Photo-thermal effects in gold nanoparticles dispersed in thermotropic nematic liquid crystals.

The last few years have seen a growing interest in the ability of metallic nanoparticles (MNPs) to control temperature at the nanoscale. Under a suitable optical radiation, MNPs feature an enhanced light absorption/scattering, thus turning into an ideal nano-source of heat, remotely controllable by means of light. In this framework, we report our recent efforts on modeling and characterizing the photo-thermal effects observed in gold nanoparticles (GNPs) dispersed in thermotropic Liquid Crystals (LCs). Photo-induced temperature variations in GNPs dispersed in Nematic LCs (NLCs) have been studied by implementing an ad hoc theoretical model based on the thermal heating equation applied to an anisotropic medium. Theoretical predictions have been verified by performing photo-heating experiments on a sample containing a small percentage of GNPs dispersed in NLCs. Both theory and experiments represent an important achievement in understanding the physics of heat transfer at the nanoscale, with applications ranging from photonics to nanomedicine.

Reversible strong coupling in silver nanoparticle arrays using photochromic molecules.

In this Letter, we demonstrate a reversible strong coupling regime between a dipolar surface plasmon resonance and a molecular excited state. This reversible state is experimentally observed on silver nanoparticle arrays embedded in a polymer film containing photochromic molecules. Extinction measurements reveal a clear Rabi splitting of 294 meV, corresponding to ~13% of the molecular transition energy. We derived an analytical model to confirm our observations, and we emphasize the importance of spectrally matching the polymer absorption with the plasmonic resonance to observe coupled states. Finally, the reversibility of this coupling is illustrated by cycling the photochromic molecules between their two isomeric forms.

Dynamic Multi-Mode Mie Model for Gain-Assisted Metal Nano-Spheres.

Coupling externally pumped gain materials with plasmonic spherical particles, even in the simplest case of a single spherical nanoparticle in a uniform gain medium, generates an incredibly rich variety of electrodynamic phenomena. The appropriate theoretical description of these systems is dictated by the quantity of the included gain and the size of the nano-particle. On the one hand, when the gain level is below the threshold separating the absorption and the emission regime, a steady-state approach is a rather adequate depiction, yet a time dynamic approach becomes fundamental when this threshold is exceeded. On the other hand, while a quasi-static approximation can be used to model nanoparticles when they are much smaller than the exciting wavelength, a more complete scattering theory is necessary to discuss larger nanoparticles. In this paper, we describe a novel method including a time-dynamical approach to the Mie scattering theory, which is able to account for all the most enticing aspects of the problem without any limitation in the particle's size. Ultimately, although the presented approach does not fully describe the emission regime yet, it does allow us to predict the transient states preceding emission and represents an essential step forward in the direction of a model able to adequately describe the full electromagnetic phenomenology of these systems.

Current practice on the use of prophylactic drain after gastrectomy in Italy: the Abdominal Drain in Gastrectomy (ADiGe) survey.

Evidence against the use of prophylactic drain after gastrectomy are increasing and ERAS guidelines suggest the benefit of drain avoidance. Nevertheless, it is unclear whether this practice is still widespread. We conducted a survey among Italian surgeons through the Italian Gastric Cancer Research Group and the Polispecialistic Society of Young Surgeons, aiming to understand the current use of prophylactic drain. A 28-item questionnaire-based survey was developed to analyze the current practice and the individual opinion about the use of prophylactic drain after gastrectomy. Groups based on age, experience and unit volume were separately analyzed. Response of 104 surgeons from 73 surgical units were collected. A standardized ERAS protocol for gastrectomy was applied by 42% of the respondents. Most of the surgeons, regardless of age, experience, or unit volume, declared to routinely place one or more drain after gastrectomy. Only 2 (1.9%) and 7 surgeons (6.7%) belonging to high volume units, do not routinely place drains after total and subtotal gastrectomy, respectively. More than 60% of the participants remove the drain on postoperative day 4-6 after performing an assessment of the anastomosis integrity. Interestingly, less than half of the surgeons believe that drain is the main tool for leak management, and this percentage further drops among younger surgeons. On the other hand, drain's role seems to be more defined for duodenal stump leak treatment, with almost 50% of the surgeons recognizing its importance. Routine use of prophylactic drain after gastrectomy is still a widespread practice even if younger surgeons are more persuaded that it could not be advantageous.

Effect of Pyloroplasty on Gastric Conduit Emptying and Patients' Quality of Life After Ivor Lewis Esophagectomy.

Background: Delayed gastric emptying (DGE) is a common complication after esophagectomy with gastric tube reconstruction. It is still unclear whether a pyloric drainage procedure might reduce the risk of DGE. Methods: We identified in our database all patients subjected to Ivor Lewis esophagectomy after neoadjuvant chemoradiotherapy in the period 2000-2012. In the period 2000-2009, we performed a routine pyloroplasty (pyloroplasty group, PP group, 15 patients), after 2009 we did not perform any type of pyloric drainage procedure (nonpyloroplasty group, NPP group, 11 patients). We compared the groups with subjective questionnaires to assess the perceived quality of life (QoL) (QLQ-C30 and OES-18) and with objective test to study the gastric tube emptying (timed barium swallow test, scintigraphy, 24 hours' pH-metry). Results: No difference was observed in questionnaires QLC-C30 and OES-18 scores: 73% of patients in PP group and 63% in NPP group scored their overall QoL as good to excellent (QLC-C30). We did not report difference in timed barium swallow test results and in scintigraphy results. Twenty-four-hour pH-metry results showed in PP group a nonsignificant higher number of acid reflux episodes (NPP group 23.2 ± 9.5 versus PP group 41.3 ± 10.7, P = .29) and a longer time with pH <4 (NPP group 0.89% ± 1.6% versus PP group 3.1% ± 2.1%, P = .24). Conclusions: In our series, pyloroplasty was not associated with improved long-term QoL nor with better gastric conduit emptying. Further studies are needed to confirm these findings.

Effectiveness of endoscopic vacuum therapy as rescue treatment in refractory leaks after gastro-esophageal surgery.

The treatment of leak after esophageal and gastric surgery is a major challenge. Over the last few years, endoscopic vacuum therapy (E-VAC) has gained popularity in the management of this life-threatening complication. We reported our initial experience on E-VAC therapy as rescue treatment in refractory anastomotic leak and perforation after gastro-esophageal surgery. From September 2017 to December 2019, a total of 8 E-VAC therapies were placed as secondary treatment in 7 patients. Six for anastomotic leak (3 cervical, 1 thoracic, 2 abdominal) and 1 for perforation of the gastric conduit. In 6 cases, E-VAC was placed intracavitary; while in the remaining 2, the sponge was positioned intraluminal (one patient was treated with both approaches). A total of 60 sponges were used in the whole cohort. The median number of sponge insertions was 10 (range: 5-14) with a median treatment duration of 41 days (range: 19-49). A complete healing was achieved in 4 intracavitary (67%) and in 1 intraluminal (50%) E-VAC. We observed only one E-VAC-related complication: a bleeding successfully managed endoscopically. E-VAC therapy seems to be a safe and effective tool in the management of leaks and perforations after upper GI surgery, although with longer healing time when it is used as secondary treatment.

Should we still use prophylactic drain in gastrectomy for cancer? A systematic review and meta-analysis.

Prophylactic drain in gastrectomy for cancer is still widely used, although some evidence has disputed this practice and spreading enhanced recovery protocol has been pushing towards surgical simplification. This study aimed at assessing the impact of drain placement on important clinical outcomes, evaluating the results of randomised controlled trials (RCTs), or cohort studies whenever information provided by the former was scarce. PubMed, PMC, Cochrane Library, CNKI and Wanfang databases were searched from January 1990 to February 2019, both for RCTs and cohort studies comparing use or avoidance of prophylactic drain in gastric cancer patients undergoing gastrectomy. All RCTs and cohort studies were rated according to Jadad score and Newcastle-Ottawa-Scale, respectively. Meta-analysis was separately performed on RCTs and cohort studies. The following clinical outcomes were considered: anastomotic leak, reoperation rate, additional drain procedure, length of stay, postoperative morbidity, postoperative mortality, readmission rate and drain related complications. Overall, 3 RCTs (330 patients) and 7 cohort studies (2897 patients) were included. Seven studies came from Eastern Countries. Meta-analysis on RCTs evidenced that drain avoidance halves overall morbidity (RR = 0.47, 95%CI 0.26-0.86, p = 0.014) and slightly reduces length of stay (SMD -0.24, 95%CI -0.51-0.03, p = 0.083). Only one postoperative death occurred in the drain group. The other outcomes were either not reported or reported just by one RCT each. Meta-analysis on cohort studies, despite higher statistical power, did not highlight any significant difference. This meta-analysis showed that prophylactic drain avoidance can reduce morbidity and length of stay, while not significantly affecting other major surgical outcomes.

Designs for electromagnetic cloaking a three-dimensional arbitrary shaped star-domain.

The design of electromagnetic cloaks based on the coordinate transformation requires a suitable geometrical definition of both the internal and the external surfaces of the cloak itself. We describe a straightforward method to design the electromagnetic cloak of a 3d-generic star domain whose surface is defined just by a set of points distributed over it. We also present numerical simulation for the ray tracing of a light beam inside the material calculated for an asymmetric three-dimensional example.

Plasmon-mediated discrete diffraction behaviour of an array of responsive waveguides.

We investigate the discrete diffraction phenomenon in a Polymer-Liquid Crystal-Polymer Slices (POLICRYPS) overlaying a random distribution of gold nanoparticles (AuNPs, plasmonic elements). We study the propagation of a CW green laser beam through the waveguide structure as a function of beam polarization, laser intensity and sample temperature. It turns out that the plasmonic field created at the interface between AuNPs and POLICRYPS waveguides enables and stabilizes the optical field propagation within the responsive nematic liquid crystal channels. The active role of the liquid crystal is pointed out by a polarization, temperature and beam divergence experimental analysis and evidenced by a peculiar trumpet-shaped discrete diffraction pattern. Theoretical simulations confirm that the observed optical behavior is governed by the interaction of the nematic liquid crystal with optical and plasmonic fields.

Polarization-dependent strong coupling between silver nanorods and photochromic molecules.

Active plasmonics is a key focus for the development of advanced plasmonic applications. By selectively exciting the localized surface plasmon resonance sustained by the short or the long axis of silver nanorods, we demonstrate a polarization-dependent strong coupling between the plasmonic resonance and the excited state of photochromic molecules. By varying the width and the length of the nanorods independently, a clear Rabi splitting appears in the dispersion curves of both resonators.

Resonant Gain Singularities in 1D and 3D Metal/Dielectric Multilayered Nanostructures.

We present a detailed study on the resonant gain (RG) phenomena occurring in two nanostructures, in which the presence of dielectric singularities is used to reach a huge amplification of the emitted photons resonantly interacting with the system. The presence of gain molecules in the considered nanoresonator systems makes it possible to obtain optical features that are able to unlock several applications. Two noticeable cases have been investigated: a 1D nanoresonator based on hyperbolic metamaterials and a 3D metal/dielectric spherical multishell. The former has been designed in the framework of the effective medium theory, in order to behave as an epsilon-near-zero-and-pole metamaterial, showing extraordinary light confinement and collimation. Such a peculiarity represents the key to lead to a RG behavior, a condition in which the system is demonstrated to behave as a self-amplifying perfect lens. Very high enhancement and spectral sharpness of 1 nm of the emitted light are demonstrated by means of a transfer matrix method simulation. The latter system consists of a metal/doped-dielectric multishell. A dedicated theoretical approach has been set up to finely engineer its doubly tunable resonant nature. The RG condition has been demonstrated also in this case. Finite element method-based simulations, together with an analytical model, clarify the electric field distribution inside the multishell and suggest the opportunity to use this device as a self-enhanced loss compensated multishell, being a favorable scenario for low-threshold SPASER action. Counterintuitively, exceeding the resonant gain amount of molecules in both systems causes a significant drop in the amplitude of the resonance.

Multipolar, time-dynamical model for the loss compensation and lasing of a spherical plasmonic nanoparticle spaser immersed in an active gain medium.

The plasmonic response of a metal nanoparticle in the presence of surrounding gain elements is studied, using a space and time-dependent model, which integrates a quantum formalism to describe the gain and a classical treatment for the metal. Our model fully takes into account the influence of the system geometry (nanosphere) and offers for the first time, the possibility to describe the temporal evolution of the fields and the coupling among the multipolar modes of the particle. We calculate the lasing threshold value for all multipoles of the spaser, and demonstrate that the dipolar one is lowest. The onset of the lasing instability, in the linear regime, is then studied both with and without external field forcing. We also study the behaviour of the system below the lasing threshold, with the external field, demonstrating the existence of an amplification regime where the nanoparticle's plasmon is strongly enhanced as the threshold is approached. Finally, a qualitative discussion is provided on later, non-linear stages of the dynamics and the approach to the steady-state of the spaser; in particular, it is shown that, for the considered geometry, the spasing is necessarily multi-modal and multipolar modes are always activated.

Blue-shifted random-laser-mode selection in gain-assisted anisotropic complex fluids.

Random laser action in organic materials is of great topical interest that is fueled by the rapid development of active compounds and new dye molecules. We propose a pure-diffusive model to describe the strong connection established between a dye-host interaction and the scattering when considering an anisotropic complex fluid. The model considers multiple scattering induced by dielectric tensor fluctuations and a suitable quantistic description for light amplification in order to explain the generation of the narrow-band blue-shifted lasing mode experimentally observed in such systems. We also find that the introduction of a strong intermolecular force field provides the condition to enhance diffusive processes. The agreement between experimental observations and simulations advances the understanding of the physical mechanism behind mode selection in these systems.

Characterization of an active control system for holographic setup stabilization.

We report on the realization and characterization of an active control system of the optical holographic setup used for fabrication of holographic gratings in liquid-crystalline composite materials. The system exploits a reference diffraction grating and a piezomirror in closed-loop feedback. The piezoelectric mirror exhibits a hysteresis that depends not only on the applied voltage, but also on the history of the mirror motion. In an open-loop configuration, the hysteresis can be reduced by adjusting the delay time between the application of two different control voltage values; in a closed-loop operation, it is possible to eliminate the residual hysteresis. By testing the system in different conditions, it has been shown that residual fluctuations are comparable to the resolution of the piezomirror operation.

In situ optical control and stabilization of the curing process of holographic gratings with a nematic film-polymer-slice sequence structure.

We report on the realization of what we believe to be a new holographic setup for the fabrication of polymer liquid-crystal polymer-slice diffraction gratings, which utilizes an optical-feedback-driven nanopositioning technique. We have increased the stability of the interference pattern by means of a simple piezomirror used in a feedback configuration to keep constant the phase of the interferometer. The feedback system is driven by a proportional, integral, derivative control software, and the stability degree is controlled by the reference signal coming from a standard test grating. A preliminary experimental characterization indicates that good control and stabilization of parasitic fluctuations of the interference pattern are obtained.

Observation of two-wave coupling during the formation of POLICRYPS diffraction gratings.

We report on the experimental observation of two-wave mixing that occurs inside a sample between the beams used for the fabrication of polymer-liquid-crystal-polymer slices (POLICRYPS) diffraction gratings. The effect depends on the phase shift between the curing interference pattern and the grating being cured. This shift can be mechanically induced by accidental vibrations of the experimental setup; thus a high setup stability is needed. We devised a mechanism that enables us to control setup vibrations in situ and used it to monitor the experiment. When the mechanically induced shift was significantly small, wave mixing was observed only if the initial intensities of the two curing beams were different from each other.

Radiative intermittent events during Fermi's stochastic acceleration.

We investigate the dynamics of a realization of Fermi's relativistic acceleration mechanism that is a charged test particle oscillating between two reflecting plates that move stochastically. By allowing the charge to radiate energy during each collision, we find that the main features of the system are (1) due to the radiation drag the energy gained by the particle is bounded, and (2) the radiated energy represents a typical realization of an on-off intermittent process, due to numerous continuous encounters with a very small emission, interrupted by short and intense bursts of radiation. This intermittent radiative process exhibits non-Gaussian statistical features.

Development of a new kind of switchable holographic grating made of liquid-crystal films separated by slices of polymeric material.

We present a new kind of UV-cured holographic grating that consists of polymer slices alternated with pure nematic films. By preventing the appearance of the nematic phase during the curing process, it is possible to avoid the formation of liquid-crystal droplets and obtain a sharp and uniform morphology, which reduces scattering losses and increases diffraction efficiency.