Studying the depolarization rate of light emerging from a turbid medium holds promise for the non-invasive characterization of its single-scattering properties, with relevant application in the quality analysis of different specimens or for diagnostic purposes in the biomedical field, to name a few. However, irrespective of sample geometry, the dynamics of light depolarization takes place on a time scale of few ps, which is too fast for traditional detection methods. Here, we present experimental results on the time-domain evolution of the depolarization ratio of light that is diffusely reflected from a scattering medium, using linearly polarized fs pulses in an all-optical gating scheme. Time-resolved reflectance curves are recorded in the parallel and perpendicular polarization channels relative to the illumination beam, granting direct access to the depolarization rate. We demonstrate our experimental approach on a lipid emulsion, fitting the data with a polarized Monte Carlo simulation to retrieve the average particle size and scattering asymmetry factor using just two time-domain reflectance measurements in a semi-infinite geometry.
Structurally anisotropic materials are ubiquitous in several application fields, yet their accurate optical characterization remains challenging due our incomplete understanding of how anisotropic light transport properties arise from the microscopic scattering coefficients. In fact, even when the dynamics of light transport is directly measured, coarse simplifications are often introduced due to a lack of established theoretical models or numerical methods. Here, we apply a general Monte Carlo implementation capable of handling direction-dependent scattering to the analysis of light transport in a sample of polytetrafluoroethylene (PTFE) tape. Using only a set of transient transmittance intensity profiles, the analysis retrieves the tensor components of the diffusive rates and the scattering coefficients along all three directions, in excellent agreement with Monte Carlo simulations.
KEYWORDS: Raman spectroscopy, Polydimethylsiloxane, Liquids, Surface enhanced Raman spectroscopy, 3D printing, Signal detection, Optical fibers, Lab on a chip, Gold
Optical detection techniques have been extensively implemented for liquid biosensing and, among all, surface enhanced Raman spectroscopy (SERS) constitutes the one of the most promising analytical method as alternative to current traditional bioassays. With the attempt to develop point-of-impact diagnostic devices, in the present study, advanced and standard manufacturing processes were successfully combined with nanoparticles (NPs) engineering for the development of multifunctional lab-on-chips (LoCs) that integrate SERS sensors for liquid optical probing. As a matter of fact, LoCs allow to handle easily micro- to nanoliters volumes of samples as well as to perform multifunctional analyses on the same restricted volumes while avoiding cross-contaminations. Furthermore, due to the exploitation of 3D printing process, the LoCs design can be rapidly prototyped to highly integrate networks of channels and detection chambers of varied size and shape smartly arranged with respect to the Raman set-up in order to optimize signal delivery and collection. Within the detection chambers, SERS functionality is achieved by the selective interaction of the target analytes with gold NPs with embedded optical fibers positioned at different excitation and collection angles. The resulting SERS-fluidic devices, characterized by different detection configurations, represent highly versatile SERS-fluidic platforms providing high repeatability, high sensitivity and speed of analysis, possibly revolutionizing liquid biopsy by making it costless, on-chip, handy, and easy to use.
Liquid biopsies represent a minimally invasive tool for the precocious diagnosis of widespread diseases as well as for routinely patients monitoring by tracking selective biomarkers. Optical detection techniques based on surface enhanced Raman spectroscopy (SERS) are capable of providing information on the molecular content of analyzed samples thus representing one of the most promising analytical method in clinical research, as alternative to traditional bioassays. With the attempt to realize point-of-impact diagnostic devices, in the present study 3D printing and soft-lithography processes were combined with plasmonic nanoparticles (NPs) synthesis for the development of multifunctional lab-onchips (LOCs) integrating SERS sensors for liquid probing. As a matter of fact, LOCs enable to easily handle small volumes of samples as well as to perform multifunctional analyses. This is crucial for pathologies whose diagnosis relies on the ratio of more than one biomarker. To this end, being based on a 3D printing process, the overall design of the devices was rapidly prototyped to integrate channels and detection chambers aligned with optical fibers and portable Raman probes for signal delivering and collection. SERS functionality was achieved by immobilization of gold NPs whose chemistry was modified to enhance NPs deposition and stability. Finally, we are exploring direct laser writing for the integration of mechanical and optical microcomponents needed for liquids control and signal delivering and collection, respectively. The final devices collecting multiple functions and detection configurations will provide high sensitivity, speed of analysis, low sample and reagent consumption, measurement automation and standardization on a highly integrated dynamic platform that will revolutionize liquid biopsy making it costless, on-chip, handy and easy to use.
Manipulating objects at the micro and nano scale is still an open fascinating challenge that scientists are addressing by proposing different approaches to obtain machines with basic or complex functions. Combining shape changing polymers that differently respond to optical stimuli on the basis of the molecular alignment, together with 3D structuration at the microscale (with nanometric features), we demonstrated synthetic microrobots entirely powered by light. The arbitrary design allowed to mimic diverse animal and even humanoid tasks as walking, grabbing or manipulating objects, even overcoming natural limitations present at such small scale. Liquid crystalline networks offer the possibility to perform different movements depending on their molecular alignment and, controlling by light their elastic deformation, wireless activation of micro-machines was obtained. We report here how tuning intrinsic parameters, as the lithographic ones, and an external setting as the actuation power, it is possible to induce diverse deformations and time responses. Such results can be exploited to tailor the working mechanism and actuation speed of different micro robots. Engineering a proper structural design and combining different time responding materials would generate not reciprocal motion, basic and necessary property to achieve swimming at the microscale. This first technical demonstration paves the way to a micro swimmer fueled by light.
Among the natural white colored photonics structures, a bio-system has become of great interest in the field of disordered optical media: the scale of the white beetle Chyphochilus. Despite its low thickness, on average 7 μm, and low refractive index, this beetle exhibits extreme high brightness and unique whiteness. These properties arise from the interaction of light with a complex network of chitin nano filaments embedded in the interior of the scales. As it’s been recently claimed, this could be a consequence of the peculiar morphology of the filaments network that, by means of high filling fraction (0.61) and structural anisotropy, optimizes the multiple scattering of light. We therefore performed a numerical analysis on the structural properties of the chitin network in order to understand their role in the enhancement of the scale scattering intensity. Modeling the filaments as interconnected rod shaped scattering centers, we numerically generated the spatial coordinates of the network components. Controlling the quantities that are claimed to play a fundamental role in the brightness and whiteness properties of the investigated system (filling fraction and average rods orientation, i.e. the anisotropy of the ensemble of scattering centers), we obtained a set of customized random networks. FDTD simulations of light transport have been performed on these systems, observing high reflectance for all the visible frequencies and proving the implemented algorithm to numerically generate the structures is suitable to investigate the dependence of reflectance by anisotropy.
In this contribution, we will report on a new adventure in the field of photonics, combining the optical control of photonic materials with that of true micro meter scale robotics. We will show how one can create complex photonic structures using polymers that respond to optical stimuli, and how this technology can be used to create moving elements, photonic skin, and even complete micro meter size robots that can walk and swim. Using light as the only source of energy. The materials that we have developed to that end can also be used to realize tunable photonic components that respond to light and adapt their photonic response on the basis of the illumination conditions.
We will review recent work on nano and micro structured polymers, in particular liquid crystalline elastomers, such as to realise small, complex, structures that deform when exposed to light. The goal is to create artificial arms, and legs, walking and swimming artificial micro creatures, and photonic components, like micro resonators that adapt to their environment.
In this paper we investigate the potentials of liquid crystalline elastomer microstructures for the realization of optically tunable photonic microstructures. While certain limitations regarding the compromise between feature size and structure warping have been observed, it turns out that the simultaneous presence of a refractive index tuning effect and of a shape tuning effect intrinsic to the LCE material can be harnessed to design tunable photonic devices with unique behavior.
Liquid Crystalline Elastomers (LCEs) are very promising smart materials that can be made sensitive to different external stimuli, such as heat, pH, humidity and light, by changing their chemical composition. In this paper we report the implementation of a nematically aligned LCE actuator able to undergo large light-induced deformations. We prove that this property is still present even when the actuator is submerged in fresh water. Thanks to the presence of azo-dye moieties, capable of going through a reversible trans-cis photo-isomerization, and by applying light with two different wavelengths we managed to control the bending of such actuator in the liquid environment. The reported results represent the first step towards swimming microdevices powered by light.
While animals have access to sugars as energy source, this option is generally not available to artificial machines and robots. Energy delivery is thus the bottleneck for creating independent robots and machines, especially on micro- and nano- meter length scales. We have found a way to produce polymeric nano-structures with local control over the molecular alignment, which allowed us to solve the above issue. By using a combination of polymers, of which part is optically sensitive, we can create complex functional structures with nanometer accuracy, responsive to light. In particular, this allowed us to realize a structure that can move autonomously over surfaces (it can “walk”) using the environmental light as its energy source. The robot is only 60 μm in total length, thereby smaller than any known terrestrial walking species, and it is capable of random, directional walking and rotating on different dry surfaces.
Continuing our ongoing investigations of random lasing, we used the Monte Carlo method to simulate random walks of photons within a multiply scattering medium. By initially applying this technique to calculate pulse-stretching in a passive disordered medium, we elucidated its agreement with analytical diffusion theory. Thereafter, we introduced conditions of optical amplification, and reproduced the experimentally observed spectral features like spectral narrowing, intensity enhancement, bichromaticity, mode competition, etc., in a random laser. After investigating diffusive and sub-diffusive regimes of scattering, we formulated our results in terms of a gain subvolume, the functioning of which depends upon local gain conditions. We then used a modified approach of this technique to study ultranarrow random lasing modes, and successfully reproduced these modes observed in a random laser. Based on our simulations, we were able to explain the origins of ultra-narrow lasing modes as excessively amplified extended modes.
People working with optics and lasers usually try to avoid dust on their equipment as much as possible. Dust
particles scatter light randomly in all directions and this is often detrimental to the performance of optical devices
and lasers. In this articles we will see that it is possible to turn this situation upside down and actually make use of
multiple light scattering to study interesting physical phenomena. In particular, we will discuss optical Lévy flights
and super diffusion, and various interference effects like weak and strong localization of light waves.
In recent years there has been an enormous development in the understanding of light transport in disordered materials, in particular in systems with optical gain. A phenomenon termed random lasing can be observed, which can be used to realize a random laser. We will describe the behaviour of these fascinating new light sources that resemble in various respects a regular laser while the lasing process is based on light diffusion.
We report on optical analogues of well-known electronic phenomena such as Bloch oscillations and electrical Zener breakdown. We describe and detail the experimental observation of Bloch oscillations and resonant Zener tunneling of light waves in static and time-resolved transmission measurements performed on optical superlattices. Optical superlattices are formed by one-dimensional photonic structures (coupled microcavities) of high optical quality and are specifically designed to represent a tilted photonic crystal band. In the tilted bands condition the miniband of degenerate cavity modes turns into an optical Wannier-Stark ladder (WSL). This allows an ultrashort light pulse to bounce between the tilted photonic band edges and hence to perform Bloch oscillations, the period of which is defined by the frequency separation of the WSL states. When the superlattice is designed such that two minibands are formed within the stop band, at a critical value of the tilt of photonic bands the two WSLs couple within the superlattice structure. This results in a formation of a resonant tunneling channel in the minigap region, where the light transmission boosts from 0.3% to over 43%. The latter case describes the resonant Zener tunneling of light waves.
We report numerical and experimental studies on multiple
scattering media with gain. We describe Monte Carlo simulations
that model the behavior of such a system through a three
dimensional random walk of photons in a disordered medium with
amplification. Two experimentally observed phenomena, viz.
temperature tunable random lasing and ultra-narrow lasing modes,
are analyzed using the model. We compare the results of our model
with previous experimental results on a disordered dielectric of
which the scattering strength could be tuned by changing the
external temperature. The agreement between the numerical and
experimental results enables us to predict the spectral features
of the emission from the tunable random laser under various
conditions. Results obtained from new experimental data are
consistent with the predictions of the simulations. The model also
explains the observation of ultra-narrow emission modes in random
lasers without requiring optical cavities. The introduction of
exponential gain in a multiple light scattering process strongly
increases the importance of very long light paths. Such long paths
are often neglected in passive disordered materials but we show
that they can dominate the emission spectrum from an amplifying
disordered system.
The dielectric constants ε′ and ε″ of a series of "liquid crystal (LC) - polymer" composites in the frequency region 10-3-105 Hz are measured. The composites are prepared by a photo-separation method with the variation of polymer content from 0 to 50 weight %. For all samples, a relaxation effect with a low relaxation frequency (10-1-20 Hz) is observed. This effect is assigned to dielectric relaxation in the near-electrode layers. The experimentally observed increase of the relaxation frequency and the electric conductivity with polymer concentration is explained by incomplete phase separation occurred in "liquid crystal - pre-polymer" mixture under irradiation.
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