications. Phase interrogation is considered in particular as a methodology to attain enhanced sensitivities, and the performance of the sensing heads as function of the metal clad and taper parameters is analyzed. General topics concerning the actual relevance of plasmonics are also presented, first in a global perspective and then when applied to sensing.
In this work we present experimental results that confirm the influence of an absorptive surrounding medium in the behavior
of conventional SPR fiber sensors. It can be observed that when a plasmon resonance coincides with a wavelength
absorbed by the medium, the depth of the dip is affected, as predicted in the simulations. The results are important from
the basic point of view, since the influence of absorptive medium in plasma waves has not been sufficiently studied up to
date and they can also be used as the basis for a new method of SPR-based refractometry selective to specific analytes
without the need of the addition of recognizing elements to the transducer.
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