A polaron approach to photorefractivity in Fe : LiNbO3

The thermally activated, incoherent hopping of small electron polarons generated by continuous illumination in iron-doped lithium niobate is simulated by a Marcus-Holstein model for which all the input parameters are known from literature. The results of the calculations are compared with a comprehensive set of data obtained from photorefractive, photogalvanic and photoconductive measurements under green light excitation on samples with different doping levels and stoichiometries in the temperature range between and room temperature. We show that the temperature and composition dependence of the photorefractive observables can be interpreted by a change in the abundance of the different hop types that a polaron performs before being captured by a deep Fe trap. Moreover, by a comparison between experimental and numerical data we obtain new insights on the initial photo-excitation part of the photorefractive process. In particular all results are consistent if a single value of the photogalvanic length is assumed for all the samples and all the temperatures. The photo-generation efficiency under green light excitation (somewhere denoted as quantum efficiency) is also estimated. It appears to decrease from 10%?15% at room temperature to about 5% at 150 K. This behavior is qualitatively interpreted in terms of a temperature-dependent re-trapping probability of the light-emitted particles from the initial Fe donor center

Colorectal Cancer Stage at Diagnosis Before vs During the COVID-19 Pandemic in Italy

IMPORTANCE Delays in screening programs and the reluctance of patients to seek medical attention because of the outbreak of SARS-CoV-2 could be associated with the risk of more advanced colorectal cancers at diagnosis. OBJECTIVE To evaluate whether the SARS-CoV-2 pandemic was associated with more advanced oncologic stage and change in clinical presentation for patients with colorectal cancer. DESIGN, SETTING, AND PARTICIPANTS This retrospective, multicenter cohort study included all 17 938 adult patients who underwent surgery for colorectal cancer from March 1, 2020, to December 31, 2021 (pandemic period), and from January 1, 2018, to February 29, 2020 (prepandemic period), in 81 participating centers in Italy, including tertiary centers and community hospitals. Follow-up was 30 days from surgery. EXPOSURES Any type of surgical procedure for colorectal cancer, including explorative surgery, palliative procedures, and atypical or segmental resections. MAIN OUTCOMES AND MEASURES The primary outcome was advanced stage of colorectal cancer at diagnosis. Secondary outcomes were distant metastasis, T4 stage, aggressive biology (defined as cancer with at least 1 of the following characteristics: signet ring cells, mucinous tumor, budding, lymphovascular invasion, perineural invasion, and lymphangitis), stenotic lesion, emergency surgery, and palliative surgery. The independent association between the pandemic period and the outcomes was assessed using multivariate random-effects logistic regression, with hospital as the cluster variable. RESULTS A total of 17 938 patients (10 007 men [55.8%]; mean [SD] age, 70.6 [12.2] years) underwent surgery for colorectal cancer: 7796 (43.5%) during the pandemic period and 10 142 (56.5%) during the prepandemic period. Logistic regression indicated that the pandemic period was significantly associated with an increased rate of advanced-stage colorectal cancer (odds ratio [OR], 1.07; 95%CI, 1.01-1.13; P = .03), aggressive biology (OR, 1.32; 95%CI, 1.15-1.53; P < .001), and stenotic lesions (OR, 1.15; 95%CI, 1.01-1.31; P = .03). CONCLUSIONS AND RELEVANCE This cohort study suggests a significant association between the SARS-CoV-2 pandemic and the risk of a more advanced oncologic stage at diagnosis among patients undergoing surgery for colorectal cancer and might indicate a potential reduction of survival for these patients

Microscopic Insights in Photo-Induced Charge Transport in Fe:LiNbO3

The description of charge transport phenomena in polar oxide materials has up to now numerous unclear points, unlike for what happens for semiconductors and conductors where the band model explain exhaustively their properties. First studies on ionic insulators highlighted the necessity to introduce new concepts and a new transport theory, without which it would not be possible to explain convincingly the photo-electrical behaviour of these materials. One of these key concepts is the polaron, introduced the first time by L. D. Landau in 1933, to explain some new properties exhibited by alkali halide. A strong electron - phonon interaction characterizes these materials thanks to which the electron can induce a local distortion through Coulombian interaction. This mechanism produces a potential well localizing the electron. A polaron can be thought as a quasi-particle composed by a charge auto-localized and the relative lattice distortion that moves as a whole. Under particular conditions, it can move by thermally assisted hopping, diffuse in the material or be trapped by point defects having an attractive potential. Moreover, the polaron constitutes a localized state in the band gap, which can absorb the light releasing the trapped charge to higher energy levels. The coexistence of all these processes, under specific conditions of illumination and temperature, determine the light-induced charge transport phenomena, which are the object of this thesis. The material chosen for this research is lithium niobate (LiNbO3). Besides its technological interest, since it is largely applied for nonlinear optic and holography, it is a prototype system for polaron study, among similar polar oxides. Another advantage provided by this material is that its electrical transport properties can be studied in a convenient way via optical measurements. The purpose of this study is the creation of a predictive model describing transport properties of the material starting from its microscopic composition and the external experimental conditions to which it is exposed. We performed a set of experiments on a series of well-characterized samples and compared them with the results of numerical modelling. The main results are a quantitative estimate of some poorly known microscopic polaron parameters and, by consequence, the development of a quantitative numerical tool able to predict the behaviour of the material, in a wide range of temperature and compositions. Several new ideas concerning a semi-analytical modelling for this system were also developed and tested, together with some interesting concepts for future research, traditionally not applied in lithium niobate community, such as the physics of anomalous diffusion of polarons among a disordered defect network.La descrizione dei fenomeni del trasporto di carica in materiali ossidi polari ha ancora numerosi punti oscuri, a differenza di quanto accade per i semiconduttori e conduttori, per i quali il modello a bande spiega in modo esaustivo le loro proprietà. I primi studi negli isolanti ionici misero in evidenza la necessità di introdurre nuovi concetti e una nuova teoria di trasporto, senza la quale non sarebbe possibile spiegare in modo convincente il comportamento foto-elettrico di questi materiali. Uno di questi concetti chiave è quello di polarone, introdotto per la prima volta da L. D. Landau nel 1933, per spiegare alcune nuove proprietà manifestate dagli alkali halidi. Una forte interazione elettrone-fonone caratterizza questi materiali, grazie alla quale l’elettrone induce una distorsione locale del reticolo tramite interazione Coulombiana. Questo meccanismo produce une potenziale che localizza l’elettrone. Un polarone può essere pensato come una quasi-particella composta da una carica auto-intrappolata e dalla relativa distorsione reticolare che si muovono come un tutt’uno. Sotto particolari condizioni il polarone può muoversi per hopping termicamente assistito, diffondere nel materiale o essere intrappolato da difetti aventi un potenziale attrattivo. Inoltre, il polarone costituisce uno stato localizzato nel band gap, che può assorbire la luce rilasciando la carica intrappolata. La coesistenza di tutti questi processi, per specifiche condizioni di illuminazione e temperatura, determina i fenomeni di trasporto fotoindotto, oggetto di questa tesi. Il materiale scelto per questa ricerca è il niobato di litio (LiNbO3). Oltre al suo grande interesse tecnologico, dato che esso è largamente utilizzato in ottica nonlineare e olografia, risulta un sistema prototipo per lo studio dei polaroni tra tutti i materiali ossidi polari. Un altro vantaggio risiede nel fatto che le proprietà elettriche del materiale possono essere studiate in modo conveniente attraverso misure ottiche. L’obiettivo di questo studio è la creazione di un modello che descriva le proprietà di trasporto del materiale, partendo dalla sua composizione microscopica e dalle condizioni sperimentali alle quali è esposto. Per fare questo sono stati condotti un set di esperimenti in campioni creati ad hoc e confrontati con i risultati di simulazioni numeriche. I risultati principali sono la stima quantitativa di alcuni parametri microscopici polaronici poco noti e, di conseguenza, lo sviluppo di uno strumento numerico capace di predire il comportamento del materiale in un grande range di temperature e composizioni. Nuove idee riguardanti un modello semi-analitico sono state anche discusse e testate, assieme a nuovi e interessanti concetti tradizionalmente non applicati al niobato di litio, come quello della diffusione anomala dei polaroni in un network di difetti disordinati

Experimental verification of quantum-optical analogies by reconfigurable waveguides structures in SBN and Fe:LN

Similarities between optic and quantum mechanics have been highlighted in many occasions. In this work, two types of quantum effect are taken into account: the two state STIRAP and the diffusions enhanced in quasi crystal. They were studied using two reconfigurable techniques: the lateral illumination technique and the photoinduced direct writing. In particular, the first quantum effect was optically demonstrated in LMOPS laboratories while the second one, in Padova, the setup permitting further the study of the quantum effect is developped

Polaron Trapping and Migration in Iron-Doped Lithium Niobate

International audiencePhotoinduced charge transport in lithium niobate for standard illumination, composition and temperature conditions occurs by means of small polaron hopping either on regular or defective lattice sites. Starting from Marcus-Holstein’s theory for polaron hopping frequency we draw a quantitative picture illustrating two underlying microscopic mechanisms besides experimental observations, namely direct trapping and migration-accelerated polaron trapping transport. Our observations will be referred to the typical outcomes of transient light induced absorption measurements, where the kinetics of a polaron population generated by a laser pulse then decaying towards deep trap sites is measured. Our results help to rationalize the observations beyond simple phenomenological models and may serve as a guide to design the material according to the desired specifications

Small Polaron Hopping in Fe:LiNbO3 as a Function of Temperature and Composition

Small-polaron hopping involved in charge transport in Fe-doped congruent lithium niobate is investigated as a function of temperature and composition by means of light-induced transient absorption spectroscopy. The relaxation dynamics of the light-induced polaron population is characterized by individual activation energies within different temperature ranges. A numerical investigation carried out by Monte Carlo simulations reveals that these findings may be understood in terms of the varying abundance of the different types of hops that the polarons may perform among regular or defective lattice sites. The role of the temperature and of the sample composition on the distribution of the different hop types is thus explored for a wide range of parameters, allowing one to preview the charge transport properties for a given set of experimental conditions

NIR-to-NIR Imaging: Extended Excitation Up to 2.2 μm Using Harmonic Nanoparticles with a Tunable hIGh EneRgy (TIGER) Widefield Microscope

Near-infrared (NIR) marker-based imaging is of growing importance for deep tissue imaging and is based on a considerable reduction of optical losses at large wavelengths. We aim to extend the range of NIR excitation wavelengths particularly to values beyond 1.6 μm in order to profit from the low loss biological windows NIR-III and NIR-IV. We address this task by studying NIR-excitation to NIR-emission conversion and imaging in the range of 1200 up to 2400 nm at the example of harmonic Mg-doped lithium niobate nanoparticles (i) using a nonlinear diffuse femtosecond-pulse reflectometer and (ii) a Tunable hIGh EneRgy (TIGER) widefield microscope. We successfully demonstrate the existence of appropriate excitation/emission configurations in this spectral region taking harmonic generation into account. Moreover, NIR-imaging using the most striking configurations NIR-III to NIR-I, based on second harmonic generation (SHG), and NIR-IV to NIR-I, based on third harmonic generation (THG), is demonstrated with excitation wavelengths from 1.6–1.8 μm and from 2.1–2.2 μm, respectively. The advantages of the approach and the potential to additionally extend the emission range up to 2400 nm, making use of sum frequency generation (SFG) and difference frequency generation (DFG), are discussed

NIR-to-NIR Imaging: Extended Excitation Up to 2.2 μm Using Harmonic Nanoparticles with a Tunable hIGh EneRgy (TIGER) Widefield Microscope

Near-infrared (NIR) marker-based imaging is of growing importance for deep tissue imaging and is based on a considerable reduction of optical losses at large wavelengths. We aim to extend the range of NIR excitation wavelengths particularly to values beyond 1.6 μm in order to profit from the low loss biological windows NIR-III and NIR-IV. We address this task by studying NIR-excitation to NIR-emission conversion and imaging in the range of 1200 up to 2400 nm at the example of harmonic Mg-doped lithium niobate nanoparticles (i) using a nonlinear diffuse femtosecond-pulse reflectometer and (ii) a Tunable hIGh EneRgy (TIGER) widefield microscope. We successfully demonstrate the existence of appropriate excitation/emission configurations in this spectral region taking harmonic generation into account. Moreover, NIR-imaging using the most striking configurations NIR-III to NIR-I, based on second harmonic generation (SHG), and NIR-IV to NIR-I, based on third harmonic generation (THG), is demonstrated with excitation wavelengths from 1.6–1.8 μm and from 2.1–2.2 μm, respectively. The advantages of the approach and the potential to additionally extend the emission range up to 2400 nm, making use of sum frequency generation (SFG) and difference frequency generation (DFG), are discussed

Equivalence classes of Fibonacci lattices and their similarity properties

We investigate, theoretically and experimentally, the properties of Fibonacci lattices with arbitrary spacings. Different from periodic structures, the reciprocal lattice and the dynamical properties of Fibonacci lattices depend strongly on the lengths of their lattice parameters, even if the sequence of long and short segment, the Fibonacci string, is the same. In this work we show that by exploiting a self-similarity property of Fibonacci strings under a suitable composition rule, it is possible to define equivalence classes of Fibonacci lattices. We show that the diffraction patterns generated by Fibonacci lattices belonging to the same equivalence class can be rescaled to a common pattern of strong diffraction peaks thus giving to this classification a precise meaning. Furthermore we show that, through the gap labeling theorem, gaps in the energy spectra of Fibonacci crystals belonging to the same class can be labeled by the same momenta (up to a proper rescaling) and that the larger gaps correspond to the strong peaks of the diffraction spectra. This observation makes the definition of equivalence classes meaningful also for the spectral and therefore dynamical and thermodynamical properties of quasicrystals. Our results apply to the more general class of quasiperiodic lattices for which similarity under a suitable deflation rule is in order